Pilot Valves

You can buy diffent types of valve for pneumatic and hydraulic use.

Basically they can be pilot operated or solenoid operated or both pilot assisted solenoid.

Most are usually spring return bu can be pilot or solenoid return. So to save confusion let's take a spring return as an example.

For a solenoid operated valve the built in solenoid is actuated by passing a current through it which actuates the valve.

For a pilot operated valve the valve is actuated by a piston which is pushed by an air pressure, this air pressure can be of a low pressure and from a low consumption airline or from other valves building up a pneumatic logic switching system.

For a combined pilot assisted valve the solenoid alone won't actuate the valve a pressure is needed to help the solenoid.

So its horses for courses, if you haven't an electrical signal then use a pilot operated valve, or if you want to have some pneumatic logic use air to switch the valves etc...

If you want low electrical power operation use a pilot assisted valve etc...

John.

John,

Just curious.... from a purpose of operation point of view, wouldn't it be just as correct to say that (with, say, positive logic in mind) a pilot valve would require constant input in order that the governed/piloted devices actuated by it will remain actuated? Hence the term, pilot? That once input to the pilot is lost or removed, other actuated devices are "turned off" or reversed? And, to the contrary, that a solenoid actuation does not require constant input once the intended, governed actuations are achieved? So that a pilot acts or can act, in effect, as a kind of failsafe: deactivating governed components when conditions demanded to actuate the pilot cease to exist?

MMMmmmmm not sure about that, as the pilot operated valve does NOT need a constant input for it to remain actuated... If the air pressure is applied the valve actuates but no air is used after actuation, so the pilot pressure could be turned off and the pressure remains to keep the valve actuated..... Unlike a solenoid which needs continuous current consumption...?

I like the analogy of a pilot light in a gas cooker or boiler, its needed to light the main flame but not to consume too much power and once the main flame is lit its not required!!

John.

Thanks for the correction, John. Indeed, I can now see that this tired ol' brain got it all turned around backwards. I too was thinking of the pilot flame analogy; due to the cerebral short circuit, however, a word was left out: generator, as in pilot generator, where constant input (from the pilot flame, in burner-off state) is necessary to enable pilot flame starting of the burner.

It would be nice if this site, like some others, allowed posts to be withdrawn &or strike-through edited by post originators--a great (verbiage-saving) tool for consensus forming, not to mention face saving or conflict avoidance.

We are all learning and making mistakes along the way.... Well, I know I am!!

I did at first, think that an edit or delete button would be useful, but the trouble is how would a thread read if people kept deleting their posts and leaving the follow up posts in place?

So I reckon its okay as it is, where you can edit your post but only for 15 minutes after you post it...

John.

This goes back to the earliest days of aviation when the pilot controlls his throttle with a hand operated valve. Later the term valve was used for vaccuum tubes, since at high attitudes where the air was much thinner & cold it was harder for the pilot to concentrate on control the throttle so he would put a brite piece of tube on it to remind him. Many crashes happen . today pilots don't have to operate valves----its automatic!

tibor

Pilot operated valves are always used on Prtessure Reducing Valves for Steam/Gas.

The function of this is that after you install the PRV on the line you have to take out a pilot operated 1/4" or 1/2" line from the designated point on the PRV to sense the down stream pressure some 3.5 ft/ 10 pipe diameter minimum. Now when your inlet pressure fluctuates say between 7-10 bar and you have set outlet pressure as say 1 bar so irrespective of your inlet pressure fluctuation the outlet remains constant at +/- 0.05%. If you do not give the pilot line during the inlet pressure fluctuations the outlet too may fluctuate +/- 7% of the set point so the safety valve on the outlet will keep popping open.

This is why you must insist on buying PRVs with pilot valve only and not standard PRVs without.

How bout a more basic definition

You really have 2 types

Direct acting- the air or electricity do all the work of opening or closing a valve

Pilot- a small valve causes a lg valve to shift using the pressure of the media to be redirected to do the work

valve called pilot operated because of use compressed air pusle to activatd the valve to change its position

the solenoid valve is the valve which use the electrical signal to activate the valve to change its position

the solenoid depend on the electo magnetic field which created when the electrical current pass throgh the coil and when the magnetic filed accure the magnetic filed push the axe and allow to air to pass to activate the valve

Eng.sherif

Solenoid valves are operated electrically. Depending on the application, you may used them for process control, logic hydraulic or pneumatic circuits and to keep close or open any valve. Once you cut the power to the solenoid, the valve should return to its deenergized state usually by means of a spring(normally open or normally close, depending on the logic of the system). The key is that the valve moves in two directions ( two ways). At this point you may have a valve that move between two limits (two positions), or centered between two limits (three way) or it will stay open at any point of is travel ( proportionally to the imput).

A pilot operated valve could perform the same features as the solenoid operated but its power source comes from the pneumatic or hydraulic system that is being controled or it may come from a mechanical device that it is being controled by the valve. Examples: hydraulic governors an mechanically driven governors, digital-hydraulic governors. At this point the level of power needed to operate the valve and the control logic together with is intrinsic safe operation is what will dictate the final operator for the valve: solenoid, pilot or any other combination

At this time the pilot name takes relevance. It may refer to a pilot operated valve as above or as a pilot control valve (spool valve) where a mechanical or pressure signal is amplified proportionally to the imput by means of a hydraulic valve (servo valve).

On the other analogy where you have a pilot light you are refering to the process starting energy source as in the case of a battery, a diesel/gas starting engine, an air/hydraulic acumulator or bladder or as a pilot light.

Regards. Luis