eiga doc 07 03 e
DESCRIPTION
Metering of Cryogenic LiquidsTRANSCRIPT
METERING OF CRYOGENIC LIQUIDS
IGC Doc 07/03/E
Revision of IGC Doc 07/76
EUROPEAN INDUSTRIAL GASES ASSOCIATION
AVENUE DES ARTS 3-5 • B – 1210 BRUSSELS Tel : +32 2 217 70 98 • Fax : +32 2 219 85 14
E-mail : [email protected] • Internet : http://www.eiga.org
EIGA 2003 - EIGA grants permission to reproduce this publication provided the Association is acknowledged as the source
EUROPEAN INDUSTRIAL GASES ASSOCIATION Avenue des Arts 3-5 B 1210 Brussels Tel +32 2 217 70 98 Fax +32 2 219 85 14
E-mail: [email protected] Internet: http://www.eiga.org
IGC Doc 07/03/E
METERING OF CRYOGENIC LIQUIDS
KEYWORDS
• CRYOGENIC
• METERING
• LIQUID
Disclaimer
All technical publications of EIGA or under EIGA's name, including Codes of practice, Safety procedures and any other technical information contained in such publications were obtained from sources believed to be reliable and are based on technical information and experience currently available from members of EIGA and others at the date of their issuance. While EIGA recommends reference to or use of its publications by its members, such reference to or use of EIGA's publications by its members or third parties are purely voluntary and not binding. Therefore, EIGA or its members make no guarantee of the results and assume no liability or responsibility in connection with the reference to or use of information or suggestions contained in EIGA's publications. EIGA has no control whatsoever as regards, performance or non performance, misinterpretation, proper or improper use of any information or suggestions contained in EIGA's publications by any person or entity (including EIGA members) and EIGA expressly disclaims any liability in connection thereto. EIGA's publications are subject to periodic review and users are cautioned to obtain the latest edition.
IGC DOC 07/03
Table of Contents
1 Introduction.......................................................................................................................................1
2 Scope ...............................................................................................................................................1
3 Definitions.........................................................................................................................................1 3.1 Ancillary equipment...................................................................................................................1 3.2 Batch .........................................................................................................................................1 3.3 Bi-phase ....................................................................................................................................1 3.4 Calculator ..................................................................................................................................1 3.5 Cavitation ..................................................................................................................................1 3.6 Conversion device.....................................................................................................................2 3.7 Cryogenic temperatures............................................................................................................2 3.8 Density compensation...............................................................................................................2 3.9 Equilibrium ................................................................................................................................2 3.10 Error.......................................................................................................................................2 3.11 Flash......................................................................................................................................2 3.12 Flow straightener...................................................................................................................2 3.13 Head, developed ...................................................................................................................2 3.14 Head, pressure......................................................................................................................2 3.15 Head, total .............................................................................................................................2 3.16 Head, velocity ........................................................................................................................2 3.17 Normal boiling point...............................................................................................................2 3.18 Master meter .........................................................................................................................3 3.19 Measuring system .................................................................................................................3 3.20 Meter .....................................................................................................................................3 3.21 Measurement transducer ......................................................................................................3 3.22 Metering section ....................................................................................................................3 3.23 Net positive suction head (NPSH).........................................................................................3 3.24 NPSH minimum.....................................................................................................................3 3.25 Pattern approval ....................................................................................................................3 3.26 Pressure raising.....................................................................................................................3 3.27 Pressure raising device .........................................................................................................3 3.28 Primary element ....................................................................................................................3 3.29 Printing mechanism...............................................................................................................3 3.30 Rangeability...........................................................................................................................4 3.31 Rated capacity.......................................................................................................................4 3.32 Recorder................................................................................................................................4 3.33 Repeatability error .................................................................................................................4 3.34 Sensor ...................................................................................................................................4 3.35 Standard conditions...............................................................................................................4 3.36 Standard volume ...................................................................................................................4 3.37 Sub-cooling............................................................................................................................4 3.38 Temperature compensation ..................................................................................................4 3.39 Totalizer.................................................................................................................................4 3.40 Traceability ............................................................................................................................4 3.41 Vapour pressure....................................................................................................................4 3.42 Vapour return.........................................................................................................................5 3.43 Vent .......................................................................................................................................5 3.44 Verification.............................................................................................................................5 3.45 Verification, initial...................................................................................................................5 3.46 Working conditions ................................................................................................................5 3.47 Zero set mechanism..............................................................................................................5
4 Properties and handling of cryogenic liquids....................................................................................5 4.1 Properties..................................................................................................................................5 4.2 Handling of medium pressure tanker ........................................................................................6 4.3 Handling of low pressure tanker ...............................................................................................7
5 Safety ...............................................................................................................................................7
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6 Units of measurement ......................................................................................................................7
7 Physical data ....................................................................................................................................8 7.1 Density ......................................................................................................................................8 7.2 Temperature..............................................................................................................................8
8 Description of meters .......................................................................................................................8 8.1 Volumetric meters .....................................................................................................................8 8.2 Volumetric meters with density compensation..........................................................................8 8.3 Direct mass meters ...................................................................................................................8 8.4 Orifice or nozzle of venturi ........................................................................................................8
9 Density compensation ......................................................................................................................8
10 Primary element and its installation..............................................................................................9 10.1 Limitation of use ....................................................................................................................9 10.2 Materials of construction .......................................................................................................9 10.3 Installation .............................................................................................................................9
