PSV calculation for exchanger cold side blocked in

[zamakaze]

I'm evaluating the thermal expansion relief scenario for a heat exchanger with LPG (cold fluid) on the tube side and a hot fluid on the shell side.

Some specific questions I’m looking for guidance on:

• For the relief scenario, should the heat input be assumed based on the exchanger’s design duty?
• During relief, should we assume that the LPG will heat up to its normal outlet temperature (e.g., from 80°F to 120°F), or is there a more conservative approach typically followed? The hot fluid enters at 248°F and leaves at 85°F.
• If the LPG is stagnant during a blocked-in condition, does that impact the overall heat transfer coefficient (U-value)? Should this change be accounted for? Does exchanger LMTD change?
• Under normal operation, LPG enters at 600 psia and is fully liquid. In the blocked-in case, should we assume its pressure will drop at all or we should assume it starts rising due to the heating right away

 

[georgeverghese]

a) If this LPG remains in liquid phase at relieving pressure, then design duty can be used as heat input
b) If the PSV is located on the LPG exit side of this HX, and given that Cp of LPG liquid will remain the same at relieving pressure, it is likely that LPG exit temp to the PSV will still be 120degF
c) Where is the PSV located on this HX ? If it is on LPG exit side, U and LMTD will remain the same as in design duty case. If this PSV is on the inlet side, then U and LMTD and Q will have to recalculated to match the flow configurations at relieving conditions.
d)Once the HX is blocked in, tube side pressure will start to rise.

 

[zamakaze]

Thanks. PSV is located on the LPG exit side in this case
What changes if its at inlet side...how does it effect U, LMTD and Q?

 

[georgeverghese]

U and Q will be much lower with PSV on tubeside inlet, since the flow path to the PSV almost completely does not pass through HX. As you said, flow in HX will be almost stagnant - it will be in natural convection mode.

 

[LittleInch]

My responses would be:

Heat input as per design data
Second one kind of depends on what happens to the LPG when overpressure occurs. At what pressure does 4it start to relieve? In theory the lpg could see 248F in a blocked in case IMO.
U value shouldn't change, but heat transfer will as the lpg warms up so lower delta T.
Blocked in case is not clear here. Do you mean isolated at both inlet and outlet or only at outlet? Or inlet only? Can you then get reverse flow out of the heater?

I suppose both ends is locked in case for which pressure rise could be very rapid.

Are we talking thermal expansion only or could this start to vapourise? I can't find much info above 200F but extrapolation gives me a boiling point of about 800psi at 240F.

Waht is relief pressure set at?