Pressure relief valve inlet sizing 3

[tbedford]

I am commissioning a dual fuel natural gas/oil hydronic boiler by Bryan Boilers. Input MBH is 1200/600 and output is 1002 MBH. Boiler MAWP is 160 psig.
I have attached a photo of PRV installed on top of boiler. Intuitively, a reduction in size from 2" to 3/4" orifice represents a significant blockage in the event of over-pressurization.
My experience and education has always been the PRV inlet size matches the boiler size.
I can't find a code reference for the PRU inlet piping... lots of reference for the PRV discharge.
I have also attached boiler nameplate.

Thank you, Tom

 

[tbedford]

Photo of installed PRV

Thank you, Tom

 

[GD2]

What Code is the Boiler? All Pressure Vessel Codes provides rules for 'Over Pressure Protection". Capacity and set pressure requirements for the PRVs are provided.

GDD
Canada

 

[mk3223]

It’s acceptable to have the smaller pipe size of the PRV inlet line than the equipment nozzle as the relief capacity and the inlet pressure drop meet the PRV sizing requirements. You may confirm it with the Engineer for the proper PRV sizing.

 

[Snickster]

Relief valves are sized in accordance with API 520 not rules of thumb of inlet size veruse service line size. The results of the API 520 calculations are to determine the orifice size of the relief valve to pass the required relieving capacity. This relieving capacity is determined also by guidelines given in API 520 and other references that uses engineering judgement to determine what would be the maximum relieving capacity under different failure scenarios.

A relief valve is then selected with the proper orifice size. Then the inlet pipe of the relief valve is sized to give no more than 3% of set pressure pressure drop. For instance for a relief valve selected that has a 2" inlet size you may need to install a 3" pipe size to maintain less than 3% pressure drop and then install a 3" x 2" reducer at the relief valve connection. See API 520 and API 521 for full discussions on relief valve sizing and other consideration of relief system design.

 

[shvet]

My experience and education has always been the PRV inlet size matches the boiler size.

You suppose PRV's inlet line shall be 2" and PRV itself shall remain 3/4" - correct?
Which part you have found suspicious - PRV or PRV inlet line or boiler nozzle?
Can you share with us your 'experience and education' sources where this point had come from?

 

[The Obturator]

From your photos, you have a hot water boiler acc. to ASME IV - it is identified by the ASME 'H' stamp in the first photograph.

The Pressure-relief Valve in the second photo, is also acc. to ASME IV as it should be. Its nameplate carries the ASME IV 'HV' stamp.

From the PRV nameplate, the PRV is set at 125 PSIG and rated for 2,639,000 BTU/h - you should be checking that this is sufficient against the boiler capacity (I don't know what is meant by your stated units of MBH).

The PRV is a 3/4" inlet Apollo Model 1060425 - data sheet attached.[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1710925518/tips/SS_RVW60_SS1032_zdgpcp.pdf[/url]

PRV's are selected on the maximum capacity required to be discharged, not the vessel/boiler outlet size.

Note that there is a separate safety relief valve forum.

Hope this helps with your commissioning.


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[LittleInch]

I can't see anything wrong here.

I suspect MBH is '000 BTU /Hr, so 1.2 million.

https://www.engineering.com/Ask@/qactid/1/qaqid/2586.aspx#:~:text=MBH = thousand BTU's per hour.

the PRV is rated for double that so it looks good.

Just looks to me like the boiler designer just allowed for a bunch of 2" connections and they've used on for a 3/4" PRV. Nothing strange in that.

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