10.3.1 Metering section.................................................................................................................9 10.3.2 Flow straightener ...............................................................................................................9 10.3.3 Filter ...................................................................................................................................9 10.3.4 Pre-cooling section ............................................................................................................9 10.3.5 Vapour return line ............................................................................................................10 10.3.6 Diversion of measured liquid ...........................................................................................10 10.3.7 Check valve .....................................................................................................................10 10.3.8 Safety valves....................................................................................................................10
11 Ancillary equipment ....................................................................................................................10 11.1 Limitation of use ..................................................................................................................10 11.2 Meter readings.....................................................................................................................10 11.3 Non-resettable summary counter ........................................................................................10 11.4 Computing / printing devices ...............................................................................................10 11.5 Equipment for density compensation ..................................................................................11 11.6 Test connection ...................................................................................................................11
12 Operating conditions...................................................................................................................11 12.1 Cool down............................................................................................................................11 12.2 Vapor elimination.................................................................................................................11 12.3 Flowrate...............................................................................................................................12 12.4 Drainage of discharge lines.................................................................................................12 12.5 Environmental conditions for ancillary equipment...............................................................12 12.6 Instruction and training of operators....................................................................................12
13 Maintenance ...............................................................................................................................12 13.1 Leak tests ............................................................................................................................12 13.2 Filter cleaning ......................................................................................................................12
14 Control of measuring devices and systems................................................................................13 14.1 Approval and verification of meters.....................................................................................13
14.1.1 Pattern approval ..............................................................................................................13 14.1.2 Provisional pattern approval ............................................................................................13 14.1.3 lnitial verification...............................................................................................................14
14.2 Approval and verification of measuring systems.................................................................14 14.2.1 Pattern approval ..............................................................................................................14 14.2.2 Initial installation...............................................................................................................14 14.2.3 Subsequent installations..................................................................................................14
14.3 Subsequent verification of measuring devices....................................................................14 15 Test facilities and procedures.....................................................................................................14
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15.1 Mass calibration...................................................................................................................15 15.2 Volumetric calibration ..........................................................................................................15 15.3 Gravimetric test facility ........................................................................................................15
15.3.1 Measuring method with flying start-stop ..........................................................................15 15.3.2 Measuring method with standing start-stop.....................................................................15 15.3.3 Measurement values .......................................................................................................15
15.4 Transfer standard ................................................................................................................16 16 Error limits...................................................................................................................................16
16.1 Maximum permissible errors at verification.........................................................................16 16.2 Minimum delivery.................................................................................................................16 16.3 Maximum permissible errors for test standards ..................................................................17
16.3.1 Gravimetric Standard (weigh scale) ................................................................................17 16.3.2 Temperature Standard.....................................................................................................17 16.3.3 Pressure Standard...........................................................................................................17 16.3.4 Time .................................................................................................................................17
16.4 Flow rates of a measuring system or a meter .....................................................................17 16.5 Minimum measured quantity ...............................................................................................17
Appendix 1: References ........................................................................................................................18
Appendix 2: List of cryogenic meters and their operating principles.....................................................19
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1 Introduction
The aim of this document is to formulate, according to present knowledge and experience, the conditions under which cryogenic liquids can be satisfactorily metered during transfer, e.g. from the supplier's delivery tanker to the customer's equipment. The products concerned are the cryogenic gases, oxygen, nitrogen, argon and the non cryogenic gas carbon dioxide according to the recommendations of OIML (1) (2). For other liquefied gases the principles of the recommendations of this document may be used but also the recommendations of OIML (2) have to be taken into account. The document includes recommendations for approval procedures and verification tests of flow-meters and measuring Systems. The recommendations are taking into account the proposal for a European Directive (3). (Some European countries do not yet have regulations controlling the metering of cryogenic liquids.) However, in the formulation of this recommendation the knowledge and experience of the European gas industry have been applied to ensure that the document is soundly based and incorporates the best available technology. In addition, account has been taken of the many practical difficulties which arise in metering due to the particular properties of these liquids. In this way the recommendations have been designed to ensure the correct functioning and accuracy of flow meters used for cryogenic liquid metering. Installation features and methods of operation are both highly significant factors in accurate metering of cryogenic liquids. These are dealt with in some detail in the document. In order to make it a document of continuing usefulness, at least in the short term, account has also been taken of such known meter operating principles as are likely to be introduced in the foreseeable future.
2 Scope
This document applies to the quantitative measurement of the cryogenic liquids oxygen, nitrogen, argon and carbon dioxide when supplied to customers using measuring devices mounted either on road tanker vehicles, on demountable-tanks, on tank-Containers or on fixed supply installations.
3 Definitions
The terms used in the document and its explanations are in conformity, as far as possible, with the available national and international Standards and recommendations, particularly the International Organisation of Legal Metrology (OIML) recommendations (1) (2) and the proposal for the directive of the Council of the Commission of the European Communities for measuring instruments (3) ).
3.1 Ancillary equipment
Secondary element (s) used to convert the output from a primary element to meaningful indication of flow rate or quantity.
3.2 Batch
A measured quantity of liquid passed through a meter for test, calibration, or delivery purposes.
3.3 Bi-phase
Containing both gas and liquid.
3.4 Calculator
A part of the meter that receives the output signal from the transducer(s), transforms it and, if appropriate, store in memory the results until they are used. Additionally, the calculator may be capable of communicating both ways with the peripheral equipment Calibration All the operation for the purpose of determining the values of the errors of a measuring instrument (and if necessary to determine other metrological properties).
3.5 Cavitation
Formation of a bi-phase mixture as a result of a pressure reduction in allowing liquid.
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3.6 Conversion device
A device that automatically converts the volume measured at metering conditions into a volume at base conditions, or into a mass by taking account of the characteristics of the measured liquid (temperature, pressure, density, relative, density etc.) using associated measuring instruments, or store in a memory, The ratio of the volume in base conditions, or the quotient of the mass, to the volume at metering conditions is called a “conversion factor”
3.7 Cryogenic temperatures
Temperatures below -153°C Note: This definition is based on the requirements of OIML. Other regulations e.g. for transport, define
higher temperatures.
3.8 Density compensation
Density compensation is the use of ancillary equipment to obtain from the output of a volumetric meter an equivalent mass flow.
3.9 Equilibrium
A state in which the fluid is at the boiling point corresponding to its pressure.
3.10 Error
The discrepancy between the result of the measurement and the value of the quantity measured.
3.11 Flash
Gas evolved from a liquid as a result of a reduction in pressure.
3.12 Flow straightener
A device used on the upstream side of a primary element to ensure freedom from swirl at entry.
