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Tank Vapor In Line Vent Valve Sizing

valve chattering tank vapor vent valve pressure control valve

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#1 kengor10

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Posted 06 January 2022 - 01:10 PM

New engineer here, first year out of college. I'm designing a blanket gas system for a Central Tank Battery.

 

There are Blanket gas regulators for the headers of each set of tanks, Brine and Oil, keeping 2 oz/in2 pressure on the header above the tanks.

There are in line dead weight loaded vent valves for the header s of each set of tanks, Brine and Oil, set to open at 8 oz/in2 of pressure in the case of an overpressure in the system. A DFA and Flare scrubber are also going to be downline form the vent valve. During routine operation the excess vapor will flow from the header to the vapor recovery compressor. 

 

Tank MAWP = 16 oz/in2

Vent Valve Set Pressure = 8 oz/in2

300' equivalent line loss from the vent valve to the Flare

I have 5 Oil Tanks totaling 5,000 BBL Storage; 850 gpm max fill rate

I have 6 Brine Tanks totaling 6,0000 BBL Storage; 700 gpm max fill rate

 

I estimated the relieving capacity needed for the in line vent valve in SCFH of air according to API 2000 7th ed.

9,600 SCFH air equivalent - Brine Tanks

15,100 SCFH air equivalent - Oil Tanks

 

I used data from the vent valve vendor to estimate pressure at the required flowrates across the vent valve and compiled a spreadsheet to analyze the system as a whole. See attached. 

 

 

I'm trying to choose the correct vent valve connection size as to not oversize the valve and cause chattering.

 

I used this formula to gauge sizing:        Flow at Set Pressure/Flow at MAWP Pressure = % Open

 

Is this a valid assumption?? I don't have any information on stem travel.

 

 

A 4"x4" connection gives me 40% Open at the required flowrate for Oil Tank Vapor and 25% Open at the required flowrate for Brine Tank Vapor.

I believe any connection larger than that will be oversized and cause chattering.

 

However with a 4" line size and the equivalent line length present, I have too much frictional loss to make it to the flare with the gas. I know I could expand the line to 6" after the Valve but I just want to make sure I'm thinking about this correctly?

 

Thanks,

KG

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#2 latexman

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Posted 06 January 2022 - 02:38 PM

I used this formula to gauge sizing:        Flow at Set Pressure/Flow at MAWP Pressure = % Open

 

Is this a valid assumption?? I don't have any information on stem travel.

 

That is either not valid or the wording is not consistent with the definition of "set pressure".  At the set pressure of a PVSV, there will not be much flow.  Think of it as "simmering".  It's more like it's leaking than it is flowing.  The pressure force and dead weight are about in balance and one second it's closed and next second it's barely open.  It's kind of unstable.  Your formula should always equal about zero because "set pressure" is a fixed number, right?  It's the set/opening pressure of the device.  And, why does the MAWP of the vessel have anything to do with the % open of a valve?  You could have a 2 oz./sq. in. PVSV on a 4 oz./sq. in. tank or a 100 psig pressure vessel.  At 4 oz./sq. in. they should both be 100% open.  I don't see what you are trying to achieve.  I think the standard flow curves for PVSVs are inadequate to figure out what % open the pallet is.  You would need to consult with a PVSV vendor.

 

At about 2X set pressure, the pallet / dead weight and spring loaded pallet on a standard / normal PVSV should be 100% open.  That's what I was taught.  On a 16 oz./sq. in. tank with a normal vent unit and a emergency vent unit, you'd have them staged.  The normal vent unit might have a 4 oz./sq. in. set pressure and be 100% open at 8 oz./sq. in. and then the emergency vent unit would have a set pressure of 8 oz./sq. in. and be `100% open at 16 oz./sq. in.  Those would be the maximum set pressures you could have on a 16 oz./sq. in. tank, but you could have lower.

