Pressurising Bypass Calculation

[juliefalce]

Does anyone know how to perform a pressurising bypass calculation?

I am wanting to pressurise a gas system from atmospheric pressure to 180barg (initial temperature is -20degC) via a 2" bypass line around an ESV. The main line is carbon steel.

Initial calculations have shown that as soon as I open the bypass valve, the temp will drop (ie due to JT effect) to about -100degC so stainless steel is required for the bypass.

I'm wanting to calculate the length of stainless steel required (ie before the metal "warms" back up to about -27degC which is the limit for carbon steel).

I've calculated this, but there are lots of different factors to consider which affects the result greatly.

Has anyone performed a calculation like this? Has anyone got an industry-standard method to do this?

Any help would be appreciated as it's beginning to use up lots of man-hours!

Thanks

 

[dcasto]

Well, I've only gone from 1800 psig and -80C. Not much difference.

The answer is it depends on rate, but then again it doesn't.

Heres what happens in real live. As the line pressures up, the temperature on the line rises, it doesn't stay at -100C, the temperature goes up. Theroretically, if your pressureized the line in 2 seconds, the steel would only see the -20C.

The same about the vent lines on a PSV. On liquid ethylene at 2280 psi, out PSV vents were carbon steel at -121 F.

So, I'd tend toward just carbon steel, but I'd need to look at the whole system and the rates.

 

[juliefalce]

Thanks, that helps me.

So, if I have a flow of about 46000kg/h starting at -20degC, 180bara passing through a 2" line for a repressurising time of about 3minutes, would this be ok still?

I'm going to propose that we put a low temp alarm on the line too just in case the repressuring time is too long and the temp drops (then the operator can stop and wait a couple of hours if the temperature of the pipe gets too low).

Thanks!

 

[dcasto]

I'd recommend about 50 psi/min or 3 bar/min. The biggest thing is to watch the pipe stress from contraction of the lines during chilling.