how to determine valve cracking pressure area?

[HarshParekh]

Hello,

Fluid flow pressure from the right side. The cylindrical valve attached to spring is supposed to open at a given pressure.

In the picture attached what diameter to select to calculate the area to determine valve cracking pressure? 16mm or 19.8mm diameter? Theoretical there are 2 cracking pressure? Sorry if the answer is obvious

I ask because test results shows something strange.

 

[SPDL310]

There is only one cracking pressure. Your seal diameter is closer to 16mm. That will be your initial plug diameter. Once you crack your seal the plug diameter increases to the ID of the red body. I'm assuming that you have an O ring or seal ring that is not shown further back on the green surface. If not the plug diameter is then 19.8 mm. What sort of strange results are you getting? When calculating your cracking pressure are you assuming static pressure? If your fluid is moving you need to use total pressure in your calculation.

 

[HarshParekh]

Hello SPDL310,

I uploaded new picture with gasket location to prevent leakage. In the test I recorded pipe wall tapping pressure gauge reading and hence static pressure. But do you think if the valve just cracks the velocity component of flow would have significant effect to consider total pressure?

You mean to say diameter 19.8mm will determine the cracking pressure?
Strange result from the test is that cracking pressure area is neither from 19.8mm nor from 16mm but from the average of above diameters. 19.8+16.0/2 = 17.9mm dia. Area equivalent to 17.9mm dia determines the cracking pressure. Am I missing something in physics? Is it obvious?

 

[SPDL310]

The velocity at cracking should not significantly affect your results if you are static prior to cracking. Just to be clear are you backing into a plug diameter from fitting pressure before your pressure drop? If so test your spring force it may not be the exact force you specified. The tolerance of your stack may be off due to over machining leading to less spring compression. What is the intended purpose of this valve?

 

[LittleInch]

Once it cracks open, where does the flow go - Into the two U shaped sections??

If so then the moment the plug starts to move, the effective area will increase from 16mm as the thin film of liquid between the gasket and the seat will still have some pressure until it decays in the U shaped area. Hence an effective area of over 17mm seems quite reasonable to me. You can't go from cracking pressure to atmospheric in 0.1mm.

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

 

[SPDL310]

I think his issue was that the valve cracked at a lower pressure than he expected it to. His calculated pressure area at cracking pressure is above what he believed his seal area to be.

 

[HarshParekh]

@SPDL310 Yes that's exactly the issue. I tested each spring in load test machine. Also parts diameters measured in CMM to be sure of stack up problems. We also tried with reduced reduced red part diameter (i.e diameter reduced from 16mm to 13mm) and still the average diameter predicted the perfect cracking pressure but I am not able to understand why? Is it something to do with pulsation (opening and closing of valve)? It's just like a relief valve to open at specified bar in fuel filter. Valve is open to sump.

@LittleInch the flow goes to sump (or its open to atmosphere). I think calculation of cracking pressure shouldn't be so complicated. Valve supposed to open at 3.5 bar and hence spring force calibrated accordingly.

 

[LittleInch]

I'm not sure why you're trying to do this. Relief valves are set by pressure, not spring force. If the pressure it lifts is too low, tighten the spring a bit. Simple.

You're not far away with average diameter to size the spring then just adjust to get the right pressure.

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

 

[SPDL310]

I definitely agree with LittleInch on this. I will offer some additional speculation. You may not have considered the force required to seat your gasket at the geometry that you have shown therefore you loose pressure early due to an unseated gasket. Regardless of the direct cause of your low cracking pressure the solution is to add a stiffer spring or the proper amount of pre loading to get the desired cracking pressure. You may want to switch to a smaller sealing surface to get a higher initial seating stress.

 

[HarshParekh]

Thanks for your feedback. Anyways with trial and error I can find the right spring but I wanted to understand the physics.

 

[gerhardl]

HarshParek: Let me just add the comment that in 'real use' you will of course have a dynamic and different situation varying with state of the valve, maintenance, wear, dirt, sealing condition, spring condition etc. etc.

The measurement of (theoretical) cracking pressure and accuracy will in lesser degree vary with theese factors and for one off brand new prototype, but will also be depending on the measurement devices accuracy, placement and the dynamic situation at the cracking moment and the instruments ability to record/react at the exact cracking moment without delay or measurement peaks.

An 'exact' area calculation might thus be colored by one or several of the points above.