8 replies to this topic

[rohollah]

Dear experts

 

There is a PSV with following condition:

Type:balanced bellows

Set pressure:106barg

Back pressure:1.8barg

The bellows is damaged and we are in lack of spare part.my question is that, can we install this PSV without bellows?

If so, should we change workshop test pressure in order to eliminate back pressure effect?

1.workshop test pressure=106-1.8=104.2barg

On the other hand since the bonnet has a vent hole then back pressure has no effect on set pressure and :

2.workshop test pressure would be=106barg

Which one is correct 1or 2?

 

Regards

Rohollah

[fallah]

Rohollah,

 

You are not allowed to use the PSV with such damaged bellows because:

 

1) The PSV would change to "unbalanced" situation...

 

2) The back pressure would be increased due to whole top surface of the disk is subject to back pressure...then the PSV will open at higher set pressure...

 

3) If the PSV outlet is connected to the flare a continuous flow would be established from PSV outlet line to the atmosphere through damaged bellows and bonnet vent...

 

One question: Is the back pressure is total (superimposed plus build up) or just superimposed one? If it is total back pressure why didn't you use the conventional type? (1.8<<10.6=106*10%)

Edited by fallah, 15 April 2013 - 03:28 AM.

[rohollah]

Dear Fallah

 

Thanks for your soon and perfect replies.

with regards to second item, since the back pressure will be vented from the bonnet vent hole.don't you think it has no effect on set pressure?

I totally agree with you.since back pressure is less than 10%set pressure.then conventional type would be ok.and I don't know why balanced type is selected! 

mentioned back pressure is superimposed one.

 

Regards

Rohollah

[fallah]

Rohollah,

 

If 1.8 barg is superimposed back pressure how much is the build up one? It might the total back pressure superimposed+build up) had been higher than 10% of the set pressure, then they have had to select the balanced bellows type...

 

Bellows vent is to be considered to prevent creating back pressure on part of top surface of the disk surrounded by the bellows due to likely minor leaks through the bellows. Then, it is a small hole for venting minor leakage to the atmosphere.

Obviously, it might not be able to vent the higher flow corresponding to back pressure due to damaged bellows rapidly and there might be pressure build up upstream of the bellows vent on top of the disk...Of course the case should be evaluated quantitatively to find if such pressure build up is possible...

Edited by fallah, 15 April 2013 - 04:54 AM.

[rohollah]

Dear Fallah

 

What if we set the PSV at lower pressure(104.2barg)

and plug the bonnet vent hole?

 

Regards

Rohollah

[fallah]

Dear Rohollah,

 

You didn't answer to my question on back pressure...Anyway if there would be total back pressure of 1.8 barg:

 

Apparently, it might be set at 104.2 barg in damaged bellows situation with closed bonnet vent but in order to evaluate the possibility of doing so in real world the matter should be carefully investigated. Obviously, replacement of damaged PSV with a new balanced bellows PSV or an equivalent conventional type (provided that total back pressure would be lower than 10% of the set pressure) would be preferable option...

[rohollah]

Dear Fallah

 

Unfortunately build-up back pressure is not given in data sheet.

anyway regardless if it is superimposed or total back pressure as you mentioned (1.8<<10.6=106*10%)

so why balanced type psv is selected?

 

Regards

Rohollah

[fallah]

Dear Rohollah,

 

As i mentioned it might the total back pressure (superimposed+build up) had been higher than 10% of the set pressure, then they have had to select the balanced bellows type...

 

Based on the limited information you provide, appears there is no another reason for selecting balanced bellows PSV in mentioned application... 

[Raj Mehta]

As mentioned in API 520, 

 

Section 2.2.1.3.5 - "Balanced PRV should be considered where the built up back pressure is too high for conventional pressure relief."

 

Section 3.3.2.2 - " Balanced PRV utilizes bellows / piston to minimize or eliminate the effect of superimposed back pressure on set pressure" 

 

I think the value of superimposed pressure in your data sheets & the later statement would be one of the reason, but now from both the above statements I have one doubt,i.e. when are balanced type PRV are used ??