Multi steps gas pressure regulation, compensate temperature drop

[pyMorty]

Hi,

In gas compression (with reciprocant compressors & combustion engines) sometimes we have no option but taking fuel gas from the discharge line. Given the most common escenario we usually encounter there is a gas feed at 70 kg/cm2, whereas our fuel stream pressure needs to be around 7 kg/cm2. Considering a discharge gas temperature of 40 deg C, this will imply, in a single step pressure reduction, that our fuel gas supply would be around 8.5 deg (using a rule of thumb of 0.5 deg C reduction for every kg/cm2 reduced).

Leeting alone possible choked flow escenarios, and the above implying we need a multi step pressure reduction (two steps) Is there any general recommendation for distance between valves in order for the gas to recover temperature before entering the following regulator valve? (we generally work with 35 deg C ambient).


Thank you in advance.

 

[MortenA]

You cant recover temperature drop from JT effect by using multiple stages or any other way. However, if you are considering "re-heating" of the gas from ambient air, usually bare pipe dosnt loose that much heat and you would need a very long piece of pipe (fairly easy to calculate how long but thats not a freebee for me anyway). You would need a HX/FG heater imo. BTW your final temperature will be different if your gas starts to drop condensate (higher). Not know your gas composition i would check this before going any further.

Best regards, Morten

 

[LittleInch]

I agree with Morten.

There is a proper way to do this - install an electric heater UPstream your first regulator to heat the gas to about 60 to 70 C, then let down.

Or you could run a few loops of pipe ( say 5m in total) downstream the hot air blast from your radiator on your engine or even take a side stream off the radiator water into a HX and run the gas through that.

Initial gas to run the engine might get a bit cold, but once the engine has warmed up away you go.

Just straight pipe in still air you would need about 100m.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pyMorty]

Great, thank you MortenA and LittleInch. Crystal clear.

 

[MortenA]

Like the ideas presented by Little inch.Only downside that i see is that untill your engine is running you will have no heating.

Best regards, Morten

 

[pyMorty]

Guys last question on this, just to confirm so what happened in the company I work for, installing two pressure regulators to reduce pressure from 70 kg/cm2 to 7 kg/cm2 was just a waste of money, since the temperature drop will be exactly the same as done with just one valve (with proper Cv), wasn't it?

 

[LittleInch]

Well a waste of money if you wanted to heat the gas , but sometimes needed to get two valves of a smaller cost.

Going from 70 down to 7 can be quite noisy and expensive.

There may be a small relaxation in temperature between the two, but not enough to make any appreciable difference unless you wrap the interconnecting pipe in electric trace heating. Even then the amount of heat you can get in is quite low - It just might stop the pipe freezing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pyMorty]

Thank you LittleInch.

 

[MortenA]

Agree with Littleinch, less noisy. Heat inter-stage is the only remedy.