6 replies to this topic

[SawsanAli311]

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,

Edited by SawsanAli311, 23 June 2020 - 08:47 AM.

[shan]

1,2.  PSV's are sized based on relief stream volumes.   The corresponding mass flows of the volumes are the calculated values with the relief considerations of temperature, pressure and relief stream properties.

 

3.  The PSV's areas are determined by the required relief rates of governing relief scenarios.

 

An undersized PSV's will not satisfy relief requirements to protect the vessel and an oversized PSV's will chatter during relieving operation to cause damage of PSV seats.  Theses are the reasons why an existing PSV is to be validated for a new governing scenario.

[breizh]

Hi,

Consider this link and download the handbook . You should find answers to your query.

https://www.leser.co...ls/engineering/

Good luck

Breizh 

[SawsanAli311]

Thanks all for your great inputs..

 

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.
 

[Jiten_process]

Hi Sawsan,

 

If you refer to the PSV sizing equation, check for example PSV sizing for gas, majority of variables in PSV area calculation is process dependent (and not manufacturer dependent) except Kd, Kb, Kc. The mol weight, density, Cp etc dependent on process and it has nothign to do with manufacturer. Again Kb is 1 as long as your back pressure is within the limit of type of PSV installed (for e.g. <30% for balanced bellow). Kc, u know if you have installed RD upstream of PSV or no. So you left with Kd. This is dischare coefficient factor which is manufacturer dependent and also varies between ASME and API. 

 

If you know the Kd for the installed valve model whic is used by manufacturer to certify the PSV rated capacity, you can probably rely on your calculation to be able to qualify the adequacy of existing PSV. In your case, with your revised process scenario and conditions if you calculated area, using vendor Kd factor, is sufficiently less than the installed area then you can rely on the installed PSV. 

 

Note that, if your relieving fluid state is changed than previous PSV scenarios which were given to vendors, then you may have to send your calc for vendor's blessings. Although, i am sure it will be ok. We here in my company, provide to PSV vendor all the scenarios where relieving fluid state is different. For e.g. two phase blocked outlet, liquid overfill, fire case etc. 

 

Hope this may clarifies your questions.

 

Lets see, what others have to say.  

Edited by Jiten_process, 08 July 2020 - 09:45 AM.

[SawsanAli311]

Thank you Jiten_Process for your great inputs.. I was actually a bit confused earlier when I saw the vendor PSV stating ASME required area and then stating selected area. The rated capacity was anyways calculating basedon the ratio of the actual area to the calculated effective ASME area (as spelled out by him). However, as you said, when comparing the PSVs adequacy for different scenarios for example: my existing PSV was designed for fire case while my evaluated adequacy scenario is gas blow by.. due to the differences in the gas properties specially the Z, MW an temperature, comparing against the rated capacity becomes incorrect and comparing the required area against the actual area is the correct approach. But before comparing.. do you think we should apply a derating factor on the API Kd i.e. 0.9 as per ASME UG-131 i.e.approximating the certified Kd of the valve in absence of vendor information..  OR shall we just compare the require area calculated by the API equation directly with the installed PSV actual area.. 

 

thanks 

[Jiten_process]

@sawsanali311

 

The coefficient of discharge (Kd) is a correction factor for the difference in behaviourof a relief valve (or rupture disc) and an ideal nozzle. It relates the predicted flow through a nozzle with a given area to the actual flow expected.   As per API-520, for preliminary sizing of relief valve vapour Kd factor of 0.975 is generally used. For liquid service this is 0.65 and for two phase this is 0.85.

 

Kd factors are different for ‘API’ type and ‘ASME’ type orifice. Additionally, this can vary from vendor to vendor depending on the actual certification. If you know the installed PSV model's actual vendor Kd while evaluating the required orifice area for revised relief conditions/flow, you dont need additional margin. This is usually available in manufacturer's datasheet if PSV is existing. We apply derating factor of 0.9 while doing the relief calc at project phase when API recommended Kd factors used as vendor information is not available. We later validate it with vendor Kd factors before IFC stage of project. 

 

hope this answers your query.