Maximum allowable pressure for copper-nickel during surge

[akupikree]

Hi all,

Currently for my transient surge analysis, my basis for the maximum allowable pressure during surge is 150% of the design pressure of the pipe. (im using Cu-Ni where design pressure is 18 bar). so my maximum limit for the surge pressure is 1.5 x 18 = 27 bar. This 150% value is actually the hydrotest pressure(ASME B31-3 section 345.4)

however, someone is arguing with me that we cannot use the hydrotest pressure as the basis for the transient surge analysis.

Can someone help me regarding this matter. appreciate if you guys can provide any info or reference about this.

thanks in advance!

Regards,

Fikree

 

[BigInch]

When you load steel rapidly, failures can occur at lower values than what you would normally get if you loaded it slowly, so of course you cannot use test pressure as a basis for transient loads. Test pressures can be far too close to (slow load) yield strengths.

Codewise, hydrotest pressures are not the basis for any allowable stresses in transient pressure loaded pipes. You will find the maximum stress allowed under transient conditions in the code, whether it be B31.3, 4, or 8. In general it will be more like 10% over the line's allowable normal operating design pressure, not test pressure, but ONLY IF certain conditions considering the length of time (number of hours) that the transient pressures occur during a year's of operation. Read the maximum design pressure clauses for the exact transient percentages over MAOP that are allowed for each expected hours of overpressure.



We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[akupikree]

thanks BigInch for the reply,

Yes you are right. Hydrotest pressure is not aplicable for the transient surge analysis since the maximum surge for the case that im working on right now is only last about 0.2 second! (where it exceed design pressure).

So far from what ive got from reading, the only code that mentioned about surge pressure is ASME 31.4. where it says the pressure cannot exceed 10% of internal design pressure for the pipe due to surge.

in ASME 31.3, the pressure can exceed 33% and 20% of its design pressure based on time length per year. however it does not mentioned any surge word.

which one shall i follow?

Thanks! really apprecite your opinion.

 

[BigInch]

OK, going with B31.3- I'm looking at the 2008 edition and it does indeed mention overpressures in paragraph 302.4(d) "Occasional variations above design conditions shall not exceed 1,000 during the life of the piping system."

Paragraph 302.4(f) gives the conditons in the following subparagraphs when such overpressures are permitted and the time limits that apply to the corresponding overpressure percentages exactly as I have outlined above.

B31.3 does not care what causes the overpressure, be it thermal heating and resulting thermal expansion, or just plain malfunction, or operator errors, or if it is the result of valve closeure, or pump start, stops. Anything causing temporary pressures over the design pressure of any kind is included.

Be careful when applying these clauses, because they have many prerequisites and conditions, or you can't do it. Read 302.2.1 and get the owner's permission in writing before you proceed with the design under any overpressure condition and limit its use only to the ductile materials as specified in 302.2.4(a) .

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[akupikree]

So youre saying that i need to follow b31.3?

Just to make you clear with my situation. Im doing the transient surge analysis for firewater system on offshore platform. The main objective is to check the integrity of copper nickel pipe when surge happened due to pump start, stop, deluge valve sudden closure and etc. Its all mainly cause by the firewater system itself and not from any external cause such as thermal expansion.

What do you think?

 

[MJCronin]

Banu...

Are you performing an analysis on a piping system where you do not know the Code of Record ?

In both B31.1 and B31.3 the hydrotest pressure is based upon the "Maximum Allowable Operating Pressure" ......which is set by the piping system designer.

MAOP means MAOP

 

[BigInch]

YOU mentioned B31.3 as the hydrotest pressure, so I assumed you are designing to B31.3, otherwise why would you throw us that red herring. If you don't know the design code ... well .... look in the project technical requirements. If it's a bid, assume B31.3 and tell them you used that as the basis, until you get an answer to your technical query.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[akupikree]

Thanks mjcronin,

Ive got your point saying that MAOP means MAOP. however the word "operating" itself means operating condition which i think is continuous operation pressure that is permissible. And that is not the same as surge pressure where it only happened due to pump start/ failed or valve closure that change the fluid velocity in the system. Correct me if im wrong.

BigInch,

Yes i dont know which code to follow. Thats why my basis for the transient analysis is using the hydrotest pressure which i think is wrong. After read the codes, im think the b31.4 fits the best with my case. Anyway, ill try to discuss with the client regarding which code to follow as per your suggestion.

Thanks!

 

[BigInch]

"Max allowable operating pressure" is the same as the "design pressure" for the pipe, the pressure you use when determing the wall thickness in the code's formula for pressure design of straight piping components, which would also be the same as you are calling maximum continuous operating pressure.

I have used B31.4 as the basis for firewater system design, and I think it would be appropriate for FW use on an offshore platform, but you really should get that square with the client at your first opportunity.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer