Pilot-operated PRV

This is a PRV in which the main valve is combined with and controlled by an auxiliary PRV. 

Process engineering and design using Visual Basic 

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Pilot-operated PRV A PRV in which the disc or piston is held closed by system pressure, and the holding pressure of that piston is controlled by a separate pilot valve actuated by system pressure (see Section 5.3). 

API Standard 526, Flanged Steel Pressure Relief Valves This is a purchase specification for flanged steel PRVs. Basic requirements, such as orifice designations and area, materials, pressure-temperature limits and centre-to-face dimensions, inlet and outlet, are given for both spring-operated and pilot-operated PRVs (Figure 4.13). 

The phenomenon of blowdown is caused by the use of springs. The valve spring balances against a pressure that equals the pressure in the protected tank minus the kinetic effects. When the PSV is open, its inlet pressure is less than the pressure in the protected vessel because of the inlet pressure drop. Blowdown is the amount by which the protected tank s pressure has to drop below the PSV s set pressure for the valve to reseat. The normal blowdown of a PRV is between 2 and 7% of set pressure. Pilot-operated PRVs can reduce the blowdown to about 2%. 

Some pilot-operated PRV uses diaphragm instead of piston to control the PRV open or close. This kind pilot PRV is usually used for low pressure service (3 inch water column to 50 psig). 

Figure 3 A typical pilot-operated PRV. [Courtesy of Tyco Valves and Controls LP]... 

Unlike spring-loaded PRV, the open or close of pilot-operated PRV is not affected by its backpressure. This means it can be used to protect equipment with higher pressure in relief valve discharge manifolds. 

There are two types of pilot valves used for pilot-operated PRV, One type will allow piston pop open at set pressure, and the other will allow piston open just enough to relieve fluid to prevent the system pressure to exceed the maximum allowed accimudation. The pop open pilot PRV is used for vapor service, while the... 

Figure 8 Typical pilot-operated PRV piston lift path versus system pressure for (a) pop open pilot PRV, (b) moduiatiiig pilot PRV. fCourtesy of Tyco Valves and Controls LP]...

Pilot-operated PRV s set pressure may be checked at field, but a field test connection should be provided. 

The best choice is modulating pilot-operated PRV, since it relieves what is necessary. Therefore, total relief is minimized. 

For service required large orifice area, spring-loaded PRV may be too heavy to install. Pilot-operated PRV may be a better choice. For medium or small size orifice, an economic comparison may be required to see which type of PRV is a better choice. 

For pilot-operated PRVs, the variation in superimposed back pressure does not affect the opening pressure. 

For a pilot-operated PRV, the blowdown can be reduced to 3% provided the pilot has a remote pressure sensing line from an independent and separate vessel nozzle. 

Typical pilot-operated PRVs are shown in Figure 4.6 and Figure 4.7. A pilot-operated PRV consists of a piston and an external pilot assembly. The piston has a larger area at the top in comparison to the bottom. Before the pressure reaches the set pressure, the balancing line maintains the same... 

One unique advantage of the pilot-operated PRV is that the lift of the piston does not depend on the back pressure of the system and the performance of the valve is not affected by the built-up back pressure. 

The performance of pilot-operated PRVs depends on the pressure balancing line. If this line is blocked, pressure balancing between the top and bottom of the piston will not be possible, and the valve will lift at comparatively low operating pressure. This is the major disadvantage of pilot-operated valves. This type of valve can be recommended only if the operating fluid is clean. The pilot-operated valve is not very popular for hydrocarbon service. 

The pilot-operated PRV is best avoided and may be considered only if other types of valves are not suitable. 

Kf, = back-pressure correction factor, obtained from Figure 4.14 = 1 for conventional and pilot-operated PRV = 1 if back pressure is less than 30% of the set pressure... 

Conventional and pilot-operated PRV The general equation of the orifice area is... 

This program has been developed for conventional and balanced-bellows PRVs. However, this program can be used for pilot-operated PRVs, too. For pilot-operated PRVs, use ... 

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