3.13 Head, developed
Difference between total heads at pump suction and delivery.
3.14 Head, pressure
Pressure expressed as the height of a column of liquid.
3.15 Head, total
The sum of pressure and velocity heads.
3.16 Head, velocity
Kinetic energy of fluid expressed as pressure or head of liquid.
3.17 Normal boiling point
That temperature at which a liquid vaporizes or boils at the pressure of 101.325 kPa
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3.18 Master meter
A working standard, traceable to national standards, used for the verification of cryogenic liquid measuring devices and systems
3.19 Measuring system
A system that is comprised of the meter itself and all ancillary devices and other equipment assembled to carry out a specified measurement task.
3.20 Meter
An instrument designed to measure continuously, memorize and display the quantity that passes through the measurement transducer Note: A meter includes at least a measurement transducer, a calculator (including adjustment or correction devices if present), conversion device (if necessary), and an indicating device. The calculator may be mechanical
3.21 Measurement transducer
A part of the meter that transforms the flow of the liquid to be measured into signal/s which are sent to the calculator. It may either be autonomous or use an external source
3.22 Metering section
A section of pipe work specially designed to accept the primary element and including other devices aimed at improving accuracy of measurement.
3.23 Net positive suction head (NPSH)
Total head of liquid at the inlet to a pump above the equilibrium pressure head.
3.24 NPSH minimum
The minimum required total head of liquid above the equilibrium pressure head at the pump section for the pump to work without cavitation.
3.25 Pattern approval
A decision taken by a competent organisation, e.g. national service of legal metrology, recognizing that the pattern of a measuring device conforms to the mandatory requirements.
3.26 Pressure raising
Raising a pressure above a liquid obtained e.g. by vaporisation of part of the liquid.
3.27 Pressure raising device
Heat exchanger used for raising gas space pressure above the liquid contained in a vessel.
3.28 Primary element
Measuring element placed in the fluid stream being measured and giving the basic indication.
3.29 Printing mechanism
A numerical quantity printing mechanism connected to the indicator of a meter.
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3.30 Rangeability
Range of maximum to minimum flowrate of a meter.
3.31 Rated capacity
Maximum flowrate specified by the manufacturer.
3.32 Recorder
A device which gives a record of the measured values.
3.33 Repeatability error
For the purposes of this document, the difference between the largest and the smallest result of successive measurement of the same quantity carried out under the same conditions.
3.34 Sensor
A device normally placed in the fluid stream to detect variations in a property of the fluid e.g. temperature, pressure.
3.35 Standard conditions
Conditions of temperature and pressure fixed as a basis for the expression of a mass of gas or liquid in volumetric units.
3.36 Standard volume
Expression in volumetric units of a mass of gas or liquid by relating it to a fixed temperature and pressure (standard conditions).
3.37 Sub-cooling
The condition of a liquid under which its temperature is lower than the temperature it would have in equilibrium at the working pressure. It may also be expressed as an excess of pressure of the liquid above its equilibrium pressure.
3.38 Temperature compensation
The use of temperature to determine density of a liquid and using this to adjust the measured volume to standard conditions.
3.39 Totalizer
A device which displays integrated flow, usually in digital form.
3.40 Traceability
The property of a result of a measurement whereby it can be related to appropriate standards, generally national or international standards, through an unbroken chain of comparisons each with a stated uncertainty.
3.41 Vapour pressure
The gas pressure over a liquid at thermodynamic equilibrium.
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3.42 Vapour return
A pipe used to return vapour to the gas space of a tank.
3.43 Vent
A pipe which allows vapour to removed from a liquid system.
3.44 Verification
All the operations carried out by a competent organisation, e.g. national service of metrology, having the object of ascertaining and confirming that the measuring device entirely satisfies the requirements of the regulations for verifications.
3.45 Verification, initial
The verification of a new measuring device which has not been verified previously.
3.46 Working conditions
Temperature and pressure at the point of metering.
3.47 Zero set mechanism
An arrangement which returns the indicator to zero either by a manual operation or by an automatic system.
4 Properties and handling of cryogenic liquids
4.1 Properties
Cryogenic* liquids are liquefied and deeply refrigerated gases with a boiling point of less than 120 K (-153°C). The atmospheric gases oxygen, nitrogen and argon which are included in this category, are liquefied by cooling to their boiling point. The following are approximate values of some of the relevant properties of these gases :
* Etymologycally cryogenics (cryos - frost, geinomai - to produce, to engender) means the science and art of producing cold, and anything directly re/ated to it, in particular below 120 K (9).
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Properties of atmospheric gases under
ambient conditions
Oxygen Nitrogen Argon
Boiling point 90 K (-183°C) 77 K (-196°C) 87 K (-186°C)
Liquid density at boiling point 1.14 kg/l 0.81 kg/l 1.39 kg/l
Gas density 1 .34 kg/m3 1.1 7 kg/m3 1.67 kg/m3
Density ratio (liquid / gas) 850 690 830
Note : The values in the above table have been rounded off. They should not be used for accurate
scientific assessments. The large ratio of liquid to gas density has been used to economic advantage since the early 1930's by distributing the products as bulk liquid in tankers. This has necessitated the development of thermally efficient insulated storage and transport vessels to minimize the amount of heat entering the cryogenic liquid from the atmosphere. Heat in leak has the effect either of increasing the liquid volume and pressure if venting of the containing vessel is restricted, or of producing evaporation of the liquid if the vessel is freely venting. For economic reasons loss of gas must be kept to a minimum and therefore in practice pressure vessels are normally used so that expansion and evaporation of the liquid can be taken care of as a rise in pressure. (A typical rate of equilibrium pressure rise for a 10.000 liter vacuum insulated liquid nitrogen transport vessel is 0.007 bar/h, equivalent to 0.04 K/h rise in temperature of the liquid. On free vent, the corresponding evaporation rate is approximately 4 m3 gas/h, measured at ambient conditions). All tanks containing cryogenic liquids are equipped with means of controlling their pressure. Liquid is normally transferred from supplier to customer using road tankers either with or without the use of transfer pumps. Since the customer's storage tank usually operates at pressures of the order of 4 to 17 bar, the tanker must either operate at similar pressures (medium pressure tanker) or be equipped with a pump (Iow pressure tanker) in order to effect the transfer. Both types of system are used for this purpose, the pump system being the more common one. (Deliveries are also made to Iow pressure storage tanks which do not require pressures available from the medium pressure tanker or centrifugal pump. The same vehicles and similar procedures are used for such transfer). Liquid and gas in a road tanker are normally in thermodynamic equilibrium on arrival at the delivery point due to movement of the liquid in transit. This is the case whether the tanker is of the medium or Iow pressure type.
