PSV Sizing Calculation (Swagelok Relief Valve)

[C26M13]

Hi All,

I am trying to size a PSV (PSV172) for a Nitrogen Supply Line to Well Control Panel as per attached file.

The PSV is installed on the N2 Supply Line with a set pressure of 1,380 kPag and Vent to Safe Location.

I have some information of the Nitrogen Regulator (BOC Regulator 8000) with it's specification and the type of installed PSV (Swagelok R4 Series).

My question is:
1. Is this PSV only protect the overpressure from Nitrogen Supply? or it protect also from Gas List Control Valve Failure Scenario? Which one is valid scenario?
2. How can calculate the rated flow of the PSV for a scenario of Nitrogen Regulator Failure (from BOC Regulator spec sheet, the flow rate at 21,000 kPag is 1,086 L/min)?

Please see the attached file of the schematic drawing with additional information of BOC Regulator and Swagelok PSV.





Much thanks

 

[LittleInch]

Depends how that three way valve works.

If there is no way to connect the two incoming gas supplies then no, it's just the n2 supply.

For flow rate you find out the max flow when regulator is wide open. That might be the 1086 figure,but double check.

 

[georgeverghese]

You've got critical flow across that BOC regulator with 21e3kPag upstream of the regulator and 1380kPag set pressure at PSV 172, so flow will remain at 1086litres/min and will be the sizing case relief load. However, this may not be with the regulator failed wide open, so in the absence of more info on this regulator, I'd add say 30-50% more flow as design case flow margin for this PSV. Temp drop will be based on isenthalpic path from 210e3kPag to 1380kPag - look up any thermo charts for N2 for this.

 

[C26M13]

Thank you Littleinch and geogeverghese fot your comment.
I took the flow of 1,086 L/min as the required capacity of PSV 172 and I will check this flow against the installed PSV (Swagelok - D -Blue).

With the below graph (estracted for swagelok R4 Serie) , I could estimate the capacity of PSV based on the Set Pressure of 1,380 kPag (220 Psig), seems the air flow given the PSV capacity less than 1,086 L/min. Just approx. 300 Air Flow, stdL/min.

 

[LittleInch]

13.8 bar is 200 psi so its even worse.

Looks like you would be up to about 250 psi / 1720kPag to flow 1000 std l/min.

So it depends on the pressure rating of the system you're trying to protect as to whether these will be good enough or not.

BTW, make sure the regulator flow is being quoted in standard l/min....

 

[RVAmeche]

Someone here may be able to explain better, but that Swagelok relief valve seems similar to other chemical feed type relief valves I've seen that aren't actually ASME rated devices. They're just spring adjustable valves basically.

Assuming this is a B31 system how does that type of device qualify as a relief device to protect the system since it doesn't meet ASME requirements for a relief valve? Not sure if there's an exemption or something that I'm not aware of

 

[C26M13]

Thanks LittleInch and agree with you, it's should be in the range 250 psi.

As per the vendor catalogue @ 21,000 kPag Inlet Pressure the flow stated as 1,086 L/min

 

[Snickster]

My question is:
1. Is this PSV only protect the overpressure from Nitrogen Supply? or it protect also from Gas List Control Valve Failure Scenario? Which one is valid scenario?

It looks like the PSV 172 is for protecting the N2 supply line only. PSV 124 looks like it is for protecting the gas lift gas supply line. One line must be a backup in case the other goes down.

2. How can calculate the rated flow of the PSV for a scenario of Nitrogen Regulator Failure (from BOC Regulator spec sheet, the flow rate at 21,000 kPag is 1,086 L/min)?

Are you sure that the BOC regulator on the N2 line is rated for 21,000 kpag at 1,086 l/min? This appears to be more of what the BOC regulator on the gas lift supply line would be rated for since it has a very high inlet pressure. I would think any regulator on the N2 line would be rated for a much less inlet pressure. Even if it is the same regulator as on the N2 line, the maximum inlet pressure would be a lot less coming from the site N2 system so under regulator failure the flowrate would be a lot less and based on the actual maximum N2 inlet pressure with regulator failed wide open. The flow through the regulator depending on the upstream pressure could be sonic or sub-sonic and should be calculated based on this to find the required PSV flowrate. If the ratio of maximum upstream pressure to downstream pressure (PSV set pressure) is greater than about 0.5 then the flow is sonic.

The flowrate curves shown for the swagelock PSV needs to be adjusted for N2 versus air. Check swagelock technical data for conversion factors and detailed calculations of PSV capacity.

 

[C26M13]

1. This PSV is to protect the overpressure from Nitrogen Supply only.

2. The N2 is supplied from N2 cylinder (with max pressure of 21,000 kPag) with regulator BOC 8000 to regulate the outlet pressure of 1,000 kPag.

Thanks for the inputs, I will double check also on the conversion factor for N2 vs Air.

 

[LittleInch]

You need to find out what the design pressure is for the Well control panel gas inlet supply.

Only then can you work out if the relief valve is satisfactory.

This is a gas supply system so could be anything in terms of design. So long as the pressure doesn't rise above the design pressure by more than 10% then you should be OK. The vendor makes it clear these are not ASME rated relief valve.

The vendor really should state standard l/min or what pressure and temperature that volume flow relates to. To assume it's at standard conditions is a bit lazy IMO.

 

[RVAmeche]

Protecting a nitrogen system, if you're using B31, still requires an ASME relief device. I'm sure the regulators/reliefs on nitrogen and propane bottles are different, but if its a piping or tubing system I don't understand how it qualifies.

 

[georgeverghese]

If there is a tear on the soft seat internals on this 3way valve, lift gas could potentially migrate into N2 - if so, at the very least, install a tubing check valve on N2 line just upstream of the 3 way valve. Ideally, you dont want a physical connection between these 2 sources.

 

[C26M13]

If there is a tear on the soft seat internals on this 3way valve, lift gas could potentially migrate into N2 - if so, at the very least, install a tubing check valve on N2 line just upstream of the 3 way valve. Ideally, you dont want a physical connection between these 2 sources.

For that scenario, PSSV-124 upstream is installed to relieve any excess gas lift coming the well control panel. Both PSVs are set at the same set pressure of 1380 kPag.

 

[C26M13]

You need to find out what the design pressure is for the Well control panel gas inlet supply.

Only then can you work out if the relief valve is satisfactory.

This is a gas supply system so could be anything in terms of design. So long as the pressure doesn't rise above the design pressure by more than 10% then you should be OK. The vendor makes it clear these are not ASME rated relief valve.

The vendor really should state standard l/min or what pressure and temperature that volume flow relates to. To assume it's at standard conditions is a bit lazy IMO.

Thanks, I can check the design pressure of that.

Dou you refer to Gas Lift Supply or N2 Gas Supply (referring to: "Well control panel gas inlet supply.")?

 

[LittleInch]

Thanks, I can check the design pressure of that.

Dou you refer to Gas Lift Supply or N2 Gas Supply (referring to: "Well control panel gas inlet supply.")?

I mean the well head control panel gas inlet design pressure whether this is N2 or natural gas.

 

[C26M13]

Hi all,

Just checked with some references for Swagelok PSV information and calculation and since it's a proportional relief device, can the rated flow the same as required flow (e.g. consider the scenario of Upstream Control Valve Failure)?



Thank you

 

[RVAmeche]

They don't have any capacity (in terms of ASME relief valve sizing) since they're not real relief valves. Flow thru the valve increases as dP across the valve rises.