Diesel PSV

[DiegoMartinez]

Gents,

I have a 2 diesel PSVs DS of a pump to protect the pump. One of them is open to Atmosphere with the set pressure of 500 KPag and the other is piped to diesel tank with the set pressure of 300 KPag. Now I want to connect the open to atm. PSV to the other PSV (300 KPag) outlet line. The back pressure on the 300 KPag is 0 KPag. In order to connect the 500 KPag PSV to 300 KPag outlet, should I change the 300 KPag PSV set pressure? If so, how much? Let me know if you require more info.

Cheers

 

[SeanB]

You say that the PSV piped to the diesel tank has zero back pressure - where is the PSV return - at the top of the tank? Normally a PSV returning to a tank would piped to one on the inlet lines. In that case the back pressure on the PSV would vary according to the level in the tank.

 

[DiegoMartinez]

SeanB,

Thanks for your message. The PSV goes back to the top of the tank. There is no variable pressure at the top of the Tank. As you said, some times PSV's are piped back to the suction side of the pump to protect the pump.

Cheers

 

[SeanB]

Are these thermal PSV's? Generally no PSV other than a thermal is allowed to relieve to grade and this is pretty much not acceptable anymore.

 

[pleckner]

If these are on the same line (which it appears to be by your post) then the problem isn't with the 300 KPag PSV but the 500 kPag PSV. The 300 KPag PSV will pop first creating a superimposed backpressure on the 500 KPag PSV. If the 500 KPag PSV is of conventional style, which are differential devices, then the 500 KPag PSV will not begin to relieve until 500 + superimposed backpressure at the time it is needed. As long as this is OK to still protect the equipment and not violate code, then you are OK at this point of the discussion.

Next to consider is the built-up backpressure on both PSVs. If the 300 KPag PSV is relieving, it will have a built-up backpressure due to the flowing liquid. Then if the 500 KPag needs to relieve, the built-up backpressure against both valves increases due to the combined flow. Therefore, you will need to increase all line sizes to minimize the built-up backpressure. If these valves are both of the conventional style, you don't want the variable backpressure against either to be greater than 10% of the set pressure. Since they will be tied together, that restricts the variable backpressure to no more than 30 KPag.
If this is not acceptable, then you will need to increase the set pressure of the 30 KPag PSV or just change them out to balanced bellows style PSVs.

 

[DiegoMartinez]

SeanB,

That's a PSV not TSV.

Pleckner,

Can you expand your explaination about "conventional type PSVS"?

I want to use a non return valve before the 500 KPag PSV tees the 300 KPag outlet. Then I have no back pressure on 500 KPag PSV however my concern is about 300 KPag PSV to get back pressure from popping 500 KPag PSV. Hence I was thinking of slight increase of 300 KPag. But have ni idea to what capacity.

Any recommendation?

M

 

[pleckner]

The conventional style PSV is a differential device. If you specified it with a pressure of say 100 psig and no backpressure, and the backpressure at the time you want it to open is now 10 psig (for one reason or another), then the valve won't fully open until the upstream pressure is 110 psig; not a good thing. This is why it is smart to only use conventional style PSVs when the backpressure is basically a known constant.

If the backpressure is not constant and can vary more than 10% of the set pressure, then you can expect to loose capacity rapidly as this backpressure exceeds 10% of set pressure.

It's all in the mechanical construction of the valve.

I don't like check valves (I believe that is what you are describing) in PSV disharge lines as there is no guarantee of their proper function; and this could affect the operation of the PSV, not only a bad thing but violates code.

My suggestion is to discuss your situation with any of the PSV vendors out there. They will be able to recommend the proper construction of a PSV that should be able to work in your environment.

 

[DiegoMartinez]

Thanks for the info Pleckner. I just want to discuss the issue of using a check valve with you a bit further. The standard is to locate all PSVs above drain header so liquid, gas can drain freely into the header. If for whatever reason the PSV can't be located above the header and it is installed below the header centreline, then we need to use a check valve on the horizental line of the psv discharge to header to avoid the outlet of the psv getting filled up. The check valve to be as close as possible to the header and at the same height.

