why acetylene pressure reliving fluid should avoid choking

[phoenixmoca]

As AIGA 022/13 CODE OF PRACTICE ACETYLENE says,

6.4.3 Pressure relief devices
Acetylene vented from pressure relief devices should be discharged outside the building to an area specifically classified for acetylene where there is no risk of ignition. All discharge pipes or orifices to the open air shall be designed and made in such a way as to avoid choking, obstruction or frictional pressure drop. Pressure relief device discharge pipes should be separate and connection to a manifold should be avoided.

My question is why and how to avoid the choked flow of PSV relieving fluid?

Thank you in advance.

 

[pierreick]

Hi,
It has to do with mass flow rate @ sonic velocity .
Consider the resource attached.
Pierre

 

[shvet]

Why?
Lee's Loss Prevention 4th ed.
11.11.2. ACETYLENE
It is a flammable and explosively unstable material ... Acetylene undergoes explosive decomposition at any pressure and temperature and even without the presence of oxygen. Acetylene decomposition may be initiated by shock, temperature, or reactive substances. The decomposition may range from a harmless puff of flame to a violent explosion.
Guess - local sonic velocity or sudden changing of flow directions may lead to decomposition.

How?
PSV - no way
piping - oversize diameters to decrease velocity and avoid obstructions e.g. use full bore valves.

 

[pierreick]

Hi ,
To add to the subject , risk of detonation of acetylene with or without air ( see extract attached) .
Pierre

 

[phoenixmoca]

Thank you,Pierre and shvet.
Maybe for the PSV it is impossilbe to avoid acetylene choked flow. But for rupture disc it is possible to prevent acetylene choked flow from occuring with the larger relieving area than the required.
Is it right?

 

[Latexman]

Is it right?

Who do you think we are, The Amazing Kreskin?

What is the sizing pressure, and the pressure the tailpipe exhausts to? And, please, calculate the critical pressure ratio for us.

Cp/Cv for C2H2 = 1.25

Good Luck,
Latexman

 

[Snickster]

In the orifice of the PSV there will be choked sonic flow and you cannot avoid it if the pressure of relief is of any significance. I don't think the code you referred to is talking about the critical pressure in the relief valve but in the discharge pipe. You can calculate the mass flow rate from the relief valve based on set pressure, relief valve orifice size, coefficient of discharge etc. From mass flowrate you can calculate pipe diameter required such that velocity in pipe does not reach sonic at the exit tip.

Basically you can solve by first determining what is sonic velocity for your case VEL = SQRT(gkRT) where VEL is in ft/sec
g= 32.2 ft/sec/sec; k=Cp/Cv; R=1545 universal gas constant; T=(2/(k+1))x(T of process)=sonic flow temperature degrees Rankine

Next determine pressure required at pipe tip to achieve mass flow of PSV using ideal gas equation PV=mRT or P(VEL)(AREA)=(m/sec)RT

P(144)(VEL)(Pipe Area in feet squared)= (mass rate pounds /sec) (1545) (2/(k+1)) (T of process)
where P is in PSIA

If P is higher than 14.7 psia (atm. pressure) then sonic flow must occur at the pipe tip to flow the mass being passed by the relief valve therefore sonic flow exists so increase pipe diameter. If less than 14.7 flow is subsonic and pipe size is OK.

 

[georgeverghese]

To minimise the risk of ignition due to static currents that may be generated by high velocity / frictional effects, would suggest that PSV and discharge piping be electrically grounded.

 

[casflo]

I think also that the reason to avoid choking at the discharge pipe exit, is to make sure that the pipe doesn´t decrease the discharge capacity of the valve.
The choking pressure in the valve, calculated with the equation (1) of the first paper attached by pierreick, considers that the expansion of the acetylene in the valve nozzle is isentropic.
To determine the pipe discharge capacity, is more realistic consider that the expansion along the pipe is intermediate between the isentropic and isenthalpic expansions, calculating the pressure at the pipe exit and the discharge capacity from the valve outlet pressure as back pressure, taking into account the pipe diameter and pipe length.
Note, that depending on the pressures, pipe size and length, it´s possible to have also choking conditions at the pipe exit, without decrease the valve discharge capacity.

 

[georgeverghese]

From what info I can gather on the net about this finicky compound, it looks like one should keep PSV discharge piping as short as possible. This is to minimise the risk of an explosion within the open discharge pipework. The flame velocity for C2H2 is an incredible 2.8m/sec ( see table 27-18 in Perry Chem Engg Handbook 7th edn.), as compared to 0.45m/sec for methane, so you'd need large amounts of N2 as purge medium to keep out accidental flames for the case where you may have long lengths of discharge piping. Hence the PSV should be mounted outdoors close to the point of final atmospheric release. A liquid seal drum would also keep out air and may be an alternative if long lengths of discharge piping cannot be avoided. Seal drum design pressure should be 10barg minimum for acetylene service, from what I read on the net.