PSV in after-cooler tube rupture case

[Bourbon103]

I have a process where an air after-cooler is fed 180# air on the tube side, cooling tower on the shell side. The MAWPs are:

Shell: design = 150# , test = 225#
Tube: design = 300# , test = 450#

As per thread124-77346, "This is a remote but possible failure that API allows you to accept up to the test pressure of the low side for a limited overpressure event." Does this hold in this case where the 180# air is less than the test pressure of the shell?

 

[Bourbon103]

I failed to mention it before, but our company is in the process of ordering API 521 and I wanted to do some preliminary analysis.

 

[EGT01]

Bourbon103,

From API RP-521, 4th edition

"The use of maximum possible system pressure instead of design pressure may be considered as the design pressure of the high-pressure side on a case-by-case basis where there is a substantial difference in the design and operating pressures for the high-pressure side of the exchanger."

You just need to make sure there is something that limits the maximum pressure on the high side to less than the test pressure on the low side. For example, a relief valve on the high side that would limit the pressure to 225 psig or less.

 

[Bourbon103]

EGT01,

Thanks for the confirmation. Unfortunetly it's one of those things where budgets are set until the end of the year before we can order the newest API 521...go figure. But thanks again for the heads up.

 

[CHD01]

You must limit the maximum operating pressure on the tube side to the MAWP which I will assume is equal to your design pressure (it isn't always that way). In your case this is 150 psig. You cannot use the 1.5X test pressure since this is a pressure vessel. For pipe you may exceed MAWP for a limited period per the applicable pipe standard. The maximum system pressure during relief is usually 10% above the relief valve set pressure.

Another situation for an exchanger is a tube leak, in this case we sometimes eliminate the shellside relief valve based on an analysis of the MAWP's and test pressures. But this does NOT allow setting a relief valve on the tube side to greater than the MAWP.



The more you learn, the less you are certain of.

 

[abeltio]

why not verify the dimensions of the shell side to 180#?

sometimes, due to manufacturing practices the vessel could be built to satisfy the 20% increase in design pressure (from 150 to 180#)

the tubesheet and flanges, exclusive of corrossion allowance, are the usual suspects... the shell itself, if made from pipe is usually more than enough for 180#.

it is not as if the 150# side would be pressurized to 300#... where the tubesheet would certainly not be enough.




saludos.
a.

 

[CHD01]

Im sorry, I meant to say that you cannot exced 300 psig on the tubeside and 150 psig on the shellside as a relief set pressure; otherwise my earlier memo is correct. The important part is that you cannot use the static test pressures to set a relief device. Also, design pressure is what the specification engineer requires the manufacturer to meet. MAWP is what the manufacturer actually builds the vessel for (equal to or greater than the design pressure). MAWP is what is stamped on the nameplate of the vessel, it is NOT the maximum normal operating pressure although they can be equal. Normally a relief valve is set equal to the MAWP of the system - but the max normal operating pressure should never be in excess of 90% of the set pressure of the relief valve - otherwise excessive leaking can occur. Also, the max normal operating pressure should account for pressure swings in operating pressure. Finally, the relief device is sized for a 10% overpressure to relieve the required flow normally (21% max for fire).

The more you learn, the less you are certain of.

 

[IR56]

Bourbon-103

API says that if the design pressure of the shell side times 1.3 is more than the design pressure of the tube side you do not need protection on the shell side.

please note that the miltiplier is not 1.5. The shell side conforms to the rules of a pressure vessel not a pipe.

In your case the DP of the shell side times 1.3 (195#) is less than MAWP of the tube side (300#) therefore you need protection on the shell side.