Relief-valve-set pressure

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. 

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. 

COOLING WATER REQUIRED RELIEF VALVE SET PRESSURE 29... 

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. 

TOTAL WETTED SURFACE AREA, ft 2. LATENT HEAT OF VAPORIZATION, Btu/lb. RELIEF VALVE SET PRESSURE, psig. ALLOWABLE OVERPRESSURE, (%). RELIEVING TEMPERATURE, oF. COMPRESSIBILITY FACTOR. 

There are two types of pressure test. The first is a tightness test, used after the equipment has been opened but not modified in any way. Typically this type of test is conducted after the item was opened for cleaning, inspection, or routine maintenance. The test ensures that the equipment is leak-free but it does not test the integrity of the vessel or piping itself. Tightness tests are never conducted at a pressure above the equipment or system design pressure or the relief valve set pressure. 

The second type of pressure test is a strength or hydro test. It is used when the equipment has been modified, say by having some welding done on it, or when structural repairs have been made. Strength test pressures are generally above the equipment or system normal design pressures or relief valve set pressure. 

As discussed above, tightness test pressures should not exceed the equipment MAWP or system design pressure or the vessel s relief valve set pressure. 

Checks — before pressure testing, a functional inspection of each system is can ied out to ensure that it has been installed coireclly — for example, correct valve types, control valves the right way round, correct instruments fitted and so on. All vulnerable equipment such as pumps and control valves are fitted with temporary strainers. The system is then blanked off at each end, filled with water and pumped up to test pressure. The line/vessel test pressures must be compatible. Relief valve set pressures are tested on-site and tagged before installation. If necessary, relief valves are blanked during pressure testing of lines. 

The inlet line to the relief valve is designed for a maximum pressure drop of 3% of the relief valve set pressure. The pressure drop should be in between the equipment being protected and the inlet to the relief valve. However, the line size should not be smaller than the inlet flange size of the relief valve. 

In the case of a conventional relief valve, general back pressure shall not exceed 10% of the relief valve set pressure. 

For conventional relief valves venting to a closed system, the line is sized to keep the back pressure within 10% of the relief valve set pressure. This is subject to a maximum velocity of 0.75 Mach. API 521 recommends a velocity up to 1 Mach. However, proper analysis of piping integrity is required for a velocity close to Mach 1 such high velocity should not be used for the design of a new system. Outlet line sizing for balanced-bellows and pilot-operated relief valves is based on a maximum velocity of 0.75 Mach. 

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