Adequacy Check For Existing PSVs

[Sawsan311]

Dear All,

For an existing PSV installed on a vessel and which was designed for fire case at 21% overpressure- orifice area given by vendor was H orifice.. the required area in the original case. Vendor provided a certified ASME capacity of 2400 Kg/hr.. MY DOUBT IS that this should not be at 21% overpressure but shall comply with ASME SEC VIII Div.1 UG-129 of having all certified relieving capacity at 10% overpressure.

as part of validating the existing PSV to handle a blocked outlet scenario.. I saw that the required capacity of 26000 Kg/hr two phase flow but due to the relatively high mass flux estimated using the homogeneous equilibrium method, the size of the existing PSV is found to be G!.

Therefore, my question is:

1- do you agree that the comparison should be based on volumetric flow and NOT mass flow.
2- when evaluating the PSV adequacy.. do we look mainly at the area being adequate since there are other factors that determine the size requirement (not only relieving rate but all gas properties, two phase flux etc).

3- Is it correct that the ASME rated capacity will differ based ont he governing scenarios and hence if my new scenario needs to be compared to the rated capacity of the valve (specially if the gas properties and Temperature are different) I will have to recheck with vendor, because I need to know if my capacity will be within 25% of the rated capacity to avoid chattering.



Vendor reported his coefficient of discharge, the calculated area as per ASME Kd and the selected area based on his valve model but the corresponding calculating maximum ASME capacity which is based on the required capacity of the valve is 2400 Kg/hr against 1818 Kg/hr required capacity. if the scenario changes to blocked outlet as a result of a new revamp but the volumetric rate of the PSV is less than the installed PSV, will I still need to request for the corresponding rated capacity.

Appreciate your views.

Regards,

 

[Latexman]

In the future, relief/PSV/RD posts may get better responses in the Safety Relief Valve Engineering forum.

1 - Given the certified flow coefficients (vapor and liquid) of the proposed/installed PSV, I calculate the flow area required for each credible scenario. Then, I compare the scenario's flow areas required to the certified flow area of the PSV.

2 - Yes, see # 1.

3 - Yes. If you know the certified area and certified flow coefficients, you should be able to calculate the adequacy of the PSV to your new scenario per # 1 without vendor involvement.

I have found that making the flow area comparison the basis, eliminates a lot of confusion.

Once I know my worst case, credible liquid, vapor, and two-phase scenarios, I evaluate each (up to 3) on the hardware (inlet pressure drop, outlet pressure drop, conventional/bellows/pilot operated, etc.).

Good Luck,
Latexman
Pats' Pub's Proprietor

 

[RVAmeche]

With regards to checking nameplate capacities, there is a useful reference called the NB-18 (known as the Red Book), that contains all the certified manufacturer information you need to calculate nameplate capacity at different set pressures for a given device. Obviously I would verify w/ a vendor prior to finalizing, but they typically match (as they should).

 

[Sawsan311]

Thank you MR.Latexman and RVAmeche for your feedback,

I would like to ask some details about vendir PSV sizing: the rated certified coefficient of discharge will be used for calculating the required ASME capacity, howeverm the rated PSV capacity can only be known after knowing the installed PSV area which differs from vendor to the other and which is given in the national book- red book? is that correct.

I was under the impression in the past that the rated coefficient of discharge is the one that directly gives the ASME rated capacity, however, after reading API 520 part I section 5.2, it states in Clause 5.2.5 that the rated capacity requires: 1) the actual orifice area AND 2) the rated coefficient of discharge AND 3) the use of equations of API 520. The rated coefficient will be used along with the required capacity in estimating the required rated area.


I got confused when one vendor estimated ASME required capacity using his Kd discharge and then calculated the rated capacity based on the actual orifice area.. do you agree that actual orifice area only depends on the valve design and the certification via the Kd will be based on the ASME performed tests and after applying the 0.9 derating factor?

thanks

regards,

 

[RVAmeche]

The standardized API orifice areas and Kd's (0.975 & 0.65 for gas/liquid w/o rupture disc) seen in API 520 & API 526 are preliminary values used to get you in the ballpark.

Once you've finalized the valve selection, or if you have an existing installation, you need to use the ASME certified area and coefficient; both area and coefficient can (and likely will) be different from the API values.

 

[Sawsan311]

Thanks RVAmeche, I just want to confirm.. is the ASME certified area calculated or it is the actual area of the valve by the vendor and the associated coefficient of discharge is the one obtained from the various tests as explained in UG-131 ASME SEC VIII(the ratio of the actual flow to the theoretical flow) and which is then derated by 0.9.

I saw some of the vendors reporting the ASME required effective area ?! using the Kd and API 520 equations.. and then stating selected area.. so now I need to know which area is the certified area.. I believe the certified area is never calculated but depends on the certification tests and the actual valve model.

Thanks

 

[MortenA]

@sawsan311, its a calculated area. The true area is not that important (and since the flow also depends on the flow coefficient it becomes rather academic).

 

[Sawsan311]

Thanks MortenA.. I saw in the vendor calculations.. that although ASME area was calculated using the Kd and the required relieving capacity.. However, there was another area called ''selected area'' which I believe is based on the vendor's PSV design. I also saw that the NB Red book provides various vendors with various Kd and Area for the several API 526 orifice designations.. are these also calculated. kindly please advise.