overpressure protection systems - lesson i

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11/06/22 11/06/22 Massimo Benedetto Lead Instrument and Automation Engineer OVERPRESSURE PROTECTION SYSTEMS

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Page 1: Overpressure Protection Systems - Lesson I

lunedì 01 maggio 2023lunedì 01 maggio 2023

Massimo Benedetto

Lead Instrument and Automation Engineer

OVERPRESSURE PROTECTION SYSTEMS

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General Characteristics of Overpressure Protection Systems

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General definition of an PRDA pressure relief device (PRD) is any device that can purge a system from an overpressure condition.

More particularly, an SRV is a pressure relief device that is self-actuated, andwhose primary purpose is the protection of life and equipment.

Through a controlled discharge of a required (rated) amount of fluid at a predetermined pressure, an SRV must prevent overpressure in pressurized vessels and systems, and it operates within limits which are determined by international codes.

The SRV must close at a predetermined pressure when the system pressure has returned to a safe level at values determined by the codes.

SRV must be designed with materials compatible with many process fluids,from simple air and water to the most corrosive and toxic media. They mustalso be designed to operate in a consistently smooth manner on a variety offluids and fluid phases. These design parameters lead to a wide array of SRVproducts available in the market today, with the one constant being that theyall must comply with the internationally recognized codes.

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Causes of OverpressureBlocked outlet.

The system input flow continues to feed the vessel while the outlet is partially or totally blocked due to personnel error, valve failure, actuator failure, lack of power to operate the valve or an operational upset in the control loops.

REMEMBER

The worst case is obviously for the outlet valve to be fully closed.A less severe case could be for the outlet valve not to be fully open, with the system input being greater than the flow through the outlet valve, ultimately causing a system overpressure.

In case there is the possibility of a blocked discharge, the rated capacity of the SRV protecting the system should be based on fully closed valve(s) and take into consideration the maximum flow capacity of the device(s) (pumps, compressors, ventilators, etc.) feeding the system.

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Causes of OverpressureBlocked outlet.

If the flow is pumped, the relief load can be calculated using the pump performance curve to determine the flow at the increased discharge pressure.Wherever possible, equipment in pump discharge systems shall be designed for the maximum closed discharge pressure to eliminate blocked outlet.Often the discharge is routed to pump suction (vessel, tank) to avoid putting liquid relief to the flare system

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Causes of OverpressureFire.

This case covers the event that the pressure vessel is exposed to external fire, which would cause the system to heat up quickly.Vapors would expand; hence there would be a faster increase in pressure above the system design pressure.

REMEMBER

The procedure used for fire sizing depends sometimes on the codes and engineering practices applied in each installation and determined by the end users. The sizing procedure, according to API RP 521 Part 4 is the most commonly used.

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Causes of OverpressureFor the purpose of API 521, fires are characterized as open pool fires, confined pool fires, or jet fires.

— Open pool fire: 50 kW/m2 to 150 kW/m2— Confined pool fire: 100 kW/m2 to 250 kW/m2 — Jet fire: 100 kW/m2 to 400 kW/m2

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Causes of Overpressure

Generally confined pool fire and jet fire cases are not considered for pressure relief devices sizing.

— Confined pool fire: in most cases it behaves like an open pool fire. NOTE: If the ratio between fire volume and confined volume becomes large, then the use of the open pool fire equations could underestimate the heat input to exposed equipment.

— Jet fire: Protection from jet fire exposure is typically addressed through means other than PRDs because failure often occurs due to localized overheating for which a PRD is ineffective.Instead of a pressure-relief system, protection against jet fires focuses on prevention of leaks through proper maintenance and/or mitigation systems such as external insulation, depressurizing systems, isolation of leaks, equipment and/or flange orientation and minimization and emergency response.

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Causes of OverpressureThermal expansion.

When a pipe or vessel is totally filled with a liquid which can be blocked in,for instance, by closing two isolation valves, the liquid in the pipe or pressurevessel can expand very slowly due to heat gain by the sun or an uncontrolledheating system. This will result in tremendous internal hydraulic forces insidethe pipe or pressure vessel, as the liquid is non-compressible and needs to beevacuated. This section of pipe then needs thermal relief.

REMEMBER

The flows required for thermal relief are very small (max. 2”).Oversizing a thermal relief valve is never a good idea, and orifice sizes preferably below API orifice D are recommended.The sizing formula is quoted in API 521, Section 3.14

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Causes of OverpressureThermal runaway.

Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.In other words, "thermal runaway" describes a process which is accelerated by increased temperature, in turn releasing energy that further increases temperature. In chemistry (and chemical engineering), this risk is associated with strongly exothermic reactions that are accelerated by temperature rise.

BHOPAL accident - 1984

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Causes of OverpressureTube rupture in heat exchangers.

Arises when a tube in a shell-and-tube exchanger suffers complete rupture, allowing the high pressure fluid to overpressure the low pressure side (API 521 - 5.19)

REMEMBER

Conservatively, the capacity of the heat exchanger can be taken as the required flow for the SRV. In that respect, ASME and API are not very specific in their recommendations and state the following:ASME VIII Division 13, Paragraph UG-133 (d):Heat exchangers and similar vessels shall be protected with a relieving device of sufficient capacity to avoid overpressure in the case of an internal failure.API RP 521 Section 3.18 states:Complete tube rupture, in which a large quantity of high pressure fluid will flow to the lower pressure exchanger side, is a remote but possible contingency.

