sizing PSV outlet around PD pump

[bukeksiansu]

Hi,

I am trying to calculate PSV that is located at a pd pump discharge, that is routed back to the suction. the psv is sized based on blocked outlet. i understand for the inlet line, i have to meet the 3% set pressure line loss. but i don't know how to calculate the outlet line. do i need to consider a change of temperature also? if so, then overtime if the liquid is increased, the fluid began to flash, then the psv would not be sized based on liquid flow, but it will be sized based on 2 phase flow. is this necessary?

thanks in advance,

bukek

 

[MortenA]

Your point is good and normally ignored. If possible maybe you should route the PSV further back? maybe if you got some kind of feeding vessel all the way back here?. If theres a TAHH then you could route upstream this?

Usually you only size for the liquid flow.

Best Regards

Morten Andersen

 

[TD2K]

I would ignore the flashing issue. If you do start to get flashing, the pump's ability to pump is going to be severely affected, thus likely eliminating the relief case.

As Morten said, I've also only seen single phase flow used for PD pump PSVs. Not to say you can't but it would be unusual situation that would make you need to do so.

 

[CHD01]

We normally try to take the discharge back to the make-up tank instead of the suction line. Over time there can be a temperature increase, but this heat is usually going to be removed to atmosphere through piping and tank vent evaporation.

Lots of area for discussion here over relief requirements for P.D. Pump versus flammability of fluid, presence of internal relief device vs. need for external r/v, HP of P.D. Pump vs. probability of injury or capital loss, type of P.D. pump, etc. The more you learn, the less you are certain of.

 

[ihg]

Bear in mind that the pressure rating of the PSV discharge must be high enough to withstand the highest pressure that the suction line can see. On a PD pump, this can often be full discharge pressure, if you have a spare pump and leaky valves. You can buy PSV with Class 1500 inlet/outlet rating but they are not inexpensive. Discharge back to a suction vessel can be preferable.

 

[MortenA]

ihg

I dont quite follow. If you sized your PSV correctly you wont se a higher pressure than SP+10%.

Best Regards

Morten

 

[22082002]

Just for personal interest, which liquid are you pumping?

 

[25362]

Bukeksiansu: PD pumps are reciprocating or rotary. Which type is your query dealing with?
A fault-tree analysis may indicate the best thing would be to stop the pump when the discharge is blocked or to unload suction valves.

 

[bukeksiansu]

Thanks for all the response.

the type of pump is a plunger triplex pump. the fluid being pumped is crude.

i never realize that this type of pump requires such a high pressure at the suction too (vendor asked for 3.5 psi - not including acceleration loss). maybe this will be another topic, because i don't have a 100% confident calculating the pd pump suction in regards to including accelaration head to the NPSHA and for calculating the suction pressure.

thanks again!

bukeksiansu

 

[elbati]

What type of PSVs uses ?
We use Balanced Pressure Safety Valve. Do you know the causes that can fail the bellows?
We are using in the suction and discharge of PD pumps and the discharge valve, releases in the suction.

 

[mucour]

MortenA, ihg is right.

He is refering to the PSV flange ratings.

The pressure class of the inlet of the PSV should be the same as the pump disharge piping class.

The outlet of the PSV would have a lower piping class, which can be similar to the suction line piping class.

The above explanations does not have any relevance to the sizing of the PSV size, which is a different issue entirely.