Safety Relief Valve (SRV)

  

1. Relief System

A pressure Relief Valve is a safety device designed to protect a pressurized vessel or system during an overpressure event.
An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP).

The primary purpose of a pressure Relief Valve is protection of life and property by venting fluid from an overpressurized vessel.

Many electronic, pneumatic and hydraulic systems exist today to control fluid system variables, such as pressure, temperature and flow. Each of these systems requires a power source of some type, such as electricity or compressed air in order to operate. A pressure Relief Valve must be capable of operating at all times, especially during a period of power failure when system controls are nonfunctional. The sole source of power for the pressure Relief Valve, therefore, is the process fluid.

Once a condition occurs that causes the pressure in a system or vessel to increase to a dangerous level, the pressure Relief Valve may be the only device remaining to prevent a catastrophic failure. Since reliability is directly related to the complexity of the device, it is important that the design of the pressure Relief Valve be as simple as possible.

The pressure Relief Valve must open at a predetermined set pressure, flow a rated capacity at a specified overpressure, and close when the system pressure has returned to a safe level. Pressure Relief Valves must be designed with materials compatible with many process fluids from simple air and water to the most corrosive media. They must also be designed to operate in a consistently smooth and stable manner on a variety of fluids and fluid phases.

2. API 521 Sizing Criteria

1. According to codes such as API 521 PSV’s shall be installed to protect equipment in case of a fire.
2. It has been recognized by the industry that fire PSV may not prevent overpressure rupture in a fire and in some cases the PSV may not even open.
3. Still fire PSV’s are installed, but they should not be considered primary protection against overpressure in a fire.
4. Primary protection is Blowdown, PFP and deluge

3. Relief System – Pressure Terminology

1. Operating pressure
2. MAWP
3. Design pressure
4. Set pressure
5. Accumulation
6. Overpressure
7. Blowdown

Superimposed Back Pressure

1. Pressure in discharge header before valve opens
2. Can be constant or variable

Built-up Back Pressure

1. Pressure in discharge header due to frictional losses after valve opens
2. Total = Superimposed + Built-up

4. Code Requirements

1. General Code requirements include:
   1. ASME Boiler & Pressure Vessel Codes
   2. ASME B31.3 / Petroleum Refinery Piping
   3. ASME B16.5 / Flanges & Flanged Fittings
2. All pressure vessels subject to overpressure shall be protected by a pressure relieving device
3. Liquid filled vessels or piping subject to thermal expansion must be protected by a thermal relief device
4. Multiple vessels may be protected by a single relief device provided there is a clear, unobstructed path to the device
5. At least one pressure relief device must be set at or below the MAWP
6. Relieving pressure shall not exceed MAWP (accumulation) by more than:
   1. 3% for fired and unfired steam boilers
   2. 10% for vessels equipped with a single pressure relief device
   3. 16% for vessels equipped with multiple pressure relief devices
   4. 21% for fire contingency

5. Pressure relieving methods

There are different methods through which the overpressure in the process can be relieved:

1. Flame arresters
2. Safety Relief valves
3. Bursting discs
4. Blowdown valves

6. Relief valves

Relief valves are characterized by:

1. Slow response times (tenths of a second up to > 1 second)
2. Risk of blockage
3. Trace leakage

Design considerations for relief valves include:

1. The pressure drop before the safety valve must be low to avoid instability
2. The design must take into consideration differences between gas and liquid duties
3. Back pressure can affect opening / closing pressures, stability and capacity
4. The relief valve usually solely determines relief capacity if appropriate piping is used

Regular proof checks are required to check lifting pressure, particularly if located in a corrosive environment. Also valve seating checks should be undertaken to ensure that the valve is not passing.

7. General Types of Safety Relief Valve Design

1. Direct acting type
   1. Oldest and most common
   2. Kept closed by a spring or weight to oppose lifting force of process pressure
2. Pilot operated type
1. Kept closed by process pressure
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   8. Advantages / Disadvantages Conventional Valve

1. Advantages
   1. Most reliable type if properly sized and operated
   2. Versatile — can be used in many services
2. Disadvantages
1. Relieving pressure affected by back pressure
2. Susceptible to chatter if built-up back pressure is too high
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4. 9. Balanced Bellows Spring Loaded Safety Relief Valve

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   10. Advantages / Disadvantages Balanced Bellows Valve

1. Advantages
   1. Relieving pressure not affected by back pressure
   2. Can handle higher built-up back pressure
   3. Protects spring from corrosion
2. Disadvantages
1. Bellows susceptible to fatigue/rupture
2. May release flammables/toxics to atmosphere
3. Requires separate venting system
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4. 11. Piston Type Pilot Operated Safety Relief Valve

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6. 12. Advantages / Disadvantages Pilot Operated Valve

1. Advantages
   1. Relieving pressure not affected by backpressure
   2. Can operate at up to 98% of set pressure
   3. Less susceptible to chatter (some models)
2. Disadvantages
1. Pilot is susceptible to plugging
2. Limited chemical and high temperature use by “O-ring” seals
3. Vapor condensation and liquid accumulation above the piston may cause problems
4. Potential for back flow
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13 .Terminology

