7 replies to this topic

[TS1979]

Hello friends,

I tried to figure out how to calculate the pressure drop through a flow control valve. This is a steam flow control valve to control 50# steam (treated as saturated steam) flow to a steam heater.

Valve type:         Butterfly

Valve size:           10 inch

Flow characteristics:       linear

Cv of the valve selected:              2930

There are some mistakes on the datasheet such as DP 301 KPa and the outlet pressure 276 KPag. Since this valve is a flow control valve, the downstream pressure cannot keep constant for different flow if the steam pressure is constant.

During normal operation, the steam flow through the valve is about 2 kg/s, which is equivalent to valve open at 20%. My question is what the pressure drop is through the control valve when the valve opening is 20%.

I tried to use the modified Darcy formula

w=1891Yd²( ΔP/K/V₁)^0.5

Where  Y = Net expansion factor

                W=steam flow, lb/hr

                d=piping inner diameter

                K=Valve resistance coefficient

                V₁=specific volume of steam, ft³/lb

                ΔP=pressure drop, psi.

But there are three unknown parameters, Y, K, ΔP,  all related to the valve opening.

In the datasheet, there is a factor “C1” used. What is this factor?

Can anyone help?

Attached Files

•  63FV-18 datasheet.pdf   352.38KB   34 downloads

[latexman]

In Fisher speak, C₁ = C_g/C_v

 

C_g is gas sizing coefficient

 

C_v is liquid sizing coefficient

 

I'd recommend downloading Fisher's valve sizing calculations in the Technical section of their on-line catalogs.

 

Edited by latexman, 19 June 2014 - 02:32 PM.

[TS1979]

Thanks Latexman. I already downloaded the file you mentioned. I try to calculate the pressure drop through the control valve with valve opening about 20%.

[katmar]

The information that you need is the Cv of the valve when it is 20% open. If the characteristic is truly linear then the Cv will be 20% of the maximum, but all control valve manufacturers supply graphs or tables of Cv vs opening for their valves.

 

My estimate is that the pressure drop would be only 8 kPa (1.2 psi) at 2 kg/s and assuming a Cv of 600.  With such a low pressure drop (relative to the upstream pressure) you could safely use the formulas for incompressible fluids (i.e. liquids).  This makes the math much easier and requires less information on the physical properties of the fluid.  You have used the word "about" several times so I guess you are not looking for 4 decimals of accuracy.  For normal plant design, and all the uncertainties that go along with that, I would be happy to go with the simplified method.

[TS1979]

katmar, I appreciate your response .  I got the similar answer. Apparently, this answer does not match the datasheet number - outlet pressure at 276 KPag. I am expecting higher pressure drop when the valve opening is reduced. I will post another query about the condensate return system design. Again, appreciate all your responses.

[Ajay S. Satpute]

TS1979,

 

For 2 kg/s flow and 345 kPag upstream pressure, you expect downstream pressure of 276 kPag. Why not put pressure gages U/S and D/S of FV and get the actual readings? As per Hysys simulation, P2 shall be 335 kPag (10 kPa delta P) for 2 kg/s steam flow (considering FV bypass valve, if any, not passing).

 

The actual normal flow rate is 2 kg/s, however, this value is very less than datasheet reported minimum flow rate (5.5 kg/h). I find it odd.

 

The mass flow rates for 4 cases in datasheet for given Cvs approximately match with Hysys values. E.g. design case flow rate calculated by Hysys is 10.5 kg/s (datasheet value is 10 kg/s), for 62% valve output and Cv of 2047. However, I failed to understand, why the selected Cv is too far away from 2047 (selected Cv is 2930). Can anyone share Fisher's valve size v/s Cv table please? Similar table is attached herewith for Flowserve (Valtek).

 

Regards.

 

Ajay S. Satpute

Attached Files

•  Flowserve Cv tables.pdf   1.18MB   42 downloads

[katmar]

In my experience it is not unusual to see the installed Cv being significantly higher than the calculated Cv, especially with butterfly valves.  The problem is that a typical 10" butterfly valve will have a Cv 60% higher than an 8" valve, which in turn is 80% higher than a 6" valve.  When you choose the next higher Cv available compared with your calculated Cv there can be a large difference.

 

The only way to match the Cv more closely would be to switch to a globe valve with restricted trim, but if the valve size is 10" the globe valve could be 10 or 20 times the price of a butterfly valve.  With modern self tuning controllers and accurate valve positioners butterfly valves can operate very stably right down to 10% (or even less) of full open.

[Neelakantan]

i understand from your other post " http://www.cheresour...-return-system/ "  the said fcv is essentially a TCV; Thus the service is for heat transfer and the pressure inside the condenser-exchanger is driven by heat transfer and the "flow control:" valve will try to reach an opening to maintain the flow and sustain the pressure drop required

 

to simplify, look at the cv of the valve and using the observed delp, find the flow rate of the steam. you will find it matching the heat load of the exchanger!

 

regards

neelakantan

Edited by Neelakantan, 24 June 2014 - 12:40 AM.