PR valves

Pressure Relief Valve - This is a generic term applying to relief valves, safety valves or safety relief valves. It is commonly abbreviated to "PR Valve". 

Spring Pressure - The spring pressure is equal to the set pressure minus the superimposed back pressure for a conventional PR valve. For a balanced bellows safety relief valve, the spring pressure equals the set pressure. 

Open Disposal System - This is discharge piping of a PR valve which releases directly to the atmosphere. 

Closed Disposal System - This is the discharge piping for a PR valve which releases to a collection system, such as a blowdown drum and flare header. However, a closed system can also be a process vessel or other equipment at a lower pressure. 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

Equipment to be Protected - All vessels subject to overpressure by fire must be protected by PR valves, with the following exceptions ... 

A PR valve is not required for protection against fire on any vessel which normally contains little or no liquid, since failure of the shell from overheating would occur even if a PR valve where provided. Examples are fuel gas knockout drums and compressor suction knockout drums. (Note Some local codes require pressure relief valve protection for "dry drum" situations.)... 

Drums and towers, 6 m and less in diameter, constructed of pipe, pipe fittings or equivalent, do not require PR valves for protection against fire, on the basis that piping is not provided with protection against overpressure from this contingency. PR valves are required on such vessels, however, if overpressure can result from contingencies other than fire. 

When the control valve size is established by the designer, for example selecting a CV, it is assumed that this size valve is installed. If the control valve size is later increased it may be necessary to recheck the PR valve relief considerations. 

Pumparound Flow Failure - The relief requirement is the vaporization rate caused by an amount of heat equal to that removed in the pumparound circuit. The latent heat of vaporization would correspond to the temperature and pressure at PR valve relieving conditions. "Pinchout" of steam heaters may be considered. 

Abnormal Process Heat Input - The required capacity is the maximum vapor generation rate at PR valve relieving conditions, including any noncondensibles produced from overheating, less the normal condensation or vapor outflow rate. In every case, one should consider the potential behavior of a system and each of its components. For example, the fuel or heating medium control valve or the mbe heat transfer may be the limiting consideration. Consistent with the practice... 

Manual Valve - Inadvertent operation of a block valve while the plant is onstream may expose equipment to a pressure that exceeds the maximum allowable working pressure. For the purpose of design, it is considered that only one manual valve is opened or closed and control devices are in their normal design position. A PR valve is required if the block valve is not locked or car sealed in the open position and if closure of such valve can result in overpressure. 

The quantity of material to be relieved should be determined at conditions corresponding to the PR valve set pressure plus overpressure, not at normal operating conditions. Frequently, there is an appreciable reduction in required PR valve capacity when this difference in conditions is considerable. The effect of friction pressure drop in the connecting line between the source of overpressure and the system being protected should also be considered in determining the capacity requirement. If the valve passes a liquid which flashes or the heat content causes vaporization of liquid, this should be considered in determining PR valve size. 

Pump and Downstream Equipment - A PR valve is required for a pump when the shutoff pressure of the pump is greater than the design pressure of the discharge piping, downstream equipment, or pump casing. Positive displacement pumps normally require such protection, while in most cases centrifugal pumps do not. 

The capacity of a pump discharge PR valve should equal the capacity of the... 

Other positive displacement pumps, such as rotary, gear, and diaphragm pumps, normally require PR valve protection for both the pump and downstream equipment. PR valves for all positive displacement pumps should have a capacity at least equal to the pump capacity. 

For any pump requiring a PR valve for its protection or for protection of downstream equipment, the PR valve set pressure should be higher than the normal pump discharge pressure by 170 kPa or 10% of the set pressure, whichever is greater. Note, however, that in some cases a higher PR valve set pressure may be desirable to assure a sufficient differential when the pump is to... 

Pump PR valves should be discharged to a closed system. In many cases they may be conveniently routed to the suction line or suction vessel. 

For positive displacement compressors, discharge PR valves are nearly always required. Reliance on stalling of a reciprocating compressor is generally not economically attractive, since driver stalling pressures are usually quite high in comparison to operating pressure. 

PR valve capacity should be equal to the compressor capacity at the emergency conditions. For centrifugal compressors, the combination of PR valve set point and relieving capacity should be such as to avoid surge conditions over... 

Steam Turbine - A PR valve is required on the steam inlet to any steam turbine if the maximum steam supply pressure is greater than the design pressure of the casing inlet. The PR valve should be set at the casing inlet design pressure and sized such that overpressure of the casing is prevented, under conditions of wide open steam supply and normal exhaust flow. 

The required relieving rates for these PR valves are based upon the steam rate to the turbine As a rough guide, the appropriate size can be checked against the following tabulation, which are recommendations of the "Standards of the Heat Exchanger Institute, Surface Condenser Section."... 

Maximum Steam PR Valve Maximum Steam FRVahe... 

It is also important to note that the lowest design pressure of any section of the casing must be specified to be no lower than the pressure which it may be subjected to under the PR valve relieving conditions. This is necessary to recognize pressure drop within the casing. The PR valve should be sized to pass the normal steam flow to the turbine, but credit may be taken for steam flow which is withdrawn from an intermediate turbine stage if it would not be blocked by the same contingency as closure of the exhaust. 

Boiler Steam Side Overpressure - All fired boilers are normally provided with PR valves sized to relieve the full steam rate in the event of closure of the normal outlet. 

Overpressure and tube failure may also result from valve closure on the inlet side of a fomace, or from feed pump failure, etc, if the coil remains pressurized by downstream equipment. In these cases, however, overpressure occurs at or below the normal operating pressure (due to overheating at no-flow conditions), and a PR valve cannot provide the necessary protection. 

If a PR valve is provided on the furnace feed line, the valve should be located upstream of the orifice which senses low furnace feed flow and actuates the fuel cut-out so that the fuel will be cut out in case the furnace should be blocked at the outlet. 

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