API521: Do you consider pipework

[jamesbanda]

Dear sirs..

I am tried to validate the sizing on several relief valves. For API 521 does the standard include piping. Some of the calculations i've seen cover the pipeing but i'm strugging to find it in the standard.

Hence i want to validate the common concusus as to how it is normally done.

 

[MortenA]

First: 521 is not for sizing - but for determination of relieving loads. Refer to API 520 for sizing (but you propably know that - just a detail).

However for springloading - regarding downstream piping API 520 sec 3.3.3.2 states that buil-up back pressure should not exceed 10% of the set pressure at 10% overpressure.

I have been leafing through the standards but have failed to find excactly where it states that the upstream piping should have less that 3% pressure drop at max flow in order to avoid piping - but im its there somewhere (but now im not sure if its 520 or 521 now :-(

Best regards

Morten

 

[jamesbanda]

sorry my question is not clear..

for setting the relief load do you need to consider the pipework area?

 

[insult2injury]

For thermal relief, you need to consider any volume of fluid that would be blocked in and subsequently heated.

I2I

 

[MortenA]

if its for api fire case you should only consider the wetted part.

I think its for normal level - but im not quite sure maybe its HHL.

Best regards

Morten

 

[CMA010]

When calculating the wetted area of a vessel or column it's not uncommon to add 10 or 15% of the wetted area to include the area of the piping of the vessel/column. Someone wrote an article about it, unfortunately i can't remember the title.

 

[jamesbanda]

thanks.. so would you just use a factor rather than the actual pipe lengths..?

the eng contactor is calculating all the lengths...where as on previous projects we have not included the lengths. hence i am trying to validate what api actually state and what is normal pratice..

 

[CMA010]

The actual additional surface created by piping is better, in my opinion. But it is however not always feasible to calculate this, in particular for existing plants. If you have access to proper isometrics it would be easy.

You can check the following articles: "Protect Plants Against Overpressure", Wing Y Wong, Chemical Engineering Magazine, june 2001 or "Fire, Vessels and the Pressure-relief Valve", Wing Y Wong, Chemical Engineering Magazine, May 2000.

 

[MortenA]

Remember that vessel (and pipes) over a certain height (15 ft i think but check that yourself) does not have to be considered accoridng to the code.

Best regards

Morten

 

[JoeWong88]

for setting the relief load do you need to consider the pipework area?

Piping area shall be included as part of the calculation...

During Basic design, includes a percentage from the calculated vessel area i.e. 10-30% subjective... as you don't have any isometrics...

During detailed design, calculated from the isometrics.

For thermal relief, you need to consider any volume of fluid that would be blocked in and subsequently heated.

For fire case...you need to consider piping area...
For thermal relief, yes and no... there are some exceptions allow in API... check out in Thermal Relief of Non-Flashing Liquid in Pipe...

3% requirements...

It is in API RP 521 section 4.2...
If the line non-recoverable line loss has exceeded 3% rule, see if this post can helps How to Resolve Issue with PSV Inlet Line Loss Exceeded 3% of Set pressure...

...I think its for normal level - but im not quite sure maybe its HHL.

Subject to approach ... For a plain vessel, maximum possible normal level (LAH) is normally considered as it is uncommon to have High-High Liquid Level (LAHH) during Fire...

For a column, LAH plus tray capture volume...

Remember that vessel (and pipes) over a certain height (15 ft i think but check that yourself) does not have to be considered accoridng to the code.

It is 25ft (7.62m) for pool fire. Check out in Extend of Pool Fire......



JoeWong
Chemical & Process Technology