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Causes of OverpressureTube rupture in heat exchangers.

API has long used the ‘ two-thirds rule ’ to identify tube rupture scenarios.This rule states that tube rupture protection is not required when the ratio of the low pressure to high pressure side design pressure is greater than two-thirds.Basically , it remains up to the design engineer and/or end user to determine whether and which type of SRV is to be used when doing a hazard and operability analysis (HAZOP).

Vapour flow into liquid will generate a transient pressure surge as the vapour accelerates the liquid.

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Fire Calculation Basic RulesAssumptions:

• Assume process unit has been shut down• Consider each vessel to be blocked in containing liquid or vapor• Overpressure arises due to vaporization of the liquid inventory or expansion

of the vapor• The relief load for liquid vaporization is a function of the wetted area of the

vessel and the latent heat of vaporization of the fluid• The relief load for vapor expansion is a function of the exposed area• The vessel is normally unaffected by fire if elevation exceeds 25 ft = 7.62 m

above ground level.

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Fire Calculation Basic RulesThe sizing procedure described by API RP 521 part 4 is the most commonly used.

I) LIQUIDS

Horizontal Cilinder with flat ends

Horizontal Cilinder with spherical ends

Spherical Vessel

Horizontal Cilinder with spherical ends

Vertical cylinder with spherical ends

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Fire Calculation Basic RulesTotal heat absorption for vessel containing liquid:

The amount of heat absorbed by a vessel exposed to an open fire is markedly affected by the type of fuel feeding the fire, the degree to which the vessel is enveloped by the flames (a function of vessel size and shape), the environment factor, firefighting, and drainage.

The following equation is used to evaluate these conditions if there are prompt firefighting efforts and drainage of flammable materials away from the vessels.

Where:

Q is the total heat absorption (input) to the wetted surface, expressed in W (Btu/h);C1 is a constant [= 43,200 in SI units (21,000 in USC units)];F is an environment factor;Aws is the total wetted surface, expressed in m2 (ft2).

NOTE : The expression, Aws^0.82, is the area exposure factor or ratio. This ratio recognizes that large vessels are less likely than small ones to be completely exposed to the flame of an open fire.

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Determine the rate of vapouror gas vaporized from the liquid.

Calculate the minimum required relieving area.

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Total heat absorption for vessel containing only gases, vapors or supercritical fluids:

A = Minimum required effective discharge area (in2).A‘= Exposed surface area of the vessel (ft 2 ).P 1 = Relieving pressure, psi, absolute (set pressure [psig] + overpressure [psi] + atmospheric pressure [psia]).

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OVERPRESSURE RELIEF DEVICES

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Safety ValveA pressure relief valve characterized by rapid opening or closing and normally used to relieve compressible fluids.

Relief ValveA pressure relief valve characterized by gradual opening or closing generally proportional to the increase or decrease in pressure. It is normally used for incompressible fluids.

Safety Relief Valve

A pressure relief valve characterized by rapid opening or closing or by gradual opening or closing, generally proportional to the increase or decrease in pressure. It can be used for compressible or incompressible fluids.

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A - Restricted lift PRV:The actual discharge area is determined by the position of the disc. The lift restricted by a mechanical stop.

B- Full lift PRV:The actual discharge area is not determined by the position of the disc. The minimum travel needed to obtain the maximum discharge coefficient.C- Reduced Bore PRV:The flow path area below the seat is less than the flow area at the inlet to the valve.

D- Full Bore PRV:The flow path area below the seat is equal than the flow area at the inlet to the valve.

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E - Direct spring loaded PRV:The disc is held closed by a spring.

F- Pilot operated PRV:Pressure relief valve in which a piston or diaphragm is held closed by system pressure and the holding pressure is controlled by a pilot valve actuated by system pressure.

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G- Conventional direct spring loaded PRV:

A direct spring loaded pressure relief valve whose operational characteristics are directly affected by changes in the back pressure.

H- Balanced direct spring loaded PRV:A direct spring loaded pressure relief valve which incorporates means of minimizing the effect of back pressure on the operational characteristics (opening pressure, closing pressure, and relieving capacity).

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I - Internal spring PRV:A direct spring loaded pressure relief valve whose spring and all or part of theoperating mechanism is exposed to the system pressure when the valve is in the closed position.

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K - Power actuated PRV (PORV):A pressure relief valve actuated by an externally powered control device. Consisting of a main valve that is the actual pressure-relieving device and anexternally powered piloting device that controls the opening and closing of the main valve by either pressurizing or depressurizing the dome of the main valve

A Power (or Pilot) Operated Relief Valve (PORV), or in some cases referred to as an Electro-Matic Relief (EMR), is a pressure relief device (valve) that activates electrically from a pressure set point usually derived from a pressure transmitter output signal. A PORV is not direct acting; rather it is actuated indirectly by a solenoid actuated pilot valve where the solenoid gets energized by the output pressure signal from the system pressure transmitter.

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PRESSURE RELIEF VALVES

• basic principles• operational characteristics• applications• selection• standards

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CODES, STANDARDS and APPROVAL AUTHORITIES

International

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Saipem standard / criteria• GUIDA ALLA SCELTA E DIMENSIONAMENTO DI PROCESSO DEI DISPOSITIVI DI

SICUREZZA - PRG.PR.GEN.0005• PSV SELECTION AND ORIFICE PRELIMINARY CALCULATION - CR-COR-ENG_PRC-

004-EApproval authorities

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Spring-loaded PRVsI) Conventional PRVs DEFINITION as per

API 520 part I

A conventional PRV is a self-actuated spring-loaded PRV that is designed to open at a predetermined pressure and protect a vessel or system from excess pressure by removing or relieving fluid from that vessel or system.

