3 replies to this topic

[jimbio]

Hi to everybody,

I'm sorry for the simply question, but I think it could be useful to others too.

I have to size a psv on the water side of an heat exchanger,
and I would like to know which is the flow rate to take into account,
but I don't know which is the relevant scenario for the water side.

In my opinion it is a phylosophical choice, because using fire scenario the valve is too big,
and using design flow rate is too small. Is there any other possible scenario? (tube rupture??)

If the scenario is the fire one I will use steam equations, if it is the closed outlet one I will use liquid equations.

Can anybody give a physical explanation of the process?

The exchanger is a refrigerant condenser:
Duty 41 kW (External surface 1,7 m^2)
Water flow rate is 5050 kg/h
Water inlet/outlet temperature 30/37 °C
Water set pressure is 10 barg and relieving pressure is 12 bar abs (tubeside design pressure is 20 barg)
Refrigerant pressure is set to 22 barg and the psv is 1E2

Let me know if some other information misses

Thanks in advance

[ankur2061]

jimbio,

PSV's on cooling medium side of a heat exchanger are sized for "thermal expansion of trapped cooling medium" due to flow blockage of the cooling medium which in your case is water. The PSV is given a specialised name as "Thermal Relief Valve" (TRV) or "Thermal Expansion Relief Valve" (TERV)

You don't take the flow rate into account but calculate the flow rate using certain equations. However, in majority of the cases the calculated flow rate is so small that the whole exercise of doing the calcualtion is a waste of time. Instead you provide a "TRV" of 3/4" X 1" (DN 20 X DN25) size and be done with it.

In english units the equation for calculating the relief flow rate is:

W = B*Q / 500*SG*Cp

where:

W = Relief flow rate, gpm (United States Gallon per minute)
B =Coefficient of cubical expansion @relieving temp 1/deg F
Q = heat duty of heat exchanger, Btu / hr
SG = specific gravity (=1 for water)
Cp =specific heat capacity of liquid @relieving temperature, Btu / lb-degF
T = relieving temperature

I have done many useless calculations for TRVs because our system demanded it and the ultimate conclusion was to provide a 3/4" x 1" TRV. The tricky part in these calculations was to determine the relieving temperature. For all heat exchangers using cooling water we used to consider the relieving temperature as 2 deg C more than the cooling water return temperature. For example, if the heat exchanger CW return temperature was say 40 deg C we used to consider the reliving temperature as 42 deg C and use the values of the cubical expansion coefficient and specific heat at that temperature. The calculated flow rates would be of the order of a few GPM.

But let me reiterate that these calculations were nothing but a waste of time considering that a 3/4" x 1" TRV would always be more than sufficient to cater to the calculated flow rate of a few gallons per minute (USGPM).

Hope this helps.

Regards,
Ankur.

[Lowflo]

Be aware that the coolant side of an exchanger can be exempt from needing a thermal expansion relief valve, if it's reasonable to assume that the coolant system will remain open and in service while the process is running. In most cases, the process won't run if the coolant system is not open and flowing. So this is usually a good assumption.

Refer to API 521, paragraph 5.14 for an explanation of when and how it's appropriate to use this exemption.

[jimbio]

Thanks to both of you for the detailed and immediate answers,
I will adopt a 3/4"x1" valve.