5 replies to this topic

[cristi90]

Dear all,

I have a theoretical question. In the typical loop of a volumetric pump on the discharge pipe there is a psv whose vent returned to the tank from which the pump has sucked. There is a particular reason for this assembly? Why for centrifugal pumps do not use this system with psv? Thank you! 

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[latexman]

The fluid picks up heat from the pump and friction.  It is returned to the tank to add this heat to the large mass in the tank  for a very small temperature increase.  If the fluid returned to the pump suction, the temperature can keep going higher and higher more quickly since the heat gain can be undiluted.

 

Centrifugal pumps are usually sized so the peak shut-off pressure does not exceed the MAWP/design pressure of the piping.  I have seen some Centrifugal pumps with a PSV to mitigate the "dead head" or blocked in scenario though.  There are several ways to mitigate this scenario, that is why some Centrifugal pumps have a PSV and some do not.

Edited by latexman, 25 September 2015 - 09:21 AM.

[fallah]

cristi90,

 

Following thread from this site, especially third post, can be helpful on your query:

 

http://www.cheresour...ntrifugal-pumpa

[shvet]

 a psv whose vent returned to the tank from which the pump has sucked. There is a particular reason for this assembly?  

 

It is not so good idea to return psv disharge back to tank which the pump has sucked.

Just imagine: you have psv, usually it is spring tipe and it discharges only after pressure drop across psv rises more than:

psv spring set pressure < pump discharge - tank normal pressure

 

Then some continguency occcurs and pressure in tank rises and tank psv dicharges. Pump remains on line but if operator closes shut-off valve then pump psv must open and protect equipment. But pressure drop across pump psv remains the same as before continguency occurs. And then pressure rises up to:

tank psv pressure (fully open) + pump shut-off head > pump psv set pressure

 

The typical system (from my personal experience) is to discharge psv in pressure independent system (tank).

Edited by shvet, 25 September 2015 - 11:03 PM.

[fallah]

shvet,

 

The pump is positive displacement type; then shut off pressure isn't applicable for it...the discharge pressure would be infinitely raised due to blocked outlet... 

 

Although simultaneous process upsets leading to suction tank pressurizing and pump discharge blockage is considered double jeopardy conditions and needless to be evaluated; by proper selection and sizing of PSV type, say modulating pilot operated one sized for full recycle flow, it can be arranged such that there would be no problem in PSV relief and suction tank operation...

[shvet]

fallah

sorry for lack of attention

 

cristi90

You can use a "rule of thumb" for psv discharge. If in equation below Δt  is negative - you can discharge liquid back to suction.

 

Δt = (860*N - K*F*(t-tair))/(K*F)

where

N - pump horsepower in kW

K - heat transfer coefficient in kcal/m2*h*°C

for insulated pipes K=0.2-0.5

for bare pipes K=5-10

F - pipe surface, m2

t - liquid temperarute, °C

tair - air temperature, °C