helisol heat transfer fluids

1.

HELISOL® –

HEAT TRANSFER FLUIDS

Safety Guidance

CREATING TOMORROW‘S SOLUTIONS

HELISOL Safety Guidance of 25 Valid from: 2021-04-08 "Print is uncontrolled"

Content

1. ................................................................................................................................................................. 1

1 Preface ................................................................................................................................................... 4

2 Scope ..................................................................................................................................................... 4

3 Normative References ......................................................................................................................... 4

4 Terms and Definitions........................................................................................................................... 5

5 Differences between the Physical and Chemical Properties of Organic Heat Transfer Fluids and Silicone-based Fluids (Polydimethylsiloxanes) ................................................................................ 6

5.1 Organic HTF ................................................................................................................................. 6

5.2 Silicone-based HTF ..................................................................................................................... 6

5.3 Comparison of BP/DPO and Silicone Fluids............................................................................ 7

5.4 High Temperature Decomposition of Heat Transfer Fluids .................................................... 7

5.4.1 Organic Fluids .......................................................................................................................... 7

5.4.2 HELISOL® Heat Transfer Fluids ............................................................................................ 7

5.5 Comparison of Safety Data ........................................................................................................ 8

6 Risk Evaluation of HELISOL® Heat Transfer Fluids ........................................................................ 9

6.1 Safety Information ........................................................................................................................ 9

6.2 Safety Relevant Parameters ...................................................................................................... 9

6.2.1 Flash Point (ISO 2719) ........................................................................................................... 9

6.2.2 Autoignition Temperature (EN 14522) .................................................................................. 9

6.2.3 Determination of the Flammability Characteristics in Contact with Hot Surfaces (ISO 20823) 10

6.2.4 Determination of the Wick Flame Persistence (ISO 14935) ............................................ 10

6.2.5 Determination of Spray Ignition Characteristics (ISO 15029) ......................................... 10

6.3 Fire Tests Under CSP Relevant Conditions ........................................................................... 10

6.3.1 Simulation of Pipe Ruptures and Leakages ...................................................................... 10

6.3.2 Simulation of Leakages into the Thermal Insulation ......................................................... 10

6.3.3 Explosion Hazards ................................................................................................................. 11

6.3.4 Hazards of Static Sparks ...................................................................................................... 12

6.3.5 Combustion Products and Burning Behavior .................................................................... 12

6.4 Corrosion Assessment .............................................................................................................. 12

7 Safety Instructions for Handling HELISOL® Heat Transfer Fluids ............................................... 13

7.1 General Recommendations when Working with HELISOL® Heat Transfer Fluids: ......... 13

7.1.1 Exposure Controls and Personal Protection ...................................................................... 14

7.1.2 First Aid Measures after Contact with HELISOL® Heat Transfer Fluids ........................ 15

7.1.3 Firefighting Measures ............................................................................................................ 15

7.2 Fluid Handling ............................................................................................................................. 15

7.2.1 Fluid Sampling ........................................................................................................................ 15

7.2.2 Accidental Release Measures ............................................................................................. 16

7.2.3 Filling and Draining of the Heat Transfer System ............................................................. 16


7.2.4 Conditions for Safe Storage, Including any Incompatibilities .......................................... 17

7.3 Fire Hazard and Explosion Risk .............................................................................................. 17

7.3.1 Ignition and Flammability Risks of Spilled Fluid and Exhaust Gasses .......................... 17

7.3.2 Recommended Precautions ................................................................................................. 17

7.3.3 Recommended Control Systems ......................................................................................... 18

7.4 Pipe System Failure/Accidents ................................................................................................ 18

7.4.1 Small Leakages (Leaky Valves, Flanges etc.) ................................................................... 18

7.4.2 Loss of Flow ............................................................................................................................ 19

7.4.3 Severe Leakages ................................................................................................................... 20

7.4.4 Leakage into Thermal Insulation ......................................................................................... 21

7.4.5 Leakage Inside the HTF/Steam Heat Exchanger ............................................................. 22

7.4.6 Leakage Inside the HTF/Thermal Energy Storage (TES) Cycle Heat Exchanger ....... 23


1 Preface

Some of the end uses of the products described in this publication must comply with local regulations, standards and requirements. This document is only intended as a guide to the use of HELISOL® heat transfer fluids. This document is the property of Wacker Chemie AG and describes technical details about HELISOL® heat transfer fluids. The enclosed information raises no claim of completeness and shall not be distributed by third parties for commercial purposes. The user is obliged to examine the technical suitability himself or herself,or by consulting a technical expert in the field. Please note that the data provided is not intended for preparing specifications. All measurements were carried out to the best of our knowledge, but WACKER assumes no liability for the accuracy of the measurement results. They only serve to inform the customer and there is no recommendation for action on the part of WACKER.

2 Scope

This document is intended to help customers consider specific properties of HELISOL® heat transfer fluids in terms of safety aspects. This document provides a detailed comparison of the properties of the new silicone based HELISOL® with organic heat transfer fluid, a eutectic mixture of biphenyl/diphenyl oxide (BP/DPO), which is currently used in parabolic trough collector applications. All the relevant characteristics are presented for both, BP/DPO and HELISOL® heat transfer fluids. The individual implications of a changeover from the state-of-the-art HTF (BP/DPO) to HELISOL® are presented in detail. The impact of equilibration and aging of HELISOL® heat transfer fluids are also described with regard to safety-relevant parameters. The information presented in this document is not only based on standards and regulations, but also on industrial-scale trials which have been performed and assessed by internationally recognized safety experts (e.g. the German Aerospace Centre [DLR], TÜV-Nord, Bundesanstalt für Materialforschung und -prüfung [BAM]). The safety and risk assesment of HELISOL® heat transfer fluids include the following trials and experiments:

• Material compatibilty with steel grades

• Critical reactions with water and molten salt

• Detailed analysis of potential hazards in combination with leakages and pipe ruptures

3 Normative References

This safety document refers to other existing standards listed below: DIN 51522 Heat transfer fluids requirements and testing DIN 4754-1 Heat transfer installations working with organic heat transfer fluids – Part 1: Safety

requirements and testing VDI Code of Practice 3033 Heat-transfer systems with organic heat-transfer media - Operation, maintenance and

repair VBG 64 German Pressure Vessels Regulation In addition, some sections refer to the following standards and regulations: EN 14522 Determination of the auto ignition temperature of gases and vapors ISO 2719:1988 Determination of flash point – Pensky-Martens closed-cup method ISO 3016 Determination of pour point DIN 51006 Determination of the boiling range DIN 51900 Heat of combustion


4 Terms and Definitions

Heat transfer fluids: This is the general term for substances used as heat carriers. Unused heat transfer fluids: Heat transfer fluids as delivered that have not been used in the heating system. Heat transfer fluids in use: Heat transfer fluids that have been used in the heating system for at least 30 days at 425 °C. Working temperature (bulk temperature): The average main flow temperature of the heat transfer fluids measured at the hot solar collector outlet under the operating conditions of the heat-transfer system. Maximum permitted working (bulk) temperature: the working temperature is the average mainstream temperature of the heat transfer fluid detected at the outlet of the hot solar collector (boiler) under the operating conditions of the heating system; the maximum permitted working temperature is the permitted maximum mainstream temperature of the heat transfer fluid at the hot solar collector outlet (boiler). Maximum permitted film temperature: The film temperature refers to the heat transfer fluid’s temperature at the heating surface of the hot solar collector outlet, namely the boundary layer temperature of heat transfer fluids in contact with the tube wall. The maximum permitted film temperature is the maximum permitted temperature in the said boundary layer. The temperature of the heat transfer fluids in any part of the heat transfer system must not exceed this temperature. Closed heating system: Heating system containing heat transfer fluids in which the expansion tank is isolated from the atmosphere. Note: closed systems generally use inert gas or cold oil seals to isolate the expansion tank from the atmosphere. Open heating system: Heating system containing heat transfer fluids in which the expansion tank connects with the atmosphere Flash point Indicates, for flammable liquids, the lowest temperature at which a vapor-air mixture which is flammable (by ignition) forms above the liquid level. Auto ignition temperature Indicates the lowest temperature of a hot surface at which the ignition of a combustible substance in the form of vapor/air mixture occurs. Lower explosion limit (LEL): The lowest concentration (volume percentage) of a gas or a vapor in air capable of producing a flash of fire in the presence of an ignition source. At a concentration in air lower than the LEL, the excess of air prevents gas mixtures from burning. Upper explosion limit (UEL): Highest concentration (percentage) of a gas or a vapor in air capable of producing a flash of fire in the presence of an ignition source (arc, flame, heat). Concentrations higher than UEL are "too rich" to burn. Limiting oxygen concentration (LOC): Indicates the highest oxygen concentration in a mixture of air, inert gas and substance vapor that will propagate a flame on a pressure release. Heat of combustion Indicates the amount of heat released during the combustion of a specified amount of substance.


