1 reply to this topic

[KS2010]

Dear All,

As we know, gases are compressible in nature and thefore their density varies with temperature and pressure.

I want to select Cv value for nitrogen blanket line flow control valve. Basically this line has inlet pressure of about 2barg and nitrogen passes through a flowmeter, 1off non return valve, 2off isolation ball valves, flow control valve [FCV] and then to the reactor. Looking to the pressure drop across all these upstream equipments on FCV, I came up with FCV inlet pressure of about 1.5 barg, 1.7barg and to 1.8 barg for let's say three different flow conditions i.e. maximum , normal and minimum flowrates which are respectively 15m3/hr, 5m3/hr and 1m3/hr. I got these 3off flowrate conditions from client. The reactor is operated at an atmospheric pressure and the length of pipe from FCV is about 5m and therefore pressure drop across pipe is negligible and therefore I have arrived downstream pressure as about 1.49 barg, 1.69 barg and 1.79 barg approximately for above 3off flow conditions.

I have estimated Cv at NTP [i.e. 15 deg C and 1.01325 bar pressure] i.e. nitrogen density of about 1.18 kg/m3. However, I think there are THREE different possibilities to calculate Cv values i.e. at NTP, STP and Actual conditions. COULD SOMEBODY CLARIFY IF MY UNDERSTANDING IS CORRECT?

I have estimated Cv at NTP as I dont know if the above flowrates [which client gave me] are at NTP i.e. Nm3/hr.

However, I think if above flowrates are measured at an actual temperature pressure conditions [i.e. at temperature of about 15 deg C and pressure of 1.5barg, 1.7barg and 1.8barg], Cv values should be calculated based on actual conditions. If I calculate Cv based on actual conditions, I have to take density of nitrogen in the range of 3kg/m3 which give different Cv value than Cv at NTP conditions. Am I correct?

Meanwhile, I confirm from client, what conditions they have measured the above flowrates, any guideline on above will be appreciated.

Thanks,

KS

Edited by KS2010, 18 April 2011 - 04:58 PM.

[daryon]

Hi KS,

You totally right to go back to the client and confirm the flow rate units. If I had to guess I'd say they were reporting Actual m³/h, just because I would typically represent standard (at 15.56°C and 1.01325 bar a) gas flowrate units with STD_m³/h or Sm³/h and flowrates at Normal conditions (0°C and 1.01325 bar a) by N_m³/h. Also direct gas flow measurements unless conpensated for temperature and pressure are reported in acutal units.

You are correct it will affect the valve sizing, if you assume the reported flowrate is alreasy reported at normal or standard conditions you will end up with a smaller required Cv. Converting Standard or Normal conditions to actual gas flow is pretty easy, using the following equation derived from the Ideal gas Law:

Q_act = Q_ref x (T_act/T_ref) x (P_ref/P_act) x (Z_act/Z_ref)

So converting 15 Sm³/h to acutal flow at 1.5 barg and say 40°C gives:

Q_act = 15 Sm³/h x (313.15/288.71) x (1.01325/2.51325) x ( 0.993/0.9994) = 6.6 A_m³/h

or conversly 15 A_m³/h (at 1.5 barg and 40°C) at Standard conditions is:

Q_std = 15 A_m³/h x (288.71/313.15) x (2.51325/1.01325) x (1.00) = 34 S_m³/h

If you calculate the Cv assuming the reported flowrate is at Standard or Normal conditions you will end up with a smaller valve.

When sizing a control valve you can calculate the Cv using a number of different units sets - but volumetric flowrate is always referenced to Standard conditions (1.01325 bar a and 15.56°C), you need to study the ISA flow equations which use a number of different constants depending on which unit set you want to use. Alternativley you can use mass flowrate to size the valve. Have a read through the gas and vapour sizing equations in the Masonelian control valve sizing handbook (attached). I like this guide. Another excellent resource is the Fisher sizing Handbook (can be downloaded), but it's not as concise as the attached.

But remeber that utimiatley it should be the control valve vendor that sizes and selects a sutiable control valve trim and body for the your application.

Hope this helps

Attached Files

•  Maoneilan CV Sizing Handbook.pdf   1018.21KB   134 downloads

Edited by daryon, 18 April 2011 - 09:25 PM.