Test, Calibration and Maintenance for Pressure Safety Devices to Prevent Explosions and Fires

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2009 DEVELOPMENT RESEARCH AND TECHNOLOGICAL PLANNING CENTER CAIRO UNIVERSITY

Pressure Relief DevicesContents Hazard and factors contribute to an accident.

Fundamentals of pressure safety relief devices.

Terminology and types of pressure relief devices.

Code and standards requirements.

Location and choosing of safety relief devices.

Installation, Inspection, and maintenance.

Chatter (principle causes and solutions).

Testing and calibration.

Accident due to missing and fail of safety devices.

Video for safety relief devices (Operation, location, maintenance).

Hazard and factors contribute to an accident : . . ( ) .

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Factors contribute to an accidentHuman errorEquipment malfunctionUpset plant conditionsFire or explosion near the apparatusImproper proceduresSevere weather conditionsThe following causes were noted for 88 incidents that occurred in refining and chemical unit operations from 1959 to 1978:28 % equipment failures28 % human error13 % faulty design11 % inadequate procedures5 % insufficient inspection2 % process upsets13 % education

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Fundamentals of pressure safety relief devices 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). Since pressure relief valves are safety devices, there are many Codes and Standards written to control their design and application. A pressure relief valve must be capable of operating at all times, especially during a period of power failure when system controls are nonfunctional.

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.

Pressure relief valve (PRV) 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. PRV must be designed with materials compatible with many process fluids from simple air and water to the most corrosive media.

Type of safety devices Spring loaded valve The basic spring loaded pressure relief valve has been developed to meet the need for a simple, reliable, system actuated device to provide overpressure protection. Figure below shows the construction of a spring loaded pressure relief valve. The valve consists of a valve inlet or nozzle mounted on the pressurized system, a disc held against the nozzle to prevent flow under normal system operating conditions, a spring to hold the disc closed, and a body/bonnet to contain the operating elements. The spring load is adjustable to vary the pressure at which the valve will open.

Spring loaded valveMaterials must also resist chemical attack by the process fluid and the local environment to ensure valve function is not impaired over long periods of exposure.

Pressure relief valves on clean non-toxic, non-corrosive systems may be vented directly to atmosphere. Pressure relief valves on corrosive, toxic or valuable recoverable fluids are vented into closed systems.

Valves that vent to the atmosphere, either directly or through short vent stacks, are not subjected to elevated back pressure conditions. For valves installed in a closed system, or when a long vent pipe is used, there is a possibility of developing high back pressure.

Back pressure which may occur in the downstream system while the valve is closed is called superimposed back pressure. This back pressure may be a result of the valve outlet being connected to a normally pressurized system or may be caused by other pressure relief valves venting into a common header. Compensation for superimposed back pressure which is constant may be provided by reducing the spring force.

Seat leakage will result in continuous loss of system fluid and may cause progressive damage to the valve seating surfaces.

Application of pressure relief valves should be assigned only to fully trained personnel and be in strict compliance with rules provided by the governing Codes and Standards.

Rupture Disk Devices A rupture disc is a thin diaphragm (generally a solid metal disc) designed to rupture (or burst) at a designated pressure. It is used as a weak element to protect vessels and piping against excessive pressure.There are five major types availableConventional tension-loaded rupture discPre-scored tension-loaded rupture discComposite rupture discReverse buckling rupture disc with knife bladesPre-scored reverse buckling rupture discThey are often used as the primary pressure relief device.Very rapid pressure rise situations like runaway reactions.When pressure relief valve cannot respond quick enough.They can also be used in conjunction with a pressure relief valve to:Provide corrosion protection for the PRV.Prevent loss of toxic or expensive process materials.Reduce fugitive emissions to meet environmental requirements

Rupture Disk DevicesWhen compared with PR valves, rupture discs have:AdvantagesReduced fugitive emissions - no simmering or leakage prior to bursting.Protect against rapid pressure rise cased by heat exchanger tube ruptures or internal deflagrations. Less expensive to provide corrosion resistance.Less tendency to foul or plug.Provide both over pressure protection and depressuring.Provide secondary protective device for lower probability contingencies requiring large relief areas.DisadvantagesDont reclose after relief.Burst pressure cannot be tested.Require periodic replacement.Greater sensitivity to mechanical damage.Greater sensitivity to temperature

Rupture Disk DevicesEvery rupture disk shall have a stamped burst pressure below within a manufacturing design at a specified disk temperature and shall be marked with a lot number.

