13 replies to this topic

[SawsanAli311]

Hi, 

 

I have been looking into the capacity certification of PSVs by ASME code and how it confirms the rated capacity of PSVs as well as the rated coefficient of discharge.

 

Considering API 520& API 526 are preliminary starting points for calculating the required and selected effective coefficient of discharge and effective area based on the required relieving capacity,

 

At the early design stage and prior to PSV procurement, selection and testing certifications, 

In order to perform the initial sizing of PSV inlet and outlet tail pipe sizes based on the 3% non-recoverable pressure losses and backpressure criteria respectively (except for modulating type pilot operated PSVs where required relieving capacity will be used), what would be the most recommended estimation of an initial PSV rated capacity in order to perform such calculations, would be assumed based on the the 10% ASME derating factor or this step mandates initial screening of actual PSV rated capacities available in the market for similar API 526 effective area designations.

 

Thanks

 

[latexman]

I am struggling to understand your question because it seems a lot of minor facts have been woven into the conversation unnecessarily.  Is your point, "what would be the most recommended estimation of an initial PSV rated capacity"?

 

If so, my first thought is, that should be done by a senior process engineer experienced in the process technology and PSV sizing methodology.  It is difficult.  Each PSV can have a different worst case scenario and each PSV can have a different controlling fluid phase - vapor, liquid, or multi-phase; no single rule of thumb can be used.

Edited by latexman, 04 January 2019 - 01:04 PM.

[fallah]

 

I have been looking into the capacity certification of PSVs by ASME code and how it confirms the rated capacity of PSVs as well as the rated coefficient of discharge.

 

Considering API 520& API 526 are preliminary starting points for calculating the required and selected effective coefficient of discharge and effective area based on the required relieving capacity,

 

At the early design stage and prior to PSV procurement, selection and testing certifications, 

In order to perform the initial sizing of PSV inlet and outlet tail pipe sizes based on the 3% non-recoverable pressure losses and backpressure criteria respectively (except for modulating type pilot operated PSVs where required relieving capacity will be used), what would be the most recommended estimation of an initial PSV rated capacity in order to perform such calculations, would be assumed based on the the 10% ASME derating factor or this step mandates initial screening of actual PSV rated capacities available in the market for similar API 526 effective area designations.

 

 

Hi,

 

Appears it's mostly informative rather than to be a query...

 

Please explain more and clarify...

[SawsanAli311]

Hi Latexman and Falhah, 

 

my question was that in order to identify the rated capacity to be used for calculating inlet pressure loss and built up backpressure checking, do we have to refer to PSV vendor data even during the design stage before procuring the PSV? 

or there is are specific rules for estimating initial rated capacity for the sake of checking PSV inlet and outlet piping?

[breizh]

hi,

You should ask yourself what is the credible scenario to size the safety valve or request support of your peers in your organization?

 

my view

Breizh

[fallah]

 

my question was that in order to identify the rated capacity to be used for calculating inlet pressure loss and built up backpressure checking, do we have to refer to PSV vendor data even during the design stage before procuring the PSV? 

or there is are specific rules for estimating initial rated capacity for the sake of checking PSV inlet and outlet piping?

 

Hi,

 

You should use the PSV rated capacity reflected in vendor data sheet for calculating inlet pressure loss and built up back pressure.

[latexman]

SawsanAli311,

 

Your approach seems to be different from mine.  I think you work for an engineering firm and I'm a client's engineer, and we execute differently because of this.  But, I've been doing this for 40 years.  I follow API 520 and 521.  Let me briefly explain my approach.  Maybe it'll help; maybe not.

 

Identify all credible cases.

Determine PSV area required for all credible cases.

Select a specific PSV that exceeds largest PSV area needed for all credible cases.

(Notice the PSV has been selected before inlet and outlet calcs are done.  No need to estimate "initial PSV rated capacity".  It is what it is!)

For worst case liquid, vapor and 2-phase scenarios, design inlet and outlet to meet dP criteria for PSV selected at rated capacity of PSV for all of the liquid, vapor and 2-phase scenarios.  Yes, it could be up to three design cases, but it's usually 1 and sometimes 2.

 

My approach usually works first time.  Sometimes there is a little recycle, especially if I have to go to a bellows or pilot PSV to make things work.

 

What do you think?

