PSV Outlet Flange Size

[steveb1111]

When we run several of our PSVs in Flarenet we get mach one at the PSV outlet flanges. Does this mean these flanges are undersized? In some cases we need to go up two sizes to avoid sonic.

 

[maddocks]

Doesn't the fact that the valve is in critical flow mean that it will be sonic?

 

[gerhardl]

Aare you indicating that you need a larger size pipeline after the valve (or larger outlet to a large, common, flareline), by selecting a larger size reduction flange stepping up from the outlet flange of the given PSV?

Now, in any case: A PSV, or the pipeline after the PSV, is undersized under two circumstances only:

1. If the pressure is higher than the design pressure for the valve, or more precise higher than the detailed design for the single parts at that particular pressuriced point, for instance for the outlet flange or pipe after the outlet flange.

As sonic velocity is just that: a velocity, not a pressure, you cant tell. As maddocs answer indicates, sonic velocity will occur at critical flow. Simplified: If P2 is less than or equal to P1/2.

2. If the capacity of the valve is not large enough to evacuate the pressure building up inside the vessel or pipeline.

Again: see point one.


However: sonic velocity could indicate that you should look both at the necessary capacity and the noice level of the valve if you do not have precise information on how the PSVs originally were sized and what they at present are reqiured to do.

You should also have a look at the internal built up pressures under release situation in your piping system.

Listening to sonic velocity or not is not the traditionol way of sizing a PSVs system, and not the safest.... ;-)

In addition: If several valves are interconnected to take part of the total load and only some obtain sonic velocity (others not) you should also have a look at dimensions and pressure balance in the system both for PSVs and pipelines.

 

[steveb1111]

My concern is that we get sonic at the outlet flange at lower than psv design throughputs. Does this suggest that the psv capacity may be affected?

 

[JoeWong88]

steveb1111,

I guess you may have interest to read "Few Concerns & Recommendations of PSV Discharge Tail pipe".

You may provide the fundamental data so that we can look into the details...



JoeWong
Chemical & Process Technology

 

[steveb1111]

PSV has a 6-inch outlet. We have input the PSV data into our Flarenet model with following result;

Flowrate = 308,136 kg/h
PSV backpressure = 14 barg.
PSV 6-inch outlet flange Mach No. = 1

If we increase to 8-inch outlet;
Flowrate = 308,136 kg/h
PSV backpressure = 14 barg.
Mach No at 8-inch outlet flange Mach No. = 0.99

We face similar issue on several other PSVs whereby we have to increase the vendor outlet flange size to avoid sonic.

 

[JoeWong88]

steveb1111,
Do you consider to split the flow and use maximum flange size ?

Choking at the tail pipe is a concern (to me) but has been left out by many designers. Whenever a section downstream of PSV is at choking flow condition, there is no additional flow can pass through this section unless there is a mean to change the properties of the the fluid. This will end-up this section is limiting the flow rather than PSV which is not the designer intended for.

JoeWong
Chemical & Process Technology

 

[steveb1111]

JoeWong88 thanks for your comments. Our current thinking is that the pressure downstream of the psv nozzle will incease. Provided the increased pressure is below the PSV critical flow pressure, relief capacity will not unaffected.

 

[JoeWong88]

Back pressure of 14 barg already very high. Further increased back pressure may helps in lesser degree.

Do you have high back pressure issue ? Is your PSV balance bellow type ?

JoeWong
Chemical & Process Technology

 

[steveb1111]

The PSV is a modulating pilot which can accept considerably higher backpressures than 14 barg. I think our initial concern has been laid to rest.