4.2 Handling of medium pressure tanker
To effect a transfer, pressure is raised in the tanker to a higher level than that of the storage tank. The additional pressure must be sufficient to overcome pressure losses in the pipe work and to ensure that liquid pressure remains above the equilibrium pressure, thus avoiding formation of gas bubbles in the liquid. This is achieved by withdrawing liquid through a pressure-raising heat exchanger and returning
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it to the tanker ullage space as gas. The pipe work System, including flow meter, is then carefully pre cooled before beginning the transfer of liquid.
4.3 Handling of low pressure tanker
Pressure is raised in the road tanker by the same means as for the medium pressure tanker, but only to a level slightly above the equilibrium pressure, typically by 0.5 bar. This is to avoid vapor bubbles entering the pump suction line and causing cavitations. The centrifugal pump and pipe work are carefully precooled using a closed circuit return line to the tanker vessel. The flowmeter situated downstream of the pump is simultaneously precooled. adequacy of precooling is checked before pump start-up by opening a vent valve on the upstream side of the meter of check for the presence of liquid. During the transfer, liquid velocity in the short length of pipe between tanker vessel and flowmeter is high. Even with an uninsulated line, which is normal practice, the rise in temperature of liquid passing along the pipe is very small. It is in any case compensated for by the initial raising of pressure in the road tanker vessel. The transfer System is operated on a drained hose principle. At completion of a delivery, the main delivery valve on the road tanker is closed and part of the contents of the flexible hose vaporizes into the storage tank due to heat in leak. The remaining gas and liquid content of the hose is vented to atmosphere before the hose is disconnected. The contents of the System upstream of the delivery valve are returned to the road tanker vessel. Reverse flow of liquid from storage tank to road tanker is prevented by the fitting of a check valve in the tanker delivery pipework. There is no separate connection between the gas phases of road tanker and storage tank. Both road tanker and storage tank Systems are designed to ensure safe conditions at all times. This is accomplished by the choice of suitable materials of construction. Safety valves are provided for relief of excessive pressure in vessels and in pipework, e.g. between stop valves where liquid may be trapped. Drivers are trained and experienced in the use of safe procedures for the transfer of cryogenic liquids.
5 Safety
Most oils, greases and organic materials constitute a fire or explosion hazard with oxygen or oxidizing gas and must on no account be used on equipment which can come into contact with it. Where contact with oxygen is possible, metallic and non-metallic materials shall be compatible with oxygen. All equipment which can come into contact with oxygen or oxidizing gas shall be specifically designed and prepared for oxygen service. Before use, all parts of the flowmeter which are normally in contact with oxygen must be degreased. The temperature of liquid nitrogen corresponding to an equilibrium pressure of 0.55 bar (gauge) or less is Iow enough to cause condensation of atmospheric air on the external surfaces of the meter or pipes containing the liquid. The liquid air so produced may be enriched with respect to oxygen (5). Materials which are incompatible with oxygen must not be used at these points. Cryogenic liquids and cold gaseous products can cause severe cold burns when in contact with the skin. Contact with uninsulated piping can also cause cold burns to the skin.
6 Units of measurement
The indicated and recorded measurement of a device shall be in the following: 1 m3 gas at a fixed reference pressure and temperature for volumetric meters with density
compensation or mass meters. 1 kg for mass meters or volumetric meters with density compensation 1 l for volumetric meters or decimal multiple or sub multiple thereof As a guide it is suggested that the value of the smallest unit indicated or recorded should be 1 m3 or 1 kg or 1 l.
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7 Physical data
7.1 Density
The latest published values of the National Institute of Standards and Technology (NIST) (4) or the “Bureau International de Metrologie Legale” (6) shall be used for specific density values.
7.2 Temperature
It is recommended that fixed temperature points based on the International Practical Temperature Scale of 1990 (ITS 90) (7) be used.
8 Description of meters
The types and operating principles of meters in general use in, or having possible application to, the cryogenic industry are listed in Appendix2.
8.1 Volumetric meters
A volumetric meter is one which measures flow in terms of volume at the working conditions. This type of meter has been commonly used for cryogenic liquid transfer to date.
8.2 Volumetric meters with density compensation
To convert the measured volume into mass a density correction is applied to the output-signal from the primary element, using a device to measure or infer liquid density close to the point of metering. This type of meter is widely used for cryogenic liquid transfer.
8.3 Direct mass meters
Due to the physical principles on which they operate, direct mass meters measure the true mass of liquid passing through the primary element, irrespective of the density of the liquid, e.g. systems based on Coriolis force.
8.4 Orifice or nozzle of venturi
Change in velocity of the fluid passing through the element causes a change in kinetic energy. Differential pressure generated is a measure of fluid velocity. An orifice plate or venturi device of known dimensions is placed in a length of pipe. Differential pressure created by flow of liquid through the restriction is measured using tappings located upstream and downstream. See also (15)
9 Density compensation
If a volumetric meter is used in order to convert the output-signal from the primary element to mass units (e.g. kg or m³ gas at 1 bar, 15°C) the actual density of the liquid at the time of metering must be determined. Any convenient method of measuring or inferring density may be used for this purpose, for example measurement of temperature or dielectric constant, or direct measurement of density. The correction based on these measurements may be applied either manually, using conversion tables or other calculation techniques, or automatically, using electronic or other computing devices. Note: In general it is not necessary to consider the influence of pressure on the density of cryogenic
liquids, due to the uncertainty of the published density data and to the fact that the dependency of the density upon temperature is very much larger than upon pressure.
