9 replies to this topic

[pasquale]

Hi guys

I have a question: in api 520 the definition of relieving pressure is: "inlet pressure of the pressure relief device at relieving conditions" .

The relieving pressure is the sum of set pressure plus overpressure (e.g. 10% of set pressure).

My question is: if at relieving conditions the inlet pressure of the PSV is greater than the set pressure (for mantaining the valve open) also the pressure of the equipment will be greater then the set pressure; but in this way the equipment will be at a pressure different from the design pressure. Is ti possible?

Thanks

[Nirav]

There are two different pressure terminologies defined for any equipment. I would like to explain in brief.

(1) Design pressure :
Equipment has to be designed to sustain the stresses imposed on it due to the maximum internal or external pressure it is subjected to. This pressure is called the design pressure.
The design pressure generally is determined from the maximum operating pressure of the vessel, which is increased by a margin to take into account probable surges of pressure during operation.

(2) MAWP (Maximum Allowable Working Pressure) : It is the maximum pressure at which the equipment is allowed to work at the specified design temperature. It is defined by "mechanical design" based on the Material of Construction (MOC) and the selected thickness of the vessel.

MAWP is decided at relatively later stage of project by equipment manufacturer or mechanical design engineer based on ASME Sec.VIII Div.1.

The difference between design pressure and maximum allowable working pressure is that the design pressure specified by "process engineer" is always less than or equal to the maximum allowable working pressure (MAWP) of the equipment which is specifed by "mechanical engineer".

PSV set pressure, in most of the cases, decided based on "design pressure". It can also be equal to MAWP but NEVER higher than that.

Now, after construction of vessel or pipe lines, they are always subjected to "hydrotest" at about 150% of it's design pressure or MAWP.

The "relieving pressure" of PSV will never be above this "hydrotest" pressure. Because overpressure is in the range of 110% to 121% of set pressure. Refer to Fig.4 of API RP521. I have attached it here for your reference. This gives fair idea about the various perssure levels with respect to PSV.

Therefore, in any circumstances, pressure rise during PSV relief will NEVER be more than the "test" pressure (which is already imposed on vessel after manufacturing).

I hope it clarifies your doubt.

Warm regards,

[gvdlans]

The short answer is: yes, during a relief pressure will be above the design pressure. Note that this should be a rare event and only for short duration. It is also limited to 10% (21% for fire case).
As explained by Nirav the equipment can handle this. For process piping, code ASME B31.3 even specifies that it is allowed to have occasional variations above the design pressure.

[pleckner]

I've written on this subject and invite you to read my ariticle published on this site for some more insight.

Nirav gave an extremely good explanation.

A couple of points:

1. The vessel pressure will exceed design pressure (or MAWP) and is allowed to by Code as long as it stays within the limitations given by Code. These limitations are defined by the allowable over pressures already stated....except for one that was not mentioned:

a. The allowable over pressure can be has high as 16% over MAWP under the conditions of using more than one relief device to protect against over pressure.

2. ASME now requires vessels to be tested at only a minimum of 130% of MAWP; reduced from 150%. However, piping must still be tested a a minimum of 150% of MAWP. If you test the piping along with the vessel, which is permissable, then both are tested at 150% of MAWP.

This raises that old age question for shell and tube heat exchangers as to what happens to the 2/3 rule. Well, it is not 2/3 anymore, it is now 10/13!

[gvdlans]

I assume that Philip Leckner refers to the following article: http://www.cheresour...asiseeit1.shtml

As for the 2/3 rule: I would recommend not to replace this 'rule' with a 10/13 rule but by:

"Pressure relief for tube rupture is not required where the low pressure exchanger side (including upstream and downstream systems) is hydrotested at or above the high pressure exchanger side design pressure."

[djack77494]

Guido,
Would you be kind enough to state the source of your quote. I've used it often and it is more or less obvious, but I don't know the source.
Thanks,
Doug

[Nirav]

Hi doug,

The 2/3 rule is based on ASME's requirmemnt of hydrotest pressure of 150% of design pressure of Low pressure side of heat exchanger.

AND

The rule of 10/13 is based on ASME's requirement of hydrotest pressure of 130% of design pressure Low presssure side of heat exchanger.

I would explain it.

[1] The 2/3 rule is equivalent to what Mr.Guido has told

"Pressure relief for tube rupture is not required where the low pressure exchanger side (including upstream and downstream systems) is hydrotested at or above the high pressure exchanger side design pressure."

This is how it is . . . Consider following.

# Low pressure side design pressure = X
# If "hydrotest pressure" = 150%, it would be = 1.5X

Now, if you want design pressure of high pressure side not more than this pressure,
# High pressure side design pressure = Y = 1.5X

Which means that ...... 2/3 Y = X
Which is 2/3 Rule !!

[2] The 10/13 rule is equivalent to what Mr.P.Leckner told as following.

2. ASME now requires vessels to be tested at only a minimum of 130% of MAWP; reduced from 150%.

Again,
# Low pressure side design pressure = X
# This time, "hydrotest pressure" = 130%, it would be = 1.3X

Now, if you want design pressure of high pressure side not more than this pressure,
# High pressure side design pressure = Y = 1.3X

Which means that ...... 10/13 Y = X
Which is 10/13 Rule !!

You can check above explanation by putting values of X & Y to any of your past or current exchanger design.

I hope now, I've explained what you asked.

Warm Regards,

[gvdlans]

Doug,

What I wrote between quotation marks " " in my previous post was not a direct quote but a summary of what is written in 4th edition (1997) of API RP 521, section 3.18.2. It is also based on answers given by the API RP 521 committee to questions asked about the 3rd edition of the RP.

My proposal is to speak about a "test pressure/design pressure" rule instead of a 2/3 or 10/13 rule...

[pleckner]

Good discussion.

As of right now, there really is no 10/13 rule because the recommended practices only talk about the 2/3 rule. However, I'm hoping to see API modify this in the next revision of RP521.

I like Guido's suggestion about using a "test pressure/design pressure rule" (shouldn't this really be design pressure/test pressure?). Anyways, if the equipment is tested at 130%, then we would use the 10/13 ratio to determine the credibility of tube rupture scenario. If the equipment is tested at 150%, then we would use the 2/3 ratio. Guido, you have me sold!

The bottom line is, know what your equipment is designed for and know what the testing constraints are and use some sound engineering judgement.

[djack77494]

Thanks to all for a good discussion and explanation of the topic.
Doug