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Calculated Capacity For Psv Deviates From Manufacture's Values


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#1 Mohammed Aljabobi

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Posted 01 August 2014 - 10:30 AM

Hi all,

 

I'm currently working on sizing a whole list of PSVs and I'm using API 520 code. I noticed that the capacities that I get are more than 30% off than those of the manufacturers'. Usually the manufacturer provides capacities of the PSV in SCFH of air equivalent and I had to covert those into vapor equivalent to make them compare to my numbers. However, when I did that I got such high discrepancies and it made me think maybe there was something wrong on my calculation.

 

After spending some time examining the problem, I realized that the standard Atmospheric pressure that manufacturers' use is 14.7 Psi which is not the same as what we use for atmospheric pressure where I am. We use 13.5 Psi. And I realized that this change in Standard conditions actually accounts for 9% increase from the manufacturer's values.

 

By the way, the formula that I use to convert vapor flow to air equivalent and vice versa is this:

 

 
image012.gif

 

Please let me know if there are other reasons that would contribute to having different values than those of the vendors.

 

Thank you,

 



#2 fallah

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Posted 01 August 2014 - 12:05 PM

Hi,

 

To be able to help, please upload the info (your calcuilated data and those of the vendor) for one PSV...



#3 Mohammed Aljabobi

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Posted 01 August 2014 - 12:43 PM

Hi Fallah,

 

here is a spreadsheet showing a sample calculation.

 

Thanks

Attached Files


Edited by Mohammed Aljabobi, 01 August 2014 - 12:56 PM.


#4 fallah

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Posted 02 August 2014 - 01:13 AM

Hi,

 

As I well understood:

 

In your calculation:

 

Required relief rate: 168494.1295 lbm/hr

Relieving pressure: 805.5 psig

MW: 19.3

Relieving temperature: 500.543 R

Kb: 1

 

Vendor info:

 

Stamped capacity based on air: 40220 SCFM

At 720 psig

 

Please confirm above mentioned data and provide below info:

 

Ratio of specific heats (k): ?

Set Pressure: ?

 

 



#5 Mohammed Aljabobi

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Posted 03 August 2014 - 01:36 AM

Yes, all the above are correct.

The set pressure is indeed 720 psig. and the ratio of specific heats is 1.66.

 

Thanks


Edited by Mohammed Aljabobi, 03 August 2014 - 01:36 AM.


#6 fallah

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Posted 03 August 2014 - 02:08 AM

Hi,

 

If set pressure to be 720 psig then the relieving pressure (assuming 10% overpressure) should be 792 psig while you did specify it equal to 805.5 psig. Please clarify...


Edited by fallah, 03 August 2014 - 02:38 AM.


#7 Mohammed Aljabobi

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Posted 03 August 2014 - 02:25 AM

The relieving pressure = set pressure + overpressure + atmospheric pressure = 720 + 0.1 * 720 + 13.5 psi = 805.5 psi. I had to add the atmospheric pressure because the set pressure was gauge pressure.


Edited by Mohammed Aljabobi, 03 August 2014 - 02:27 AM.


#8 Mohammed Aljabobi

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Posted 03 August 2014 - 02:32 AM

Please note that the unit for pressure that I'm using is psi not barg as you mentioned in your previous response.



#9 fallah

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Posted 03 August 2014 - 02:48 AM

Hi,

 

Ok, I corrected the pressure unit but to prevent any misunderstanding please always try using "g" or "a" at the end of pressure unit to specify if the pressure is in gauge or absolute value.

 

Anyway, I am trying to check the matter and I'll let you know about the result...



#10 fallah

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Posted 03 August 2014 - 03:33 AM

Hi,

 

As I did calculate per the info you provided, the required equivalent air flow at pressure of 805.5 psig is 46622.3 SCFM. Then proposed PSV with stamped capacity of 40220 SCFM air is less than required value and cannot be accepted.



#11 Mohammed Aljabobi

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Posted 03 August 2014 - 11:54 AM

Hi Fallah,

 

Thanks a lot for your response. 

I was just wondering how come your number is much different than mine? Did you use the same equation I used?

If you can submit a calculation of what you did there, that would be great. 

 

Also I would like to know if you used the same equation I indicated in my original post to convert from vapor flow rate to air equivalent flow rate.

 

Thanks again.

 

Mohammed



#12 xavio

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Posted 03 August 2014 - 10:33 PM

Mohammed,

 

If you want to compare your SCFH with vendor's SCFH, you MUST use the same pressure and temperature basis.

SCFH as per PSV vendor's definition (=ASME) is evaluated at 60F and 14.7psia.

It is unrelated to your actual atm pressure, it is a convention.