Will be happy to hear your point of view on this.

Rgds

 

[Ashereng]

pleckner,

If back pressure is a concern (either too much, variable, etc), you can also go to a bellows design. I believe bellowed PSV are still considered conventional.

If bellows doesn't work, then yes, try pilot operated PSVs.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Ashereng]

London2002,

A check valve after the PSV? I have never seen a setup like that.

I am not sure ASME Section VIII allows that, although someone will need to check on that.

My concern is if the check does not open, you just lost your pressure relieving capability.

As a general design guide, PSV straight to header, no obstructions.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[djack77494]

Ashereng,
I agree with Phil's terminology in that he is not including balanced bellows PSV in the set of "conventional" PSV's. This is a good thing because a key performance characteristic, the ability to cope with backpressure, is different. I agree with you and Phil in having a serious aversion to using check valves in a PSV discharge line.
Doug

 

[Ashereng]

djack,

Fair enough.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[pleckner]

ASME and API don't specifically restrict the use of valves in the PSV discharge line, check or otherwise. They most definitely do not allow ANYTHING that might interfere with the proper operation of the PSV and I think we all agree that a check cannot be trusted to allow full open flow when needed and thus fits the definition of a possible interference of the proper operation of the PSV. So, don't use it!

The header must be designed properly to eliminate the possibility of liquid build-up in the discharge piping. The liquid head will cause more problems with or without a check valve.

 

[DiegoMartinez]

Thanks Guys,

Ashereng& Pleckner,

Can you please explain what the balanced bellow PSV is?

Sean B,

In the valve data sheet, the design code is mentioned "Thermal" and the discharge case is mentioned "Thermal relief". But it is a PSV, Set pressure of 500 KPag and 0 back pressure.

 

[Ashereng]

A bellows PSV is a design whereby there is a flexible piece of "bellow" above the PSV's piston, thereby keeping the pressure from the outlet nozzle from acting on top of the piston (can't push down on the area).

If you google "bellow PSV" you may find some pictures.

As an aside, my impression is that you may be inexpereienced with PSVs. If this is true, you may want to get someone with more experience to help you with your application.

No offense meant, but a PSV is not to be trifled with.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[SeanB]

So it sounds like this is a thermal relief valve (still a PSV but the required relieving rate is based on thermal expansion of a blocked in fluid - in your case, diesel). It is probably a very small PSV; 3/4" x 1" or maybe 1" x 2"? A lot of older refineries when they were built allowed thermal PSV's to discharge to the atmosphere as the relief rates are very tiny. However, this philosophy has changed and this is no longer acceptable. A lot of refineries are now piping these existing PSV's to a closed system - sounds like that is what you are doing here as well.

 

[DiegoMartinez]

Asherang,

I am not inexperienced with the PSVs; you might have more experience. Industry is not classified in codes and standards. Sometimes field experience has a lot more to suggest. Hope it is not patronizing. If the PSV you are gonna use can't be at a higher elevation than the flare header, even if it is a process line, you may consider using a check valve on the horizental line to the main header (at the same level); nothing will happen. Trust me!

SeanB,

That PSV is sized 1"x1".

 

[Ashereng]

London2002,

I am putting my thoughts and views out there. You do with it as you will.

If I have offended you and your level of experience, my apologies. I did not mean to offend.

You asked for definition of what a bellows PSV is. I responded. This is not a question that I expected from an experienced I&C person on PSV. My apologies.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[pleckner]

London2002:

I have to second "Ashereng"'s sentiments.

And, take the free discussion and advice based on many years of experience in working with the codes and recommended practices for what it is worth to you.

 

[DiegoMartinez]

Asherang,

There is no need to apologise; it has just been a misunderstanding. Both you and I have worked in the industry for number of years; however area of expertises might be different. I respect you and your experience and would like to hear your point of views on different questions further more.

Rgds