Vessel minimum design pressure

[Sawsan311]

Dear All,

Various engineering standards consider a minimum design pressure of 3.5 barg for various reasons such as in order to have a practical estimation of the PSV size to the flare and its associated tail pipe sizes(since at lower pressure, we will end up with larger area and large rated capacity--> tail pipe size).
There is also another driver discussed for this minimum design pressure which is the steam out conditions requirement for vessels and hence the need to have its design pressure high enough to withstand the external stresses when vessels cools down post a steam out operation...

My question is..do all pressure vessels (irrespective to the operating pressure)shall be designed for steam out operation at 150 C and 3.5 barg, or this is only limited to vessels operating at low pressure or containing steam only?? I saw in almost all projects, all ASME SEC VIII vessels/filters and exchangers have the requirement to be designed for Full vacuum and steam out conditions at 150 C & 3.5 barg.

Thanks

 

[MJCronin]

Not all industrial Pressure Vessels are subject to "steamout" .... but your concern about a minimum design pressure is well founded.

In other segments of industry, low pressure designed vessels can sometimes lead to very thin shells, especially if designed to ASME VIII.

For some services, such as power plant deaerators and condensate collection tanks, it is desirable to use the robust rules of ASME VIII for design, inspection and testing, but use either a minimum wall thickness or a minimum design pressure. 50 psig is suggested.

It is important to note that this question always comes up when a LP vessel is specified with a design pressure less than 15 psig. Technically, this falls outside of the rules of ASME VIII but it is nonetheless wise to use ASME rules in the design.

Additionally, I completely agree with a minimum temperature of 150C

MJCronin
Sr. Process Engineer

 

[Sawsan311]

Thanks Mr.MJCronin, actually almost multiple vessels engineering specifications dictate the requirement for designing the vessels for steam out operation at full vacuum and 150 C.I believe the driver which is vessel cleaning will directly impact the minimum design pressure selection for the vessels due to external pressure effects post steam out operation as a result of quenching by fast cooling.
For 150 C, do you think that the requirement is already covered by the selected vessel's flange rating and thickness since the coincident pressure (full vacuum with 150 C) is low enough compared to the vessel's design temperature at the higher design pressure (which is based on the maximum operating conditions).

Thank you,
Regards,

 

[georgeverghese]

3.5barg is a bit too low to handle deflagration pressures generated as a result of failure of flare purge - 7barg would be better. FV to handle vacuum conditions post steam out is also a good idea. If the vessel dia is not very large, you may get FV "for free" if you were to use 7barg as the upper design pressure - check with the pressure vessel design engineer.

 

[MortenA]

The essence of the advice given by MJCronin is spot-on, however, since you refer to barg (and not psig) you may live outside USA - and ASME is only valid here. PED is e.g. valid in EU an while it dosnt cover the same as ASME you may not live up to PED if you specify a ASME vessel. Refer to your local rules/standards - but the advice is still valid!

WRT to _3.5_ barg (and 150ºC) - this may be a company standard that shall cover the risk of vessel implosion as a result of vacuum after steam-out (may be what you are referring to). IMO you can set the s.p. of a PSV below the design pressure if you wish to limit your peak discharge rate - but given the same design case i would assume that this would lead to a larger valve?

I think i would specify the design pressure require by the process or for steam-out and if steam-out is needed i would also specify full vacuum.

Best regards, Morten

 

[Sawsan311]

Hi Mr.georgeverghese , I totally agree with you that a vessel design pressure of 3.5 barg will not be suitable for withstanding the flashback effects resulting from internal deflagration. In fact API 14 J states that a minimum design pressure of 125 Psig (8.6 barg) is required to mitigate /absorb the flashback effects. However, detonation is still a phenomena which is developed from an initiated unstable deflagration and which can be miniimized by having sufficient inert purging to keep the process media outside its flammability and explosive limits.

Mr.MortenA , I totally agree with you that the selection of a minimum design pressure directly affects the practical sizing of thePSV tail pipes and flare capacity. In fact, some design specifications state that the minimum 3.5 barg to have an economic limit of the PSV tail pipe since lower than 3.5 barg, not only the PSV area would be large, its rated capacity would correspondingly be high as well as the backpressure on the PSV (i.e. high volumetric flow).

I think dictating full vacuum and 150 C as sub-requirements for steam out conditions should be covered under the selected vessel's design pressure (higher design pressure) and design temperature.

Appreciate your views.

Regards,