 

Also, most vent valves (PVSV), I know of, do not have a "huddling chamber", so they do not "chatter" at a high, destructive frequency like a safety valves (PSV).  Normal vent valves are more like a modulating valve.  But, be careful, there is at least one PVSV on the market, Protego, that does have a "huddling chamber" and they claim the valve is 100% open at 10% over pressure (10% above set pressure).  So, you have to know what is going to be purchased before you size the equipment.



#3 kengor10

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Posted 06 January 2022 - 04:17 PM

Thank you for your response.

 

Sorry, Yes I worded that wrong.

 

At 8 oz Set Pressure and 9600 SCFH flowrate a 4"x4" valve will have a outlet pressure of 8.48 Oz

 

(Flowrate at Set Pressure (i.e.=0) ) / (Flowrate at MAWP) = 0 % Open

 

(Required Flowrate yields a pressure of XX) / (Flowrate at MAWP) = % Open 

 

9600 SCFH / 37500 SCFH = 0.256 Open

 

But I think you are right it is still not valid. 1. Because the relationship isn't linear. 2. Because the MAWP isn't always 2X Set pressure (just so happens to be in this case).

 

 

The in line vent valve going to flare is the first line of defense (8oz/sq in.). There is a thief hatch that will open at 12 oz/sq in. as the second line of defense. 

 

 

So by your rule of thumb 100% open at 2X set pressure, if I used a 2"X3" valve @ 16 Oz it has a flowrate of 21,000 SCFH. That would be 100% Open.

Similarly a 3"X3" valve has a flowrate of 32,700 SCFH at 16 oz     100% Open

               a 4"X4" valve has a flowrate of 37,500 SCFH at 16 oz      100% Open

 

So, how would I go about sizing in relation to my required flowrate. Is there a method? Or do I just put my trust in the vendor and give him the required flowrates and see what he returns?



#4 latexman

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Posted 06 January 2022 - 07:26 PM

At 8 oz Set Pressure and 9600 SCFH flowrate a 4"x4" valve will have a outlet pressure of 8.48 Oz


Are you sure about that? How about:

A 4"x4" valve with 8 oz Set Pressure and 8.48 oz Sizing Pressure flows 9600 SCFH.

Sizing Pressure is the pressure at the PVSV inlet flange. Every flow curve/table I’ve seen for a PVSV has the outlet flange at atmospheric pressure.

#5 latexman

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Posted 06 January 2022 - 08:01 PM

So, how would I go about sizing in relation to my required flowrate. Is there a method? Or do I just put my trust in the vendor and give him the required flowrates and see what he returns?


Yes there is a method. The vendors have it programmed. They may have it on their website or will let you have a copy. My company has their own program. It calculates inlet and outlet line pressure drop at the sizing flow, so it can calculate the actual Sizing Pressure at the PVSV inlet flange and the actual Back Pressure at the PVSV outlet flange. From this, they know the differential pressure across the valve, which is exactly the form of their data since their capacity data is with atmospheric pressure (0 oz) at the outlet flange.

This should help you with the methodology.

Also, Google a good PVSV design guide from a/your favorite vendor and learn the terminology.



#6 breizh

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Posted 07 January 2022 - 02:47 AM

Hi,

To me you should prepare a spec sheet and submit to reputable vendors to get a design and quotation .

More important is to provide the right information for them to perform the task .

Good luck

Breizh 



#7 jayari

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Posted 07 January 2022 - 08:16 AM

If your ultimate goal is to avoid a chattering PSV, I don't think the inlet connection flange size is the most critical factor. Also, it seems to me that you're trying to pick the tank nozzle size and make the PSV conform to nozzle size but I think it should be the other way around unless the tank is existing and only has one nozzle left to work with.

 

I'd agree with breizh that you should contact a PSV vendor your company has a good track record with and discuss your application. You've compiled most of the information they would need so you're off to a good start.



#8 kengor10

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Posted 07 January 2022 - 12:32 PM

Thank you all for your help. I have a better understanding but going to talk with the vendor for sure.






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