Hydraulics - Cracking Pressure? 2

[TrippL]

What is the definition of the cracking pressure for a check valve in a 1500 psi (operating pressure)hydraulic system? Would the cracking pressure be 1500 psi?

 

[hydtools]

No. 1500psi would be full open full flow pressure. Cracking pressure is that pressure at which the valve just begins to open and will be lower than full flow pressure.

Ted

 

[TrippL]

Hydtools, thanks. Is there a certain cracking pressure I should specify for the spring check valve for this system? How do I determine what cracking pressure to specify?

 

[TrippL]

Ok, after some research I found out that 5 psi is the standard cracking pressure for hydraulic check valves. Thanks for your help.

 

[IceStationZebra]

The actual cracking pressure for a valve in use will depend on a combination of its design (poppet design, surface area, etc.) and the flow forces acting on it.

ISZ

 

[Oldhydroman]

Sorry ICZ, flow force doesn’t really count in this instance. Since the “cracking pressure” of the valve is the pressure needed to open the valve the tiniest amount (open just a “crack”) then, by definition, the flow will be infinitesimally small and the “flow forces” at the cracking pressure will be similarly infinitesimal. The principal factors determining the cracking pressure of a valve are the area of the poppet (or ball) on which the inlet pressure acts and the strength of the spring (spring rate and pre-compression) pushing the poppet (or ball) onto its seat. I’m assuming here that the outlet pressure is zero.

The complication is that, for practical purposes, some manufacturers redefine the cracking pressure to be the pressure differential at a real and readily measurable flow, such as 2 in³/min. And although this sounds like a cheat, it is much more pragmatic than waiting around to see when a drop forms on the open outlet of the valve and trying to decide if that drop came through the valve or just gathered itself together from the [already wet] insides of the valve. Also note that, again for practical purposes, some manufacturers define the allowable leakage across the closed valve as a [small] figure which is actually greater than zero; say, 1 drop/min (there being 250 drops in one cubic inch).

Regarding the question about which cracking pressure you should choose, the answer is, as always, it depends:

If you ever want to be able to PULL oil across the check valve with just tank pressure (atmospheric pressure) on the inlet then you need quite a low cracking pressure otherwise the oil will cavitate. Some manufacturers consider a 4 psi cracking pressure suitable for this. This value is a nice compromise between ensuring rapid and secure closure and still allowing an anti-cavitation function.

If the anti-cavitation flow has to pass through several valves and/or pipe runs of significant length then you might want to choose an even lower cracking pressure, say 1 psi. If your total pressure drops exceed 14.7 psi then you cannot pull the oil through at the rate you intend. To be on the safe side you might want to ensure your total pressure drop does not exceed 10 psi – and do the calculation at your lowest temperature/highest viscosity condition.

For the lowest cracking pressure of all, you would choose a valve with no spring - but this valve has to be installed vertically so that gravity will give the initial closure of the valve. You can’t really use this technique on mobile equipment since “vertically down” will not be a fixed direction. Not all manufacturers offer the no-spring version, but some offer a choice of springs and sometimes the springs are separate items for you to fit yourself. The no-spring version then would be simple to make – take out the original spring and don’t fit a replacement. Do check, however, that the poppet (or ball) will still be properly guided when there is no spring behind it.

As you have discovered TrippL, most manufacturers have a cracking pressure that they consider to be “standard”, for example: 4 psi, 5 psi, 7 psi, 15 psi or, for some, 30 psi. Some manufacturers give the cracking pressure as a range, e.g., 4-7 psi, which suggests that: they have no control over their manufacturing process, or they don’t want to be tied down to unnecessary details, or they have different values for all the sizes in that style of valve and want to simplify the published data. Most of the time it really doesn’t matter much: just look at the graphs of pressure against flow and note the pressure at zero flow … that’s the “cracking pressure”. If it is critical then ask the manufacturer for a proper graph (as opposed to marketing hype). If the manufacturer can’t or won’t send one then they are probably not the right people to supply components for critical applications.