A further method of density compensation is to relate the delivery amount to a fixed density value based on a mean liquid temperature which must be determined by operational tests. This method may be used if it is found that the actual error of the measuring system, including the error due to the
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temperature increase between first and last deliveries during a journey is smaller than the maximum permissible error.
10 Primary element and its installation
10.1 Limitation of use
If a meter is intended to measure accurately only liquids having particular properties, or to measure accurately only under specific installation or Operation conditions, or to measure accurately only when used in conjunction with specific ancillary equipment, these limitations shall be clearly and permanently stated in the appropriate operating manual. In addition the specific operating conditions shall be clearly and permanently stated close to the operating point.
10.2 Materials of construction
Materials of parts of the measuring System in contact with the cryogenic liquid must be suitable for the lowest operation temperature of the gas e.g. 77 K (-196°C) for air gases . Where contact with oxygen or oxidizing gas is possible, metallic and non-metallic materials shall be compatible with oxygen.
10.3 Installation
10.3.1 Metering section
To ensure correct Operation most flow meters require undisturbed conditions at the inlet, usually achieved by specifying minimum straight lengths of pipe upstream and downstream of the primary element. Manufacturer's specifications shall be observed for particular meters, but in the absence of such data ten pipe diameters upstream and five downstream should be regarded as a minimum. It is important that these lengths of pipe are of the same bore as the meter inlet and outlet respectively. Neither of these requirements is necessary in the case of positive displacement meters.
10.3.2 Flow straightener
Many types of flow meters are severely affected by a swirling flow. Control valves, pumps and some pipework configurations are all possible sources of swirl which may persist over long runs of pipe. The fitting of a flow straightener at the upstream and of the straight pipe upstream of the primary element is good practice. In the absence of manufacturer's specifications it is recommended that the length of a flow straightener should be not less than two pipe diameters.
10.3.3 Filter
If foreign bodies can affect the normal functioning of the primary element, a filter shall be provided upstream of the metering section. Take care not to install filters that interfere with correct operation or accuracy of the flow measuring device.
10.3.4 Pre-cooling section
It is essential that the whole of the installation between the tank and the primary element be cooled to liquid temperature before a delivery is started. For this purpose a pipe, with a manual control valve, should be fitted to the line immediately upstream of the meter, and should return to the gas space of the tank. It is important for this pipe to be installed so that the normal flow of liquid to the meter is disturbed as little as possible. It may be necessary to provide a manual test valve on this pipe at a point close to the meter so that the presence of liquid may be confirmed.
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10.3.5 Vapour return line
If a vapour return line between the supplier's tank and the customer's tank is used the quantity of the gas return has to be measured and taken into account.
10.3.6 Diversion of measured liquid
No means shall be provided by which any measured liquid can be diverted from the measuring chamber of the primary element or the discharge line there from. However a manually controlled outlet to atmosphere may be provided for purging or draining or for the purpose of pre cooling the system.
10.3.7 Check valve
An automatic check valve operating in the direction of flow must be provided on the outlet side of the primary element to prevent reversal of flow.
10.3.8 Safety valves
Safety valves installed after the primary element must be adjusted to a set pressure which is not Iower than the maximum working pressure of the transfer system. These valves shall be installed so that it can be clearly seen if they open. This is normally achieved by observing the escape of gas from this valve outlet.
11 Ancillary equipment
11.1 Limitation of use
If ancillary equipment is intended to measure accurately only under specific installation or operating conditions or over a fixed range, these limitations shall be clearly and permanently stated in the appropriate operating manual. In addition the specific operating conditions shall be clearly and permanently stated close to the operating point.
11.2 Meter readings
Readout devices of a meter must be clearly and conspicuously marked with the units of measurement. In the case of volumetric meters the reference conditions, and whether expressed in terms of gas or liquid, shall also be clearly stated on the readout device. For example :
kg l (liquid, at NBP*) m³ (gas at 15°C and 1 bar) * Normal Boiling Point
11.3 Non-resettable summary counter
In addition to the normal resettable counter for indicating the delivery amount, all meters shall be equipped with a non-resettable counter.
11.4 Computing / printing devices
If a computing/printing device is included in the measuring system, any printed ticket issued by the device shall indicate the total amount of product metered in the units given in clause 6, together with the product concerned and, in the case of volume indication, the reference conditions whether expressed in terms of gas or liquid. The unit indication (m³, kg, I) must also be printed in case of m³ and l together with the reference conditions.
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11.5 Equipment for density compensation
Density compensation equipment should be designed for the temperature range over which the measuring System operates. Note: Typical temperature ranges are:
-165 to -185°C for oxygen/argon -175 to -195°C for nitrogen.
A preferred method of temperature measurement is to use a platinum thermometer Pt 100.
11.6 Test connection
A test connection may be provided for checking measuring device outputs by means of test instruments.
12 Operating conditions
12.1 Cool down
In order to prevent vaporization of the liquid in the metering section, the pipe system from the tank to the primary element must be thoroughly cooled down and the pipes upstream of the primary element completely filled with liquid. It is necessary to check the presence of liquid, e.g. using the manually operated test valve on the pre cooling line.
12.2 Vapor elimination
A measuring system equipped with a volumetric device shall be so operated that the product being measured remains in the liquid state during passage through the primary element. Before starting the delivery it is necessary to raise the pressure so that the pressure in the whole system is higher than that of the liquid in its equilibrium state and the amount of sub cooling is sufficient to keep the primary element within its maximum permissible error. This pressure increase has to be maintained and checked during delivery. Note: Most meters show an increase in error when sub cooling drops below a certain level. Pressure
differences equivalent to a typical sub cooling figure of 8 K are shown below: Liquid nitrogen Liquid oxygen Liquid argon
Equilibrium pressure (bar) 1 10 1 10 1 10
Pressure difference equivalent to sub cooling of 8K (bar)
1.3 6.2 1.2 5.5 1.2 5.5
It is recommended that the actual boundary level be established for the type of meter concerned and that minimum pressures be specified to ensure that the required amount of sub cooling is achieved. When using an external centrifugal pump it is recommended that the upstream pressure be increased by at least 0.5 bar in view of the pressure losses caused by the usual pipe and pump installations. The pressure increase in the centrifugal pump provides additional sub cooling of the liquid passing through the primary element. In the case of fixed installations the head of liquid above the pump can be taken into account when providing the necessary sub cooling.
