33 replies to this topic

[wojtar]

 

 

The area will be the actual orifice area.

 

For a Q orifice valve the API effective discharge area is 7129mm2. The ASME actual discharge area is 8742mm2

 

JRudd,

 

If so, as per API 526 there would be no room to increase the size of PSV outlet flange. Then, it might increasing the discharge line size or using the balanced bellows valve be the applicable options...

 

This is what I'm doing. Unfortunatelly those balanced bellows are not acceptable by EndClient who sees their price.

Understand my situation: Some valve Vendor told the Client that he could be cheaper than others because stupid engineer did his work wrong.

According to Vendor balanced bellows could be avoided because backpressure calculated by me is wrong - it is much lower even for longer pipes!

Now I'm wasting my time and yours, trying to explain them that they are wrong.

 

Additionally they say: this medium has a tendency to create polymers, What would happen to our ballanced bellows when PSV opens.

[PaoloPemi]

if the PSV allows a backpressure of about 0.8 bar.g and the line is short I do not see problems to size the line accordingly,

a problem could have been a PSV not properly selected (for example with dout << 200 mm),

in that case the discharge pressure could have been >> 0.8 bar.g due to critical flow limits,

in this case it seems that discharge pressure cannot be reduced below about 0.4 bar.g (for the same reasons)

but I hope that this is an acceptable limit.

Edited by PaoloPemi, 04 December 2013 - 06:47 AM.

[latexman]

A 6Q8 PSV is noted for excessive tailpipe (8") pressure drop issues in our company design manual.  Flow chokes at the end of the tailpipe is common.  A 6R8 has even more issues.

[PaoloPemi]

I agree,

according a rough estimate with PR model (see post #25)

the mach number at PSV outlet is about 0.8 with P = 0.8 bar.g T = 45 C

if we reduce the discharging pressure (and density) the flow

chokes below about 0.4 bar.g indicating possible problems

(of course numbers should be verified with exact process data)

[wojtar]

I agree,

according a rough estimate with PR model (see post #25)

the mach number at PSV outlet is about 0.8 with P = 0.8 bar.g T = 45 C

if we reduce the discharging pressure (and density) the flow

chokes below about 0.4 bar.g indicating possible problems

(of course numbers should be verified with exact process data)

On the other hand:

Let's assume that the flow at the end of discharge piping is critical (I made a simulation where my 1m long pipe is divided into 10 segments -> showing that critical flow is present at the end of piping).

Is that critical flow a problem - it is still higher than required.

Maybe I'm asking stupid questions but I'm just curious and want to learn more.

Edited by wojtar, 04 December 2013 - 10:42 AM.

[PaoloPemi]

if you are close to the limit (speed of sound)  a small variation may introduce a increase in pressure (cocked flow limit),

you can design for mach number about 1.0 (not considering higher noise, vibrations etc.) as far as you have verified that operating pressure range doesn't  limit the specified mass flux.

[wojtar]

if you are close to the limit (speed of sound)  a small variation may introduce a increase in pressure (cocked flow limit),

you can design for mach number about 1.0 (not considering higher noise, vibrations etc.) as far as you have verified that operating pressure range doesn't  limit the specified mass flux.

So from process/valve capacity point of view: critical flow in piping is not a problem if it's still above required flow?

We have still backpressure at PSV flange but if it's still in range of  balanced belows use everything's OK.

The problem is: piping vibrations and noise. Am I right?

[PaoloPemi]

yes, as far as you can allow to increase the pressure to keep in account the speed of sound limit,

(for compressible flow, for incompressible flow behaviour is different)

for a PSV you have also to consider that Pin / Pout should be > about 2

(the exact value depending from fluid characterstics and conditions),

not considering here mechanical aspects (influence on set point etc.),

finally, if you have been close to a large PSV when it opens (by chance I did that)

you have an idea of the noise which produces :-))

Edited by PaoloPemi, 04 December 2013 - 12:22 PM.

[latexman]

and thrust.