Check PSV sizing isn't too low for normal operation

[mintgreen]

Folks,

I've been asked to check the PSV sizing for a temporary chemical injection skid our company has proposed. It's been a while since I've looked at pump sizing and there is one thing bothering me.

The setup is a Checkpoint 1500, 1" plunger pump injecting 25L/h into a system operating at 1450kPag. There is a 75m 0,5" diameter temporary hose running from the pump discharge to the injection point where there is a block valve before it enters the system. They've calculated approx 50kPag pressure drop across the temporary line based an injection pressure at the injection point 50kPag above the operating pressure, 1500kPag. There is a PSV installed on a tee from the pump discharge that relieves to the pump suction at 3900kPaA. The tee is between the pump discharge and a check valve. The temporary hose is connected downstream of check valve.

Looking at the pump curve and using the available 100psig air supply, the discharge pressure on the pump will be around 2100psig (14479kPag) which will certainly lift the PSV if the check valve jams closed or the block valve on the injection point is closed whilst the pump is running.

Under normal operation these conditions should not cause the PSV to lift continuously, correct?

Thanks

 

[LittleInch]

This is why I don't like KPa, but from your data normal pump discharge pressure is 15 bar and the relief valve set at 39bar Given this is a PD pump then it would appear that there is a lot of leeway between NOP and relief set point.

Pump curve for a PD pump sounds a little odd, but the pump CAN produce 114 barg (not "will"), but only if there is that back pressure.

you need to make sure the isolation valve and the "temporary" (HATE that word) hose is good for 39 bar. Remove the word "temporary" from anything you write or draw. implication is that it can be lower quality, inspection etc, but I've seen "temporary" items still there 15 years later.....\rant over

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

 

[mintgreen]

Thanks for the reply LittleInch. I think you've uncovered my confusion which I appreciate.

What will the pump discharge pressure be in this instance? See attached the flow curve for the pump.

Certainly with you on the "temporary" rant BUT I suspect this customer will make sure it stays temporary.

 

[mintgreen]

Scratch that last post.

I've already answered my own question. The discharge pressure will be equal to system back pressure (1450kPag) plus any frictional loss + pressure head in the temporary hose which in this case equals 1450kPag + 50kPag = 1500kPa. That means the PSV is well high enough but low enough to lift before the temporary hose rating of 4300kPag.

Not sure why I was confusing the max output pressure from the flow curve for the actual operating discharge pressure. As I said, been a while since I sized a pump :-(

Some related and great info that helped jog my memory:

http://www.eng-tips.com/viewthread.cfm?qid=85162