Using MAOP in lieu of DP for inservice API 570/574 assessment

[Ammrp]

Hi Guys,

I just started a new job and came across a general practice here in the company which seems wrong to me.

We are doing simple integrity assessment and inspection planning for piping. In the software provided by the client it uses MAOP and DP to calculate the Remaining Life(s) then decide the inspection interval as per the max value. I have seen difference of as much as 8 MPa between the two pressures. In general MAOP is 10 to 15% lower than DP. Which means we are selecting lower minimum required thickness, specifying higher inspection intervals and higher remaining life for each.

I understand we could do that if we had a derating of the system or something along that lines , but I dont think we can apply this rule as a general rule to decide the minimum required thickness.

I have gone through the codes and could not find any clause which allows us to do that (use MAOP instead of DP) even if the client is allowing us. As far as i know the clients wants us to comply as per API 570 requirements as the piping is designed as per B31.3.

P.S. Could someone also explain to me who decides the MAOP?

 

[Dhurjati Sen]

Dear Ammrp,

RBI softwares (like Shell-RBI, Meridian, etc.) are intended to reduce inspection frequency by taking advantage of the excess thickness (more than the design) of piping.

The design pressure is chosen by the designer at the 'FEED' stage of the project. The maximum allowable working pressure is a code requirement.

Regards.

DHURJATI SEN
Kolkata, India

 

[SJones]

Since this is piping, is it a classic case of the client using ASME pressure ratings and identifying the class rated pressure as the 'design pressure,' with the "MAOP" actually being a design pressure by the true definition of B31.3?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Ammrp]

Hi Dhurjati,

I am talking about MAOP not MAWP. The point about MAWP is completly understanable because you have excess thickness so you could potentially use that to your advantage.

But what I have seen is that an MAOP is used to calculate the required thickness rather than calculating the MAWP from actual thickness.

 

[Ammrp]

SJones,

So you are saying the design pressure is from the pipe specs and not necessarily that specific line design pressure? hmm... interesting...

does MAOP include all the possible conditions which the pipe is expected to be exposed to (as considered by the design pressure)?
Who and how is the MAOP set ? by your definition it is just a different way to represent a specific line design pressure?

 

[TGS4]

What pressure is the relief device set at? That should govern your calculations.

 

[Ammrp]

TGS4,

I agree. So is MAOP defined as the pressure where the relief devices are set?
I cannot check each pipe as there are thousands of them but I am sure one way or the other they would have a relief attached to the pipe.

The software client uses, does not take any of theses consideration. there is a simple comparison between MAOP thickness and DP thickness. The minimum of which is selected.
and consequently a larger value of remaining life is selected.

As a principle, I do not understand that, unless it is the case as mentioned by SJones.

 

[SJones]

@Ammrp - that is exactly what I am saying: Class 150 - DP 20 bar; Class 300 - DP 50 bar and so on, and so forth. DP is set by class rating irrespective of the actual maximum pressure that the system will ever see. A lazy way to set DP and drives corrosion engineers crazy when it comes to calculating partial pressures of acid gases believe me. The process engineer will then quote the actual absolute maximum pressure that the system can achieve as the MAOP that to all other parties is the design pressure.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Dhurjati Sen]

Dear Ammrp,

Entirely agree with Steve Jones regarding MAOP aka Design Pressure.

Regards.

DHURJATI SEN
Kolkata, India

 

[Ammrp]

Thanks SJones!

Yes it is frustrating!

Thanks everyone for clearing up the confusion!

 

[KevinNZ]

For piping the design pressure is the maximum allowable pressure and is often much lower than the flange class or what the nominal pipe thickness can take.

Once the design pressure is set the piping designer then uses the excess pipe wall for other stress calculations. sustained, occasional thermal, local stress on supports etc. After design you can not use the excess wall thickness to increase the design/maximum pressure without redoing all the stress calculations.