HX Design Pressure and Design Temperature Decision for a Pumped System.

Pavan Kumar · Jul 26, 2022

Hi All,

I know there has been a lot of discussion on this forum regarding Design Pressure and Design Temperature of a pressure vessel. I went through a few threads but could not get a straight forward answer. Let me explain my problem to determine the Design Pressure and Temperature that I need to specify for my system that I need to procure.

I need to specify procure a Shell and Tube water-water Heat Exchanger whose Thermal Design is complete. On the shell side is Tepid water which is being cooled from 57 Deg C to 47 Deg C and on the tube side is Cooling water whose temperature is increasing from 25 to 32 Deg C.

1. The Tepid water is supplied from a Tepid Tank through a centrifugal pump whose shut-off pressure I calculated is 46.5 psig. So for shell side the Design Pressure be 1.25*46.5 = 58.125 psig at the maximum possible coincident temperature of 57 Deg C during normal operation. The piping is 12" Sch 40 CS piping with 150 # flanges.

2. For the tube side the the cooling water is supplied from a Cooling Tower through a centrifugal pump that is dedicated to this Heat Exchanger alone. Pump shut-off pressure = 55 psig, so the design pressure is 1.25*55= 68.75 psig at the maximum coincident temperature of 32 Deg C. The piping is 12" Sch 40 piping with 150 # class flanges.

My question are there any other factors I should consider ( or I missed) to determine the Design Pressure and Design Temperature. The Heat exchanger is in a non-fire zone and there are no PSVs installed on the lines.

ASME BPVC says this "Each element of a pressure vessel shall be designed for at least the most severe condition of coincident pressure (including coincident static head in the operating position)and temperature expected in normal operation. For this condition, the maximum difference in pressure between the inside and outside of a vessel, or between any two chambers of a combination unit, shall be considered [see UG-98 and 3-2]. See also U-2(a)."

I want to make sure that I have considered the most severe condition of coincident pressure and temperature for estimating the design pressure and temperature for my HX.

Thanks and Regards,
Pavan Kumar

danschwind · Jul 26, 2022

Define your maximum operating temperature for the shell side, which should include any upstream process upset that may increase the temperature above your normal operating temperature of 57C.

Then, add say 10C to that and round it up to the nearest 5C increment.

Set this temperature as design temperature for both the shell and tube to simplify safeguarding.

If you plan to steam out this vessel, then other considerations might apply. I would consider a different design case.

For this low pressures I would add 1 bar to the highest operation pressure (which I'm assuming you calculated right) and define this as the design pressure. I would also use the same design pressure for both the shell and tube because I can't see much economy in optimizing this on your vessel.

Daniel
Rio de Janeiro - Brazil

georgeverghese · Jul 27, 2022

Why have you got a 1.25x factor for design pressure. 1.1x should be adequate ? Are you accounting for some future pumping pressure increase ?
Assuming your 1.25x factor is justified, then a design pressure of 60psig for both sides should be adequate, while design temp could be set at 70degC / 0degC.
Some national pressure vessel codes require a firecase PSV even if the unit is not in a fire zone (like ASME), so check for shellside. If tubeside volume is <500litres, a tubeside firecase PSV wont be required.
You can expect considerable microbial corrosion with open loop cooling water on the tubeside with this HX. Tubes will leak after some time, so check if migration of dirty CW into tepid water will be possible and what the consequences would be with this contamination.

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HX Design Pressure and Design Temperature Decision for a Pumped System.

Pavan Kumar

Chemical

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