A cracking pressure of 30 psi is good for ensuring the valve closes firmly and quickly when using high viscosity fluids (or normal fluids but at low temperatures). Without a substantial contact pressure between the poppet and the seat it’s hard for the valve to close completely against the tenacious oil film sitting on the valve seat. It is possible for there to be a microscopic flow through this film so if you are looking for a completely leak free closure over a matter of days and weeks a high cracking pressure would be better.

Also look at the pressure in your system – if yours is a low pressure lubrication circuit, say 150 psi, then inadvertently putting in a 30 psi check valve will rob 20% of your power input.

Very high cracking pressures are also available, e.g. 75 psi, 100 psi and 200 psi. These are useful when you want to use the check valve as a cheap, robust, non-adjustable “relief” valve to control the maximum differential pressure round a cooler or filter, or to sequence the operation of some pilot operated valves.

For cracking pressures higher than this you would need the actual “relief” valve – which, in its direct operated form, isn’t that different from a check valve. The spring force may be adjustable and there might be some attention to detail with regard to cancelling out the flow forces, or damping the movement of the poppet etc.

So, if you’ve no particular need to pull oil across the valve, if your viscosity isn’t particularly high, if your pressures are typical of industrial/mobile hydraulic systems (1500 psi is) then the manufacturer’s “standard” valve will probably be OK for you.

There are, however, other issues which you should be aware of:

1) Do you want your valve to have a soft seat or a metal-to-metal seat? there are advantages and disadvantages to both - but you didn't ask that question so I won't answer it here.

2) Is the size correct? An inline valve is typically sized to match the pipework or port to which it is attached – but that does not guarantee that the flow capacity of the valve is actually correct for your application. A check valve which is far too big will only open a fraction at full flow – it is possible for a pulsating flow (such as from a slow running gear pump, or a radial piston pump with only 3 cylinders) will cause the check valve to chatter in sympathy. The poppet oscillates at the same frequency as the pulses in the flow but, because the gap is so small, the poppet actually touches the seat during every trough in the pulse train. Converseley, if the check valve is too small then the flow will push the poppet fully open and the pressure drop will follow the same basic quadratic relationship regardless of the particular spring you chose. Check the graphs of flow (x) against pressure again and you will see a series of sloping lines representing the different spring rates – but on their right hand end they all join the same curved line representing the flow restriction of the fully open check valve.

DOL

 

[nmiller127]

TrippL,

To your original question, the definition of spring cracking pressure is the differential pressure required to begin to open a valve. For instance if you have a 1.5 psi cracking pressure, the valve will begin to open up when there is 1.5 psi more pressure on the inlet side of the valve than the outlet side.

ie inlet = 1.5 psi
outlet = 0 psi

inlet = 101.5 psi
outlet = 100 psi

Hopefully that will give you the idea.

Now keep in mind that a spring loaded valve only begins to open at the cracking pressure as Oldhydroman said. To be able to fully open a valve it typically takes a certain amount of calculated pressure drop, more than the spring, to be able to open it fully. This can be 2 times to 5 times the spring cracking pressure (check with the company, keep in mind some may not publish this data). So let's say for instance it takes 4 times the spring cracking pressure in calculated pressure drop across the valve to fully open the valve. If you have a 1.5 psi spring, you are going to need 4 times that or 6 psi in calculated pressure drop. This does not mean you only need 6 psi of inlet pressure. It means that based on your media, temperature, inlet flow, inlet pressure, valve Cv value, you are getting 6 psi of drop through the valve.

It is important to get the valve fully open for reasons like Oldhydroman said. Also, a partially open valve can potentially rattle and chatter which can cause premature wear. Additionally you can not calculate the projected pressure drop on a partially open valve because the Cv value of a partially open valve will constantly vary. The listed Cv values for all valves are only valid if the valve is fully open.

As to your question of what spring cracking pressure to specify, typically you want to go with the lightest spring possible (or no spring for vertical flow up as Oldhydroman said) for your application. This will require the least pressure drop to fully open the valve (ie 1 psi spring needs 4 psi drop as compared to .5 psi spring needs 2 psi drop).

