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Transient pressure

Transient pressure

Transient pressure

(OP)
Hi,
I have a pressure vessel designed for 16 Bar, but it will see a transient pressure of 18 Bar for a short period of time due to a pressure surge on pump shutdown.

Do I have to design the whole vessel for the 18 Bar pressure or is there some guidance on allowable transient pressures ?

Any help is appreciated.
Thanks
 

RE: Transient pressure

Are you suggesting that its ok for the safety valve to blow only occasionally?

If this is an ASME VIII vessel, thou shalt design for all forseable conditions, not just normal operating. Since you are forseeing 18 bar, the DP on this vessel should be 18 bar plus a reasonable margin.

Piping on the other hand... if its B31.3 there is an allowance for short term loadings.

jt

RE: Transient pressure

(OP)
jt
The vessel code is PD5500, but I have seen some transient allowances on pipework and in particular plastic pipes.

I just wanted to know if there was any similar guidance on vessels in general, or if as you say thou shalt ......

 

RE: Transient pressure

You have to meet the rules for design for the Code you are designing to.

There is very little safety margin in the assigned allowable stresses for pressure vessels built using using the PED pressure equipment directive, compared to the safety margins provided 40 yrs ago. The code may allow some overage during safety valve lifting events, but the exact amount is defined by code, and includes other pressure inaccuracies such as relief valve accumulation and error in setting the relief pressure. So, except for steam hammer ( 0.5 second duration)type events, overpressures above the code permitted relief vale overage is not acceptable.

 

RE: Transient pressure

Design for the maximum foreseeable pressure (including upset events) + some margin and provide pressure relief at that pressure.

RE: Transient pressure

UG-22(e): "cyclic and dynamic reactions due to pressure or thermal variations..."

Regards,

Mike

RE: Transient pressure

In PD5500 3.2.1 "Consideration of loads"

Quote:


Consideration shall be given to the effect of the following loads where it is not possible to demonstrate the adequacy of the proposed design, e.g. by comparison with the behaviour of other vessels....

...
2) shock loads caused by water hammer or surging of the vessel contents;
....
5)fluctuations of pressure and temperature

If the vessel also has to conform to the PED then you have to consider all possible loads to comply with the Essebtial Safety Requirements (of course being design to PD5500 does not automatically mean that it has to be to the PED as well, only if it is being used in an EU member state).

UG-22 is not relevant to PD5500, that's an ASME reference.

RE: Transient pressure

PD5500 is similar to other vessel codes in requiring that at least one safety valve be set no higher than the vessel design pressure.

RE: Transient pressure

Operationally, is there any way to mitigate the pressure transient at pump shutdown?

RE: Transient pressure

If it is a waterhammer event, one can install a compressible , gas filled bladder  in a tee connection, or there may be available a check valve that has a dampened closure mechanism.

RE: Transient pressure

(OP)
The vessel is the surge vessel, the pressure occurs once on the return surge and last only for a few seconds, the subsequent return surges are lower than 16 Bar.
The surge specialist can't seem to design out the return surge with a return bypass, as the vessel is considerably more expensive if we go over 16 Bar I was looking for a way out.
 

RE: Transient pressure

Get a second opinion from another surge analyst?

Where does the surge eminate from? perhaps if a pump you could add a flywheel and increase the moent of inertia. If from a check valve change to a non slam type or if valve closing time then consider vave stroking times to change the system dynamics.

Check out www.pipingdesign.com for some papers on mitigating surge.

RE: Transient pressure

If the vessel is a surge vessel then it will always see the peak pressure, that's what it is designed for and so the maximum allowable pressure has to be at least the maximum surge pressure.

I would get more quotes, what size vessel is it?

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