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A measuring system equipped with a direct mass meter shall be operated so that the proportion of gas bubbles to liquid is within the known or approved operating capability of the device.
12.3 Flowrate
The actual flow rate must be kept within the flow range of the primary element. This is achieved by proper matching of the flow range to the flow capability of the transfer system. In the special case of the medium pressure transfer system without pump the meter should be operated normally at between half and full output. The operator has to ensure that the correct flow conditions are maintained during the transfer.
12.4 Drainage of discharge lines
It is common practice to use a "drained hose" system for transferring cryogenic liquids. Due to the physical properties of these liquids, it is not possible to transfer the whole contents of the hose to the customer at the end of a delivery. Steps must be taken to ensure that at the end of a delivery the amount of liquid which has passed through the meter but does not reach the receiving tank (e.g. liquid discharged to atmosphere from the hose) is in accordance with para 16.2. In practice this may be achieved e.g. by suitable design of the pipe work system, by specifying appropriate minimum delivery amounts, by applying a fixed credit allowance to invoice totals, or by partially evaporating the liquid into the customer's tank by pressure increase in the hose.
12.5 Environmental conditions for ancillary equipment
The ancillary equipment used in conjunction with a primary element should be suitable for the environment in which it operates. The primary feature of a typical cryogenic installation is that equipment should be capable of withstanding cold, damp conditions in proximity to pipe work which is uninsulated and carrying liquids at cryogenic temperatures. The ancillary equipment should be installed so that the working temperature is within the specified temperature limits.
12.6 Instruction and training of operators
It is important that the operators of measuring Systems (portable or fixed) should receive proper training, not only in measuring system operation but also in the general problems of handling cryogenic liquids. Written operating instructions must be made available to the operator, and these should cover e.g. the action to be taken in the event of an Interruption of transfer. Annual refresher training courses are advisable.
13 Maintenance
Regular routine checks should be made in situ to confirm satisfactory condition of the measuring System.
13.1 Leak tests
Screwed and flanged joints and vent valves before and after the primary element should be tested for leaks.
13.2 Filter cleaning
Regular inspection and cleaning of filters is necessary and is particularly important after commissioning new tanks or pipework or after their repair.
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14 Control of measuring devices and systems
14.1 Approval and verification of meters
Based on legal rules it is established practice in many areas of custody transfer that meters are controlled by a competent organisation e.g. a national service of legal metrology. The stages of control (1) are: • pattern approval of a new meter type • initial verification of individual meters to confirm that limits of error are within the
permitted maximum • and subsequent verification at stated intervals to check that the requisite stability of
measurement is maintained. The request for approval and initial verification is the responsibility of flowmeter manufacturers or their authorized representatives. The manufacturer of a measuring device must submit his product to a competent organisation before it may be used in custody transfer. This also gives user, supplier and customer a guarantee concerning accurate and reliable operation. Subsequent verification at stated intervals is effected under the responsibility of the competent organisation. The controls may be done either by the user himself or by an association of users by agreement with the competent organisation. Based on appropriate tests the competent organisation shall determine the parts of a verified measuring device - primary element, ancillary equipment or parts of them -which may be replaced in service by examined parts without immediately requiring verification of the measuring device as a whole. For practical reasons the control of the whole measuring system shall be effected during the next periodic subsequent verification.
14.1.1 Pattern approval
Pattern approval is defined as a decision taken by a competent organisation recognizing that the pattern of a measuring device conforms to the mandatory requirements. The manufacturer submits one to three measuring devices of the same type to the competent organisation. Examination comprises the tests necessary for the purpose of approving the pattern and should include the influence of the properties of the different cryogenic liquids to be used. The tests must establish the criteria to be observed for each separate element of the measuring device in such a way that the total error of the measuring device with a cryogenic liquid does not exceed its maximum permissible error.
14.1.2 Provisional pattern approval
Provisional pattern approval is defined as an approval, of a pattern of measuring device for a limited period, in order to allow comprehensive and prolonged tests, under normal conditions of use, of a sufficiently large number of Instruments conforming to the pattern. Since endurance testing is impracticable with cryogenic liquids - due to complexity of the test equipment and the difficulty of reproducing typical operating conditions - provisional pattern approval of a new type of meter may be necessary in order to allow both user and competent organisation to gain sufficient operating experience and establish a satisfactory working life of the device by testing it under normal operating conditions. The manufacturer shall, in agreement with the competent organisation, place two or more measuring devices which have been given provisional approval at the disposal of the user for actual operational tests. It is recommended that by agreement with the competent organisation the first three devices of a new type of meter should be put into service and examined after six months operation. Final pattern approval should be given when the results are found to be satisfactory.
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14.1.3 lnitial verification
The manufacturer of a measuring device shall be required to submit it for initial verification. The tests for verification may be carried out on an approved test facility using one of the cryogenic liquids for which the device is designed. In this case, preliminary tests by agreement with the competent organisation will be necessary in order to establish the differences between the verification made with the cryogenic liquid and that made with the substitute liquid.
14.2 Approval and verification of measuring systems
14.2.1 Pattern approval
The flow sheet of the measuring system designed for the installation of a verified measuring device and the operating instructions must be submitted by the manufacturer of the measuring system or his authorized representative to the competent organisation for approval. Each modification of the measuring system has to be submitted for approval in advance.
14.2.2 Initial installation
A verification test with one of the cryogenic liquids to be used is required. This test may be made with a fixed or mobile approved standard at the user's option.