5 Differences between the Physical and Chemical Properties of Organic Heat Transfer Fluids and Silicone-based Fluids (Polydimethylsiloxanes)

5.1 Organic HTF

In the high temperature range over 300 °C, aromatic hydrocarbons and their derivatives predominate, this is because almost all the other aliphatic mineral oils that are widely used as heat-transfer fluids, already decompose in the 250 - 350 °C range. The most common heat transfer medium used at high temperatures is the eutectic mixture of biphenyl with diphenyl oxide (BP/DPO). The biphenyl serves to lower the melting point of the diphenyl ether from 27 °C to 12 °C, which considerably simplifies the operation of outdoor plants (general commercial composition: 73% DPO + 27% BP). Due to its dominant position, BP/DPO is the benchmark for every newly developed HTF.

Figure 1: Representation of the molecular structure of biphenyl (BP) and diphenyl oxide (DPO) BP/DPO can operate up to 400 °C in a liquid and gaseous state. Its classification as "irritant" and "dangerous for the environment" in the unused state as well as its unpleasant smell, do not play a role in the normally closed systems but are relevant during handling and accidents. The formation of benzene (toxic and carcinogenic) is a particularly serious disadvantage during thermal decomposition. Other decomposition products are phenol (caustic) and hydrogen (extremely flammable).

5.2 Silicone-based HTF

HELISOL® heat transfer fluids are linear, non-reactive polydimethylsiloxanes with a viscosity of approx. 5 – 35 mm2/s (at room temperature). They are clear, odorless and colorless liquids as supplied. At temperatures above 200 °C, the HELISOL® heat transfer fluids undergo a rearrangement reaction (equilibration) of their silicone-oxygen bonds when used as intended (under inert conditions). The rate of this molecular rearrangement is directly related to the temperature applied. As a result, low molecular-weight linear and cyclic siloxanes are formed until an equilibrium fluid composition is reached that remains stable. This rearrangement reaction is not a degradation reaction and does not affect the fluid’s lifetime. The newly formed low molecular-weight linear and cyclic siloxanes are part of the heat transfer fluid itself and therefore require operation in closed heating systems under inert conditions. There is no need to separate these low-boiling components from the fluid.

Figure 2: Representation of the equilibrium reaction of HELISOL® heat transfer fluids at high temperatures Because of the rearrangement reaction, some physical properties of the fluids will change during service. Especially, the viscosity and flashpoint decrease, while the vapor pressure increases. Once the equilibrium fluid composition is reached, the physical properties remain stable (in general this is completed after 30 days’ operation at the desired temperature). For comparison purposes, the typical product data and the physical property tables are provided in the technically relevant pressure/temperature range, both for HELISOL® heat transfer fluids unused and in use.


5.3 Comparison of BP/DPO and Silicone Fluids

Table 1: Overview of chemical and physical properties of BP/DPO compared to silicone fluids

5.4 High Temperature Decomposition of Heat Transfer Fluids

Generally, heat transfer fluids change their chemical composition and consequently their properties during extended operation. Thus, the properties of the “HTF in use” must be considered rather than those of the unused HTF, as the period during which unused HTF is operated is comparably short. Concerning HELISOL® two conditions are defined: “unused” and "in use". The first describes the fluid in the state it is delivered, whereas the term “in use” refers to the state the fluid reaches after 30 days of uninterrupted operation at the desired operating temperature. For the aged eutectic mixture of BP/DPO, only limited information concerning the “in use” conditions is available, while information about HELISOL® is generally available and included in this document.

5.4.1 Organic Fluids

BP/DPO is a eutectic mixture of two pure substances. At high temperatures, the individual components of BP/DPO undergo decomposition reactions (e.g. carbon-carbon cleavage, oxidation) which ultimately lead to decomposition products (e.g. methane, carbon monoxide, carbon dioxide, benzene and other carbon rich substances). Therefore, decomposition in the case of DP/DPO is clearly defined – everything except the BP/DPO itself is a decomposition product. These decomposition products can be classified into two different groups: low boilers and high boilers. While low boilers reduce the flash point and increase the volatility of the mixture, high boilers tend to increase the viscosity.

5.4.2 HELISOL® Heat Transfer Fluids

In contrast, HELISOL® and silicone fluids in general are mixtures of large numbers of molecules of different chain lengths (polydimethylsiloxanes). The molecules can be interconverted by depolymerization reactions and can also form other, new oligomeric components which, however, do not represent decomposition products. Accordingly, with silicone fluids, the "degree of decomposition" is not as easy to determine as in the case of BP/DPO, since the definition of DIN 51528 “Testing of mineral oils and related products - Determination of thermostability of unused heat transfer fluids” is not applicable. The thermostability and suitability of silicone fluids as HTF is therefore less a matter of their material composition rather than of the relevant properties such as their viscosity and degree of branching.

BP/DPO Silicone Fluids (e.g. HELISOL®)

Eutectic mixture of two pure substances (biphenyl/diphenyl oxide)

Multi-component mixture of molecules with different chain lengths (classification: polymer) Monomeric unit: Siloxane [Me2SiO2/2]

Fluid composition remains constant well below the

maximum permitted working temperature.

Fluid composition changes due to a rearrangement reaction until equilibrium is reached. Equilibrium is highly temperature dependent.

Specific freezing point Depending on the fluid composition:

• Cloud point refers to the temperature below which parts of the fluid already form a cloudy appearance.

• Pour point refers to the temperature below which the liquid loses its flow characteristics.

Specific boiling point Depending on the fluid composition some parts of the fluid can be separated by distillation, while others remain liquid.

Specific vapor pressure In a multicomponent system, where vapor and liquid consist of more than one type of compounds “vapor pressure” is described by the partial pressures of the single compounds or their vapor and liquid mole fractions.


5.5 Comparison of Safety Data

Table 2: Comparison of safety-related parameters of the HELISOL® heat transfer fluids unused and in use with BP/DPO unused*

Parameter Method HELISOL® 5A HELISOL® 10A HELISOL® XA HELISOL®

XLP BP/DPO*

Freeze point or pour point (in

use) ISO 3016 -65 °C < -55 °C

-36 °C (cloud point)

-45 °C (cloud point)

12 °C

Flash point (unused) closed cup

(ISO 2719)

120 °C 175 °C 225 °C 222 °C 113 °C

Flash point (in use)

51 °C 61 °C 60 °C 67 °C no data

available

Auto ignition temperature

(unused) EN 14522

359 °C 365 °C 369 °C 376 °C 599 °C

Auto ignition temperature

(In use) 358 °C 370 °C 364 °C 366 °C

no data available

Limiting oxygen

concentration

EN 1839-B (200 °C)

9.2%** not applicable not applicable not applicable no data

available

Explosion limits in %

EN 1839 not applicable not applicable not applicable not applicable LEL: 0,8% UEL: 7,0%

Heat of combustion

(In Use) DIN 51900 26.3 MJ/kg 25.5 MJ/kg 23.9 MJ/kg 24.6 MJ/kg 36.05 MJ/kg

Equilibrium vapor pressure

(400 °C)

in accordance with OECD Guideline

104

15.8 bar 13.0 bar 12.7 bar 10.3 bar 11.0 bar

Equilibrium vapor pressure

(425 °C) 19.5 bar 16.3 bar 16.0 bar 12.7 bar not applicable

These data are based upon samples tested in the laboratory and are not guaranteed for all samples. A comparison of safety relevant parameters for both systems in use is not possible as those values are not listed in the TDS of the BP/DPO systems. *Lit. values, Data for BP/DPO unused not available **The oxygen concentration limit of HELISOL® 5A was determined at the German Federal Institute for Materials Research (Bundesanstalt für Materialforschung BAM).


