Dear All

In a case which I am faced there is a Depropaniser reboiler into which Hydrocarbon (in shell) is heated by LP steam. The design and operating conditions are as follow:

• TEMA Type: BKU
• Design Temp. Tube side: 270˚ C Shell side: 160 ˚C
• Desigs pressure of steam system: 8 barg
• Operating pressure of steam system: 4 barg
• Design pressure of shell side: 28.5 barg

The engineering contractor has considered one PSV on the tube side (SP = 28.5 barg) whose Tail Pipe is conducted to a safe location (not to Flare). He also ignoring design pressure of steam system, considered the design pressure of the tube side equal to 28.5 barg (for external pressure) and FV (Full Vacuum for internal pressure). My questions are :

1-In the cases same as above (steam in low pressure side) we must consider ΔP of the tube side equal to shell side?
2-Is the reason that: ΔP = 28.5 (ext. press.)-FV(int. press.)=28.5 barg for the tube side in worst case?
3-Must the value of ΔP (28.5 barg) of tube side be extended to entire relevant LP steam system (piping and header)?
4-Why is the tail pipe of the PSV conducted to a safe location, since we know that in the case of PSV action on 28.5 barg, HC along with steam will certainly be released ?
5-Can we conclude in the cases using steam in tube and given as low pressure side , the 2/3 or 10/13 rules are ignored and ΔP of two sides are considered rqual?

REGARDS

Fallah,
I guess the answer are pretty obvious and i would be interested to respond. However, i felt that you alreay have the answers.

You raised above questions as you believe there may be problem with the design. Have you discussed above issues with contractor and what was the responses ? Don't you mind to share the background here to avoid preliminary discussion without touching on the real issue ?

You may ready some recent postings related to tube rupture, 2/3 or 10/13 rule and pressure relief device type...

Hi,
As I found from your statements, the steam distribution network design pressure is 8 barg but the tube side design pressure up to nearest first isolation valves(this is my guess) are considered to be equal to shell side design pressure. This is one solution to protect low pressure side in S&T heat exchangers but it needs economical justification.
Anyway I guess that you should have control valve in condensate line? If so it is clear that the psv is considered to protect the tube side from blocked outlet scenario and consequently the psv tail pipe can be routed to safe location. But in this case you may need to provide silencer.
I hope this help.

With a shellside design pressure of 28.5 barg, and assuming the "10/13 rule" can be applied, you can design your tubeside for 10/13*28.5 or about 22 barg. Then you can dispense altogether with the need for a PSV for the tube rupture case. If that is your goal, I can see no justification in increasing the TS design pressure even higher to 28.5 barg. However, as jp points out, even that step needs economic justification. Since you did not point out the TS design pressure, I assume you are in the process of trying to determine it. Another option is to set it for 8 barg and then to provide adequate pressure relief so that you will not exceed 1.1*8 even in the event of a tube rupture. I suspect that that would be the low cost solution.
Doug