A rupture disk device may be installed on the outlet side of a spring loaded safety relief valve which is opened by direct action of the pressure in the vessel.

Terminology - Pressure Relief Valve - PRV Relief Valve. A relief valve is a pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure.

Safety Valve. A safety valve is a pressure relief valve actuated by inlet static pressure and characterized by rapid opening or pop action.

Safety Relief Valve. A safety relief valve is a pressure relief valve characterized by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure.

Advantages of PRVMost reliable type if properly sized and operatedCan be used in many services

Disadvantages of PRVRelieving pressure affected by back pressureSusceptible to chatter if built-up back pressure is too high

Non-Reclosing Pressure Relief Devices. Rupture Disc Device. A rupture disc device is a non-reclosing pressure relief device actuated by inlet static pressure and designed to function by the bursting of a pressure containing disc.Rupture Pin Device. A rupture pin is designed to be a non-reclosing pressure relief device, similar to a rupture discA piston is held in the closed position with a buckling pin which will fail at a set pressure according to Euler's LawAn o-ring on the piston is used to make a bubble tight seal

Rupture Discs Are Well Suited For Some ApplicationsWhen compared with PR valves, rupture discs have: Advantages + Reduced fugitive emissions - no simmering or leakage prior to bursting.Protect against rapid pressure rise cased by heat exchanger tube ruptures or internal deflagrations. Less expensive to provide corrosion resistance.Less tendency to foul or plug.Provide both over pressure protection and depressuring.Provide secondary protective device for lower probability contingencies requiring large relief areas.

Rupture Discs Are Less Well Suited For Other ApplicationsWhen compared with PR valves, rupture discs have: Disadvantages

Dont reclose after relief.Burst pressure cannot be tested.Require periodic replacement.Greater sensitivity to mechanical damage.Greater sensitivity to temperature

Code and standards requirementsGeneral Code requirements include:ASME Boiler & Pressure Vessel CodesASME B31.3 / Petroleum Refinery PipingASME B16.5 / Flanges & Flanged FittingsAll pressure vessels subject to overpressure shall be protected by a pressure relieving deviceLiquid filled vessels or piping subject to thermal expansion must be protected by a thermal relief deviceMultiple vessels may be protected by a single relief device provided there is a clear, unobstructed path to the deviceAt least one pressure relief device must be set at or below the MAWP. Relieving pressure Relieving pressure shall not exceed MAWP (accumulation) by more than: - 3% for fired and unfired steam boilers10% for vessels equipped with a single pressure relief device 16% for vessels equipped with multiple pressure relief devices

Location and choosing of safety relief devices.Overpressure protection is a primary consideration in the design of any piping system. The objective of overpressure protection is to maintain the pressure downstream of a regulator at a safe maximum value. In the system shown below, a high-pressure transmission system delivers natural gas through a pressure reducing regulator to a lower pressure system that distributes gas to individual customers. The regulators, the piping, and the devices that consume gas are protected from overpressure by relief valves.

choosing of safety relief devices.Selection and Sizing CriteriaThere are a number of common steps in the relief valve selection and sizing process. For every application, the maximum pressure conditions, the wide-open regulator flow capacity, and constant downstream demand should be determined. Finally, this information is used to select an appropriate relief valve for the application. The following is a suggested list of service conditions which must be provided in order to properly size and select a pressure relief valve;1.Fluid Properties: a. Fluid and State, b. Molecular Weight, c. Viscosity, d. Specific Gravity, Liquid (referred to water), Gas (referred to air), e. Ratio of Specific Heats (k), f. Compressibility Factor (Z)2. Operating Conditions: a. Operating Pressure (psig maximum), b. Operating Temperature (F maximum), c. Max. Allowable Working Pressure (psig)3. Relieving Conditions: a. Required Relieving Capacity Gas or Vapor (lb/hr)Gas or Vapor (scfm), Liquid (gpm), b. Set Pressure (psig), c. Allowable Overpressure %, d. Superimposed Back Pressure (psig), e. Built-Up Back Pressure (psig), f. Relieving Temperature (F)