6 Risk Evaluation of HELISOL® Heat Transfer Fluids

6.1 Safety Information

The Material Safety Data Sheet (MSDS) for each of the HELISOL® heat transfer fluids is available upon request. The MSDS contains complete health and safety information regarding the use of this product. Read and understand the MSDS before handling or otherwise using this product. The MSDS contains the following information for the use of HELISOL® heat transfer fluids:

• First aid measures

• Firefighting measures

• Accidental release measures

• Handling and storage

• Exposure controls/personal protection

• Stability and reactivity

• Toxicological information

• Ecological information

• Disposal considerations

• Transport information

• Regulatory information The MSDS of HELISOL® heat transfer fluids refer to unused HELISOL®. The unused product requires no labeling according to GHS classification. Some of the low molecular-weight linear and cyclic siloxanes that result from rearrangement in the fluid are classified as hazardous substances. This must be considered when handling (sampling, leakages, filling, emptying) HELISOL® heat transfer fluids. For further information regarding the hazard identification and classification of HELISOL® heat transfer fluids “In Use” please contact your WACKER representative.

6.2 Safety Relevant Parameters

To further address safety issues, WACKER has conducted an extensive fire and ignition risk assessment together with safety experts and institutes. The testing and assessment of these properties are standardized in Germany and Europe. In addition, there are international regulations which define such test methods.

6.2.1 Flash Point (ISO 2719)

The rearrangement reaction described in section 5.2 results in the formation of volatile siloxanes with lower flash points. Thus, the flash point of HELISOL® heat transfer fluids in use is significantly decreased compared to the values of unused HELISOL® (see Table 1). During normal operation at high temperatures additional gaseous degradation products such as methane, ethane, hydrogen, tetramethylsilane and trimethylsilane may be formed in low concentrations, which may further depress the flash point. Depending on the venting conditions during plant operation, the flash point may be depressed even below room temperature. The actual flash point will vary from system to system. Appropriate precautions must therefore be taken. As the volatile materials in the vapor space of the expansion tank will typically be classified as flammable at ambient temperatures, vapor vents and safety relief lines must be vented to safe areas and the exhaust should be discharged to a safe area away from open flames and other potential sources of ignition. Mists resulting from minor leaks could also present a flammability hazard even if no ignition source is present because an explosive atmosphere may be formed. Any leaks that generate a mist cloud should therefore be treated as significant flammability hazards.

6.2.2 Autoignition Temperature (EN 14522)

The autoignition temperature refers to the temperature at which the heat transfer fluid self-ignites without any external ignition source. From the theory of auto-ignition it is known that, beside other factors, the ignition temperature changes with the size of the ignition vessel (more precisely, with changing surface-volume ratio). This especially must be taken into account in the safety assessment of large-scale technical equipment. The autoignition temperature of unused HELISOL® heat transfer fluid is typically between 355 and 380 °C (EN 14522). This value remains nearly constant for HELISOL® unused and in use. As the autoignition temperature is generally below the service temperature, blanketing with an inert gas (nitrogen, argon) is required to help maintain a safe operating environment. Therefore, leakages or spills at above 350 °C are potential hazards. However, compared to an actual parabolic trough collector facility, there are some major differences, which make it very difficult to translate the findings according to EN 14522 to results at a larger scale. The determination method in EN 14522 is isobaric (atmospheric conditions), while the fluid in industrial-scale facilities is under high pressure. When released to the environment, it is believed that the HELISOL® heat transfer fluids cool down below their autoignition temperature in case of a leakage due to Joule-Thomson expansion.


6.2.3 Determination of the Flammability Characteristics in Contact with Hot Surfaces (ISO 20823)

Contrary to the determination of the autoignition temperature this test takes place under standard environmental conditions. The fluid's tendency to self-ignite on a hot surface was tested according to EN ISO 20823. During the test, a small amount of HELISOL® 5A was dropped onto a preheated surface and its reaction assessed. HELISOL® 5A only ignited on the surface above temperatures of 475 °C, which is far above the maximum recommended working temperature. Contrary to the determination of the autoignition temperature this test is performed under standard environmental conditions. This test was carried out with HELISOL® 5A as the representative of the HELISOL® heat transfer fluids with the most critical values for flash point, auto-ignition temperature and volatility in terms of safety aspects.

6.2.4 Determination of the Wick Flame Persistence (ISO 14935)

During the test according to EN ISO 14935 a small rectangular piece of aluminum silicate was immersed in HELISOL® 5A and then used as a wick. The soaked wick was ignited and the burning behavior on the wick was assessed (time of afterburning). The fire behavior of HELISOL® 5A (flame appearance, flame temperature, smoke release and burning duration) can be described as less energetic compared to conventional organic heat transfer fluids. This result is supported by the heat of combustion (DIN 51900), which is 26 MJ/kg for HELISOL® 5A and therefore considerably lower than for comparable organic heat transfer fluids. This test was carried out with HELISOL® 5A as the representative of the HELISOL® heat transfer fluids with the most critical values for flash point, auto-ignition temperature and volatility in terms of safety aspects.

6.2.5 Determination of Spray Ignition Characteristics (ISO 15029)

To further estimate the hazard potential of leakages and mists a spray ignition tests according to EN ISO 15029 was carried out. Here, the fluid is dispensed under high pressure and then impinged with an ignition flame. The evaluation criteria are after flame time, flammability and flame length index as well as smoke density. Compared to conventional organic heat transfer fluids HELISOL® 5A is less sensitive to ignition. A shorter afterburning time of the fluid mist was observed and even the ignition source had to be placed closer to the release nozzle. The evolved mist and white smoke are mostly unchanged fluid, but oxidation of the hot vapor in air may produce some toxic by-products, including carbon monoxide and formaldehyde. This test was carried out with HELISOL® 5A as the representative of the HELISOL® heat transfer fluids with the most critical values for flash point, auto-ignition temperature and volatility in terms of safety aspects.

6.3 Fire Tests Under CSP Relevant Conditions

The ignition and fire behavior of HELISOL® heat transfer fluids were studied in laboratory tests in order to gain a detailed understanding of associated risks and subsequently design counter measures. Additionally, under carefully controlled test conditions used HELISOL® heat transfer fluids were subjected to a series of industrial-scale release experiments to gain a better insight into the flammability behavior in case of pipe ruptures or leakages. These experiments were also assessed by an independent testing laboratory (TÜV Nord).

6.3.1 Simulation of Pipe Ruptures and Leakages

Under carefully controlled test conditions HELISOL® heat transfer fluids were subject to a series of industrial-scale release experiments to gain further insight into the flammability behavior in case of pipe ruptures or leakages. These experiments were also assessed by an independent testing laboratory. These trials were carried out with HELISOL® 5A as the representative of the HELISOL® heat transfer fluids with the most critical values for flash point, auto-ignition temperature and volatility in terms of safety aspects.

6.3.2 Simulation of Leakages into the Thermal Insulation

In general, materials such as aluminum/calcium silicate, rock wool or foam glass wool can be used as thermal insulation for heat transfer systems operated with HELISOL® HTF. But heat transfer fluid leaking from pipelines, valves or joints may soak into the thermal insulation layer. Due to the large contact area in combination with atmospheric oxygen, any thermal insulation material that has been soaked with the heat transfer fluid has a potential risk of catching fire at operating conditions and therefore needs to be replaced accordingly. According to studies, it usually takes several hours for the heat transfer fluid to heat up to ignition temperature inside the insulation. Organic substances that have seeped into the insulation for example have already developed small smoldering fires that were not recognizable from the outside. In general, if you observe any leaks in a part of the system that is at elevated temperatures, quickly shut down the heating system (e.g. PT-mirrors should be defocused in CSP applications). The affected part should be remedied immediately and all insulation material that has come into contact with the product replaced. This is especially relevant for smaller leaks, that are often only noticed after hours or days. Note, that if the insulation is removed immediately, such smoldering fires can quickly grow to an open fire due to the increased air access after removal of the sheet metal jacket. For this type of work, fire extinguishers should therefore be at hand and the staff should


wear leather gloves and fire-resistant clothing. Observe the following recommendations to minimize the possibility of any ignition in the insulation material:

• Always follow DIN 4754 (Heat transfer installations working with organic heat transfer fluids - Part 1: Safety requirements, test).

• Avoid leakages in the pipe system.

• Minimize flanges and other mechanical joints in the system design,

• Use recommended pipeline specifications.

• In case of a leakage, the contaminated thermal insulation shall be replaced as soon as possible, and the leakage repaired.

• Consult the supplier of the thermal insulation material and the insurance companies about other suggestions on the reduction of fire hazards.

