Pilot Operated Relief Valve

Hi,

I'm gonna tell you what is pilot operating of valves. It works not only with relief valves, but also for shutoff valves, pressure regulating valves, whatever you imagine.

When we are talking about current domestic, or industrial low flow equipment, we encounter a number of valves that are actuated directly by hand, by solenoid, by torque motors in a PID loop, etc. However, when we are talking about high flow, we are talking about valves that have large parts and considerable reaction loads, that cannot be directly actuated by a small solenoid or torque motor, nor a manual handle, specially if they are located far from the ground in a plant. You still could use an electric actuated valve, but it implies in having a special control circuit monitoring your process and sending a signal to the valve (or someone permanently looking at a manometer and pushing buttons... for each valve in your plant...).

Another solution, that works fine and gives somewhat precise results, is the use of piloted valves, specially if you are talking about compressed air. These valves use an actuator that is fed from your process and/or from a remote regulator, depending on the valve function, and actuates proportional to the process variable you are trying to control. One side is connected to the process, the other is loaded with a spring and/or pressurized by a small pressure regulator or solenoid valve (there you can use small ones, the one you need, thus, converting your large valve to respond to a small solenoid). The following sketch will show you my vision of a piloted relief valve:

The pneumatic representation of the above structure would be:

It's easy to see that the sensing line can be connected to the valve body inlet, in the upstream pipe, or in a remote reservoir, depends on the distance the valve is from the critical point where you must control the pressure. If you have a very hot fluid in a reservoir, for example, you would have problems of valve damage if installing it at the reservoir. Extending a long pipe and signal line can drop the temperature enough to allow the valve to operate safely. It can also be connected to a small solenoid for automatic discharge, to a shutoff valve for manual remote actuation, and, if you replace the spring by (or add) a pressure regulator, manual or electronically controlled, you can also have remotely variable relief pressure set. Cool, ins't it?

Another important application is when we talk about pressure regulating valves. Imagine now the following sketch. It differs from the other by the place where the pneumatic signal enters the actuator:

As the fluid flows and pressure builds up at valve outlet, also the pressure in the upper side of actuator increases, until the valve reaches an equilibrium between downstream pressure and remote signal. If you increases the signal pressure, the valve moves to a position more opened, thus, increasing the downstream regulated pressure. If the pressure in the inlet of the valve drops, for example, it will also cause a drop in the downstream pressure. So, the valve will be more opened. Of course it will not give you a pressure at downstream greater that at upstream...

With these ideas, you can image a plant with several valves commanded by solenoids, by PIDs, or by a single operator in a remote location.

I hope it helps.

1. I presume you are referring to hydraulic relief valves. By conventional, do you mean direct operated? If so, then they are used for quick response since pressure is acting on a poppet held on a seat by a spring. They will crack at the adjusted pressure, and continue to open to accept flow while the pressure increases above the cracking pressure. This is called pressure rise. You get quick response at the expense of rising pressure. The pressure has to decrease below the setting, or cracking pressure for the poppet to reseat. This can present a problem if the load won't allow the pressure to decrease so the poppet can reseat. This will result in loss of actuator speed. 2. A pilot operated uses a small spring loaded poppet to relieve pressure from the top side of a larger lightly spring loaded poppet to create a pressure imbalance. The pressure differential acting on the poppet area compresses the lite spring to open, and bypass oil to the tank. The small poppet can have an alternate port in parallel with it going to a remote direct operated relief much the same as the small poppet. The remote poppet can reduce the releving pressure below that of the main poppet, but not above it. This question was not asked, but be aware that any pressure drop on the tank port of the relief of a direct, or pilot operated type add to the relief setting.

1. When the location of a relief valve makes it difficult or dangerous to reach to adjust manually a pilot operated relief valve could be used. A pilot regulator located in a safe and convenient location may send pressure to the pilot port of the relief valve and control the set point.
2. Remote sensing: If a tube or pipe is connected to a down-steam position as well as the pilot port of the relief valve it would allow the pilot operated relief valve to open at a lower pressure (Thereby feeding more fluid forward) when the pressure drops down steam at the sensing position. This can be done with a direct connection or by using a "feed back sensing pilot regulator".

Additionally, the small pilot valve is easier to finely adjust the setting, and much less costly to replace.

How it works:

1. A "direct" sensing system has a large relief valve that opens when it senses excessive pressure where it is located.

2. In a pilot system, the pilot valve is actually a small relief valve. It's exhaust then operates a large diaphram actuated globe valve (typically an inverse operated valve). Another method is: The pilot is a pressure sswitch, which then operates a solenoid valve to relieve the pressure.

One advantage of the pilot system is that the remote operated valve is a common item, not requiring calibration. The precision part that requires sensitive relief settings is a much smaller valve. The cost for this smaller valve will be less. You will be able to adjust it more precisely.

The one disadvantage of the pilot system is the purchase cost and installation labor will be a little higher than the single valve system

Without going into detail on the many types of relief valves, piloted or non-piloted, the major difference from a control standpoint is that in a piloted valve, the force or pressure required to control the valve is much less than in a non-piloted (direct acting) valve.

Remote sensing allows the relief valve to be controlled by a condition that exists remotely, hence its description as remote sensing. An example would be in a process system where a problem or condition in one part of the system would trigger the relief of pressure in some other part.