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I) Conventional PRVs

Although the principal elements of a conventional safety valve are similar, the design details can vary considerably. In general, the DIN style valves (commonly used throughout Europe) tend to use a simpler construction with a fixed skirt (or hood) arrangement whereas the ASME style valves have a more complex design that includes one or two adjustable blowdown rings. The position of these rings can be used to fine-tune the overpressure and blowdown values of the valve.

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I) Conventional PRVs

The valve inlet (or approach channel) design can be either a full-nozzle or a semi-nozzle type. A full-nozzle design has the entire 'wetted' inlet tract formed from one piece. The approach channel is the only part of the safety valve that is exposed to the process fluid during normal operation, other than the disc, unless the valve is discharging.Full-nozzles are usually incorporated in safety valves designed for process and high pressure applications, especially when the fluid is corrosive.

Conversely, the semi-nozzle design consists of a seating ring fitted into the body, the top of which forms the seat of the valve.

The advantage of this arrangement is that the seat can easily be replaced, without replacing the whole inlet.

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I) Conventional PRVs

Standard safety valves are generally fitted with a lever, which enables the valve to be lifted manually in order to ensure that it is operational at pressures in excess of 75% of set pressure. This is usually done as part of routine safety checks, or during maintenance to prevent seizing. The fitting of a lever is usually a requirement of national standards and insurance companies for steam and hot water applications. For example, the ASME Boiler and Pressure Vessel Code states that pressure relief valves must be fitted with a lever if they are to be used on air, water over 60°C, and steam.

The plain or open lever is a simple construction which does not provide any seal to the atmosphere, so it can only be used when the system is allowed to vent to atmosphere. It should not be used on corrosive, inflammable or toxic products. It is also not recommended when the valve is used on a system withbackpressure.

The packed lift lever design usually requires a little less lever force to operate. It is packed and provides a tight seal to the atmosphere; therefore it can be used for corrosive or toxic products. It can also be used if there is backpressure because the fluids are contained in the cap.Usually, it has a bolted design with a graphoil, soft iron, TFE or other type of gasket, depending on the product it needs to contain.

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I) Conventional PRVs

• For service where a lever is not required, a cap can be used to simply protect the adjustment screw. If used in conjunction with a gasket, it can be used to prevent emissions to the atmosphere.

• A test gag may be used to prevent the valve from opening at the set pressure during hydraulic testing when commissioning a system. Once tested, the gag screw is removed and replaced with a short blanking plug before the valve is placed in service.

The screwed cap is the most common, economic and simple design.It usually has a soft iron or other gasket for sealing the cap in case the valve opens, as vapours might be present in the cap during the opening of a conventional SRV.The disadvantage is that it can come loose due to vibrations on thesystem. It could also become difficult to remove due to corrosion of the threads, especially in corrosive environments.

Test Gag physically blocks the spindle from moving upwards, preventing the valve from opening. This device is frequently misused and dangerous if not removed after hydraulic tests or before a new installation.

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I) Conventional PRVs

• Unless bellows or diaphragm sealing is used, process fluid will enter the spring housing (or bonnet).

• If emission of this fluid into the atmosphere is acceptable, the spring housing may be vented to the atmosphere - an open bonnet.

• This is usually advantageous when the safety valve is used on high temperature fluids or for boiler applications as, otherwise, high temperatures can relax the spring, altering the set pressure of the valve.

• However, using an open bonnet exposes the valve spring and internals to environmental conditions, which can lead to damage and corrosion of the spring.

• When the fluid must be completely contained by the safety valve (and the discharge system), it is necessary to use a closed bonnet, which is not vented to the atmosphere.

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I) Conventional PRVs

• Some safety valves, most commonly those used for water applications, incorporate a flexible diaphragm or bellows to isolate the safety valve spring and upper chamber from the process fluid.

• An elastomer bellows or diaphragm is commonly used in hot water or heating applications, whereas a stainless steel one would be used on process applications employing hazardous fluids.

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TerminologyDischarge Area.

CURTAIN AREAArea of the cylindrical or conical discharge opening between the seating surfaces created by the lift of the disc above the seat

EFFECTIVE DISCHARGE OR FLOW AREAThe minimum cross-sectional area between the inlet and the seat, at its narrowest point.

ACTUAL DISCHARGE AREAThe lesser of the curtain and effective discharge or flow areas. The measured minimum net discharge area determines the flow through a valve.

API 520 Definition: A nominal or computed area used with an effective discharge coefficient to calculate the minimum required relieving capacityfor a pressure relief valve per the preliminary sizing equations contained in API Standard 520. API Standard 526 provides effective discharge area for a range of sizes in terms of letter designations D through T.API 526 identify the EDA with term Orifice Area

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When the inlet static pressure rises above the set pressure of the safety valve, the disc will begin to lift off its seat. However, as soon as the spring starts to compress, the spring force will increase; this means that the pressure would have to continue to rise before any further lift can occur, and for there to be any significant flow through the valve.

The additional pressure rise required before the safety valve will discharge at its rated capacity is called the overpressure. The allowable overpressure depends on the standards being followed and the particular application.

