Relief piping iced up

[NH3ISFORME]

I had heard that if a relief valve were to "relieve" the piping from the identified valve would ice up. I am relatively new to NH3 so maybe someone would mind breaking this down for me a little. On temp/pressure controlled vessels i could see how that is possible liquid flashing off during a release, but is that also true for a screw compressor barrel if the relief(s) gave on the discharge pressure side how would superheated vapor escaping through a relief cause the piping to ice up?

I swear I won't post this anywhere else...my bad.:confused:

[HallsEngineer]

There is an expansion of gas as the valve exits the relief valve at high pressure. The energy for this expansion must come from some where so it takes it from the environment ie the pipe. Liquid is just more efficient at expanding because it uses latent and sensible energy to cool itself to the required temp thats why the fridge system works.
When you pressure test with nitro your reg sweats or even freezes and thats superheated about 120'c

[glenn1340]

If you`re talking about the inside of the relief pipe freezing up (and possibly blocking) it wouldn`t happen as it would require moisture in the discharging refrigerant for it to do so.
It`s a big problem we get on air compressors: the expanding air on vavle exhausts freezing up and causing blockages. Most places fit now fridge or, for really dry air dessicant, dryers

[zolimer]

I will give this a shot! I have never seen a pressure relief pop on the discharge side of a compressor, but in my plant we do have some lower temp flooded evaporators fed by surge drums! Imagine that you have a solenoid valve that is fried! After a while your liquid and suction line will lose they're frost! Even if you had a uninsulated pipe with an isolation valve closed, you would see no frost because there is no pressure drop across that valve! Your refrigerant is dead heading against that closed valve!

In order to get frost, there has to be a drop in pressure!
This could be possible if there were a piece of slag on the seat or something. The resulting pressure drop could bring your system down to condensing pressure, therefore
ice up! This is my theory! Old schoolers does this make sense?

[717NH3]

NH3,
What kind of compressor do you have? Is it high stage / low stage? I’ve seen relief valves leak by & not have any frost for days or until the leaking of the valve becomes enough to condense to liquid & even then you might see frost in a low spot of the header. If the relief valve was on the thermosypon oil cooler it may leak only oil into your header depending on what side of the heat exchange. If the valve is leaking by only a small amount you may not see evidence of frost at all.

[NH3ISFORME]

Sorry all I wasn't clear enough. This is a single stage system running at 35# suction and 125# discharge Frick RWB's. The relief piping I refer to does not have a possibility of moisture inside of them. I was just trying to better understand the nature of ammonia in a relief relieving. I guess I don't understand how a relief pipe can frost up on a compressor at the end of the barrel where we have dual reliefs next to the coalenscing manway. High pressure high temperature discharge vapor at 125# PSIG is a temperature of 75deg F. When this vapor were to relieve at 300# (which is the relief valve pressure to relief) the temperature at 300 PSIG would be over 120 deg F. How does vapor of this pressure/temperature going through relief piping cause frost to buildup on a relief pipe at 120 degrees?

These thoughts are reflective of information I have heard at work not my experiences. I am poting this statement here to verify the validity of this information.
:D

[717NH3]

The pressure on the inlet side of the releif valve may be 125psi & the releif valve is set to 300psi. The releif header goes to atmosphere (I assume). The header is at 0psi always. If gas goes from 125 psi @ 75*F to 0psi the releif valve leaking by acts as an expansion or metering device allowign the vapor become 0psi refrigerant, 0psi equals -26*F. A frost patern may not always be present depending on the rate of gas leaking by a releif valve.

[HallsEngineer]

thats what i said

[IceMan_4000]

Yup what Hallsengineer said.

Huge pressure change also = huge temp change.

[simplygold]

Releif piping can also ice up if the vessel has become flooded beyond capacity. Especially prevalent in our Low Pressure Receiver! (Feed solenoid malfunction)

[gwapa]

In NH3 I don't see how a relif valve can frost inside in an oppenig.

In CO2 it is a problem due to the fact the CO2 becomes dry ice when the liquid low the pressure under tripple point