Overpressure protection

There are contingencies that need not necessarily be considered for the design of a particular PRV but will have an impact in the design of the plant relieving system (flare or cold vent). Relieving contingencies under all possible scenarios need to be established for the following situations  

Process engineering and design using Visual Basic 

There are a large number of contingencies that influence the design of a particular PRV, but may have no impact on the design of the overall plant. The controlling contingencies need to be established and reported for the following situations  

Check valve failure Blocked discharge Control valve failure Thermal expansion of liquid Heat exchanger tube rupture Reflux failure and overhead system Loss of reboiler heat Venting of storage tank Failure of individual motor Accidental closure of valve 

Terms commonly used in the design of PRV systems and their definitions are presented in the following subsections. 

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Protection against explosions is typically provided by explosion-venting, using panels or membranes which vent an incipient explosion before it can develop dangerous pressures (11,60). Protection from explosions can be provided by isolation, either by distance or barricades. Because of the destmctive effects of explosions, improvement in explosion-prevention instmmentation, control systems, or overpressure protection should receive high priority. 

Pressure Relief Devices The most common method of overpressure protection is through the use of safety rehef valves and/or rupture disks which discharge into a containment vessel, a disposal system, or directly to the atmosphere (Fig. 26-13). Table 26-8 summarizes some of the device characteristics and the advantages. 

For flammable and/or toxic materials all of the precautions for a pressurized system should be considered. For example, when a centrifuge is pressurized, overpressure protection is required, even if the pressurization is an inert gas. Relieving of the pressure to a closed system or safe location must be considered. 

Blockage of relief device by solids deposition (polymerization, solidification). Possible loss of overpressure protection. 

Selection of Pressure Relief Device - From the range of available pressure rehef valves and other devices, selection is made of the appropriate type for each item of equipment subject to overpressure. Instrumentation, check valves, and similar devices are generally not acceptable as means of overpressure protection. 

Design for overpressure protection in most cases consists of providing pressure-rehef devices sized to handle the calcidated relieving rates necessary to prevent emergency pressures from rising above the design pressure (plus allowable accumulation). 

Now let us consider utility failure as a cause of overpressure. Failure of the utility supphes (e.g., electric power, cooling water, steam, instrument air or instrument power, or fuel) to refinery plant facihties wiU in many instances result in emergency conditions with potential for overpressuring equipment. Although utility supply systems are designed for reliability by the appropriate selection of multiple generation and distribution systems, spare equipment, backup systems, etc., the possibility of failure still remains. Possible failure mechanisms of each utility must, therefore, be examined and evaluated to determine the associated requirements for overpressure protection. The basic rules for these considerations are as follows ... 

Startup. Shutdown and Alternate Operations - Not only design steady-state conditions, but also startup, shutdown, washout, regeneration, alternate feed stocks, blocked operations and other possible different conditions must be evaluated for overpressure protection. 

Low-pressure stage casings and interstage circuits on both centrifugal and positive displacement multi-stage compressors are not normally designed for full discharge pressure and must also be provided with overpressure protection. 

If the control valve size is critical to the overpressure protection of the downstream equipment, and must not be increased, then this is clearly noted in all relevant documentation (specification sheets, flow diagram, operating manual, etc.,) and a warning notice plate is welded to the valve body. In such cases, an actual check of the valve installed or purchased should be made during the startup review. 

Vessels should be provided with overpressure protection as required. Vents and relief valve vent piping should be so arranged Uiat Uie vented vapors will not constitute a liazard. Relief valves must be kept free from corrosion or fouling and should be operable at all Umes. 

A single rupture disk can be used as the only overpressure protection on a vessel or system (Figure 7-10). The disk must be stamped by the manufacturer with the guaranteed bursting pressure at a specific temperature. The disk must rupture within +5% of its stampied bursting pressure at its specified burst temperature of operation. The expected burst temperature may need to be determined by calculation or extrapolation to be consistent with the selected pressure. 

Figure 7-14, Operational Check Sheet [25], lists 16 possible causes of overpressure in a process system. There are many others, and each system should be reviewed for its peculiarities. System evaluation is the heart of a realisdc, safe and yet economJcal overpressure protection installation on any single equipment or any group of equipment. Solving formulas with the wrong basis and/or data can be disastrous. The following should be reviewed ...

Turbine pumps are positive-displacement pumps They cannot be started with a closed discharge valve and require the fitting of a pressure relief valve to provide overpressure protection. 

The ASME code provides the basic requirements for overpressure protection. Section I, Power Boilers, covers fired and unfired steam boilers. All other vessels including exchanger shells and similar pressure-containing equipment fall under Section VIII, Pressure Vessels. API RP 520 and lesser API documents supplement the ASME code. These codes specify allowable accumulation, which is the difference between relieving pressure at which the valve reaches full rated flow and set pressure at which the valve starts to open. Accumulation is expressed as percentage of set pressure in Table 1-9. 

Dispersion, Flaring, Scrubbing, and Containment An example of an overpressure protection system designed to reduce emissions to the atmosphere and at the same time provide adequate protection to the equipment has been described [234]. The equipment indicated is used for the manufacture of ethylene-vinyl acetate-vinyl chloride polymer emulsions. The design pressures are up to 100 bar. 

NFPA 30 and API Standard 2000 provide guidance for design of overpressure protection involving storage tanks that operate at or near atmospheric pressure. In particular, NFPA 30 focuses on flammability issues, while API 2000 addresses both pressure and vacuum requirements. The ASME code (Sections I and VIII) and API RP 520 are the primary references for pressure relief device sizing requirements. 

Overpressure protection Compressible relief Compressible relief... 

This code included rules for the overpressure protection of boilers, based on the best industry practice of the time The principles of today s code rules for overpressure protection is little changed from the first code. 

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