8 replies to this topic

[atiq124]

Inlet Nitrogen pressure to the regulator is 6.4 barg and outlet is adjusted to 1.4 barg to get around 20 psig in the plant. On the graph, looking at 3rd line from bottom to check the flow rate (m3/h), it does not intersect at 1.4 barg pressure. So, how do I know what is the flow rate of N2 through this pressure regulator when I know the inlet and outlet pressure across it ?

Thanks

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Edited by atiq124, 11 March 2015 - 11:03 AM.

[Pilesar]

The circled area on your graph represents the phenomenon called "choked flow" where there is no increase in flow rate even when the downstream pressure is reduced further. As a rule of thumb, this point is generally reached when the downstream pressure is about half of the upstream pressure.

[fallah]

atiq124,

 

No need the flow curve intersects the constant pressure line of 1.4 barg to obtain the flow rate of N2 in your case...

 

Actually as the critical pressure ratio of N2 is 0.55, then the downstream pressure at or below which there are chocked flow condition is 7.4 (bara)*0.55=4.07 (bara or 3.07 barg); then in your case because the pressure is below the critical pressure, chocked flow would ocuur and based on the graph you provided the flow rate of N2 would be around 212 dm3/s...

[atiq124]

Thanks a lot for your guidance.

What happens if the pressure is above the critical pressure, will the flow still chock and how would it look on the graph ?

[fallah]

atiq124.

 

If downstream pressure is above the critical pressure, flow will not be choked and the corresponding flow of the pressure will be readable on the graph... 

[breizh]

http://www.mankenber...D/pdf/s33_1.pdf

 

This may help you and others 

 

Breizh

[atiq124]

Hi Fallah, you have already given me the answer but trying to understand a bit more. We know that the critical pressure of Nitrogen is 34 bar and below we have calculated critical pressure of Nitrogen as 3.52 bar, what is the difference between these two critical pressure ?

 

 

Pc / Pi = ( 2 / (k + 1))^(k/(k-1))

where

Pc =  critical pressure

Pi =  inlet pressure = 6.4 barg

k  = specific heat capacity ratio

k  = Cp/Cv (Cp= heat capacity at constant pressure, Cv= heat capacity at constant volume)

 

Cp= 0.873 kJ/kg K (www.apithailand.com)

Cv= 0.684 kJ/kg K (www.apithailand.com)

k = 1.2763

 

Pc/Pi = (2/2.2763) ^ (1.2763/0.2763)

Pc/Pi = 0.55  (Critical Pressure Ratio)

Pc = 0.55 x 6.4 = 3.52 bara

Pc = 3.52 +1.01 = 4.53 barg 

[fallah]

atiq124,

 

First i have to correct the value of Pc you calculated in barg. In fact it would be 2.53 barg rather than 4.53 barg....

 

The critical pressure of N2 of 34 bar you mentioned is a thermodynamic properties of N2 and indicates the end point of phase equilibrium (critical point) for N2 under which liquid N2 and its vapor can coexist. The critical flow pressure, on the other hand, indicates the absolute pressure at nozzle exit at critical flow rate passing through that nozzle which corresponds to the nozzle inlet pressure ..

Edited by fallah, 16 March 2015 - 06:51 AM.

[ahmadikh]

 atiq124,

 

Please follow exactly what has been clearly explained by Mr. Fallah. The critical pressure downstream of a hole is what  has been clearly clarified above.

 

In the case of a control valve, the ratio of the critical pressure to the upstream pressure also depends on the type of the valve which defines the pressure profile in the vena contracta. In the method recently used to size control valves, AKA "Standardization Method", which has replaced the traditional ones, it is specified that for compressible fluids such as nitrogen in your case, the flow chokes across the control valve if: 

 

X > (F_p) * (X_T) , where X = (P_upstream - P_downstream) / (Absolute P_upstream),   F_p = (Cp/Cv)/1.4,  and X_T is the based on the type of the valve

 

For more information, refer to ISA-750101.

 

Best,

Milad