Silly PSV Question

[polyroly]

Good Morning Everyone!

It has been a long time since I have had to size a PSV and I am a little rusty My scenario is blocked outlet from a pump (centrifugal) capable of 110m of head at shutoff. My questions is............what do I use for a flowrate for the relief case? I can't use the design point that the pump is operating at or I get a rediculous orifice size and that can't be correct.

Any help would be appreciated!

Regards,
polyroly

 

[LittleInch]

This sounds really quite odd. Normally a centrifugal operating pressure or head is so close to shut off head that a PSV is the wrong thing to use. You really need to give us much more info - pump head is onlyone part, what is the design pressur eof the system, normal operaitng etc etc.

The issue for me is that if the pump doesn't stop then the flow won't stop. Reliefing the flow won't change the head by very much (10% at most) so your PSV isn't doing what you think it is.

Post more info AND a digram / schematic as this really doesn't stack up at the moment....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[polyroly]

After looking a little more into it, a control valve will be a better choice for what we need. Thanks for your response LittleInch.

 

[LittleInch]

Are you sure? A blocked outlet needs more than a control valve to protect your pump and stop it disintegrating... Control valves only work with flow or are you using it to create a minimum glow through the pump?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Latexman]

If a centrifugal pump is left running with inadequate flow through it, heating and subsequent expansion of the fluid is inevitable. Flow might be low or zero because of closed downstream valves, closed upstream valves, even control valves, or fouled piping or equipment. The consequences that could result from a deadheaded pump situation need to be evaluated. There is more potential for damage when handling flammable, toxic or reactive materials. However, even a water pump can self-destruct if allowed to run deadheaded for an extended time. A brittle material of construction (cast metals) are a concern also. A risk based analysis is recommended.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[polyroly]

Here are the rest of the details on the system. There is a nitrogen blanket on the expansion tank for the system that is on the suction side of the pump. There is a PSV on the tank that is set at 150 psi though normal operating pressure is only 20 psi.

Given that possible 150 psi nitrogen pressure combined with the possible 110m of head (204 psi fluid pressure because its 30% CaCl brine) we exceed the pressure rating of the 150# flanges.

 

[SNORGY]

The other thing you will need to be aware of is that at whatever flow and pressure you come up with for the relief valve, make sure that you know the corresponding pumping efficiency and, hence, power required. Otherwise you might end up burning ouyt your motor in a valiant effort to safeguard the pump.

 

[BenThayer]

this sounds a bit like double jeopardy. how likely is it to have non-normal/max pressure at the suction side and blocked outlet?

do you need the full 150 psig inlet blaketing capability or can you can you reduce that?

 

[LittleInch]

I think you're double dipping here, but is difficult to see without a diagram.

If I read this right, you have a pump with a relief valve or control valve, which relieves this liquid back to the inlet tank which sits at 20 psig gas blanket. What ever pressure or head this liquid is at at the discharge of the pump will dissipate completely once it goes through the valve and into a tank with a gas phase. In addition so long as the tank is feeding liquid into the pump which then relieves back into the SAME tank, then there is no net increase in liquid level in the tank other than whatever is being forced into it from the upstream incoming pipe.

Therefore unless your gas blanket is a very low percent of the tank volume, I doubt you'll ever get close to 150 psig, but only you know what your system consists of.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way