 

After converting your mass flow to the proper SCFH, proceed to convert the SCFH of gas to SCFH of air.

You shoud compare two values on equal basis.

I see Mr. Fallah has done the calculation and find out that the proposed PSV is too small and is not acceptable.

Contact your vendor for replacement.

 

Dealing with gas at "standard conditions" is always troublesome, use with care.

If you have used the same P, T for  both SCFHs, and then yes, your formula for converting SCFH of gas to SCFH of air is correct.

 

Good luck.

 

xavio



#13 Marc-Andre Leblanc

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Posted 04 August 2014 - 09:20 AM

Hello,

 

First of all, get your hand on a PSV sizing software, I suggest Pentair (PRV2SIZE), It is free and can be used for preliminary calculation with good accuracy (I always double check with a handmade API 520 excel tool, and in general the value of both are equal or extremely close).

 

Second thing, use standard value for your gage pressure (14.7 psia), there is no reason to complicate things your own gage pressure (13.5) for PSV calculation because it is under standard atmospheric. The impact will only be a small to negligable increase of the capacity of your PSV,  this approach is conservative.

 

Third thing, the capacity you see on the nameplate of the PSV is not an air equivalent rated capacity.

 

This capacity is called the "nameplate capacity" and its the capacity you will get with the PSV on the test bench with the test gas or liquid depending in the PSV trim (water, air or steam) at testing conditions.

 

The proper way of finding the rated capacity of any PSV is to use the equations provided in API 520 and calculate the capacity you will get according to your own process conditions. 

 

In the case you provided, I made calculation with API 520 formula (homemade tool) and with PRV2SIZE to show nameplate and rated capacity :

 

L orifice

Set pressure : 720 psig

Relief pressure : 806.7 psia (10% overpressure)

 

Preliminary nameplate capacity (air at 60 F) : 41 154 SCFM (using preliminary discharge coefficient of 0.975) IN PRV2SIZE / API 520

 

Farris as discharge coefficient of 0.953 according to their documentation for this PSV, so their nameplate capacity would drop to 40 225 SCFM (41 154 / 0.975 X 0.953 )

 

Final nameplate capacity : 40 225 SCFM (you report stamped of 40220 SCFM)

 

Rated Capacity calculation using PRV2SIZE and API 520 :

 

Edit : MW :19.3

Ratio of specific heats : 1.66

Compressibility factor (Z) : 0.798

Relief temperature : 40.83 F

Coefficient of discharge (preliminary) : 0.975

 

Preliminary rated capacity : 186 024 lb / hr using PRV2SIZE - 185 996 using API 520 equations 

 

Farris discharge coefficient : 0.953

 

Rated capacity (using API value) : 181 799 lb / hr

 

Since your required relief capacity is 146000 lb/hr (Edit : Updated with op value), A L orifice is the correct size for your PSV.

 

Moral of story : DO NOT CONVERT NAMEPLATE CAPACITY TO RATED CAPACITY, IT IS FAR MORE RELIABLE TO CALCULATE BOTH INDEPENDANTLY ! 

 

Nameplate cacacity should not be used for anything beside the testing of the PSV at the test bench. 

 

Regards

 

Marc-Andre


Edited by Marc-Andre Leblanc, 05 August 2014 - 12:05 PM.


#14 fallah

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Posted 04 August 2014 - 02:32 PM

Hi,

 

From where has the required relief capacity of 168494 lb/hr come from?



#15 Marc-Andre Leblanc

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Posted 04 August 2014 - 02:48 PM

Hello, 

 

Fallah, I quoted your previous post (dated august 2 2014) where you said the required capacity is 168 494 lb/hr and where the original poster replied that these values where correct.

 

If required capacity is different than 168 494 lb/hr it can be compared to the rated capacity I reported and i can edit my post with the new value

 

regards

 

Marc-Andre



#16 fallah

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Posted 04 August 2014 - 03:26 PM

Marc-Andre,

 

Yes, you are right and it might i made a mistake in my post dated Augost 2 about the required relief capacity. Of course, in the post #10 i did calculate based on required relief capacity of 185102.8365 lb/hr. Anyway, i think the OP should come here and clarify about the correct value...



#17 Marc-Andre Leblanc

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Posted 04 August 2014 - 04:03 PM

Hello, 

 

I made correction to my original post to show that required capacity was not clear and needed to be confirmed by OP.

 

I also wanted to add, just to clarify the difference between nameplate capacity and rated capacity a good exemple where these value will have a good difference:

 

In fire case, you will evaluate the rated capacity with 21% overpressure for the selection of the valve. The certified capacity on the nameplate will be evaluated for 10% overpressure or 3 psig, whichever is greater as per ASME sec. VIII div 1 : UG-131 © (1). It will not be tested at 21% overpressure and the nameplate capacity will only be for 10% overpressure (or 3 psig if it is the higher value)

 

 

A simpler way of putting it is : Nameplate capacity is the "Mechanical capacity" while the Rated Capacity is the "Process capacity".