To investigate the flammability of the HELISOL® heat transfer fluids, when leaking and slowly accumulating inside the thermal insulation in cases of an undiscovered leak the hot fluid was released into the insulation of an insulated pipe section. The small pipe leakage experiment was executed under typical operating conditions. Thus, the relevant part of the leaking pipe section was heated up to maximum working temperature (425 °C) to simulate a hot tube section during regular operation and the fluid itself was released from a pressure vessel directly into the insulation at the same temperature. It was than observed whether ignition takes place at the prevailing temperature. According to DIN 4754, in general, avoid fully covering potential sources of leakage, such as flanges with thermal insulation to prevent unnoticed accumulation of the heat transfer fluid. As a result, after releasing the hot fluid into the insulation only white dense mist of the fluid and no ignition was observed. Only, when the metal cover sheets, and the insulation was partly opened (introduction of oxygen into the insulation material) the material was ignited. This is also supported by the literature, since it is well known that a large surface-area material may reduce the ignition temperature when oxygen is present. These trials were carried out with HELISOL® 5A as the representative of the HELISOL® heat transfer fluids with the most critical values for flash point, auto-ignition temperature and volatility in terms of safety aspects.

Figure 3: Schematic representation of the piping section with thermal insulation and the fluid heating vessel (autoclave).

Therefore, in cases where the release of a white mist is observed in a running system (e.g. leaks or possible leaky connections) it is necessary to stop the system and let it cool down below autoignition temperature before opening the affected part of the piping section.

6.3.3 Explosion Hazards

HELISOL® heat transfer fluids have very high boiling points above 270 °C. The data necessary for explosion protection can therefore only be determined at high temperatures (to analyze explosive vapor mixtures it must be

Key1. Temperature sensors (e.g. thermocouples)2. Test specimen (heated fluid)3. Fluid heating vessel (autoclave) with electrical heaters4. Ball valve5. Stainless-steel feed pipe6. Thermal insulation blanket (Rockwool®)

1

1

Pipe section

3

4

5

6

7

89

2

Fluid flow direction

7. High density thermal insulation (Rockwool®)8. Steel pipe (D60x1.5mm, 1500 mm in 1.4301)9. Electrical heating unit with temperature regulation10. Inlet opening for fluid release11. Metal tight end cap12. Metal shell

1011

1500 mm

60

mm

d =

26

0 m

m

12

11


possible to evaporate the test specimen at the desired temperature). For this purpose, ignition tests were carried out in a pressure-resistant autoclave in which defined mixtures of the test substance, nitrogen and air are produced and ignited. The composition of the gas mixtures is systematically varied until the highest air content is determined at which the mixtures are no longer explosive. In each case test substance or oxygen fractions were determined at which the mixture is no longer explosive. The experimentally determined limiting oxygen concentration (LOC) according to EN 1839-B is only measurable for HELISOL® 5A unused and HELISOL® 5A in use (9.2 mol-% at 200 °C, 1 bar atmospheric pressure). The oxygen concentration limit of HELISOL® 5A was determined at the German Federal Institute for Materials Research (Bundesanstalt für Materialforschung BAM).

6.3.4 Hazards of Static Sparks

Static charges and static electricity discharges within vessels during filling or draining or when loading and unloading aged fluid may occur as HELISOL® heat transfer fluids, like most silicone fluids, are poor electrical conductors. Precautions such as grounding and exclusion of air through inert blanketing should be taken.

6.3.5 Combustion Products and Burning Behavior

When ignited, the main combustion product of the HELISOL® heat transfer fluids are macroscopic powders of amorphous silica (SiO2). According to spectroscopic analyses no traces of crystalline structures can be found. Instead, the combustion products are very similar to synthetic amorphous silica, which is not classified as hazardous. Also, during the determination of the wick flame persistence, a barrier of solid amorphous silica was formed by the combustion products of HELISOL® resulting in the suffocation of the flame even though unburned fluid remained.

6.4 Corrosion Assessment

Up to the maximum permitted working (bulk) temperature HELISOL® heat transfer fluids are noncorrosive

towards common metals and alloys if contamination of the fluid is avoided. In material compatibility test series, the

corrosivity of HELISOL® towards common steel qualities at temperatures up to 425 °C was analyzed. In general,

carbon steel grades which are used predominantly like stainless austenitic steel 1.4541 (X6CrNiTi18-10), as well

as non-alloy quality steel 1.0425 (P265GH) have been assessed as consistent under specific test conditions. As

copper is one of the most sensitive materials regarding corrosivity issues, a copper strip test series according to EN

ISO 2160 (ASTM D4048) was performed. As a result, no corrosivity towards HELISOL® heat transfer fluids were

observed. Corrosion problems may be caused by trace chemicals or residues which are introduced into the fluid

during cleaning or other processes which require opening the system. If other specific construction materials are

used (e.g. in instrumentation), extra precautions should be taken. To avoid complications, please contact your

regional WACKER representative for further information.


7 Safety Instructions for Handling HELISOL® Heat Transfer Fluids

The following paragraphs describe environmental, health and safety related issues, as well as safety instructions regarding fire and explosion hazards of the HELISOL® heat transfer fluids. To obtain detailed test reports on safety and fire hazard-related topics, contact your local WACKER representative. The details in this document are based on the state of our knowledge at the time of revision. They do not constitute an assurance of the described product properties in terms of statutory warranty requirements. Providing this document to a recipient does not absolve the recipient from his or her responsibility for compliance with all laws and stipulations applicable to the product. This applies in particular to the further sale or distribution of the product or substances or items containing the product, in other jurisdictions and with regard to the protection of third-party intellectual property rights. If the described product is processed or mixed with other substances or materials, the details stated in this document cannot be conferred to the resultant new product. If the product is repackaged, the recipient is obligated to additionally provide the required safety-related information.

7.1 General Recommendations when Working with HELISOL® Heat Transfer Fluids:

Do not handle HELISOL® heat transfer fluids until all safety precautions have been read and understood. Most important, keep away from heat, hot surfaces, sparks, open flames and other ignition sources. Flammable vapors may accumulate unnoticed and form explosive mixtures with air in containers, process vessels, including partial, empty and uncleaned containers and vessels, or other enclosed spaces. Keep away from sources of ignition and do not smoke. Take precautionary measures against electrostatic charging. Cool endangered containers with water. Unused HELISOL® heat transfer fluids do not contain any ingredients above the permitted limits. However, ensure adequate ventilation. Fluid vapors must be syphoned off in situ. Avoid formation of aerosols. In case of aerosol formation special protective measures are required (exhausting by suction, respiratory protection). Spilled substance increases risk of slipping. Regulations for safe storage vary by country; check with your local WACKER representative for further information. Due to the rearrangement reaction described in section 5.2 when used as intended, the HELISOL® heat transfer fluids in use contain octamethylcyclotetrasiloxane D4 (CAS No. 556-67-2), hexamethyldisiloxane D5 (CAS No. 107-46-0), dodecamethylhexasiloxane D6 (CAS No. 540-97-6) and hexamethyldisiloxane Si2 (CAS No. 107-46-0) in various quantities. These volatiles will have different toxicological and flammability ratings from the unused fluid. Therefore, appropriate handling precautions must be taken. HELISOL heat transfer fluids in use contain substances ≥ 0.1% that have been subjected to the SVHC process according to REACH regulation (EC) No 1907/2006 Art. 57 as fulfilling the PBT and/or vPvB criteria according to REACH regulation (EC) No 1907/2006 Annex XIII. Table 3: Hazardous ingredients of HELISOL® heat transfer fluids in use

EC-No. CAS No. Substance Content [%]

209-136-7 556-67-2 Octamethylcyclotetrasiloxane ≥ 0.1%

203-492-7 107-46-0 Hexamethyldisiloxane ≥ 0.1%

Table 4: The product contains the following substances of very high concern (Regulation (EC) No. 1907/2006 (REACH), Article 57) in amounts ≥0.1%

CAS No. Substance Content [%]

556-67-2 Octamethylcyclotetrasiloxane ≥ 0.1%

541-02-6 Decamethylcyclopentasiloxane ≥ 0.1%

540-97-6 Dodecamethylcyclohexasiloxane ≥ 0.1%

Table 5: Classification of HELISOL® heat transfer fluids in use

Hazard Class Hazard Category

Flammable Liquid Category 3

Reproductivity toxicity Category 2 (impair fertility)

Long-term (chronic) aquatic hazard Category 4


Table 6: Label Elements of HELISOL® heat transfer fluids in use

H-Code Hazard Statements

H226 Flammable liquid and vapor1

H361f Suspected of damaging fertility

H413 May cause long lasting harmful effects to aquatic life

7.1.1 Exposure Controls and Personal Protection

In general, avoid exposure. Avoid contact with eyes and skin. Do not inhale gases/vapors/aerosols. Do not eat, drink or smoke when handling. Wash hands at the end of work and before eating. Keep working clothes separately. Remove contaminated, soaked clothing immediately. Clean work areas regularly. Use with adequate ventilation.