The following definitions are taken from DIN 3320 but it should be noted that many of the terms and associated definitions used are universal and appear in many other standards. Where commonly used terms are not defined in DIN 3320 then ASME PTC25.3 has been used as the source of reference. This list is not exhaustive and is intended as a guide only; it should not be used in place of the relevant current issue standard:

• OPERATING PRESSURE (WORKING PRESSURE)
  is the gauge pressure existing at normal operating conditions within the system to be protected.
• SET PRESSURE
  is the gauge pressure at which under operating conditions direct loaded safety Valves commence to lift.
• TEST PRESSURE
  is the gauge pressure at which under test stand conditions (atmospheric backpressure) direct loaded safety Valves commence to lift.
• OPENING PRESSURE
  is the gauge pressure at which the lift is sufficient to discharge the predetermined flowing capacity. It is equal to the set pressure plus opening pressure difference.
• RESEATING PRESSURE
  is the gauge pressure at which the direct loaded safety Valve is re-closed.
• BUILT-UP BACKPRESSURE
  is the gauge pressure built up at the outlet side by blowing.
• SUPERIMPOSED BACKPRESSURE
  is the gauge pressure on the outlet side of the closed Valve.
• BACKPRESSURE
  is the gauge pressure built up on the outlet side during blowing (built-up backpressure + superimposed backpressure).
• ACCUMULATION
  is the increase in pressure over the maximum allowable working gauge pressure of the system to be protected.
• OPENING PRESSURE DIFFERENCE
  is the pressure rise over the set pressure necessary for a lift suitable to permit the predetermined flowing capacity.
• RESEATING PRESSURE DIFFERENCE
  is the difference between set pressure and reseating pressure.
• FUNCTIONAL PRESSURE DIFFERENCE
  is the sum of opening pressure difference and reseating pressure difference.
• OPERATING PRESSURE DIFFERENCE
  is the pressure difference between set pressure and operating pressure.
• LIFT
  is the travel of the disc away from the closed position.
• COMMENCEMENT OF LIFT (OPENING)
  is the first measurable movement of the disc or the perception of discharge noise.
• FLOW AREA
  is the cross sectional area upstream or downstream of the body seat calculated from the minimum diameter which is used to calculate the flow capacity without any deduction for obstructions.
• FLOW DIAMETER
  is the minimum geometrical diameter upstream or downstream of the body seat.
• NOMINAL SIZE DESIGNATION
  of a safety Valve is the nominal size of the inlet.
• THEORETICAL FLOWING CAPACITY
  is the calculated mass flow from an orifice having a cross sectional area equal to the flow area of the safety Valve without regard to flow losses of the Valve.
• ACTUAL FLOWING CAPACITYis the flowing capacity determined by measurement.
• CERTIFIED FLOWING CAPACITY
  is actual flowing capacity reduced by 10%.
• COEFFICIENT OF DISCHARGE
  is the ratio of actual to the theoretical discharge capacity.
• CERTIFIED COEFFICIENT OF DISCHARGE
  is the coefficient of discharge reduced by 10% (also known as derated coefficient of discharge).

The following terms are not defined in DIN 3320 and are taken from ASME PTC25.3:

• BLOWDOWN (RESEATING PRESSURE DIFFERENCE) -
  difference between actual popping pressure and actual reseating pressure, usually expressed as a percentage of set pressure or in pressure units.
• COLD DIFFERENTIAL TEST PRESSURE
  the pressure at which a Valve is set on a test rig using a test fluid at ambient temperature. This test pressure includes corrections for service conditions e.g. backpressure or high temperatures.
• FLOW RATING PRESSURE
  is the inlet static pressure at which the relieving capacity of a pressure relief device is measured.
• LEAK TEST PRESSURE
  is the specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure.
• MEASURED RELIEVING CAPACITY
  is the relieving capacity of a pressure relief device measured at the flow rating pressure.
• RATED RELIEVING CAPACITY
  is that portion of the measured relieving capacity permitted by the applicable code or regulation to be used as a basis for the application of a pressure relieving device.
• OVERPRESSURE
  is a pressure increase over the set pressure of a pressure Relief Valve, usually expressed as a percentage of set pressure.
• POPPING PRESSURE
  is the value of increasing static inlet pressure of a pressure Relief Valve at which there is a measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling or hearing.
• RELIEVING PRESSURE
  is set pressure plus overpressure.
• SIMMER
  is the pressure zone between the set pressure and popping pressure.
• MAXIMUM OPERATING PRESSURE
  is the maximum pressure expected during system operation.
• MAXIMUM ALLOWABLE WORKING PRESSURE (MAWP)
  is the maximum gauge pressure permissible at the top of a completed vessel in its operating position for a designated temperature.
• MAXIMUM ALLOWABLE ACCUMULATED PRESSURE (MAAP)
  is the maximum allowable working pressure plus the accumulation as established by reference to the applicable codes for operating or fire contingencies.

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