Valve set pressure

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. 

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... 

Set Pressure - The set pressure (the pressure at whieh the PR valve is designed to open) is speeified in accordance with Code requirements. In most vessel apphcations, the set pressure of at least one PR valve is equal to the design pressure. However, this set pressure is adjusted (up or down) for any effect of static pressure and friction pressure drop that may apply when the valve is installed elsewhere than directly on the vessel. For example, if a PR valve is installed in a non-flowing line above a liquid-filled vessel, the PR valve set pressure would be reduced sufficiently to allow for the liquid static head between the vessel and the valve. 

Other types of pressure-relief valves do not depend upon the back pressure for their performances. However, to ensure that the safety valves work at their maximum capacity, back pressure is limited to 50 percent of the relief valve set pressure. In the balanced bellows type valve, the spring does not act directly on the disk. Instead, it serves on a bellows first, which in turn acts on the disk. In case of the piston type, it works on the same principle as the bellows type, except that the bellows is replaced by a piston (see Figure 17B). The cross-sectional area of both the piston and the bellows is the same as the inlet nozzle of the valve and the effect of the back pressure on the top and the bottom of the disk creates equal balancing forces. That is, P,A is always equal to F, as shown in Figure 17B. 

With all nonconventional valves, the maximum allowable back pressure may be taken as high as 50 percent of the valve set pressure. This pressure value... 

Suction throttle valves are common in gas-lift service to minimi/c the action of the flare valve. Flow from gas-lift wells decreases with increased back-pressure. If there were no suction valve, the flare valve may have to be set at a low pressure to protect the compressor. With a suction valve it may be possible to set the flare valve at a much higher pressure slightly below the working pressure of the low-pressure separator. The difference between the suction valve set pressure and the flare valve set pressure provides a surge volume for gas and helps even the flow to the compressor. 

Figure 13-1 shows the various relationships between MAWP and the relief valve set pressure. The primary relief valve should be set to open at no more than 100% of MAWP and to relieve the worst case flow lates,... 

Figure 13-1. Various relationships between MAWPs and relief valve set pressure. [Reprinted with permission from API RP 521.)...

The calculation method can be selected by application of the decision tree in Figure 9.2. The liquid temperature is believed to be about 339 K, which is the temperature equivalent to the relief valve set pressure. The superheat limit temperatures of propane and butane, the constituents of LPG, can be found in Table 6.1. For propane, T, = 326 K, and for butane, T i = 377 K. The figure specifies that, if the liquid is above its critical superheat limit temperature, the explosively flashing liquid method must be chosen. However, because the temperature of the LPG is below the superheat limit temperature (T i) for butane and above it for propane, it is uncertain whether the liquid will flash. Therefore, the calculation will first be performed with the inclusion of vapor energy only, then with the combined energy of vapor and liquid. 

As system pressure increases and approaches valve set pressure, it acts upward on the sense diaphragm, with the feedback piston moving upward to close the inlet seat, thus sealing in the main valve dome pressure, as the outlet seat is also closed. A small, further increase in system pressure opens the outlet seat, venting the main valve dome pressure. This reduced dome pressure acts on the unbalanced feedback piston to reduce feedback piston lift, tending to lock in" the dome pressure. Thus, at any stable inlet pressure there will be no pilot flow (i.e. zero leakage). 

Safety relief valves are normally tested in the shop, or even on the equipment at atmospheric temperature. The set tolerances on the valves as manufactured are established by die Code as discussed earlier. In order to recognize the difference between the test temperature and the actual operating temperature at actual relief, the corrections shown in Table 7-lA and 7-lB are applied. An increase in temperature above design causes a reduction in valve set pressure due to the effects of temperature on the spring and body. 

Inlet piping sized so that pressure drop from vessel to pressure relief valve inlet flange does not exceed 3 percent of valve set pressure... 

This type of valve may be used when the variations in backpressure on the valve discharge connection do not exceed 10% of the valve set pressure, and provided this backpressure variation does not adversely affect the set pressure. 

Safety valves are normally manually tested at periods between 1 and 6 months and pressure-tested every 12 months. Boiler operating pressures are typically 5 to 10% less than safety valve set pressures to avoid valve weeping. 

COOLING WATER REQUIRED RELIEF VALVE SET PRESSURE 29... 

Figure 5-P62 illustrates two relief valves. The valve disk is designed to lift when the upstream pressure in the vessel (Pi) reaches the valve set pressure. Valve A has a disk that diverts the fluid leaving the valve by 90° (i.e., to the horizontal direction), whereas the disk in valve B diverts the fluid to a direction that is 60° downward from the horizontal. The diameter of the valve nozzle is 3 in., and... 

Naturally, we do not want to overpressure the tower, and pop open the safety relief valve. Alternatively, if the tower pressure gets too low, we could not condense the reflux. Then, the liquid level in the reflux drum would fall and the reflux pump would lose suction and cavitate. But assuming that we have plenty of condensing capacity, and are operating well below the relief valve set pressure, why do we attempt to fix the tower pressure Further, how do we know what pressure target to select ... 

Relief valves were invented to prevent steam boilers from blowing up as a result of excessive steam drum pressure. This was a distressingly common occurrence in the nineteenth century. The relief valve is also called a safety (or pop ) valve. When the pressure in a vessel exceeds a preset amount, the relief valve is supposed to pop or spring open. Gas will then be vented from the vessel, until the pressure in the vessel drops by 10 to 20 psi below its relief-valve-set pressure. We usually operate pressure vessels 25 psig or 10 percent below the relief-valve setting. 

A rupture disk is a thin sheet of metal installed below the valve, intended to protect the relief valve from plugging. The rupture disk ruptures at the relief-valve-set pressure. A better approach to retard this plugging problem is to maintain a steam purge, or inert-gas bleed, below the relief valve to prevent the accumulation of solids below the valve. 

P = Valve set pressure (barg) X Required percentage pressure drop p = Specific volume of fluid at pressure P (m3/kg)... 

The superimposed backpressure varies, so the set pressure of the conventional valve will vary proportionally. This is acceptable if the valve set pressure increased by the maximum backpressure is equal to or bebw the maximum albwable pressure of the protected installation. 

For pilot-operated valves, set pressure testing on pilot only is allowed. Functional tests of the full unit are required. 

In a conventional relief valve, the pressure force acts on a disk that is held against a seating surface by a spring. The compression of the spring can be adjusted using an adjusting screw so that the spring force is equal to the pressure force at the valve set pressure. 

Fig. 14 shows an example of a closed pilot-operated valve. Operation of the main valve is subject to operation of the pilot, which is itself a spring-loaded direct-acting relief valve, except that the valve is held open by the spring to allow system pressure to be bled to a chamber above the main valve disk. As the system pressure rises, so too does the pressure in the chamber above the main valve disk (and as this acts downward on a larger area than the main valve bore, it keeps the valve firmly clamped shut). The pilot valve is set to commence lifting at (say) 97% of the intended main valve set pressure. With a short travel, it reaches the seat and hence seals off the bleed line to the chamber above the main valve disk at (say) 98%i of the set pressure. Further, increase in system pressure causes the pilot valve spindle to lift clear off its disk, opening a central drain to relieve pressure in the chamber above the main valve disk (see Fig. 15). Hence, the main valve lifts, and fully.

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