Poppet Check Valves: Does low cracking pressure reduce response time?

[Bambie]

Two poppet check valves have the same Cv but different cracking pressures.

Does the lower cracking pressure result in lower opening response time?

We would like to reduce opening time of a solenoid valve handling air that has a poppet check valve downstream.

The Cv is acceptable for steady state operation.

 

[moltenmetal]

The cracking pressure is determined by a spring on the poppet, holding it closed until the forces balance and the pressure differential overcomes the force of the spring.

The higher the cracking pressure, the more securely the valve will seal in the reverse direction especially under low differential pressures.

I would imagine that the lower the cracking pressure, the quicker the valve would be to open in the forward direction.

 

[Bambie]

moltenmetal,

Thank you for replying and I agree, but where could I find a technical basis for this opinion?

 

[MikeHalloran]

IF the poppets weigh the same; (not necessarily true)
The valve with lower cracking pressure uses a weaker spring, e.g., smaller wire, so the last turn and a half or so effectively adds less to the poppet's mass.
The valve with the lighter (poppet + part of spring) will move faster in response to the reversed pressure because of Newton's Second Law.

If you're looking for really fast response, consider a reed valve.


Mike Halloran
Pembroke Pines, FL, USA

 

[moltenmetal]

There is a volume between the solenoid valve and the check valve.

As the solenoid valve begins to open, gas flows through the valve trim and gradually pressurizes the volume between the solenoid discharge and check valve inlet.

The check valve begins to open when the pressure in that volume exceeds its cracking pressure.

The lower the cracking pressure, the sooner that pressure is reached, and hence the sooner the check valve opens.

This assumes that other dynamic factors (i.e. the mass of the poppet) are equal between the two, as Mike said.

Any other proof needed? Seems self-evident to me.

 

[Bambie]

MikeHalloran,

With reference to the graphs attached, could the argument be made that, as the delta P across the poppet check valve with the lower cracking pressure increases (moving along the ordinate), flow is initiated sooner and never loses this advantage (moving along the abscissa)?

 

[Bambie]

moltenmetal,

Poppet mass effect between the valves could be eliminated (or not) by the Vendor.
The current valve is 1"dia (16C) with 1 psid cracking pressure and the transient imposes 70 psid for 1 second.
A 1/3 cracking pressure (not shown on these graphs) is being considered.

 

[MikeHalloran]

The easiest way to improve the response time is to remove the poppet check valve.
Why is it needed?

If it's just needed for servicing the solenoid valve, replace the poppet check with a ball valve.



Mike Halloran
Pembroke Pines, FL, USA

 

[Bambie]

mikehalloran,

Please see the sketch attached.
Either solenoid A or B can vent the steam valve actuator but not both.
The conundrum is that low cracking pressure may speed venting through solenoid B but impede venting through solenoid A with increased leakage due to lighter seating load.

 

[MikeHalloran]

You're assuming that solenoid B will leak.
Maybe it does; maybe it always did.
Maybe you need to replace both solenoid valves with double block and bleed assemblies.
... or something even more complicated.

We here cannot see why you need to slam the steam valve open.
... or why you have two valves controlling it.

Maybe you need to back up and figure out the logic of your control strategy,
and what component failures, like leakage through valve B,
your strategy must defend against.



Mike Halloran
Pembroke Pines, FL, USA

 

[Bambie]

MikeHalloran,

The sketch shows the non-venting positions of valves A and B.
When valve A vents, valve B continues to supply air up to the closed check valve and to the (subsequently) closed port on valve A. Check valve leakage increases venting time.
When valve B vents, its supply port closes and air opens the check valve.
Valve B is not assumed to leak.
The process valve is a pilot operated safety valve and needs to open quickly.
Two solenoids provides control redundancy.

 

[gerhardl]

How fast an air (pilot operated) actuator will open is dependent on several factors. From the sketch attached the actuator seems to be opening by spring(s) with air relieved on opposite side. The main problem is how to open the valve fastest possible. Secondary eliminate capacity and operational problems caused by possible air leakage by pilot- valves and at the same time secure sufficient air to close.

This is a classical problem, and have a number of ready assembled commercially available standard systems as solution, many including SIL certification for secure long-time operation. I would suggest you replace existing system and/or consider following for faster operation:

Size of safety-valve actuator
Number/strength of springs of safety-valve actuator
Increased air opening size out of actuator
Increased pipeline out of actuator
Minimal opening of air relieving valve (note: real smallest orifice through any solenoid valve might be far less than the connecting pipeline nominal size)
Ready assembled solutions for actuators or presses.

Safety will cost, but less with correct equipment than by large damages caused by failing equipment.

Sorry about last 'moralization', but could not resist!

Best wishes!

 

[Bambie]

gerhardl,

Thanks for your suggestions but complete replacement is not possible at this time.

A comparison of stroke times using solenoid A vs B indicates that the check valve is limiting flow and switching out with a lower cracking pressure would be the easiest change proposal to get approval for.

 

[gerhardl]

OK! Have you checked if the check-valve could be replaced with another type with lower cracking pressure and possible larger/better throughflow?

 

[Bambie]

gerhardl,

Yes, testing is underway to determine if lower cracking pressure reduces venting time through solenoid B without increasing venting time through solenoid A due to increased seat leakage.