Installation, Inspection, and maintenance.It is essential that routine inspection and maintenance occurs on these important valves. In time of emergency, can you be sure your pressure relief valve will operate properly?A number of guidelines for pressure relief valve service and maintenance exist to ensure that pressure relief valves work correctly. Some tips in brief include the following; Atmospheric discharge lines should have adequate rain and moisture protection, and be capable of draining condensate and rainwater. Check to make sure the relief valve exit stays unobstructed. Routine maintenance of pressure relief valves should include visual inspection of the relief valve and discharge piping every six months. Relief valves should be replaced at intervals of no longer than five years of service. Maintain pressure relief valve data in an inventory record, including location, size, set pressure, manufacturer, capacity, date installed, dates of inspections, and latest date for replacement.

Installation, Inspection, and maintenance.Pressure relief valves should not be discharged during installation or start-up.Replace pressure relief valves once they have discharged. Never expose your face or body to a connected relief valve exit. Avoid trapped ice build up between valves and other equipment. Reduce inlet pressure to zero before attempting to install or replace any pressure relief valve. Preferably, and as required by most codes, Use a three-way dual shut-off valve to isolate relief valves for individual inspection or replacement. Check the nameplate or installation date tag to be sure the time-in-service does not exceed five years. Look for corrosion and leaks. If there is any doubt about the internal condition of a relief valve, remove and inspect it internally. If there is any question about a valves condition, replace it. Shut down gas supply, drain gas from relief valve and system before any maintenance.When using a vise or wrench on the valve, use only enough torque to hold or move the part.

Periodic MaintenanceIf the valve is sits for long periods it may not perform as expected. Disks may stick in seats, setpoints can shift over time, and small passages can become clogged with pipeline debris. Therefore, periodic maintenance and inspection is recommended.

Installation, Inspection, and maintenance.Undersizedinlet pipingConsider the pressure dropfrom all these connections

Installation, Inspection, and maintenance.Discharge directeddownwardDischarge toonear deck

Installation, Inspection, and maintenance.Long moment armShipping plug stillin bellows vent

Installation, Inspection, and maintenance.Will these bolts holdwhen the PRV relieves?

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Pressure Relief Valves) Safety) : ( ) . . . . . . . . : . . . 10- .11- .12- .

Chatter ( ) (principle causes and solutions)Chattering is the rapid, alternating opening and closing of a PR Valve.Resulting vibration may cause misalignment, valve seat damage and, if prolonged, can cause mechanical failure of valve internals and associated piping. Chatter may occur in either liquid or vapor servicesChatter - Principal CausesExcessive inlet pressure dropExcessive built-up back pressureOversized valveValve handling widely differing rates

Chatter ( ) (principle causes and solutions)Chatter Mechanism Excessive Inlet Pressure DropChatter Solutions Excessive Inlet Pressure Drop

Venting - The venting end shall be at least 6 m above the floor level and at least 2.5 m above the compressor enclosure. Furthermore, as regards own and neighboring constructions, it shall comply with the following formula: h > H D + 2.5where h = venting height (m)H = neighboring or own construction height (m) (the highest one)D = neighboring construction horizontal distance (m)- Venting pipe shall have a noise dampener system that shall be activated when noise exceeds the admissible levels.- Discharge of all relief systems inside the compressor enclosure shall be delivered into a shock absorber tank which design and construction shall be approved by Gas del Estado or of any other system enabling a safe and clean operation.- Service venting normally closed with manual valves and plugs shall be admitted to be located in suitable pipe runs.- Permanent or periodical venting of gas either due to operation, lack of maintenance of installations or others shall not be accepted except

(Testing and Calibration) ( ) :

. (ASME) 3% . . (ASME). . . . .

ConventionalRelief Valve

Accident due to missing and fail of safety devicesOn November 19, 1984, a series of explosions at the PEMEX petroleum storage facility (LPG) at San Juan Ixhuatepec in Mexico City ignited a major fire and killed about 500 people. The cause of explosion was safety relief valve missing.

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