Pilot operated solenoid valves (2 way) - what happens after they are open?

[Rodger Furey]

Many (most? all?) pilot operated solenoid valves require a 5 psi pressure differential to open.
An example of the type of solenoid valve I am referring to is here ...

https://www.grainger.com/product/4E...p.ds&msclkid=5a48497735b11814f8382f06b130d937

The question(s) I've submitted to two manufacturers (ASCO, Plastomatic) is - what happens after the valve is open? Does the valve need to maintain the 5 psi diff in order to remain open and close properly?
Surprisingly, I have not received a definitive answer from either. ASCO tech support suggests that their valves might not close properly, Plastomatic just doesn't know.

The Cv of the valve will determine how much flow is required to create a 5 psi differential across the valve.
What happens to the valve when the actual flow is below that value, resulting in a greatly reduced pressure diff? ... like, 0.1 psi.

Does anyone work with pilot operated solenoid valves like this, and how do you deal with this situation?

Thx!

 

[Heaviside1925]

OP,
See if the attached will help.

 

[LittleInch]

My understanding is that yes it does need a constant DP as this is basically what holds the diapragm open. If you lose flow then the DP reduces and the spring over the diaphragm closes it.

The ASCO diagram for me has no difference between Fig 2A and 3A which is not correct.

https://www.ato.com/direct-acting-vs-pilot-operated-solenoid-valve

However there are clearly valves which have no or very small DP across the valve when flowing or not flowing.
either direct or where the solenoid lifts the diaphram off the seal.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Heaviside1925]

OP,
From the pdf I attached.

It depends on the type of pilot valve you select.

 

[Rodger Furey]

Thanks guys.
I've just read both attachments.
For the sake of this discussion, a pilot operated valve (POV) would be the floating diaphragm valve design.

According to the lit, it sounds like the POV starts to close if the flow through the valve is low enough to result in < 5 psi diff.
I think a semi-closed diaphragm would result in a lower actual Cv ... I wonder if the valve achieves some sort of equilibrium with whatever the (low) flow rate is, so there is enough of a pressure differential to keep the diaphragm partially open to pass the needed flow. If the flow were to increase, the pressure diff would also increase, the diaphragm would open a bit, and the valve would now be operating with a new actual Cv.

If this is true, then I probably don't really care if the valve is partially closed due to a low pressure differential. As long as it passes the required flow.

Yes? No? Maybe?

My main concern is - will the POV valve close with a < 5 psi diff? I didn't see the literature mentioning that.

Continuing my wondering ... A POV valve is in low flow equilibrium ... the solenoid is de-energized ... The diaphragm closes a little, the actual Cv goes down a little, psi diff goes up a little ... this continues until there is adequate psi diff to fully close the valve.

If this is true, then I really shouldn't be too worried about POV valves not closing completely in low flow situations. ... but, is it?

It would be nice if I could find an authority on the topic who could confirm or reject my musing.

 

[LittleInch]

I think you're about right but the whole purpose of the valve is to stop flow, not stop flow when it wants to, maybe, some time later.

Just get a direct acting one instead.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Rodger Furey]

The one "problem" with direct acting valves is the 5 f/s velocity limit for PVC solenoid valves.
I've run across it with two manufacturers. Plastomatic and Hayward.
I haven't been able to determine if that is a characteristic of direct acting valves in general or just those specific manufacturers.

So far, it seems that ...

Pilot operated valves: I have to contend with the 5 psi diff. There doesn't appear to be any velocity limit.
Direct acting valves: I have to contend with the 5 f/s velocity limit. There is no minimum pressure diff.

 

[Heaviside1925]

OP,
From the pdf as well. The force needed to open the valve is proportional
to the orifice size and fluid pressure. As the orifice
size increases, so does the force required. To open
large orifices while keeping solenoid size small, a
Pilot Operated construction is used.

It's a matter of orifice size and pressure as to whether you need a pilot or direct solenoid. If spring force is sufficient to act against the force generated by the pressure gradient, then direct, if not pilot.

 

[Heaviside1925]

OP,
To get back to your original question Does anyone work with pilot operated solenoid valves like this, and how do you deal with this situation? depending on what you are trying to achieve, a solenoid valve may or may not be the preferred selection. Even though other factors may dictate, trying to shoehorn a certain valve type into a specific application may not be the best approach. I don't know your situation and I am just saying this for consideration.

 

[LittleInch]

Why not just look up small 1 inch electrically actuated valves. They come in pretty cheap and are very low DP when open (ball valves).

The direct acting valves when you look at them have quite a restriction in them so the actual velocity in the middle of valve is even higher.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Snickster]

The one "problem" with direct acting valves is the 5 f/s velocity limit for PVC solenoid valves.
I've run across it with two manufacturers. Plastomatic and Hayward.
I haven't been able to determine if that is a characteristic of direct acting valves in general or just those specific manufacturers.

I believe the issue with PVC piping quick opening/closing valves is water hammer pressure surges. In previous thread it was discussed that limiting velocity of 5 ft/sec in PVC piping was due to water hammer pressure surges. When flow suddenly comes to a stop a pressure surge occurs that is much higher than the normal operaing pressure and could damage the piping. A solenoid operating valve itself is a source of water hammer so if you have one in the system you WILL have water hammer if the velocity is greater than 5 ft/sec. It may be that a pilot operated solenoid valve in which the solenoid does not open the valve directly but opens internal ports to allow system pressure slower to a diaphragm to open the valve opens and closes more slowly than a direct acting valve. With a more slowly opening non-direct acting solenoid valve then velocities higher than 5 ft/sec are possible without water hammer. In fact this may be the purpose for the development of non-direct acting solenoid valves in the first place.

So far, it seems that ...

Pilot operated valves: I have to contend with the 5 psi diff. There doesn't appear to be any velocity limit.

I believe there is higher velocity limits due to slower opening and closing of the main diaphram as described above; I dont't think tere are no velocity limits whatsoever. I believe with these valves you must maintain at least 5 psi pressure in the upstream port of the valve not 5 psi differential since the 5 psi is what acts on the diaphram to open the valve not a 5 psi differential. However when the valve is closed then that would require 5 psi on the inlet with no pressure on the outlet if you are discharging say to atmosphere, and in esscence when valve is close you would need 5 psig period upstream since only the upstream port is open to the diaphragm/disc when the valve is closed, so upstream pressure needs to be 5 psig regardless to open the main valve. I think that as long as you maintain 5 psig in the pipe not a differential then the valve will stay open as there will be enough force to overcome the spring keeping the diaphram/disc closed. However if you are dumping into an nonpresurized system then you will quickly loose pressure since there is hardly any pressure drop across the valve and piping. So if you add the downstream pressure you are discharging to, plus pressure drop in line, plus pressure drop in valve it will need to give you greater than 5 psig or the valve will start closing. However as you indicated in your post, as the valve starts closing the Cv will get greater and the pressure drop across the valve will increase until there is an equillibrium flow where you do get 5 psig on the upstream side, assuming you have more than 5 psig pressure availble upstream. But his this equillibrium point may be below your required point. Note this is all my own observations and am not a manufacture's rep or have direct experience with these pilot operated solenoid valves.

Direct acting valves: I have to contend with the 5 f/s velocity limit. There is no minimum pressure diff.

As discussed I believe this is due to the quick acting nature of direct acting valves so 5 ft/ssec is to protect the valve and PVC piping from water hammer pressure surges.

See pilot operated valve manual in this link:

https://www.manualslib.com/manual/1586043/Asco-Redhat-8321-Series.html#manual