Pilot-operated valves

Various features may be included in a pilot-operated valve, such as backflow preventers, field testing connections, pilot line filters, manual or remote opening for depressuring purposes, etc. Details may be obtained from manufacturers literature. 

Pilot-operated valves may be satisfactorily used in vapor or liquid services up to a maximum back pressure (superimposed plus built-up) of 50% of set pressure, provided that the back pressure is incorporated into the sizing calculation. At higher back pressures, capacity becomes increasingly sensitive to small changes in back pressure. As an exception, back pressure up to 7% of set pressure may be used, provided that this disadvantage is recognized. 

Remote Depressuring - A pilot operated valve is sufficiently positive in action to be used as a depressuring device. By using a hand valve, a control valve or a solenoid valve to exhaust the piston chamber, one can open the pilot-operated valve and close it at pressures below its set point from any remote location, without affecting its operation as a pressure relief valve. 

For applications involving unusually high superimposed back pressure, a pilot operated valve may be the only possible balanced valve that is commercially available, because of the mechanical limitations which apply to bellows. 

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 ... 

Typical discharge coefficients of pilot-operated valves are in the range 0.84 to 0.92. If the exact value is not known, use the lower coefficient. 

Pilot-operated valves have limitations in liquid service and reference should be made to the manufacturer for advice on sizing procedures. 

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. 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

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... 

Pilot-operated valves have the advantage of allowing operations n the set point v/ith no leakage, and the set position is not affected by ba pressure. However, they will not function if the pilot fails. If the sens line fills with hydrates or solids, the valve will open at 25% over pressure trapped above the disc (usually the normal operating pre the vessel). For this reason they should be used with care in dirty vice and liquid service. They are used extensively offshore where all platform relief valves are tied into a single header because up to 5 back-pressure will not affect the valve capacity. 

A disadvantage of pilot operated valves is that, if there is no pressure in the vessel, back-pressure could cause the disc to lift. This could occur if t was shut-in and depressured for maintenance, the relief val ve wa led in a header, and another valve in the header was opened,... 

Percent absolute back-pressure is 43%, which is less than the 50% limit for pilot-operated valves. 

WThen the pressure rise in a system is gradual and not explosive in nature, a safety or safety relief valve is the proper detdce, but when it is critical to completely depressure a system or the rate of pressure increase might be expected to be rapid, then a rupture disk is the proper device. Properly designed a pilot operated valve may be selected after checking its performance with the manufacturer. 

For bellows valves with superimposed or variable backpressure see Appendix B. For pilot-operated valves see note below. 

As we indicated, we can compensate for the effects of backpressure by selecting the correct valves such as balanced bellows - or pilot-operated valves but the backpressure also has an effect on the rated capacity of the valve and therefore larger valves could be required if backpressure exists (Figure 3.10). 

Dome pressure The pressure at the dome connection of a pilot-operated valve, which is usually the same as the inlet pressure. It is the pressure that is exercised on top of the unbalanced piston in the main valve and which, in normal operating conditions, is the force that keeps the valve closed. 

The main objectives achieved with some pilot-operated valves on the market... 

The main valve is basically a simple valve body design with a free-moving piston on a nozzle (semi or full). The main valve represents the capacity of the valve and is designed to flow the required rated flow. The pilot represents the quality of the valve and makes sure the valve works conectly. A pilot is usually the same for all sizes and pressures for a certain range of pilot-operated valves and is relatively small. The pilot is the intelligence of the valve and is where the set pressure is adjusted. 

Function of Anderson, Greenwood Co. pop action pilot-operated valve... 

Blockage of supply lines in pilot-operated valves... 

If there is a risk of blockage, this risk should be eliminated and that applies to a spring-operated valve as well as for a pilot-operated valve. Some manufacturers have acted on this and can propose many different accessories and configurations to achieve this which have been proven to work. Pilot valves can be protected against dirt with a variety of options, and most spring valve suppliers can supply such items as steam or electrical jackets, to avoid, for instance, polymerization or formation of hydrates. 

Pilot-operated valves are used primarily in the following services ... 

A pilot-operated valve with the pilot vented to atmosphere is fully balanced... 

In highly viscous liquid service. Pilot-operated valves have relatively small orifices which can become plugged by viscous liquids. 

In particular, vibrations and pulsations due to positive-displacement compressors can cause premature opening when the forces within the valve are anywhere near equilibrium and are acting as another upward force. For pilot-operated valves, some manufacturers provide pulsation dampeners in their pressure pick up lines so that the effect of these pressure spikes are compensated. As can be seen in Figure 6.21, for spring-operated SRVs, unfortunately, no provisions can be taken to compensate for this effect. 

Long blowdown required due to inlet pressure losses above 3% On gas service, a pop or modulating action POSRV can be used. On some pilot valves, blowdowns between 3% and 25% can be achieved. In case of very high pressure losses, pilot-operated valves with a remote sensor should be considered. 

Below —70°C and no backpressures Pilot-operated valves are still preferred, but spring valves could be used, preferably soft-seated ones. It should be noted that metal-seated valves always present a high risk of freezing at these temperatures. 

Fully cryogenic licjuid service Pilot-operated, soft-seated valves with a type of vaporizer which (relatively) warms up the fluid entering the pilot or high-performance, soft-seated, spring-operated SRVs. The vaporizer and other accessories of a typical cryogenic configuration on a pilot-operated valve keep the pilot warm, which then works on vapour. In any case, these are applications that should be discussed with your SRV supplier. Some suppliers have done extensive tests on cryogenic applications and have experience to share on this specific application. 

---