VALVE MACH NO FOR OUTLET PRESSEURE -14.7 PSIG 1

[rahipars]

I have problem with a valve. I have service that outlet pressure should be -14.7 psig.Inlet pressure is 84.8 and pressure drop is 99.49 and line size is 0.75" and i can changed it to 1". media is nitrogen ( gas). Flow rate is 25 lb/h.
I run the sizing program. The program result say me the MACH # is 1 and sonic diameter 6.114 and also software say me you should chose larger line size but i can not.
please advised me what shall i do for this application?

 

[bingbingwang]

MACH=1.0, means your valve will work at choked condition. the velocity at valve outlet will be more than 400m/s (depends on the density of fluid), it is just simply too high. working at choked condition means this valve will not change the flow when you try to open/close the valve.
suggest you to call your process engineer, to make the line larger, for process design, the gas velocity in the pipe should be lower than 50m/s in design, and 6-9m/s for liquid......
if you have to use 0.75" line, go check the pressure drop on the pipe line and on the control valve, i pretty sure the pressure drop should be lower than what you have got from your process guy---99.49 psi-d???

Get help and return with helps!!.

 

[rahipars]

Thank you.
You know outlet pressure should be -14.69 psig and in this case inlet pressure is 84.8 psi, my line size is 3/4". So, I would like to control flow woth flow control valve and Flow rate is ( min = 25 ib/hr) and max= 100 lb/hr.
in this case I try to select a valve that it can be keep pressure at - 14.69 psig.
As you see we have to rach to Outlet pressure = -14.7 psig.

 

[bingbingwang]

your process data are wrong,,,,, it is not practical to get -14.69psig=-101.36kpag ,,,,,,which is absolute vacuum?????? no machine can make this!!!! and you donot have the temperature, so is the N2 liquid?

Get help and return with helps!!.

 

[bingbingwang]

i am new on choked flow too, just read a article says you can still control the mass flow when operating at choked flow condition. ...google it!

Get help and return with helps!!.