For compressible fluids, this is normally between 3% and 10%, and for liquids between 10% and 25%.

In order to achieve full opening from this small overpressure, the disc arrangement has to be specially designed to provide rapid opening. This is usually done by placing a shroud, skirt or hood around the disc. The volume contained within this shroud is known as the control or huddling chamber.

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Full lift, high lift and low lift safety valves.

• The terms full lift, high lift and low lift refer to the amount of travel the disc undergoes as it moves from its closed position to the position required to produce the certified discharge capacity, and how this affects the discharge capacity of the valve.

• A full lift safety valve is one in which the disc lifts sufficiently, so that the curtain area no longer influences the discharge area. The discharge area, and therefore the capacity of the valve are subsequently determined by the bore area. This occurs when the disc lifts a distance of at least ¼ of the bore diameter.

• The disc of a high lift safety valve lifts a distance of at least 1/12 of the bore diameter.

This means that the curtain area, and ultimately the position of the disc, determines the discharge area. The discharge capacities of high lift valves tend to be significantly lower than full lift, and for a given discharge capacity, it is usually possible to select a full lift valve that has a nominal size several times smaller than a corresponding high lift valve, which usually incurs cost advantages. Furthermore, high lift valves tend to be used on compressible fluids where their action is more proportional.

• In low lift valves, the disc only lifts a distance of 1/24 of the bore diameter. The discharge area is determined entirely by the position of the disc, and since the disc only lifts a small amount, the capacities tend to be much lower than full or high lift valves.

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Spring-loaded PRVsPRV Operation—Vapor/Gas Service

• The spring load is preset to equal the force exerted on the closed disc by the inlet fluid when the system pressure is at the set pressure of the valve.

• When the inlet pressure is below the set pressure, the disc remains seated on the nozzle in the closed position.

• When the inlet pressure exceeds set pressure, the pressure force on the disc overcomes the spring force and the valve opens.

• When the inlet pressure is reduced to the closing pressure, the valve recloses.

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Maximum Allowable Working Pressure.

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PRV discharge cycle.

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PRV discharge cycle.

Simmer (def. API 520)The audible or visible escape of compressible fluid between the seat and disc of a pressure-relief valve that may occur at an inlet static pressure below the set pressure prior to opening.

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PRV discharge cycle.

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PRV discharge cycle.

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PRV discharge cycle.

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Popping and blowdown.

The opening and closing characteristics of the valve can be controlled by the adjustment of a blowdown ring, as its position affects the shape and volume of the huddling chamber. When the blowdown ring is adjusted to its top position, the exit area from the huddling chamber is restricted to its minimum. The valve will pop distinctly with a short simmer and long blowdown. Conversely, when the blowdown ring is in its lowest position there is a maximum exit area from the huddlingchamber and the valve will have a longer simmer with a shorter blowdown. The blowdown ring can be positioned between these two extremes to give the required performance, but it is usually factory set to achieve re-seating 7-10% below set pressure.

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As lift begins (Figure b), and fluid enters the chamber, a larger area of the shroud is exposedto the fluid pressure. Since the magnitude of the lifting force (F) is proportional to the product of the pressure (P) and the area exposed to the fluid (A); (F = P x A), the opening force is increased. This incremental increase in opening force overcompensates for the increase in spring force, causing rapid opening. At the same time, the shroud reverses the direction of the flow, which provides a reaction force, further enhancing the lift.These combined effects allow the valve to achieve its designed lift within a relatively small percentage overpressure. For compressible fluids, an additional contributory factor is the rapid expansion as the fluid volume increases from a higher to a lower pressure area. This plays a major role in ensuring that the valve opens fully within the small overpressure limit. For liquids, this effect is more proportional and subsequently, the overpressure is typically greater; 25% is common.

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Popping and blowdown.

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Blowdown rings.

The blowdown rings found on most ASME type safety valves are used to make fine adjustments to the overpressure and blowdown values of the valves.

The lower blowdown ring is a common feature on many valves where the tighter overpressure and blowdown requirements require a more sophisticated designed solution. The upper blowdown ring is usually factory set and essentially takes out the manufacturing tolerances which affect the geometry of the huddling chamber.

The lower blowdown ring is also factory set to achieve the appropriate code performance requirements but under certain circumstances can be altered.

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TerminologyMaximum Allowable Working Pressure (MAWP) and Set Pressure (SP)API 520 Definition:

MAWP:

The maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure.

The pressure is the least of the values for the internal or external pressure as determined by the vessel design rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure.

The MAWP is the basis for the pressure setting of the pressure-relief devices that protect the vessel.

The MAWP is normally greater than the design pressure but can be equal to the design pressure when the design rules are used only to calculate the minimum thickness for each element and calculations are not made to determine the value of the MAWP.

SP:

The inlet gauge pressure at which the pressure-relief device is set to open under service conditions.

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TerminologyDesign Pressure and Operating Pressure

API 520 Definition:

Design Pressure: Pressure, together with the design temperature, used to determine the minimum permissible thickness or physical characteristic of each vessel component as determined by the vessel design rules. The design pressure is selected by the user to provide a suitable margin above the most severe pressure expected during normal operation at a coincident temperature. It is the pressure specified on the purchase order.

This pressure may be used in place of the MAWP in all cases where the MAWP has not been established. The design pressure is equal to or less than the MAWP.

Operating Pressure:The inlet gauge pressure at which the pressure-relief device is set to open under service conditions.

