Setting Relief Valve

[twhite]

Hello,

I am learning as I go here, so please excuse any sign of ignorance.

I have designed and am now troubleshooting a lubrication power unit for a large piece of equipment. The power unit is designed to flow up to 53GPM of ISO 150 (750 SSU) weight oil. In this circuit is a large oil cooler that has a pressure limit of 200psi. Therefore, we have incorporated a pressure relief valve into the lube manifold. We have tried 2 different relief valves now (a Sterling A06G2HZN and Parker RAH161S10). Each one is rated for a much lower viscosity weight oil (150 SSU), but have the flow rate we need even though it is listed at the lower viscosities.

Our problem is that the relief valve is opening and reducing our flow to the equipment. Instead of the fixed displacement pump generated 53GPM, we are getting more like 35GPM when cold and climbing to 45GPM when oil hits about 110F. This happens when we set the relief valve by deadheading the manifold and set the valve to exactly 200psi to match the oil cooler limit. Instead, if I set the relief valve closer to 225psi through the deadheading method, we get almost 53GPM during normal operation. It appears that the relief valve is not capable or dumping all 53GPM back to tank without creating lots of backpressure. Could this backpressure be causing me to set the relief pressure unnecessarily low?

Is there a proper way to set relief pressure other than capping off the lube-out port and dialing in my 200psi? In reality, we should never see a complete blockage of the lube line causing the relief valve to have to flow 53GPM through it. Instead, we should only see a partial blockage. We have investigated a larger valve, but haven't had much success in finding one. Any suggestions there also?

I really appreciate the help.

 

[TD2K]

I don't understand what you mean by the following:

<start>
This happens when we set the relief valve by deadheading the manifold and set the valve to exactly 200psi to match the oil cooler limit. Instead, if I set the relief valve closer to 225psi through the deadheading method, we get almost 53GPM during normal operation. It appears that the relief valve is not capable or dumping all 53GPM back to tank without creating lots of backpressure. Could this backpressure be causing me to set the relief pressure unnecessarily low?
<end>


If I understand you right, you can't get the 53 gpm through your lube oil system without the PSV relieving some of the flow. Have you done a pressure survey of the system because it sounds like to me that you are taking too much pressure drop somewhere and the pressure at the inlet to your lube oil cooler is high enough to cause the PSV to operate (as it should). This matches why you get more flow through the system as the oil gets warms, its viscosity decreases and pressure drop changes. When you raise its set pressure to 225 psig, the additional available pressure drop allows you to get design oil flow through your system but at the expense of overpressuring your lube oil cooler, not a good thing.

Take a single pressure gauge and get readings to allow you to see what the oil pressure is across each piece of equipment and see how that compares to design.

200 psig seems like a lot of oil pressure to me though this isn't my field. If I've misunderstood your post, please correct me.

 

[twhite]

You did interpret my description correctly, the relief valve is opening under normal operating conditions therefore not allowing all 53GPM from the pump to go to the equipment. However, I believe the relief valve isn't really set at 200psi. This is why: When we run the power unit and it is only delivering roughly 45GPM, the pressure of the lube oil (as read on the lube out line) is only 110psi. So, that tells me that the relief valve is relieving at 110psi, not 200psi. The relief valve is very closely located to the pressure gauge I am using as my reference, so there should be little to no pressure drop. My guess is that because the relief valve can't relieve the 53GPM without creating major backpressure, that we are mistakenly setting the relief setting too low.

It appears that the relief valve cracking pressure is a function of flow in this instance because it can't relieve all the flow. Instead of a nice linear cracking pressure, we are getting some non-linear cracking pressure depending on the flow rate. We may start setting the cracking pressure using a small hand pump with very low flow because of this.

Any more thoughts? I appreciate the response. 200psi is fine for this application by the way.

 

[StoneCold]

twhite
The set pressure for most relief valves is set at a bench. Compressed air is appied to the valve and the set pressure is adjusted until you can maintain the desired pressure. Then the system pressure is raised to make sure the relief valve goes off. The process is repeated until the desired result is achieved.
In your case it appears that you need the relief valve to be fully open at 200psig. This means your "cracking pressure" will be between 180 and 190 psig depending on the valve.
Your valve needs to be big enough to relieve the entire 53 GPM incase you have a closed valve or a complete blockage.

My two cents

StoneCold

 

[twhite]

StoneCold,

Thank you for confirming what I am now starting to believe. Setting the relief valve on anything other than a test bench is poor planning. I will work on building a test bench.

As far as the 53GPM ever going fully through the relief valve, I doubt that would ever happen. We have never had a complete blockage to our equipment, only partial.