7.1.1.1 General Protection and Hygiene Measures

Observe standard industrial hygiene practices for the handling of chemical substances. Do not eat or drink when handling. Where inhalative exposure above the occupational exposure limit cannot be excluded, adequate respiratory protection equipment must be used (limits can be found in the safety data sheets of the HELISOL® heat transfer fluids):

• Suitable respiratory equipment: Respirator with a full-face mask, according to acknowledged standards such as EN 136.

• Recommended filter type: gas filter type ABEK (certain inorganic, organic and acidic gases and vapors; ammonia/amines), according to acknowledged standards such as EN 14387

In case of mist, spray or aerosol exposure wear suitable personal respiratory protection and protective suit:

• Suitable respiratory equipment: Respirator with a full-face mask, according to acknowledged standards such as EN 136.

• Recommended filter type: Combined filter type ABEK-P2 (certain inorganic, organic and acidic gases and vapors; ammonia/amines; particles), according to acknowledged standards such as EN 14387

Observe the equipment manufacturer's information and wear-time limits for respirators.

7.1.1.2 Personal Protection Equipment

If handled uncovered: Chemical protective clothing, full-body liquid-tight protection if necessary. Please observe the instructions regarding permeability time which are provided by the supplier. Recommended eye protection: protective goggles. Gloves are required at all times when handling the material:

• Recommended glove types: Protective gloves made of nitrile rubber o thickness of the material: > 0,10 mm o Breakthrough time: 30 - 60 min

• Recommended glove types: Protective gloves made of nitrile rubber: o thickness of the material: > 0,1 mm o Breakthrough time: > 480 min

• Recommended glove types: Protective gloves made of butyl rubber o thickness of the material: > 0,3 mm o Breakthrough time: > 480 min

Please observe the instructions regarding permeability and breakthrough time which are provided by the supplier of the gloves. Also take into consideration the specific local conditions under which the product is used, such as the danger of cuts, abrasion, and the contact time. Note that, due to the numerous external influences (such as temperature), a chemically resistant protective glove in daily use may have a service life that is considerably shorter than the measured break through time.

1 Classification as flammable liquid is only applicable if the sample has a flashpoint ≤ 60 °C.

Signal Word: Warning

Pictograms1:


7.1.2 First Aid Measures after Contact with HELISOL® Heat Transfer Fluids

7.1.2.1 General Information

Take persons to a safe place. Observe self-protection for first aid. Pregnant women exposed to this substance must seek medical advice. Product contains reproductive toxins (may cause harm to the unborn child and/or impairs male or female reproductive function). After exposure, it is recommended that you seek specialist medical advice.

7.1.2.2 After Contact with the Eyes

Rinse immediately with plenty of water for 10 − 15 minutes. Keep eyelids well open to rinse the entire eye surface and eyelids with water. Seek medical advice in case of continuous irritation.

7.1.2.3 After Contact with the Skin

Remove contaminated or soaked clothing. Immediately rinse with plenty of soap and water. In serious cases, use emergency shower immediately. In the event of a visible skin change or other complaints, seek medical advice (show label or safety datasheet where possible).

7.1.2.4 After Inhalation

Keep the patient calm. If unconscious place in stable sideways position. Protect against loss of body heat. Seek medical advice and clearly identify substance.

7.1.2.5 After Swallowing

If patient is conscious, give several small portions of water to drink. Do not induce vomiting. Seek medical advice and clearly identify substance.

7.1.3 Firefighting Measures

Consider making a specially trained team available for firefighting on site. Special protective equipment for firefighting is recommended. Use respiratory protection independent of recirculated air. Keep unprotected persons away. Suitable extinguishing media are water mist, sprinkler system. If necessary, also alcohol-resistant foam, carbon dioxide, sand or extinguishing powder may be used. However, fire tests at TÜV Nord have shown that foam/powder extinguishers are less effective compared to water spray devices. Risk of hazardous gases or fumes in the event of fire. Exposure to combustion products may be a health hazard! Hazardous combustion products: toxic and very toxic fumes. Detailed test reports on safety and fire hazard related topics can be received by contacting your local WACKER representative.

7.2 Fluid Handling

7.2.1 Fluid Sampling

According to DIN 4754 heat transfer fluid must be checked for its serviceability once a year or on demand.

7.2.1.1 Sampling Procedure

• Use appropriate personal protection equipment (for further information see section 7.1.1.2)

• The samples sent for analysis should be representative, i.e. they should preferably be taken from the main circulating line of the system e.g. from the cold side of the loop, not from the expansion vessel (risk of mixture). Occasionally, additional samples may have to be taken from other parts of the system if specific problems exist.

• The heat transfer fluid should be sampled while cold below its flash point (<< 50 °C). Avoid contact with the environment to prevent the escape of low boilers and gases which otherwise would be lost from the sample.

• In order to evaluate the quality of the HELISOL® heat transfer fluid, the sample should be at least 1 kg.

• Before sampling, the sampling container should be thoroughly flushed with inert gas.

• The sampling container should be filled with HELISOL® heat transfer fluid. The person who fills the package with hazardous materials is responsible for its safety on dispatch. WACKER recommends sampling bottles of the following type which are in compliance with UNE 206015 standard (Heat transfer fluids for solar thermal power plants with parabolic trough collector technology. Requirements and tests):


• Pure aluminum (material AL 99.5, e.g. https://www.buerkle.de/en/aluminium-bottle)

• With tamper-evident screw cap made of PP with aluminum inner seal

• With UN approval (1B1/X/250)

• Packaging group I (X)

7.2.1.2 Transport Information

National and local regulations must be observed. Sample bottles should be shipped accordingly: Valuation: Dangerous Goods UN no.: 1993 Proper Shipping Name: Flammable liquid, n.o.s. (contains hexamethyldisiloxane and

octamethyltrisiloxane) Class: 3 Packaging Group: III

7.2.2 Accidental Release Measures

7.2.2.1 Personal Precautions, Protective Equipment and Emergency Procedures in the Event of an Accidental Release

Secure the area. Wear personal protection equipment (see section 7.1.1.2). Keep unprotected persons away. Avoid contact with eyes and skin. Do not inhale gases/vapors/aerosols. If material is released indicate risk of slipping. Do not walk through spilled material.

7.2.2.2 Environmental Precautions

Prevent material from entering surface waters, drains or sewers and soil. Close leak if possible, without risk. Contain any fluid that runs out using suitable material (e.g. earth). Retain contaminated water/extinguishing water. Dispose of in prescribed marked containers. Inform authorities if substance leaks into surface waters, sewerage or ground.

7.2.2.3 Methods and Material for Containment and Cleaning up

Take up mechanically and dispose of according to local/state/federal regulations. Do not flush away with water. For small amounts: Absorb with a neutral (non-acidic/non-basic) liquid binding material such as diatomaceous earth and dispose of according to government regulations. For large amounts: Liquids may be recovered using suction devices or pumps. If flammable, only air driven or properly rated electrical equipment should be used. Clean any slippery coating that remains using a detergent/soap solution or another biodegradable cleaner. Silicone fluids are slippery; spills are a safety hazard. Apply sand or other inert granular material to improve traction.

7.2.2.4 Disposal Considerations

Waste of HELISOL® heat transfer fluids in use that cannot be re-used, reprocessed or recycled should be disposed of in accordance with Federal, State, and local regulations at an approved facility. Depending on the regulations, waste treatment methods may include, e.g., landfill or incineration. Regarding uncleaned packaging, completely discharge containers before further use (no tear drops, no powder rest, scraped carefully). Containers may be recycled or re-used. Observe local/state/federal regulations. Uncleaned packaging should be treated with the same precautions as the material itself.

7.2.3 Filling and Draining of the Heat Transfer System

The following recommendations can ensure ease of filling, topping off and system recharging.

• HELISOL® heat transfer fluids are generally operated at very high temperatures (up to 425 °C). To avoid contact with hot fluid always ensure the HTF system is properly cooled down (shut down) before opening the system in the case of filling, draining or maintenance works.