14.2.3 Subsequent installations
The competent organisation checks that subsequent installations are identical with the initial Installation. Each installation of a verified measuring device must be in accordance with the drawings of the approved measuring system. A verified meter may by agreement with the competent organisation be mounted on a tanker without further tests, provided that it has once been previously shown by approved tests that the total error of the measuring system does not exceed the maximum permissible error for such a system.
14.3 Subsequent verification of measuring devices
The period between subsequent verifications is determined in agreement with the competent organisation. It would normally be one year but may be prolonged provided sufficient subsequent verification results prove the stability of the meter. Subsequent verification may be carried out • on an approved fixed test facility, the tanker with its measuring system being brought to the place
of testing; • or with an approved mobile standard (transfer standard) transported to the place where the
subsequent verification is to be effected; • or replacing by another verified element. Ancillary equipment may be tested by simulation without using cryogenic liquids and may be replaced if defective by other verified elements. Replacement of any defective element may be effected with another verified examined element by specially qualified personnel in agreement with the competent organisation. For practical reasons the examination of the whole measuring system shall be effected during the next periodic subsequent verification.
15 Test facilities and procedures
Any test facility used for verification purposes shall be approved by a competent organisation. Test facilities generally required for approval and verification purposes may be based on two different principles, mass or volumetric. They may be either fixed or mobile.
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15.1 Mass calibration
A known mass of liquid is passed through the primary element and compared with the mass indicated by the meter-readout. This mass may be expressed either directly in the mass unit or in equivalent Standard volume units of gas. If the primary element operates on a volume basis, the mass of liquid is converted into volume of liquid by taking the density (expressed as a function of temperature and pressure or by direct measurement) into account. Master-meter based on Corriolis-Force or turbine meter with density correction are commonly used. The master meters are used as a Transfer Standard according to clause 15.4. The measuring method is similar to the method described in clause 15.3.
15.2 Volumetric calibration
The present calibration experience is based exclusively on mass calibration systems. There is however no objection in principle to volumetric calibration, e.g. passing a quantity of liquid through the primary element and comparing the indicated output with a direct volumetric measurement of this quantity. Because of lack of experience with this type of test facility, no details are provided in this document.
15.3 Gravimetric test facility
On a gravimetric test facility, normally fixed, measuring devices for determination of the volume and mass of cryogenic liquids can be tested and verified. The cryogenic liquid is transferred through the measuring device from a storage tank by means of a centrifugal pump and is delivered through a flexible connection to a tank fixed on a weigh scale.
15.3.1 Measuring method with flying start-stop
For the meter verification the method using a flying start-stop is recommended. The flow rate should be kept as constant as possible. The Standard measured quantity results from the difference of two weighings; the systematic error due to the connection between meter and weigh-tank can be neglected with this method. The measuring device under test shall be connected to the weigh tank in such a way that the free motion of the weigh scale is guaranteed.
15.3.2 Measuring method with standing start-stop
The test facility shall also be capable of effecting minimum delivery measurements. These measurements can only be effected with the standing start-stop method. The flexible connection pipe is disconnected from the weigh tank prior to each weighing, after the remaining liquid has been largely transferred into the weigh tank in the case of the final weighing. At the time of the first weighing the hose is empty. In the case of minimum delivery measurement it is necessary to use the same type of hose as is used in normal operation. The error of the measuring system, related to the minimum delivery quantity, can thus be determined.
15.3.3 Measurement values
The following Standard measurement values should be available for the error determination: • Volumetric measuring devices with or without density compensation
Mass (kg) Temperature (K) Pressure (bar) Time (s)
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• Mass measuring device Mass (kg) Time (s)
• Determination of density during verification of volumetric meters To determine the error of volumetric meters, the density of the liquid in the primary element being tested must be known. For this reason, both temperature and pressure measuring devices shall be provided close to the primary element. The standard volume for the error determination is calculated from the mass of the liquid measured by weighing and from the density, determined by the temperature and pressure values measured.
15.4 Transfer standard
A transfer standard is a device designed for use in field testing measuring systems. It normally consists of one or more measuring devices complete with piping, valves and instrumentation, mounted on a frame. It is designed to be capable of being transported without damage to any of the equipment and without the need for recalibration of any of the components after transportation. Its primary purpose is the checking of meters by competent organisations in cases where it is either inconvenient or impracticable to move the meters from their points of use, for example for re-verification of meters in situ on tankers. The transfer standard itself is subject to initial verification and subsequent verification at intervals. Proven experiences have shown that gravimetric test facilities are the most suitable method for the verification tests of the master meters
16 Error limits
16.1 Maximum permissible errors at verification
Errors specified in the following table refer to the measured values within the intended measuring range. Type of gases cryogenic gases liquefied gases, e.g. CO2 Measuring system at pattern approval
±2.5%
± 1.5%
Measuring system at initial or subsequent verification under in service conditions
±2.5%
± 1.5%
Meter at pattern approval ± 1.5%
± 1 %
Components at pattern approval: Temperature sensor ± 1 K ± 0.5 K Pressure sensor ± 50 kPa ± 50 kPa Density sensor ± 5 kg/m³ ± 2 kg/m³ Measurement transducer ± 1 % ± 1 % calculator ± 0.25 % ± 0.25 % Conversion device ± 1 % ± 1 % The repeatability error of the measuring system
not greater than 0.2% of the measured quantity
not greater than 0.2% of the measured quantity
16.2 Minimum delivery
The minimum delivery amount shall be fixed such that the maximum possible error at minimum delivery, including losses due to the dry hose system, does not exceed twice the maximum permissible error of the measuring system.
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16.3 Maximum permissible errors for test standards
16.3.1 Gravimetric Standard (weigh scale)
The gravimetric standard should be examined by the competent organisation and have a permissible error of not more than ± 1 kg over the whole measuring range. The total error due to the applied measuring method should not be greater than 1/5 of the error of the tested unit. But when checking the minimum delivery the total error may be max.± 1% of the measured quantity.