 

Regards

 

Marc-Andre



#18 xavio

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Posted 04 August 2014 - 07:21 PM

Hello all,

 

Valuable input by Marc-Andre!

I've just opened Mohammed's excel file, the required flow is 185102 lb/h, that's why the L orifice is deemed insufficient.

 

It is correct to ask the vendor to perform valve sizing based on actual gas properties, instead of buyer trying to match his requirement to vendor's list of valve capacity.

It is also best to use mass flowrate in the calculation, so that such confusion can be avoided.

 

Anyway, vendor's list of "nameplate air capacity" (or any other name) can still be used for first approximation.

For purchasing, however, proper sizing is required.

 

Good day!

 

xavio



#19 fallah

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Posted 05 August 2014 - 12:43 AM

Marc-Andre,

 

It's not related to the PSV rated capacity and relates to the over pressure percentage by which the valve to be tested. The main point is that the PSV for fire case is to be sized based on 21% over pressure; but is tested at 10% over pressure per ASME. Then the PSV will be a little bit conservative for operating at 21% over pressure and will result in no problem in this regard.

 

We still await for OP coming to clarify about the required relief capacity...


Edited by fallah, 05 August 2014 - 02:13 AM.


#20 Mohammed Aljabobi

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Posted 05 August 2014 - 09:26 AM

Thanks guys for your valuable input.

The available capacity for an L orifice from my calculations was 185,102 Ib/hr. This was based on the API 520 equation shown in the excel file I posted earlier.

 

The required capacity that I need is around 146000 Ibm/hr for a blocked flow basis. So an L orifice is the right size.

 

My initial question here was this, is it ok to convert my calculated capacity to an air equivalent capacity (using the equation in the my original post) and then compare it to the vendor's at that set pressure?

So from what I gathered from you guys, the vendor's data for capacities should not be used to compare the numbers we get from the API 520 calculations?



#21 fallah

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Posted 05 August 2014 - 11:53 AM

Hi,

 

Based on required capacity of 146000 lbm/hr, proposed PSV by vendor with stamped capacity of 40220 SCFM air can do the job then is accepted for required application...



#22 Marc-Andre Leblanc

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Posted 05 August 2014 - 12:04 PM   Best Answer

Hello

 

To your question : is it ok to convert my calculated capacity to an air equivalent capacity (using the equation in the my original post) and then compare it to the vendor's at that set pressure?

 

No it is not!

 

Because the air capacity the vendor give you is not representative of the other conditions applicable to your PSV.

 

This capacity, normally called "nameplate capacity" will not take into account things like: fire case 21% allowable overpressure, cold differential test pressure of the PSV is not equal to set pressure, vessel maximum allowable working pressure is not equal to set pressure, multiple PSV application, high back-pressure with bellow application, multi-phase flow ... 

 

Another problem can be that the vendor will not have other access to standard you use in your plant or for your clients. 

 

For exemple, the standard of one of my clients is that a thermal PRV for liquid espansion should at least have a minimum standard orifice size API D (0.110 in2) regardless of the required capacity if the required orifice size is under API D. 

 

Countless time Farris tried to sell their 0.068in2, Crosby their 0.078in2 and Consolidated their 0.096in2 because in term of capacities, these PRV could be fine, but they were not a correct selection because of the client standard of minimum 0.110in2.

 

To sum up, from my experience :

 

It is always better to first get all the data you need for a PSV with the API 520 calculations using the preliminary coefficient of discharge. (0.975 gas, 0.65 liquid ....)

 

This should give you the minimum required standard API orifice size for your PSV, along side with a preliminary rated capacity.

 

Then you contact the vendor for a PSV with the standard API orifice size you calculated and with your relevant process data. 

 

The vendor should then give you a PSV with the orifice size you requested and the final rated capacity and nameplate capacity considering the coefficient of discharge of his PSV. If the vendor coefficient of discharge is smaller then the preliminary and not sufficient to reach your required capacity - avoid it! ( Edit : find an other vendor whose coefficient will be enought to reach your required capacity), multiple problem will arise from selecting larger orifice size. 

 

 

If you are checking your psv to see if their capacity are correct after a change in the plant or operation, get the standard orifice size required and compare it to the orifice size installed, take into account variation because of coefficient of dicharge if applicable.

 

Regards

 

Marc-Andre


Edited by Marc-Andre Leblanc, 05 August 2014 - 12:09 PM.





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