• The vapor pressure of HELISOL® heat transfer fluids at room temperature and under normal environmental conditions is negligible. However, when HELISOL heat transfer fluids are heated up to high temperatures, take measures to avoid harm to human respiration caused by the increased concentration of HTF vapor in the air and to avoid undesired vapor release when opening the system.

• Use technical measures and personal protective equipment to avoid exposure (for further information see section 7.1.1.2).

• Ensure adequate ventilation. Avoid formation of aerosols. In case of aerosol formation special protective measures are required (exhausting by suction, respiratory protection).

• Spilled substance increases risk of slipping.


• Flammable vapors may accumulate and form explosive mixtures with air in containers, process vessels, including partial, empty and uncleaned containers and vessels, or other enclosed spaces.

• Keep away from sources of ignition and do not smoke.

• Take precautionary measures against electrostatic charging. Cool down endangered containers with water.

7.2.4 Conditions for Safe Storage, Including any Incompatibilities

Evaluate storage and observe local/state/federal regulations. Keep container tightly closed. Store in a dry and cool place. Maximum temperature allowed during storage and transportation is 40 °C. Store container in a well-ventilated place where they should be protected from exposure to direct sunlight and/or precipitation. If stored and handled in accordance with standard industrial practices no hazardous reactions are known. Heat, open flames, and other sources of ignition must be avoided. Measurements have indicated that small amounts of formaldehyde are formed at temperatures above about 150 °C (302 °F) through oxidation. Compensate for high humidity. If you're in a high-humidity environment, consider not filling your system with partially filled containers of fluid. Humid air can work itself into partially filled containers and develop condensation inside, which is then added to your system.

The 'Best use before end' date of each batch is shown on the product label. Storage beyond the date specified on the label does not necessarily mean that the product is no longer usable. In this case, however, the properties required for the intended use must be checked for quality assurance reasons. For further information please contact your local WACKER representative.

7.3 Fire Hazard and Explosion Risk

7.3.1 Ignition and Flammability Risks of Spilled Fluid and Exhaust Gasses

During normal operation at high temperatures additional gaseous degradation products such as methane and hydrogen, as well as low boiling silanes and siloxanes may be formed in low concentrations which remain in the fluid. These substances may depress the flash point of the HELISOL® heat transfer fluids in use. Depending on the venting conditions during plant operation, the flash point may even be depressed below room temperature. But take into account that the actual flash point will vary from system to system. In general, HELISOL® heat transfer fluids are commonly operated at temperatures above their flash points and auto ignition temperature. In the presence of an ignition source, the vapor above the surface of the spilled flammable-liquid can ignite and cause a flashfire or a vapor cloud explosion if the vapor cloud is formed in a partially confined space. The ignition of a vapor cloud can also lead to a pool fire, which can last for an extended period. As the volatile materials in the vapor space of the expansion tank will be typically classified as flammable at ambient temperatures, vapor vents and safety relief lines must be vented to safe areas and the exhaust should be discharged to a safe area away from open flame and other potential sources of ignition (heated surfaces, sparks). Mists resulting from minor leaks also present a flammability hazard if other flammables are present. Any large leaks that generate a mist cloud should be treated as significant flammability hazards.

7.3.2 Recommended Precautions

Therefore, precautions must be taken for the plant design and the design of safety valves for exhaust vapor.

• The HTF system must be durable and technically leakproof and the plant should only be operated with an inert gas blanket.

• Good engineering design of the system and practice to prevent spills and leaks (e.g. use of correct high-temperature screws with defined torque).

• Always consider national codes and safety standards (see section 3).

• Maintenance should include daily and weekly inspections for signs of white vapor from potential leak points, especially valves, flanges, welds, instrument ports and threaded fittings. Prevent overheating and sparking due to mechanical equipment failure.

• Install insulation in accordance with DIN 4754-1 (Heat transfer installations working with organic heat transfer fluids - Part 1: Safety requirements, test) so that potential leak points like flanges and valves are free of insulation material.

• Exhaust gases must be discharged to a safe area away from open flames and other potential sources of ignition. The area surrounding the exhaust gas outlet must be declared as an explosive zone.

• Consider bleed valves in the piping system so maintenance can be performed without draining the entire system.

• Identify and eliminate all ignition sources that would ignite the flammable atmosphere under both normal and foreseeable abnormal operations especially in areas where spills might occur (heat sources, friction/impact sparks, electrical arcs and sparks, and electrostatic discharges).

• Additionally, insulate hot surfaces.

• Prevent accumulation of static charges on conductive materials with effective bonding and grounding so


that the items remain at zero electrical potential.

• Consider automatic control systems for oxidant concentration (inert gas), pressure or temperature.

• Consider explosion protection (definition of explosion areas in accordance with ATEX Directive 1999/92/EG, EN 1127-1 etc.).

• Ensure that there are exit paths for plant personnel.

• Relevant information in other sections must be considered. This applies in particular for information given on personal protective equipment (section 7.1.1.2) and on disposal (section 7.2.2.4).

7.3.3 Recommended Control Systems

The following control systems are strongly recommended:

• High-temperature control system: if the outlet temperature of the collector exceeds the maximum permitted operating temperature, the collectors should be defocused to prevent overheating of the HTF.

• Low-flow control system: If the circulation flow is lower than the set value, or the heat transfer fluid stops flowing due to mechanical fault or failure of the circulating pump, the collectors should be defocused to prevent overheating of the HTF.

• Safety relief valve: Safety valves should be placed in such a way that no workers can be injured when activated and there are no ignition sources nearby.

• Expansion tank level control device: If the HTF level in the expansion tank falls below the minimum level or exceeds the maximum level, an alarm is required, and the system must be checked immediately, and appropriate measures taken.

• Pressure control system: Consider using alarms for high-and low pressure. An alarm system for high pressure is especially important to avoid overpressure in the system.

• Additional measures: In general, the use of fail-safe instruments and control systems in the event of a power failure is highly recommended.

7.4 Pipe System Failure/Accidents

Even though these tests provide useful data, none should be applied as the only selection criterion. Under actual conditions, leaking fluid will cool down quickly when exposed to air, dropping below the flash point. Any vapors produced will turn to white vapor if the area has adequate ventilation. This white vapor will be most noticeable around small-volume leaks and can be used to quickly discover and eliminate potentially dangerous situations. Relatively few fires have originated in thermal fluid systems. Most of the incidents that do occur are insulation fires, or are caused by loss of flow, cracked heater tubes or leakages and most of these incidents can be avoided due to proper maintenance or inspections of the HTF system. Additionally, few design recommendations must be taken into account:

• For the piping of heat transfer systems operated with HELISOL® heat transfer fluids, it is recommended that the pipes be of such a size/diameter to maintain the flow required by the normal heating load at the specific economic pressure drop.

• The system must withstand temperature changes, so thermal expansion and contraction stress of pipelines must be considered for the design.

• HELISOL® heat transfer fluids (or any other HTF) tend to leak from joints or flanges unless these sections are closely connected. These are potential leak points and the best way to avoid pipeline leakage is to weld all connecting parts.

• Insulation layer which is soaked by the HTF has a greater fire risk than leaking HTF itself. It is therefore very important to prevent any fluid from leaking into the insulation (for more information see section 7.4.4 regarding thermal insulation).

In the following section the most possible scenarios of dangerous situations, possible precautions to avoid and measures to handle them will be described.

7.4.1 Small Leakages (Leaky Valves, Flanges etc.)

Situation: HTF systems are prone to small leakages at potential leak points such as valves, flanges, instrument ports and threaded fittings, especially at operating temperatures or during rapid temperature changes. The exact location of the leak may be identified by dripping fluid or the formation of a white vapor (at corresponding temperatures). In most cases, the leaking fluid cools down quickly below the flash point. Even a small leakage is a potential safety risk and should be resolved immediately.

Cause: In most cases leakage incidents can be traced back to inadequate mounting procedures or

selection of incorrect materials or components. For example, a flange connection may start leaking at one point if no tension screws were used. Furthermore, small leakages may also be a


result of incorrectly applied tightening torques, both too high and too low. Prevention: The HTF system must be designed so as to withstand temperature changes. Therefore, keep in

mind the thermal expansion and contraction stress of pipelines or flange screws. All used material must be suitable for the system conditions (e.g. temperature range, operation pressure, material compatibility). Establish a safety documentation to enable practical measures for prevention, protection and isolation of such incidents. Specification for potential leak points must be in accordance with the manufacturer’s requirements. Regular maintenance should include daily and/or weekly inspections for signs of white vapor from potential leak points or the formation of fouling structures, especially at valves, flanges, welds, instrument ports and threaded fittings. To repair smaller leakages always shut down/cool down the HTF system before pipeline works and consider proper personal protective equipment (section 7.1.1.2). Regarding the disposal of spilled fluid please observe section 7.2.2.4. The risk of leakages is drastically diminished if the installation is designed and manufactured according to the VDI Code of Practice.