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TerminologyRelieving Pressure

API 520 Definition:

Relieving Pressure: Relieving pressure, shown as P1 in the various sizing equations, is the inlet pressure of the pressure-relief device at relieving conditions. The relieving pressure is the total of set pressure plus overpressure. The effects of inlet pressure drop on specification of relieving pressure for PRV sizing can be neglected if the inlet pressure drop does not exceed 3 % of set pressure

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TerminologyOverpressure, Accumulation, BackpressureAPI 520 Definition:

Overpressure: The pressure increase over the set pressure of the relieving device.

Overpressure is expressed in pressure units or as a percentage of set pressure.

Overpressure is the same as accumulation only when the relieving device is set to open at the MAWP of the vessel.

Accumulation:The pressure increase over the MAWP of the vessel, expressed in pressure units or as a percentage ofMAWP or design pressure.

Maximum allowable accumulations are established by applicable codes for emergency operating and fire contingencies.

Backpressure:The pressure that exists at the outlet of a pressure-relief device as a result of the pressure in the discharge system.

Backpressure is the sum of the superimposed and built-up backpressures.

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TerminologySuperimposed and Built Up Backpressure

The static pressure which exists at theoutlet of an SRV due to existing pressure in the discharge system. It is the sum of superimposed and built-up backpressure, and potentially influences the set pressure and the operation of thevalve.Based on backpressure existing at the outlet of a spring-operated SRV, we use the conventional SRV for backpressures typically under 10% of set pressure.

We must use a balanced bellows SRV or a pilot-operated SRV for instable backpressures or backpressures over10%

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TerminologySuperimposed and Built Up Backpressure

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TerminologySuperimposed and Built Up BackpressureAPI 520 Definition:

Superimposed backpressure:The static pressure that exists at the outlet of a pressure-relief device at the time the device is required to operate. Superimposed backpressure is the result of pressure in the discharge system coming from other sources and may be constant or variable.It is the sum of Costant Superimposed Backpressure and Variable Superimposed Backpressure.

Built-up backpressure:The increase in pressure at the outlet of a pressure-relief device that develops as a result of flow after the pressure-relief device opens.

Built-up backpressure occurs when the safety valve is open and flowing due to:

■ The rate of flow through the PRV■ The size and/or configuration of the discharge piping■ Other sources of pressure into the discharge header

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Effect of Built-Up Backpressure.

Pressure losses occur in piping configurations that are attached to the outlet connection of a pressure relief valve that has opened and is discharging the service fluid. This occurs whether the PRV is discharging to atmosphere via a tailpipe or into a closed header system.

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Effect of Superimposed BackpressureWhen the PRV outlet is connected to a closed disposal system, it is a good possibility that there will be pressure in this outlet piping before the PRV may be called upon to relieve. This type of back pressure is called superimposed.A PRV datasheet will normally have two fields for superimposed back pressure, one that would list “constant” and one that would list “variable.”An example of constant superimposed back pressure might be a relief valve protecting the discharge of a pump.It is common to send the discharge piping of a pump relief valve back into the suction side of the equipment. This suction pressure may be costant or variable.

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TerminologyConstant Superimposed Backpressure

Costant Superimposed backpressure:Usually backpressures that occur when a safety valve outlet is connected to a static pressure source and doesn’t change appreciably under any conditions of operation.

In this case, conventional valves may be used if the valve spring setting is reduced by the amount of the constant backpressure.

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TerminologyVariable Superimposed Backpressure

Variable Superimposed backpressure:Usually the result of one or more SRVs discharging into a common header. The backpressures may be different at each moment and at each relief cycle.

Bellows or pilot design is always required since no predetermined set pressure is possible when the outlet pressure is acting on the trim of the valve, therefore directly influencing the set pressure, and the set point will vary with backpressure.

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How we can compensate for the effects of backpressure.

In a conventional safety valve, only the superimposed backpressure will affect the opening characteristic and set value, but the combined backpressure will alter the blowdown characteristic and re-seat value.

Conventional safety relief valve - The spring housing is vented to the discharge side, hence operational characteristics are directly affected by changes in the backpressure to the valve.

Balanced safety relief valve - A balanced valve incorporates a means of minimizing the effect of backpressure on the operational characteristics of the valve.

Pilot operated pressure relief valve - The major relieving device is combined with, and is controlled by, a self-actuated auxiliary pressure relief device.

Page 65: Overpressure Protection Systems - Lesson I

65lunedì 01 maggio 2023

Bonnet Vented to Valve Discharge – Effect of Backpressure.

PV = Fluid inlet pressureAN = Nozzle area FS = Spring force PB = BackpressureAD = Disc area

da cui:

Any superimposed backpressure will tend to increase the closing force and the inlet pressure required to lift the disc is greater.

Page 66: Overpressure Protection Systems - Lesson I

66lunedì 01 maggio 2023

Bonnet Vented to Valve Atmosphere – Effect of Backpressure.

PV = Fluid inlet pressureAN = Nozzle area FS = Spring force PB = BackpressureAD = Disc area

𝑃𝑉 𝐴𝑁=𝐹𝑆−𝑃𝐵 ( 𝐴𝐷−𝐴𝑁 )

Any superimposed backpressure acts with the vessel pressure to overcome the spring force, and the opening pressure will be less than expected.