16.3.2 Temperature Standard
The uncertainty of measurement of the complete temperature measuring device, including the indication, should not be greater than ± 0.5 K. One preferred method of temperature measurement is to use a platinum resistance thermometer Pt 100 according to (9). The error of the resistance thermometer should not exceed one-third of the admissible deviation, according to the latest published temperature Standards, e.g. International Temperature Scale of 1990 (ITS90) (7) . Examination of the temperature measuring device should be effected using a bath of boiling oxygen (fixed point of ITS 90) and/or a bath of boiling nitrogen (auxiliary point of ITS 90).
16.3.3 Pressure Standard
The uncertainty of measurement of the complete pressure measuring device should not be greater than ± 0.5 bar.
16.3.4 Time
The accuracy of commercially available mechanical or electrical time counters with second’s indication is sufficiently high to make it unnecessary to specify a maximum permissible error.
16.4 Flow rates of a measuring system or a meter
The maximum and the minimum authorized flow rates are specified for a measuring system by the manufacturer. The ratio between the maximum and the minimum flow rates of a meter shall be at least equal to 5
16.5 Minimum measured quantity
The minimum measured quantity of the system shall be specified by the manufacturer. The minimum measured quantity shall not be less the 100 scale intervals. The value of the minimum measured quantity shall be in the form 1*10n, 2*10n, 5*10n authorized units, where “n” being a positive or negative whole number or zero.
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Appendix 1: References
(1) Dynamic measuring devices and systems for cryogenic liquids (including tables of density for liquid argon, helium, hydrogen, nitrogen and oxygen),OIML R81, 1998 International Bureau of Legal Metrology, 11, rue Turgot - F-75009 Paris - France
(2) Measuring systems for liquids other than water, OIML R 117,1995, International Bureau of
Legal Metrology, 11, rue Turgot - F-75009 Paris - France (3) Commission of the European Communities
“Proposal for a Directive of the European Parliament and of the Council on measuring instruments”, COM(2000)566 final, Brussels 15.09.2000
(4) http://webbook.nist.gov/chemistry/fluid/, link to the density values based on NIST (National
Institute of Standards and Technology) (5) Eiga Document "Prevention of accidents arising from enrichment or deficiency of oxygen in the atmosphere" (6) Bureau International de Métrologie Légal
„Cryogenic liquids density tables“ (7) Supplementary Information for the ITS-90” and “Techniques for Approximating the ITS-90".
prepared by the Comité Consultatif de Thermométrie published by the BIPM (Bureau International des Poids et Measures) http://www.bipm.frNPL, Ministry of Technology
(8) EN 60751, Industrial platinum resistance thermometer sensors (9) International Institute of Refrigeration, Paris “Recommendations on low temperature terminology” (10) Mann, Douglas "Flow Measurement Instrumentation" ASRDI, 1974 (11) Brennan, Stokes, Mann and Kneeborne "An evaluation of several cryogenic turbine flow meters" NBS Technical Note 624, 1972 (12) Windgassen "Eichfähiges Mengenmeßgerät für tiefkalte Druckgase" Gas aktuell, 1972 (13) Brennan, Stokes, Kneeborne and Mann
"An evaluation of selected angular momentum, vortex shedding and orifice cryogenic flow meters" NBS Technical Note 650, 1974
(14) Wenzel "Improvements of flow measurement of cryogens" International Congress of Refrigeration Washington DC 1971 (15) EN ISO 5167-1:2003, ' Measurement of fluid flow by means of pressure differential devices
inserted in circular cross-section conduits running full'. 'Part 1: General principles and requirements'
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Appendix 2: List of cryogenic meters and their operating principles
The meters listed in the following table represent a selection of those which have been used or thought to be suitable for cryogenic service. They are typical only, are not intended as a recommendation, and do not necessarily fall within the scope of the document.
Item Type Classification Operating principles Description of operation 1 Turbine
(10,11) Volumetric (indirect) velocity
A freely rotating element in the fluid stream measures mean velocity, volume being proportional to rotor revolutions.
A rotor with blades inclined at an angle to the axis of flow of the fluid is mounted on bearings inside a chamber. The number of revolutions of the impeller due to the passage of the liquid into meter is detected by means of a coil mounted on the chamber.
2 Turbine with density
compensation *)
(11,12)
Mass (indirect) As item 1 with the addition of liquid temperature measurement to give an inferred density. The volumetric output from the turbine is modified according to variations in the liquid density to give flow in mass units.
The meter normally consists of a turbine as item 1, a temperature sensor and an electronic package. Signals from turbine pick -up and temperature sensor are converted by electronic circuits to a mass reading.
*)The principle of using density correction to give a reading of flow in mass units may equally be applied to volumetric primary elements other than the turbine.
Item Type Classification Operating principles Description of operation 3 Coriolis force Mass Measure of mass, in
liquid or vapour form, no influence of temperature, no need for straight piping upstream and downstream
A mass flow dependent Coriolis force occurs when a moving mass is subjected to an oscillation perpendicular to the flow direction. The measuring system accurately determines and evaluates the resulting effects on the measuring tubes.
4 Vortex shedding
(13)
Volumetric (indirect)
Velocity or Mass
An obstruction in the fluid stream sheds vortices at a frequency which is a measure of the velocity
The sensing element is in a stationary bluff body located across the flow stream. The sensor detects vortices shed alternately on cither side in a periodic fashion. The signal produced is directly proportional to volumetric flow rate.
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5 Orifice or nozzle or venturi (10,14)
Volumetric (indirect)
Velocity or Mass
Change in velocity of the fluid passing through the element causes a change in kinetic energy. Differential pressure generated is a measure of fluid velocity.
An orifice plate or venturi device of known dimensions is placed in a length of pipe. Differential pressure created by flow of liquid through the restriction is measured using tappings located upstream and downstream.
6 Ultrasonic Volumetric (indirect)
Velocity or Mass ( indirect)
Determination of fluid velocity by observing the effect of fluid flow on the apparent velocity of propagation of sound waves in the fluid.
A number of alternative forms using different transducer and electronic arrangements are used.