N Miller

 

[IceStationZebra]

"Sorry ICZ, flow force doesn’t really count in this instance. Since the “cracking pressure” of the valve is the pressure needed to open the valve the tiniest amount (open just a “crack”) then, by definition, the flow will be infinitesimally small and the “flow forces” at the cracking pressure will be similarly infinitesimal. The principal factors determining the cracking pressure of a valve are the area of the poppet (or ball) on which the inlet pressure acts and the strength of the spring (spring rate and pre-compression) pushing the poppet (or ball) onto its seat. I’m assuming here that the outlet pressure is zero."

I agree with you, but what I meant was momentum-induced flow forces and potentially a change from laminar-to-turbulent flow. The magnitude obviously depends on the valve design, housing design and installation; but you should see a slight difference in cracking pressure with a change in flow.

For one of my past projects I was responsible for tuning the hydraulic valves for a whole series of forklifts. The valve manufacturer set the relief valves in a plain block and then installed them into the main control valve. Once installed in the trucks the relief pressure was checked and it was off from what they set it to, so we had to develop a "correction" factor for their manufacturing rig. I also saw differences between low and high GPM.

ISZ

 

[Raspberry]

Thank you very much everyone for the thorough and exhaustive treatment you have given the subject of cracking pressures here in this thread. However, for completion I would like to comment that cracking pressures are in-fact static pressures and nothing to do with flow forces whatsoever - dynamic pressures (those forces responsible for pressure losses across the valve).
To explain - when a system is first turned on, the pump/compressor pressurises the fluid to operating pressure. If this pressure is less than the cracking pressure the check valve will remain fully closed and the fluid will dead-head behind it. This is highly unlikely - you have 1500 psi and a cracking pressure of 5 psi (much lower cracking pressures are available).
As soon as the valve cracks, flow initiates and the dynamic forces maintain the valve open.

 

[nmiller127]

Rasberry, can you explain your thoughts behind your statement "This is highly unlikely, you have 1500 psi and a cracking pressure of 5 psi"?

N Miller

 

[hydromech]

It's highly unlikely, although not impossible.

The 5 PSI is the hydraulic force opening the valve, the remaining 1495 PSI would have to be a function of the gap across valve as flow passes through.

 

[nmiller127]

My understanding of spring cracking pressure is that in theory you can choose just about any spring cracking pressure you want (although it is difficult to find a manufacturer that offers more than a couple spring cracking pressures). Also valve cracking pressure is the differential pressure required to begin to crack open the valve. So if you have a 5 psi spring, the valve would start to crack open at 5 psi of differential pressure, but it would take more pressure and flow to fully open the valve.

Also, as far as how much pressure is being lost through the valve (ie 5 of the 1500 psi) that loss would be considered the pressure drop across the valve. So for instance if you do have 1500 psi of inlet pressure, and a spring cracking pressure of 5 psi, you would not necessarily have a pressure drop of 5 psi. It will depend on the specific gravity of your media, temperature, flow rate, and Cv value of your valve to determine the actual pressure drop you will see across the valve.

dP for a gas (pressure drop in psi) = P1 - sqrt(((P1)^2)-2GT((Q/1360Cv)^2))

where P1 = Inlet pressure (psia)
G = Specific Gravity of the media
T = Temperature of media (deg Rankine)
Q = Flow rate (scfh)
Cv = Valve coefficient (should be listed on the valve cut sheet)

dP for liquid (pressure drop in psi) = ((V/Cv)^2)G

where V = Flow rate (gpm)
Cv = Valve coefficient (should be listed on valve cut sheet)
G = Specific Gravity of the media

N Miller

 

[Raspberry]

Sorry for not having posted a reply sooner than this. I think there may be some confusion here; to re-inforce comments from my previous posting, cracking pressure is a static pressure and pressure exerted by a flowing fluid is a dynamic pressure - they are quite distinct. We certainly should not consider subtracting one from the other.

During valve opening the operating pressure must exceed the cracking pressure, once flow is established there will be a pressure recovery, the extent of which will depend on the design of the valve (this is a matter of thermodynamics and perhaps shouldn't be discussed here).

 

[nmiller127]

Rasberry,

Thank you for the clarification. You are correct on your description of static vs dynamic pressures. Additionally, to determine full open pressure drop, you do take into consideration both pressures.

Thanks again,

N. Miller