Page 67: Overpressure Protection Systems - Lesson I

67lunedì 01 maggio 2023

Effect of Built-Up Backpressure.PV = Fluid inlet pressureAN = Nozzle area FS = Spring force PB = BackpressureAD = Disc area𝑃𝑉 𝐴𝑁=𝐹𝑆+𝑃 𝐵𝐴𝑁

Therefore, if the backpressure is greater than the overpressure, the valve will tend to close, reducing the flow. This can lead to instability within the system and can result in flutter or chatter of the valve.

Once the valve starts to open, the inlet pressure Pv is the sum of the set pressure, PS, and the overpressure, PO.

(𝑃 𝑆+𝑃𝑂 ) 𝐴𝑁=𝐹𝑆+𝑃𝐵 𝐴𝑁

Then:

𝑃𝑆 𝐴𝑁=𝐹𝑆+𝐴𝑁 (𝑃𝐵−𝑃𝑂 )

Page 68: Overpressure Protection Systems - Lesson I

68lunedì 01 maggio 2023

Pressure Level relationship for PRVs as per API 520.

MAWP

Page 69: Overpressure Protection Systems - Lesson I

69lunedì 01 maggio 2023

Pressure Level relationship for PRVs as per ASME Boiler and Pressure Vessel Code– section I (single PRV)

• the maximum accumulation allowed during an overpressure event must be limited to 3%

• the Code will allow the highest Set Pressure to be equal to Maximum Allowable Working Pressure (MAWP).

• the design of the valve must allow adequate lift to obtain the needed capacity within 3% overpressure.

Page 70: Overpressure Protection Systems - Lesson I

70lunedì 01 maggio 2023

Pressure Level relationship for PRVs as per ASME Boiler and Pressure Vessel Code– section I (Multiple PRV)

• When multiple PRVs are used, the allowable accumulation for a fired vessel can be 6%.

• the Code will allow for a staggered or variable set pressure regime for the valves. This helps to avoid interaction between the safety valves during their open and closing cycle.

• One of the multiple valves, sometimes called the primary pressure relief valve, must still be set no higher than the MAWP but the additional or supplemental pressure relief valve can be set up to a maximum of 3% above the MAWP. In this case, the same valve design criteria, obtaining the needed valve lift with 3% overpressure, is still required.

• The Code requires that the overall range of set pressures for a multiple valve installation not exceed 10% of the highest set pressure PRV.

Page 71: Overpressure Protection Systems - Lesson I

71lunedì 01 maggio 2023

Pressure Level relationship for PRVs as per ASME Boiler and Pressure Vessel Code– section VIII div.1 (Single PRV)

• The requirements for ASME Section VIII are less stringent than those in Section I. It is permissible to use a PRV certified for Section I in any Section VIII application provided than the design will meet all of the requirements of the application.

• When the source of overpressure is not an external fire, the vessel is allowed to experience an accumulation in pressure, during an upset condition, up to 10% over the maximum allowable working pressure (MAWP).

• The design of a pressure relief valve must allow adequate lift to obtain the needed capacity within 10% overpressure.

Page 72: Overpressure Protection Systems - Lesson I

72lunedì 01 maggio 2023

Pressure Level relationship for PRVs as per ASME Boiler and Pressure Vessel Code– section VIII div.1 (Single PRV)

• Code does allow the PRV to be set below the MAWP.

• When the device is set to open below the MAWP, it may be sized using the overpressure (the difference between the set or burst pressure and the maximum allowable accumulation).

Page 73: Overpressure Protection Systems - Lesson I

73lunedì 01 maggio 2023

Pressure Level relationship for PRVs as per ASME Boiler and Pressure Vessel Code– section VIII div.1 (Single PRV)

• When a pressure vessel can experience an external fire that would cause an overpressure condition, the Code allows for a maximum accumulation of 21%.

• The rule is the same as the non-fire condition, in that the maximum set or burst pressure for a single device installation cannot be higher than the MAWP of the vessel.

• If a PRV is selected, it typically will have the same operational characteristics as the one selected for a non-fire relieving case. An overpressure of 21% can be used to size this valve.

Page 74: Overpressure Protection Systems - Lesson I

74lunedì 01 maggio 2023

Pressure Level relationship for PRVs as per ASME Boiler and Pressure Vessel Code– section VIII div.1 (Multiple PRV)

• If more than one device is needed, the accumulation, for a non-fire generated overpressure scenario, is to not exceed 16% above the MAWP.

• This additional accumulation will allow for the multiple pressure relief valves to be set at different pressures.This staggered set point regime will help to avoid interaction between the multiple PRVs.

• A primary PRV can be set no higher than the MAWP of the vessel.

• Any additional or supplemental PRV can be set above the MAWP, but at a level no higher than 5% above the MAWP.

Page 75: Overpressure Protection Systems - Lesson I

75lunedì 01 maggio 2023

Pressure Level relationship for PRVs as per ASME Boiler and Pressure Vessel Code– section VIII div.1 (Multiple PRV)

• When multiple PRVs are required when the relieving case contingency is heat input from an external source, such as a fire, the primary valve can again be set no higher than the MAWP

• Any supplemental valve can be set to open at a pressure 10% above the MAWP.

• The overall vessel accumulation that is allowed by the Code is 21%.

Page 76: Overpressure Protection Systems - Lesson I

76lunedì 01 maggio 2023

Effect of Built-Up Backpressure.

In general, if conventional safety valves are used in applications, where there is an excessive built-up backpressure, they will not perform as expected.

