Operation pilot valves

If a governor starts to cycle after operating for some time, this is generally the resiilt of wear which causes dead band or sticking. Also, all pilot valves should be inspec ted for the effects of dirt in the oil. 

A typical pilot-operated PR valve is illustrated in Figure 5. Under normal operating conditions, the vessel pressure acts on the main valve seat at the bottom of the free-floating differential area piston, and by means of the pilot supply line is also applied to the top of the piston and under the pilot valve disc. Since the top area of the piston is larger than the nozzle area at the lower end of the piston, there is a large net load holding the piston down on the nozzle. Under static... 

When the pilot exhausts to the atmosphere, a pilot-operated PR valve is fully balanced. Like the balanced bellows valve, therefore, its opening pressure is unaffected by back pressure, and high built-up back pressure does not result in chattering. 

Pilot-operated PR valves can be specified for blowdown as low as 2%. This is an advantage for main gas pipeline and pressure storage applications, where the narrow range of pressure cycling minimizes product losses resulting from a release. 

Pilot-operated PR valves are subject to the following disadvantages ... 

The application of pilot-operated PR valves is limited to clean services, where they are an acceptable alternative to the bellows valve if a balanced characteristic is required. Their particular advantage is freedom from "simmer", which may permit a closer operating pressure/set pressure margin than normal. 

In the case of a pilot-operated valve, provided that the pilot valve exhausts to the atmosphere, the main piston is independent of back pressure and is thus also considered as a balanced valve. Balanced PR valves can be characterized by the following ... 

If the pilot valve exhausts to the atmosphere, a pilot-operated valve may be considered as a balanced valve. 

Chattering caused by undersized inlet piping may sometimes be eliminated on pilot operated PR valves if the pilot valve pressure tapping is taken directly from the vessel being protected. However, it is recommended that the above inlet pressure drop limitations still be applied, to avoid the capacity reduction that would result from excessive inlet losses and to ensure freedom from chatter. 

Pilot-operated valves have a pilot valve combined with the main valve. The spring of the main valve provides 75 percent loading on the disk and the remaining 25 percent is offered by the gas or vapor through the pilot valve. When the vessel reaches the maximum allowable working pressure, the pilot valve relieves the gas pressure, which contributes to the disk load. Thus the safety valve becomes wide open. This is illustrated in Figure 18. 

PORV - Pilot (or power) Operated Relief Valve. 

Pilot-operated relief valves use the pressure in the vessel rather than a spring to seal the valve and a pilot to activate the mechanism. Figure 13-6 is a schematic of a typical pilot-operated valve. A piece of tubing communicates pressure between the relief valve inlet and pilot. When this pressure is below the set pressure of the pilot, the pilot valve is in the position... 

All relief valves are affected by reaching critical flow, which corre-spond.s to a back-pressure of about 50% of the set pressure. Pilot-operated relief valves can handle up to 50% back-pressure without any significant effect on valve capacity. Back-pressure correction factors can be obtained from the relief valve manufacturers for back-pre.ssures above 50%. API RP 520 gives a generic method for sizing a pilot-operated relief valve for sub-critical flow. 

Calculate orifice size for pilot-operated relief valve. 

The set pressure tolerances of pressure relief valves are not to exceed +2 psi for pressures up to and including 70 psig and +3% for pressures above 70 psig. Indirect operation of safety valves, for example, by pilot valve, is not acceptable unless the primary unloading valve will automatically open at not over the set pressure and will operate fully in accordance with design relieving capacity conditions if some essential part of the pilot or auxiliary device should fail [1]. 

The Code [26] allows pilot operated vent control valves provided the vent valve can operate automatically in case the pilot valves/system failed. 

Describe the advantages and disadvantages of a pilot-operated safety valve. See Grossel and Louvar (2000). 

Radioactive decay contributed to further heat buildup. This caused water in the primary circuit to boil. The pilot-operated relief valve on the... 

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