2 replies to this topic

[ChemEng01]

Hey guys

 

I've noticed that on FLARENET 7.3 no matter how high you set the flowrate, the pressure at the flare tip inlet is atmospheric pressure. (mach 1 flow still shows atmospheric pressure at tip)

 

If the required velocity through the tailpipe is above mach 1 (of the fluid), there will be a pressure build up such that the volume decreases and the required mass flow can flow through the tailpipe. The pressure calculated at the tip can be used for reaction force calculations.

 

FLARENET 7.2 gave you the pressure at the flare tip when the flow was choked which you could use for reaction force calculations.

 

 

Anyone else having this issue 7.3?

 

cheers

 

[flarenuf]

hello process no 1

 

you are correct in saying that at mach 1 , choked flow teh pressure will be sufficient to allow the mass flow to pass down the line. Flarenet will then claculate the downstream pressure depending on built up backpressure, if any and line size.. This will then show up as a pressure discontinuity. ie high pressure dropping suddenly down to a low presure across a tee piece say.

 

so this fact should point you to look at the tip settings and see that as you have defined the P downstream of the tip as atmospheric

and you are seeing the inlet P as atmospheric then you are seeing no dP across the tip at aall. This is obviously incorrect ... so have you input a 0 k factor for the tip?  or have you input a P vs Flow curve and not activaated it?

 

flarenuf

[ChemEng01]

Thanks flarenuf

 

If I change the fitting loss coefficient basis to static pressure (instead of total pressure) I get a pressure at the tail pipe exit and my built-up backpressure is less.

 

If I set the fitting loss coefficient basis to total pressure I get atmospheric pressure at the tailpipe exit (no matter how much I increase my flow) and my built-up backpressure is more.

 

My tailpipe is a short section (Approx 1.5m) going directly to atmosphere.

 

I just used a fitting loss coefficient of 1.00 for fitting loss.

 

Question is do I use static pressure or total pressure for the  fitting loss coefficient basis?

 

It seems to me using static pressure basis is more realistic because I should have choked flow hence a pressure at my tip from which I can determine a reaction force.

 

However maybe using the total pressure basis for built-up backpressure is better as more conservative.

 

Flarenet help doesn't really give much help on what to use.

 

Really appreciate your help

 

Thanks