According to the API 520 :

A conventional pressure relief valve should typically not be used when the built-up backpressure is greater than 10% of the set pressure at 10% overpressure. A higher maximum allowable built-up backpressure may be used for overpressure greater than 10%.

The British Standard BS 6759, however, states that the built-up backpressure should be limited to 12% of the set pressure when the valve is discharging at the certified capacity.

If, however, if it is not possible to reduce the backpressure, then it may be necessary to use a balanced safety valve.

Page 77: Overpressure Protection Systems - Lesson I

77lunedì 01 maggio 2023

Balanced Safety Valves (Piston and Bellows).

Page 78: Overpressure Protection Systems - Lesson I

78lunedì 01 maggio 2023

Balanced Safety Valves (Piston).

• Generally consist of a piston type disc whose movement is constrained by a vented guide.

• The area of the top face of the piston, AP, and the nozzle seat area, AN, are designed to be equal.

• The effective area of both the top and bottom surfaces of the disc exposed to the backpressure are equal, and therefore any additional forces are balanced.

• The spring bonnet is vented such that the top face of the piston is subjected to atmospheric pressure

Page 79: Overpressure Protection Systems - Lesson I

79lunedì 01 maggio 2023

Balanced Safety Valves (Piston).PV = Fluid inlet pressureAN = Nozzle area FS = Spring force PB = BackpressureAD = Disc areaAP = Piston area

being AN = AP

Page 80: Overpressure Protection Systems - Lesson I

80lunedì 01 maggio 2023

Balanced Safety Valves (Bellows).

• A bellows with an effective area (AB) equivalent to the nozzle seat area (AN) is attached to the upper surface of the disc and to the spindle guide.

• The disc area extending beyond the bellows and the opposing disc area are equal, and so the forces acting on the disc are balanced, and the backpressure has little effect on the valve opening pressure.

• The bellows vent allows air to flow freely in and out of the bellows as they expand or contract.

• The spring bonnet is vented such that the top face of the piston is subjected to atmospheric pressure

• The bellows also serve to isolate the spindle guide and the spring from the process fluid, this is important when the fluid is corrosive.

Page 81: Overpressure Protection Systems - Lesson I

81lunedì 01 maggio 2023

Balanced Safety Valves (Bellows).• This type of safety valve is

usually only used on critical applications in the oil and petrochemical industries.

• In addition to reducing the effects of backpressure, the bellows also serve to isolate the spindle guide and the spring from the process fluid, this is important when the fluid is corrosive.

• Since balanced pressure relief valves are typically more expensive than their unbalanced counterparts, they are commonly only used where high pressure manifolds are unavoidable, or in critical applications where a very precise set pressure or blowdown is required.

Page 82: Overpressure Protection Systems - Lesson I

82lunedì 01 maggio 2023

Balanced Safety Valves (Bellows with Auxiliary Balanced Piston).Bellows failure is an important concern when using a bellows balanced safety valve, as this may affect the set pressure and capacity of the valve. It is important, therefore, that there is some mechanism for detecting any uncharacteristic fluid flow through the bellows vents. In addition, some bellows balanced safety valves include an auxiliary piston that is used to overcome the effects of backpressure in the case of bellows failure. This type of safety valve is usually only used on critical applications in the oil and petrochemical industries.required.

Page 83: Overpressure Protection Systems - Lesson I

83lunedì 01 maggio 2023

Pilot operated safety relief valve (POSRV)

• Backpressure reduces the rated capacity of the valve and therefore larger valves could be required if backpressure exists.

• A conventional SRV is best suited for simple discharge via a tail pipe into atmosphere. Its ability to tolerate built-up backpressure is very limited.

• The loss of lift and resultant capacity is caused by the backpressure acting on the external surfaces of the bellows, attempting to lengthen it, which produces an increased spring rate of the bellows.

• To maintain the bellows’ structural integrity and resist to instability, conventional SRV are normally limited to 50% of backpressure (as a percentage of set pressure) or less. Above that value of backpressure, pilot valves should be considered.

Page 84: Overpressure Protection Systems - Lesson I

84lunedì 01 maggio 2023

Pilot operated relief valve (PORV)

A pilot-operated PRV consists of the main valve, which normally encloses a floating unbalanced piston assembly, and an external pilot.

Page 85: Overpressure Protection Systems - Lesson I

85lunedì 01 maggio 2023

Pilot operated relief valve (PORV)

The piston is designed to have a larger area on the top than on the bottom. Up to the set pressure, the top and bottom areas are exposed to the same inlet operating pressure. Because of the larger area on the top of the piston, the net force holds the piston tightly against the main valve nozzle. As the operating pressure increases, the net seating force increases and tends to make the valve tighter. This feature allows most pilot-operated valves to be used where the maximum expected operating pressure is higher than the expected.

REMEMBER: The lift of the main valve piston or diaphragm, unlike a conventional or balanced spring-loaded valve, is not affected by built-up backpressure. This allows for even higher pressures in the relief dischargemanifolds.

Page 86: Overpressure Protection Systems - Lesson I

86lunedì 01 maggio 2023

Pilot operated relief valve (PORV)

At the set pressure, the pilot vents the pressure from the top of the piston; the resulting net force is now upward causing the piston to lift, and process flow is established through the main valve. After the overpressure incident, the pilot will close the vent from the top of the piston, thereby reestablishing pressure, and the net force will cause the piston to reseat.

1- Below set pressure: normal operation

During normal operation, the system pressure is picked up at the main valve inlet and routed to the dome (see illustration).

