Enlarger before PSV? 1

[foxymophandlpapa]

ASME states that you should not use an enlarger before a PSV on a vessel. Can someone give me an explanation to why this is?

 

[dcasto]

pressure drop through a sudden expansion.

 

[foxymophandlpapa]

You'll have to convince me of that because I can't believe there will be a any kind of a pressure drop in a pipe going from 3" to 4" when you're coming from a 3000 gallon vessel.

 

[dcasto]

If need a 4" inlet valve, then do the pressure drop on the reduced 3" port and add the pressure drop through the expansion back to 4". It may be greater than 3%.

 

[foxymophandlpapa]

I think we may be confusing each other. I have a vessel with a 3" nozzle but have sized my PSV as a 4" x 6". I want to know why I cannot stick a 4" by 3" enlarger inbetween my PSV and vessel nozzle.

 

[dcasto]

What capacity do you need? What is the sonic flow through a 3" orifice? If sonic flow is less than capacity, then your 4" relief will not flow at capacity.

 

[foxymophandlpapa]

What I'm searching for is the reason why ASME states that the connection to the inlet size of a PSV must be at least equal to the inlet size of the PSV.

I currently have a 3" and need to upsize to a 4". The 4" is not a limiting factor. I'm trying to figure out why ASME will not allow using enlargers inbetween the vessel and PSV.

 

[insult2injury]

It's poor engineering practice. Is your SRV orifice area less than the 3" pipe area?

I2I

 

[Latexman]

The reason you cannot use an enlarger before a PSV is ASME Code requires the nominal inlet pipe size to be equal or greater than the nominal inlet flange size of the PSV (UG-135-b(1)). If you need an enlarger right before the PSV to physically connect it, then you do not comply with this mandatory requirement.

Good luck,
Latexman

 

[pleckner]

@foxymophandlpapa:

As @Latexman wrote, it is the Code (ASME) and if your location has adapted ASME as Code then the answer is simply "you have to follow what is written". As to why, you can send a request to ASME as to the explanation for this. Perhaps someone on the ASME committee will write in with the answer. I would think that if the line is only 3" but you need a 4" inlet to the PSV then @dcasto probably has it correct that your 3" line will be a constriction to the required relieving rate. It may also not meet the 3% Rule. Before discounting the pressure drop, perhaps you should go through the calculation.

 

[foxymophandlpapa]

We are well below the 3%, but because of standard PSV sizes we would need a change in piping.

I understand what the code states, thats my reason for posting. I'll see if I can contact ASME and find a solid reason. Thanks for the responses.

 

[pleckner]

@foxymophandlpapa:

Aplogy in advance if this is an obvious question to you but when you calculated the pressure drop for the 3% Rule, did you use the stamped capacity of the PSV as you are required to do so? Perhaps you can feed us some details.

 

[djack77494]

Don't forget that the entrance loss will be based on accelerating your fluid to its velocity in a 3" rather than a 4" pipe. With a pretty close coupled PSV, your pressure recovery will probably be minimal.
Doug

 

[pleckner]

Just to throw what may be a curve ball, if the expansion from 3" to 4" results in a pressure recover (as it should) AND the 3% Rule ONLY requires non-recoverable losses to be evaluated, THEN, we don't even need to consider the addition of the expander.

 

[Latexman]

Just to throw what may be a curve ball, if the expansion from 3" to 4" results in a pressure recover (as it should) AND the 3% Rule ONLY requires non-recoverable losses to be evaluated, THEN, we don't even need to consider the addition of the expander.

I'm not so sure we can do that. If you look at Bernoulli’s theorem and strike out the elevation terms (because we always do that with gas/vapor flow) and strike out the velocity terms (because that’s where the pressure recovery comes from), we are left with something like:

144P₁/[ρ]₁ - 144P₂/[ρ]₂ = h_L

It looks to me we can only ignore an expander when recoverable and non-recoverable losses are to be evaluated, or am I missing something.


Good luck,
Latexman

 

[pleckner]

I was thinking about it after I wrote that and realized that since we don't take the pressure recovery into account then the frictional loss part of the expander still needs to be included.

 

[zerosum]

Could it be that diverging flow is subject to separation and vortexing and that is not something we want going into an orifice.

 

[Latexman]

There is no orifice. This is going into a PSV which is best modeled as a converging flow nozzle.

Good luck,
Latexman

 

[zerosum]

That may be Latexman, but ASME calls it an orifice.

 

[pleckner]

And calling a PSV an orifice was be best thing ASME could ever have done to create massive confusion! We've had this discussion many times before in this (and other) Forum(s) and a PSV IS NOT AN ORIFICE but is in actuality a nozzle. There is no "hole" to speak of. It is (as Latexman stated) a converging nozzle with a short but definite straight flow path before exiting into an open cavity if you will (the pressure relief valve's body).