Table 7: Description of incidents with leaking HELISOL® 5A. Most small incidents can be avoided by precisely implementing the requirements from the applied standard, this includes third party supervision and a complete documentation. During operation regular inspections and maintenance works are indispensable.

Location: flange connection at a 4” flowmeter (thermal insulation removed) Occurrence: the leakage was discovered by the formation of white mist during an inspection. The facility was operated at 240 °C/24 bar, while the temperature had been increased at the time of the leakage. The facility was shut down/cooled down and the incident could be handled properly. Cause of the situation: the operating temperature and pressure require the use of high-temperature tension screws. Instead, regular machine screws were used.

Location: outer flange at the spiral casing of a HTF pump Occurrence: the leakage was discovered by the formation of white residue at the through-hole of the casing flange (cover) during a regular inspection. The facility was shut off, cooled down, locally drained, the stud bolts and the gasket were replaced and a high temperature screw lubricant (anti-seize) was applied. Cause of the situation: an excessive torque was applied to the nuts (based on incorrect documentation)

7.4.2 Loss of Flow

Situation: As the burner or electrical heating element continues to heat the system while the fluid remains stagnant, the temperature may increase above the maximum recommended working (425 °C) or film temperature (450 °C). This could lead to serious situations, for example if a pipe rupture happens and the now overheated fluid is released to the environment. Otherwise, if the pipe system remains intact, there is a strong possibility of the fluid rapidly decomposing, and high-viscosity material being formed.

Cause: Loss of flow can occur when a series of equipment failures interrupts the flow of the HTF in the

heat transfer cycle. This could be either a pump motor failure, blocked filters/pipes or any other system pressure control failure (e.g. fouling induced).

by courtesy of CIEMAT/DLR by courtesy of CIEMAT/DLR


Prevention: To avoid incidents resulting from loss of flow, a low-flow shutdown should be included in the heater

safety system, which is connected to pressure sensors. Overpressure must be reduced via safety valves. Along with a pressure release and a detailed analysis of irregularities in the process that could harm the heat transfer fluid, check that the overheated fluid is suitable for further use.

7.4.3 Severe Leakages

Situation: Serious fires caused by cracked pipes are rare but are still possible. Large-volume leakage from the thermal fluid system might be a direct cause of fire if the hot fluid contacts an ignition source. If the ignition source is part of the failing component or is the source of the leak, a significant fire may occur.

Cause: Cracks in pipelines are formed by thermal stress or near hot spots that may develop from internal

fouling. Most major leaks also result from component failure. Expansion joints, flexible hose and rotary unions are among the components that might fail. Worst of all, pipeline cracking due to material failure may be a rare but very severe incident. The demolition of a pipeline especially for CSP systems can also be carried out by the collector movement in relation to the rigid pipe system. Although the collector movement is compensated by rotation and expansion performing assemblies (REPAs) or ball joints, it is a potential risk.

Prevention: Although, the risk assessment proved that HELISOL® heat transfer fluids do not show any auto

ignition during release tests, directly on heated surfaces (see Figure 4), as well as in large-scale releases (see Figure 5), any cracking must nevertheless be treated as a serious threat to facility and personnel safety. Most large-scale incidents may be prevented by proper system design, regular maintenance and inspection of pipes and equipment. Avoidance of ignition sources, regular inspections, flame detectors and flammable gas detectors are strongly recommended. Detectors must be compatible with silicone vapors.

Figure 4: Series of pictures taken during a fluid release experiment with HELISOL® 5A. The hot fluid (425 °C) is directly released on a heated steel pipe (also at 425 °C). During the release test no ignition was observed (duration of the release: 30 sec). These fluid release experiments were carried out in cooperation with TÜV Nord. Any large-scale incident ought to be identified by a sudden loss of internal system pressure.

Proper instrumentation and sensors monitoring internal process data, such as pressure, temperature and mass flow, should therefore be integrated. If any leak is observed via loss of system pressure in a part of the system, quickly shut down the heating system (e.g. PT-mirrors should be defocused in CSP applications). If possible, also isolate the affected part of the facility by means of safety valves so that only a small portion of the fluid can contaminate the


environment. In addition, special safety systems can be installed to further provide safety protection. The area should be largely closed off due to the danger of auto ignition and explosion risks. If combustible vapor extensively leaks into the atmosphere from a pressurized system, the gas condenses in contact with cold air, forming aerial fog composed of small droplets with a high surface area. The combustible medium aerial fog in air may burn if it reaches a certain concentration and is in contact with ignition sources (electrical discharges, heated surfaces, flames). The limiting oxygen concentration for HELISOL® 5A in use for example is 9.2% at 200 °C. Even if the overall temperature of said aerial fog is lower than the flash point of the HELISOL® heat transfer fluids and the saturated concentration of combustible vapor is lower than the ignition limit, the aerial fog may still be combustible. This aerial fog mixture is very similar to the mixture of combustible gas and air, and combustion may also lead to explosion. Therefore, evacuate the affected part of the facility and further access for personnel should only be allowed once it has been ensured that the fluid has cooled down and the system part has been completely emptied. Always minimize further danger and consider proper personal protective equipment (section 7.1.1.2). Regarding the disposal of the spilled fluid please observe section 7.2.2.4.

Figure 5: Series of pictures taken during a large-scale fluid release experiment with HELISOL® 5A (3 pictures per second). Release nozzle spec: perforated disc with 2” borehole diameter. HTF temperature: 425 °C. During the release test no ignition was observed. These large-scale fluid release experiments were carried out in cooperation with the German Aerospace Centre (DLR), Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT) and TÜV Nord on the Plataforma Solar de Almería, Spain.

7.4.4 Leakage into Thermal Insulation

Situation: Insulation fires occur when HTF leaks from valves, gaskets, welds or instrument ports and infiltrates the porous insulation material (commonly rockwool, aluminum and calcium silicates or fiberglass wool). These materials have a very porous structure with high surface areas which allows the HTF to spread throughout the insulation. Additionally, high surface areas tend to drastically decrease the onset temperature for auto ignition of the HTF. Therefore, spontaneous ignition might result upon the fluids being sudden exposed to oxygen if, for example, the protective covering is punctured or removed.

Cause: Insulation fires often occur if the insulation is not constructed in accordance with DIN 4754-1 (Heat

transfer installations working with organic heat transfer fluids - Part 1: Safety requirements, test). In most cases also inadequate or missing documentation about the specification of valves, flanges or screws is the major cause of such incidents. However, compared to small leakages outside the insulation these incidents can become even worse due to the accumulation of fluid and a potential ignition upon the fluid sudden exposure to oxygen. Some industrial plants have reported fire incidents with organic HTF as well, when the metal sheets and the insulation is removed from the pipe.

Prevention: Comparable to small leakages (section 7.4.1), leakages into the insulation can often be detected

by the formation of white vapor which is released between the alumina sheet covers. This is not a sign of ignition, but the normal evaporation of the leaking fluid (see Figure 6).


Figure 6: Series of pictures taken during a fluid release experiment into thermal insulation with HELISOL® 5A. Hot HELISOL® 5A (at 425 °C) is released into the thermal insulation layer (Rockwool® ProRox) around a heated steel pipe (also at 425 °C). The HTF evaporates immediately (white vapor) and the insulation acts as a condenser (liquid HTF leaks into the insulation layer and finally escapes from the steel sheet in drops). For this kind of leakage, no ignition is observed due to a lack of oxygen. These fluid release experiments were carried out in cooperation with TÜV Nord.

The most effective precaution against insulation fires is the identification of all potential leak points and proper installation of high-temperature insulation. If possible, always consider closed-cell insulation sections to prevent the fluid from spreading throughout the entire insulation. Flanges and valves must not be covered with insulation in accordance with DIN 4754-1 (Heat transfer installations working with organic heat transfer fluids - Part 1: Safety requirements, test). Also minimize flanges and other mechanical joints in the system design and observe the recommended pipeline specifications.

