9 replies to this topic

[MFAHOS]

Hi everybody

my quesion is about the ratio (Pressure drop in line upstream PSV) / (PSV set pressure) which shall not exceed 3% as per API RP 520,

shall it be calculated based on Absolute or Relative basis?

Exemple:

Pressure Drop = 0.3 bar
Set Pressure = 8 barg = 9.01 bara

the ratio is (0.3/8) or (0.3/9.01) ??


thanks

[NGL Licensor]

Hi everybody

my quesion is about the ratio (Pressure drop in line upstream PSV) / (PSV set pressure) which shall not exceed 3% as per API RP 520,

shall it be calculated based on Absolute or Relative basis?

Exemple:

Pressure Drop = 0.3 bar
Set Pressure = 8 barg = 9.01 bara

the ratio is (0.3/8) or (0.3/9.01) ??

Hi,

Pressure drop is absolute in nature (pressure diffrence). Hence the ratio should be in absolute terms (logically speaking)

A.King


thanks

[fallah]

Hi everybody

my quesion is about the ratio (Pressure drop in line upstream PSV) / (PSV set pressure) which shall not exceed 3% as per API RP 520,

shall it be calculated based on Absolute or Relative basis?

Exemple:

Pressure Drop = 0.3 bar
Set Pressure = 8 barg = 9.01 bara

the ratio is (0.3/8) or (0.3/9.01) ??


thanks

As far as i know 0.3/8 would be correct.

[JoeWong]

Pressure drop is absolute in nature (pressure diffrence). Hence the ratio should be in absolute terms (logically speaking)

What is the meaning of "pressure drop absolute in nature" ?

3% rule refer to set pressure in gauge. Change in barometric pressure will not affect the ratio for a specific PSV...

[NGL Licensor]

Pressure drop is absolute in nature (pressure diffrence). Hence the ratio should be in absolute terms (logically speaking)

What is the meaning of "pressure drop absolute in nature" ?

3% rule refer to set pressure in gauge. Change in barometric pressure will not affect the ratio for a specific PSV...

Pressure drop is in absolute units. If the 3 % rule refers to set pressure in gauge, as you stated, so it should be 0.3/8

[JoeWong]

Can you elaborate a bit more what do you mean by "Pressure drop is in absolute units" ?
I have heard of pressure in absolute unit... i have heard of pressure drop is pressure difference between two points and it can the difference between two absolute pressure or two gauge pressure...I am sorry. I still can not appreciate above sentence "Pressure drop is in absolute units".

[NGL Licensor]

Can you elaborate a bit more what do you mean by "Pressure drop is in absolute units" ?
I have heard of pressure in absolute unit... i have heard of pressure drop is pressure difference between two points and it can the difference between two absolute pressure or two gauge pressure...I am sorry. I still can not appreciate above sentence "Pressure drop is in absolute units".

I meant in both cases you shall end-up with pressure difference in absolute not in gauge since it is a difference between two values. Example, for a pump, if the suction pressure is 10 bar and discharge is 25 bara then the pressure difference is 10 bar. The same difference is true even if the above values are in gauge units i.e barag. Hope you appreciate now

[fallah]

I meant in both cases you shall end-up with pressure difference in absolute not in gauge since it is a difference between two values. Example, for a pump, if the suction pressure is 10 bar and discharge is 25 bara then the pressure difference is 10 bar. The same difference is true even if the above values are in gauge units i.e barag. Hope you appreciate now

Joe Wong is absolutely going on right track.
In calculating pressure difference two pressure values should be considered on the same basis,both in gauge or both in absolute.

IMO,the pressure difference is nominated neither absolute nor gauge.

I think you meant the pressure difference for the pump you mentioned would be 15 bar.Am i right?

[demank]

the answer of the thread topic already done by fallah, which is (0.3/8).
In my opinion, its already clear.

The state is come from API RP 520 part II.
RP = Recommended Practice.
So it is just recommended from API 520. You can follow it, otherwise not follow the 3% rule also not wrong (for special condition maybe).

[skearse]

The 3% originated in the ASME Code (Section VIII); however, it's only specified as '3%' in the non-madatory Appendix M (Paragraph M-6), and then is only applicable to compressible service (by the letter of the Code, but not necessarly the intent). It's based on an old study performed (at the U of Michigan, I believe) where it was determined that most PSVs would blowdown about 7-10%, so, to prevent chattering of the valve, 3% was sort of arbitrarily chosen. It is calculated on 3% of the gauge opening pressure.

As far as compliance with it goes, many companies have internal standards that allow higher values, particulatly on incompressible service (5% seems to be fairly common). However, API RP 520 Pt 2 does state that using a higher value should be supported with an 'engineering analysis.' As far as being 'only' an RP, well, that's up to you, but obviously, if anything were to ever happened, you're on the hook for explaining why you deviated from a recognized industry standard. 520 Pt 1 was recently reissued as a Standard rather than an RP, and I would imagine that the next revision of Pt. 2 would also include issuance as a Standard rather than an RP.