Oversizing PSV for HEX tube rupture case

[pastyl]

Dear all

I sized a pressure relief valve for an Heat Exchanger and for tube rupture case.
The problem is that I considered 3 tubes to be ruptured and as result I oversized the relief. I have 2 questions:

1) Is forbidden from API to consider 3 tubes ruptured and you have to consider only one ?

2) Were is the problem if I oversized the relief and it can handles more mass flow than the minimum required ?

Because my client complained about the above problem I really appreciate a quick response

Best Regards

 

[CMA010]

1) What's the reason for considering 3 ruptured tubes? Usually a single rupture is considered.

2) Oversized valves can lead to a number of problems, in my eyes the most important one being chattering. It will also result in higher capital costs due to larger relief valves and flare system.

 

[pastyl]

Dear CMA010

1) Agreed that usually one tube rupture is taken into consideration. I have read that in such cases more than one is considered and API521 does not forbid that.

2) From my point of view I had to sizing the reliefs for the HEX I constructed from a NG project and I consider good practice to take 3 tube ruptures. I am not responsible for the dns flare system and if it is designed for one rupture case the bigger valve does not allow greater mass flow to pass from a smaller one.
As far as the chatter problem I think that relief valve mfr can help me by make lower the valve lift. I think that it is a solution that will allow the valve to work in low flow.

Please advise

 

[gr2vessels]

pastyl,
Let me guess;- your exchanger design pressure ratio is less than 10/13, so you need a relief vale.
If liquid or gas is flowing in both sides, then you might get away with a pilot operated relief valve, sized for one or many tubes rupture, the limit is your pocket size.
If the service is mixed, I'm afraid you might be in trouble, because it will be very difficult to size the valve for a range of gas/liquid mixture. However, is not impossible.
However, what you consider a good practice in fact is not a recommended practice and it is the least economical. By the way, would you be kind to share with us who recommended you to design a relief system for several tubes rupture simultaneously (not progressively!).
Best regards,
gr2vessels

 

[pastyl]

Dear gr2vessels

The wrong sizing of the relief, as I understand, is my mistake but it has been arisen due to extensive under pressure and unacceptable environment that I try to work in a country were engineering is an unknown word (without this to be an excuse).
However I take one moment a wrong decision and now the reliefs are going to be delivered, so I cannot do anything this moment.
I try to find a solution in order not to have problem with the oversize reliefs.
I read that you can make the lift shorter in order to avoid chattering. I will advise my relief mfr and I come back.
Finally if you think that any very bad situation may occur with the oversized reliefs please let me know asap


Regards

 

[gr2vessels]

pastyl,
You have to understand that in this forum, you get what you are asking for;- help. You need to tell if it is gas both sides or liquid both sides;- I'm not interested to name the place or the process or any other confidential detail.
The pilot driven relief valve will open in full, regardless of the flow for the set pressure, provided the valve is sized for the maximum flow. It is impervious to the flow, to a certain extent.
The only thing bad is the unnecessary extra expenditure for this over sized valve, otherwise it will handle your 3 tube rupture without problem.
Please advise what valve did you order, so we can be of some assistance to you.
C'mon, is not that bad, we'll fixit.
Cheers,
gr2vessels

 

[pastyl]

Dear gr2vessel
The valve is a spring loaded safety valve. The HEX is a BEU steam condenser and in the tubes flows natural gas.
I fix the problem by ordered a new valves sized for one tube rupture, because I was afraid for chattering problem if I used the big valves. So we are ok
One issue is that I used below equation for finding critical pressure ratio rc:

(rc^2-1)/(K+ln(rc^2))-1=0, K=1+fL/D, rc=P1/Pc. For Pc>P2 critical flow occurs

and then: W=0.031*d^2*Pc*sqrt(MW*k/T*Z) for mass flow

"Process Engineering and Design Using Visual Basic" by Arun Data pp301.

from above equation you take a pressure ratio larger than appr. 2 you take if you use the classic equation for isentropic expansion and less mass flow than from the classic orifice equation. I want to know if you are familiar with the above equations or you based at classic orifice mass flow equation with isentropic expansion for finding rupture mass flow

Best Regards

 

[CMA010]

1) My interpretation is that API STD 521 talks about single ruptured tube, unless you made an engineering analysis concluding that a multiple tube rupture is a credible scenario.

2) The engineers who will size the flare system will do so based on the information you'll give them, such as relief load and rated capacity of the valve. I can't tell you if reducing the lift of the safety valve will eliminate possible chattering, you are right to ask that to the valve manufacturer.

As for your relief load calculation, the assumption is that the tube ruptures at the tube sheet, resulting in two open ends. One being the stub in the tube sheet and the other being the remaining tube (see API STD 521, §5.19.3). For the flow through the tube sheet you can use the classic orifice equation. For the flow through the tube you can use the formula from Process Engineering and Design Using Visual Basic. However, the company i work in and the clients we work for usually take the classic orifice equation times two to calculate the total relief load.

 

[pastyl]

As far as the kind of rupture acc. spec. is a tube rupture and not tubesheet rupture. So I think that I shall use the no linear equation for rc considering as L the half of the tube length. Then I must multiply by 2 the flow since gas flows at both sides of rupture (up and down) acc. API 520.
In fact the equation I describe gives almost equal results with the classic equation for isentropic expansion if you put L=0. It make sense because for L=0 you will not have pressure loss before the rupture and then you can say that the expansion is isentropic. This can be done if you have a rupture at the tubesheet ( as gas come from the channel) . But then you cannot multiply the result by 2 because to the shell side gas will flow only to one direction (only up).
CMA010 you have very right about 2)

Regards