Since the dome area is larger than the area of the main valve seat, the closing force is greater than the opening force. This keeps the main valve tightly closed.

Page 87: Overpressure Protection Systems - Lesson I

87lunedì 01 maggio 2023

Pilot operated relief valve (PORV)

2- At set pressure: actuating state

At set pressure, the pilot valve actuates. The medium is no longer routed to the dome (see illustration). This prevents a further rise in dome pressure.

Also, the dome is vented. As a result, the closing force ceases as a precondition for the system overpressure to push the main valve open.

Page 88: Overpressure Protection Systems - Lesson I

88lunedì 01 maggio 2023

Pilot operated relief valve (PORV)

3- Main valve opening

The main valve opens. Depending on the design of the pilot valve, this opening is either rapid and complete (Pop Action) or gradual and partial following system pressure (Modulate Action).

Page 89: Overpressure Protection Systems - Lesson I

89lunedì 01 maggio 2023

Pilot operated relief valve (PORV)

4- At closing pressure: refilling the dome

If system pressure drops to closing pressure, the pilot valve actuates and again routes the medium to the dome.

The pressure in the dome builds up and the main valve recloses either rapid and complete (Pop Action) or gradual and partial following system pressure (Modulate Action).

Page 90: Overpressure Protection Systems - Lesson I

90lunedì 01 maggio 2023

Pilot operated relief valve (PORV)

Page 91: Overpressure Protection Systems - Lesson I

91lunedì 01 maggio 2023

Pilot operated relief valve (PORV) The main valve of the pilot operated PRV can use a diaphragm in lieu of a piston to provide the unbalanced moving component of the valve.

A disc, which normally closes the main valve inlet, is integral with a flexible diaphragm.

The external pilot serves the same function to sense processpressure, vent the top of the diaphragm at set pressure, and reload the diaphragm once the process pressure is reduced.

As with the piston valve, the seating force increases proportionally with the operatingpressure because of the differential exposed area of the diaphragm.

Page 92: Overpressure Protection Systems - Lesson I

92lunedì 01 maggio 2023

Pilot operated safety relief valve (POSRV) – Flowing Type

The pilot vent can be either directly exhausted to atmosphere or to the main valve outlet depending upon the pilot’s design and user’s requirement.

Only a balanced type of pilot, where set pressure in unaffected by backpressure, should be installed with its exhaust connected to a location with varying pressure (such as to the main valve outlet). Slight variations in backpressure may be acceptable for unbalanced pilots.The flowing type pilot allows process fluid to

continuously flow through the pilot when the main valve is open.

Page 93: Overpressure Protection Systems - Lesson I

93lunedì 01 maggio 2023

Pilot operated safety relief valve (POSRV) – Non Flowing Type

A backflow preventer is required when the possibility exists of developing a pressure on the discharge side of the valve that exceeds the inlet pressure of the valve. The higher discharge pressure can cause sufficient upward force on the diaphragm or piston to open the valve and cause flow reversal. The backflow preventer allows the discharge pressure to provide a net downward force on the diaphragm or piston to keep the valveclosed.

The Non flowing type pilot not allows process fluid to continuously flow through the pilot when the main valve is open. It is generally recommended for most services to reduce the possibility of hydrate formation (icing) or solids in the lading fluid affecting the pilot’s performance.

Page 94: Overpressure Protection Systems - Lesson I

94lunedì 01 maggio 2023

Pilot Types – Pop action typeThe pop-action pilot causes the main valve to lift fully at set pressure without overpressure. This immediate release of pressure provides extremely high opening and closing forces on the main valve seat. This opening action is typically not recommended for liquid services to avoid valve instability.

Page 95: Overpressure Protection Systems - Lesson I

95lunedì 01 maggio 2023

Pilot Types – Modulating typeThe modulating pilot opens the main valve only enough to satisfy the required relieving capacity and can be used in gas, liquid, or two-phase flow applications.

A modulating pilot-operated valve limits the amount of relieving fluid to only the amount required to prevent the pressure from exceeding the allowable accumulation.

The modulating pilot valve also can reduce interaction with other pressure control equipment in the system during an upset condition, reduce unwanted atmospheric emissions, and reduce the noise level associated with discharge to the atmosphere.

Page 96: Overpressure Protection Systems - Lesson I

96lunedì 01 maggio 2023

Bibliography

• The Safety Relief Valve Handbook - Marc Hellemans - Butterworth-Heinemann, first ed. 2009

• Pentair Pressure Relief Valve Engineering Handbook – Pentair Valves & Control – rev.12 – 2012

• Introduction to Safety Valves – Spirax Sarco – Module 9.1• Types of Safety Valves – Spirax Sarco – Module 9.2• Safety Valve Selection – Spirax Sarco – Module 9.3• Snamprogetti - Guida alla scelta e dimensionamento di processo dei

dispositivi di sicurezza - PRG.PR.GEN.0005 REV.1 – 2005• Saipem - PSV selection and orifice preliminary calculation - CR-COR-

ENG_PRC-004-E rev. 1 – 2013• Pressure Relief Valve Engineering Handbook – Crosby Valve Inc. – 1997• API 520 part. I - Sizing, Selection, and Installation of Pressure-

relieving Devices• API 520 part. II - Sizing, Selection, and Installation of Pressure-

relieving Devices• API 521 - Pressure-relieving and Depressuring Systems• API 526 - Flanged Steel Pressure relief Valves