Teflon valve seat softening caused by gas compression through valves?? 1

[Raspberry]

I have a process gas operating at around 210 deg C in the pipeline. This will then pass into a valve, which has a PTFE lined seat. The flow area in the valve is somewhat less than the full bore of the pipe and so there must be a degree of compression. Now if compression takes places - we must see a temperature rise, and I am already close to the softening point of 230 deg C.

Does anyone think this is a realistic chain of events?

My concern is that my calculated temp increase according to:
T/To = (p/po)^((y-1)/y)
suggests we may see an increase of 50 degrees or more!

Thank you for any help.

 

[Compositepro]

How can you have compression in a valve? We don't need compressors any more?

PTFE is soft long before 230C. That is its sintering temperature where is gets really-really soft.

 

[Raspberry]

'How can you have compression in a valve?'
Well the cross-sectional area inside the valve is less than that of the pipe. So The unit volume must be less. And if the volume is less than the pressure must be correspondingly more.

'We don't need compressors any more?'
This act of compression in the valve is of course the pressure drop, which the upstream compressor is there to overcome.

230C is a temperature quoted by valve manufacturers as a maximum workable temperature for teflon seats.Sintering temperature suggests to me we are talking about a powder? We have formed teflon pieces. Could these be too soft at 230C?

 

[monkeydog]

Take a look at AMS 3294 para 7.2.1

Ask you supplier if they are talking deg F or deg C.

 

[monkeydog]

That is ASTM 3294.

 

[dgallup]

"I have a process gas operating at around 210 deg C in the pipeline. This will then pass into a valve, which has a PTFE lined seat. The flow area in the valve is somewhat less than the full bore of the pipe and so there must be a degree of compression. Now if compression takes places - we must see a temperature rise, and I am already close to the softening point of 230 deg C.

Does anyone think this is a realistic chain of events?"

Absolutely not. As CompositePro said "How can you have compression in a valve." You need to take Thermo 101 over again. If the pressure in the valve is higher than the upstream pressure would not the flow reverse?

 

[Raspberry]

Thank you for the help Monkeydog - the data in the standard has been of great assistance.

dgallup - I may have been confusing you. It may be simpler to consider the problem as a vena contracta. If the fluid entering is compressible (a gas) then it will squeeze its way into the contraction (which is what I meant by the compression). Now in its compressed state it has a higher pressure than it previously had. The gas is flowing and its pressure never comes to steady state (hence it never gets reverse flow) - it almost immediatly expands again back to full bore (probably experiencing JT effect - cooling down.

 

[IRstuff]

"it will squeeze its way into the contraction "

No, it won't. Liquids, compressible or not, do not compress themselves, and certainly, will not exhibit a higher pressure than the inlet pressure without thermal or mechanical energy inputs. Compression requires work, therefore, in a conservative system with no energy inputs, there is no additional compression.

TTFN

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[cloa]

Its a process gas not a liquid

 

[btrueblood]

"The flow area in the valve is somewhat less than the full bore of the pipe and so there must be a degree of compression"...

Please review Bernoulli's equation.

 

[bcd]

Amen btrueblood. Was waiting until someone saw the error in the compression theory. Smaller area means greater flow velocity to fit the same volume. Higher velocity means lower pressure. For gas, reducing pressure means lower temperature. Just the opposite of what was initially stated.

So it is a combination of Bernoulli and Joule-Thompson that defines what the net effect will be.