In general, if any leaks are observed in a part of the system that is at elevated temperatures, quickly shut down the heating system (e.g. PT-mirrors should be defocused in CSP applications). The affected part should be remedied immediately after cooling down the system and all insulation material that has come into contact with the product replaced. This is especially relevant for smaller leaks that are often only noticed after hours or days. Note, that if the insulation is removed immediately, such smoldering fires can quickly grow into an open fire due to the increased air access after removal of the sheet metal jacket. Even if the heat transfer system is cooled down (e. g. at room temperature), for this type of work, fire extinguishers should therefore be at hand and the staff should wear protective clothing as mentioned in section 7.1.1.2.

7.4.5 Leakage Inside the HTF/Steam Heat Exchanger

In contact with HELISOL® heat transfer fluids, water is consumed at elevated temperatures due to the formation of methane and branching of the siloxane molecules, which ultimately results in an increase of viscosity. Unwanted contact with water is possible in two cases: Minor leakages (e.g. residual water, humidity):

Minor water contaminations do not affect the properties of HELISOL® heat transfer fluids significantly. Low quantities of water may be removed from the HTF by boiling out at elevated temperatures (120 - 200 °C). However, depending on the amount of contamination, there may be time-independent and unwanted aging of the heat transfer fluid. The mass percentage of water contaminating HELISOL® and the amount of degradation products is directly correlated. Contamination of HELISOL® heat transfer fluids with 1% of water corresponds to thermal aging of 25 years of operation at 425 °C in a CSP plant. Depending on the magnitude of the contamination, measures must be taken to maintain the fluid properties, for example a partial exchange of the heat transfer medium.

Major leakages (e.g. pipe failure inside the heat exchanger between HTF cycle and steam cycle):

In case of a leakage in the heat exchanger between the heat transfer fluid and the water/steam cycle hot HELISOL® heat transfer fluid may come into contact with water. Due to the higher pressure in the water/steam cycle, it is expected that water will be compressed in the heat transfer


fluid cycle. The pressure in the heat transfer fluid cycle will increase immediately due to the evaporation of water (pressure peak, detection of the leakage). Therefore, the system must be immediately cooled down (e.g. automatic pressure shut-off). In general, water can be removed from the heat transfer cycle by phase separation at room temperature as the densities of the HELISOL® heat transfer fluids are lower than that of water.

7.4.6 Leakage Inside the HTF/Thermal Energy Storage (TES) Cycle Heat Exchanger

Figure 7: Schematic representation of a concentrated solar thermal trough power plant with integrated thermal storage system (1: hot storage tank; 2: cold storage tank). The heat exchanger between heat transfer circuit and thermal storage system is marked in red (see Figure 8). In general, modern PTC (parabolic trough collector) power plants are combined with a thermal storage system. A proven form of high-temperature thermal storage system operates with two tanks and a binary mixture of molten salt NaNO3/KNO3 as storage medium. The excess heat of the solar collector field heats up the molten salt. Figure 1 shows the principle of the parabolic trough power plant with integrated thermal storage system. In principal, the molten salt from the cold storage tank (at 292 °C) flows through the heat exchanger, where it is heated by the high-temperature heat-transfer fluid (425 °C). The molten salt then flows to the hot storage tank and is heated up to 386 °C. The heat transfer fluid exits this heat exchanger at a lower temperature and returns to the solar collector or receiver, where it is heated back to 425 °C. If the solar collector field cannot produce enough heat to drive the turbine, the molten salt is pumped back from the hot storage tank and heats up the heat transfer fluid to 425 °C. The molten salt itself exits the heat exchanger at a lower temperature and returns to the low temperature tank. This system is used in the most of the commercial parabolic power plants worldwide.

Figure 8: Schematic representation of the heat exchanger between the heat transfer circuit and the TES during normal plant operation (storage of thermal energy in the hot tank). In the event of a leakage in this specific heat exchanger the hot HELISOL® heat transfer fluids (425 °C) will rapidly come into contact with the heat storage medium (solar salt NaNO3/KNO3, 60/40 wt.%, ca. 390 °C) which could lead

Solar collector

field

Turbine

Cooling towerThermal storage system

Heat transfer fluid circuit

Generator

Grid

1

2

TES heat exchanger

from hot collector outlet

425 °C

386 °C292 °C

Cold storage tank Hot storage tank

Back to collector field

390 °C

Heat exchanger


to a critical reaction. In general, the TES cycle is not pressurized but an inert gas (nitrogen blanket) is used to prevent any decomposition of the molten salt. It is known that oxidizing nitrate salts reduce the onset of the decomposition temperature of the heat transfer fluid HELISOL® 5A. Therefore, an exothermic reaction between the two compounds is to be expected. In an assessment study, the influence of the solar salt on the decomposition of HELISOL® 5A was determined by measuring 1:1 mixtures of HTF and molten salt. The onset temperature at which the exothermic decomposition of the mixture starts was determined and compared to the onset temperature of the pure components (via differential scanning calorimetry, DSC). In the presence of solar salt, the onset temperature for exothermic decomposition of HELISOL® 5A was measured at 541 °C. According to TRAS 410 (identification and control of exothermic chemical reactions), the probability of an immediate reaction of HELISOL® 5A with the molten salt after contact is to be classified as low, as the operating temperature (425 °C) differs from the onset temperature (541 °C) by more than 100 K. A detailed test report of the assessment described above may be ordered by contacting your local WACKER representative. The principal intrinsic reaction of decomposition in the equimolar binary NaNO3 / KNO3 system over the temperature range 500 – 620 °C is the thermal decomposition of the nitrate ion to form nitrite and oxygen. Equation 1+2: Nitrate/Nitrite equilibrium constant

The ratio of nitrate to nitrite has been shown to be proportional to the partial pressure of oxygen (p02). The summarized results which can be found in the literature are:

• At 220 – 450 °C only small amounts of nitrite can be found. Therefore, no significant oxygen formation is observed either.

• In the temperature range 400 – 500 °C the kinetics of the thermal decomposition of nitrate to form nitrite and oxygen could be ignored due to the slow reaction rate.

• At 450 – 600 °C the main thermal decomposition is the partial dissociation of the nitrate ion to nitrite ion and oxygen (see Eq. (1)). But this reaction is also very slow, for example it takes 500 h continuous operation at 550 °C to change the molar ratio of NO2

−/NO3− to 0.1.

Additional results:

• The oxygen formation of molten salt which was heated at 425 °C for 240 hours is in the range of the detection limit and therefore not significant.

• If HELISOL® heat transfer fluids are added to molten salt at 425 °C no reaction could be observed. Instead, the silicone HTF evaporates quickly and enriches the surrounding atmosphere.

If a leakage in the heat exchanger occurs, the HELISOL® heat transfer fluid will be forced into the storage medium due to a higher pressure in the HTF circuit. The temperature in the HTF circuit is 425 °C, the TES cycle is operated at lower temperatures. Therefore, the molten salt could only be heated up to the maximum recommended operating temperature of 425 °C for a short time. According to the summarized results, below 500 °C the thermal decomposition of nitrate to form nitrite and oxygen could be ignored. Therefore, it is not possible to reach even the limiting oxygen concentration of 9.2% of HELISOL® 5A in the short event of a leakage in the TES heat exchanger. This would take at least >500 h of an uninterrupted decomposition at 550 °C according to Olivares2 et al. HELISOL® heat transfer fluids do not react with the molten salt, instead they quickly evaporate. This for example could be recognized by an increase of the internal pressure so that appropriate countermeasures may be taken.

2 R.I. Olivares, SolarEnergy 86 (2012), 2576–2583

(1) NO3− = NO2

− + ½O2 (2) Keq = [NO2

−]/[NO3−] × pO2


Wacker Chemie AG Hanns-Seidel-Platz 4 81737 Munich, Germany Phone: +49 89 6279-0 www.wacker.com/contact www.wacker.com Follow us on:

The data presented in this medium are in accordance with the present state of our knowledge but do not absolve the user from carefully checking all supplies immediately on receipt. We reserve the right to alter product constants within the scope of technical progress or new developments. The recommendations made in this medium should be checked by preliminary trials because of conditions during processing over which we have no control, especially where other companies’ raw materials are also being used. The information provided by us does not absolve the user from the obligation of investigating the possibility of infringement of third parties’ rights and, if necessary, clarifying he position. Recommendations for use do not constitute a warranty, either express or implied, of the fitness or suitability of the product for a particular purpose.

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