[SawsanAli311]

Hi Latexman, 

 

Actually your approach is not widely different from the conventional PSV design approach, my question was exclusively addressing the use of the '' rate capacity of PSVs'' in checking that your considered PSV inlet line sizes during the initial design stage of the project (based on the inlet pressure loss criteria) is OK. The same applies to the built up backpressure condition with respect to the selected PSV type and the code allowable overpressure for the specific scenarios/contingencies.

Quote:

''For worst case liquid, vapor and 2-phase scenarios, design inlet and outlet to meet dP criteria for PSV selected at rated capacity of PSV for all of the liquid, vapor and 2-phase scenarios.  Yes, it could be up to three design cases, but it's usually 1 and sometimes 2.'' 

Unquote:

In your PSV initial sizing calculations, you have used the required relieving capacity. However, as Fallah states, the use of the rated capacity for inlet DP and built up backpressure calculation  shall be the rated capacity which can be obtained from vendor data sheet which confirms the actual rated capacity of the valve (and not the API capacity which corresponds to the effective coefficient of discharge).

Regards, 

[latexman]

 

In your PSV initial sizing calculations, you have used the required relieving capacity. However, as Fallah states, the use of the rated capacity for inlet DP and built up backpressure calculation  shall be the rated capacity which can be obtained from vendor data sheet which confirms the actual rated capacity of the valve (and not the API capacity which corresponds to the effective coefficient of discharge).

 

 

I believe you have read too much or too little into my words.  I said "For worst case liquid, vapor and 2-phase scenarios, design inlet and outlet to meet dP criteria for PSV selected at rated capacity of PSV for all of the liquid, vapor and 2-phase scenarios."  Rated capacity of PSV; not "required relieving capacity" or "API capacity".  I think Fallah and I are saying the same thing.  Rated capacity can be taken from vendor data sheet or calculated using certified  nozzle area (which is the actual nozzle area derated about 16%) and flow coefficient.  Certified areas and flow coefficients can come from the vendor or the "ASME Red Book".  I never mentioned API capacity (or API nozzle area).  Those are obsolete terms from the history of sizing relief valves.

[SawsanAli311]

Thank you Latexman,

 

I was mainly inquiring on the source of the rated capacity to be considered in reference to the vendor data sheets and the ASME certified capacities...  All other calculation steps you have mentioned exactly match with the approach we tend to use in our company.

 

Regards,

[PRVBoss]

I'm a bit late here, but are you asking, "Since I need to use the rated relief capacity to size my inlet line and determine my built-up backpressure, how do I determine the rated capacity when I do not yet know the PRV vendor? Early in the design stage and before I've selected Consolidated, Crosby, Farris, etc. what area and discharge coefficient combination do I use?" 

 

If so, API 520 has language to clarify that the effective area and effective coefficient of discharge are for preliminary sizing to select valve configurations from API 526 and design the piping. See §5.2. Currently API 520 states, "It is important to remember that the effective area and the effective coefficient of discharge are used only for the initial selection. The actual orifice area and the rated coefficient of discharge shall always be used to verify the actual capacity of the PRV." 

 

The subcommittee is looking to soften this language because it is not always possible (two-phase relief being an obvious violator...certified 2p Kd???). 

[SawsanAli311]

Thank you PRV Boss for your reply, in fact that was my query, I was under the impression that in order to validate the rated capacity used for the inlet and lateral pipe sizing during the early design, we would have to refer to vendor catalogs of similar API /ASME designated PRVs. 

 

I agree with you that the certified liquid and two phase PSVs require vendor inputs to confirm their rated capacities and rated coefficient of discharge. I feel that the rated capacity utilization for inlet PRV line sizing and tail pipe sizing would still require vendor data since the effective areas and coefficient are used for estimating PSV area with the required relieving capacity. 

[PRVBoss]

Hello, SawsanAli311. 

 

In the preliminary stage I wouldn't worry too much about using the API preliminary effective orifice areas and discharge coefficients for the line sizing as they are generally pretty close to certified combinations. Remember that after you select a vendor and finish piping design the calculations and documentation should be updated to reflect the as-builts.

 

Below is a table of the NB-18 certified vapor discharge coefficients and corresponding areas for three major US vendors and a comparison to the API preliminary combinations (table looked ugly, inserted as snapshot instead). 

 

 

 

Hope this helps!

Attached Files

•  Kd x A.png   423.96KB   13 downloads

Edited by PRVBoss, 31 January 2019 - 11:49 AM.

[SawsanAli311]

Thank you very much PRV Boss