10 replies to this topic

[Arsal]

We have multiple PSV installed in a Crude distillation column and all having the same set pressure and overpressure criteria is 10% that is contradicting with API-520 criteria for multiple PSVs.However, the datasheet refer basis of seletion to  RP API- 520,The data sheet has been devolped in 1974. 

 

So, my question is that, criteria mentioned in TABLE 1,2,3, & 4 of API-520 (7th Edition) has established after 1974? I dont have a 1st edition.

 

Furtermore, releiving pressure and set pressure are also same with 10% overpressure which is again contradicting with API-520.

Relieving Pressure = set preesure + over pressure.

Edited by Arsal, 28 June 2016 - 03:40 AM.

[fallah]

 

So, my question is that, criteria mentioned in TABLE 1,2,3, & 4 of API-520 (7th Edition) has established after 1974? I dont have a 1st edition.

 

 

Arsal,

 

It might to be as you thinking...

 

Please let's know if in history of the plant you mentioned, had ever been there any accident/PSV damage/maintenance issue....due to such equalization of set points in multiple PSV's?
 

[Arsal]

We are having turaround plan at every 3 years and no such maintenance and other issue have found.and each turnaround all PSVs has been calibrated.
Furthermore no accident has been reported since the plant commissioned.
In whole plant; all multiple PSVs; the same criteria has been followed as I mentioned above.
That is quite confusing for me.

[fallah]

Arsal,

 

Would you please upload part of a PID in which the multiple PSV's has been shown?

[shvet]

We have multiple PSV installed in a Crude distillation column and all having the same set pressure and overpressure criteria is 10%

 

API 520 does not (did not) establish set pressure and allowable accumulation requirements. This criteria refers to ASME BPVC VIII code. You should check column's passport - what is mechanical design code mentioned in passport? Different intenational codes (ASME, EN, ISO, DIN etc.) establish different critera to overpressure and set pressure.

 

 

 Furtermore, releiving pressure and set pressure are also same with 10% overpressure which is again contradicting with API-520.

Relieving Pressure = set preesure + over pressure.

 

Don't mix together different terms. Overpressure refers to protected equipment and allowable accumulation refers to relieving device. Accumulation shall be no more overpressure. In your case accumulation = 10% and overpressure can be = 6 or 10 or 15 or 16 or 21% according to design code requirements. There is no mistake. You look to relieving device passport and see accumulation. For overpressure you should find out what meachanical design code is (was) and what overpressure is required.

Edited by shvet, 29 June 2016 - 04:08 AM.

[Arsal]

Please find attach P & ID and detail of PSV.

Attached Files

•  P & ID.pdf   267.14KB   30 downloads

Edited by Arsal, 29 June 2016 - 03:47 AM.

[Arsal]

shvet 

I m not clear.

 

Is it possible that the releiving pressure and set pressure are same? however Overpressure is mentioned 10%, and electric power failure case is mentioned in attached file that means accumulation is 10 %.

[shvet]

In your case it looks like a mistake. Set pressure can't be equal relieving pressure. As I think there should be written:

 

"press. psig. norm | relieving | 140 | 171.6"

 

If in your case (there is no clear sign) mech design pressure = set pressure than

relieving pressure < or = accumulation < or = overpressure

 

In no case relieving pressure can be > or = set pressure.

Actually there shall be some margin between set pressure and relieving pressure to allow disc to travel to fully open position.

Edited by shvet, 29 June 2016 - 04:07 AM.

[Arsal]

What about the criteria of multiple PSVs??

As it should follow overpressure criteria of 116 Instead of 110.further we have all PSV having same set point which is again contradictory.

Furthermore;one more question;if existing PSV has been undersized and need to replace in new conditions.so what would you suggest;
1)to replace PSV with larger size.
2)to add in parallel with existing one.

Edited by Arsal, 29 June 2016 - 04:59 AM.

[AlertO]

Arsal

I don't know why the existing PSVe are desgined like that but I don't advise you to replace the existing PSVs as per following reasons;

1. The existing PSVs have adequate area and not undersized. To design with your original basis (same % overpressure at 10%) usually gives the larger required area than the basis of 16%.
2. To change the % overpressure, it means you change the relieving pressure too. It requires to recalculate the relief load and if the governing scenario is some kind of complicated one i.e. power failure in distillaiton column, you may need help from the vendor who has designed this equipment.
3. You also need to check your inlet / outlet pipe again.
4. Of course, you need to pay more money.

I know your attention is to follow the code but i think to do the periodic maintanance as you did is more important.

Anyway, I just want to share my idea and to change the PSVs accordingly to the code is the thing which noone can blame you.

[shvet]

What about the criteria of multiple PSVs??
 

 

All criteria about multiple PSVs refer to mechanical design code, not to API 520. As per your document in attachment PSV calculation was done by API RP 520 but what is the vessel design code?

Anyway as my memory talks ASME VIII criteria for multiple PSV are recommended. You should check vessel design code requirements.

 

 

 
Furthermore;one more question;if existing PSV has been undersized and need to replace in new conditions.so what would you suggest;
1)to replace PSV with larger size.
2)to add in parallel with existing one.

 

Second one.

You should be consulted by PSV manufacturer. May be you can just adjust lift-stop device or remove bore reduction ring.