Pressure drop and flow rate calculation, help? 3

[dwyer]

Hi,

I have a question regarding pressure drop and flow rate in a steam line.

I have an existing steam line with a 100 psig at inlet and 12 psig at oulet of a pressure reducing valve. I need to figure out what is the maximum flow fate across a wide open globe valve if the pressure reducing valve is by-passed. The globe valve is 2" and I have the resistance co-efficient for this valve.

How do I calculate the flowrate?


I am trying to answer this question for a friend and the above information is all that is supplied.

Do I have enough information?

Please help if you can.

Thanks,

dwyer

 

[TD2K]

If it's an control valve, I'd check with the vendor's catalogue as they'll have equations in there. If it's a Fisher valve, I can run it using their sizing equations (let me know the valve model as well as the full open Cg or Cv). You'll also need to know whether the inlet steam is saturated or superheated to determine the density.

Another less exact way is to use Crane (I really suggest you get a copy of Crane's TP-410, it's an excellent reference).

Anyway, I'd use the following formula out of Crane for compressible flow:

W = 1891*Y*d1^2*(dP*rho/K)^0.5

W is the flowrate, lb/hr
Y is the expansion factor, you'll need to supply the K factor for the valve for me to give this to you, it's a function of the ratio of the dP to the inlet pressure and the K factor.
d1 is the inside diameter of the piping, inches.
dP is the pressure drop, psi. This is also a high enough dP that I expect you will have chocked flow which is another wrinkle in the calculation (essentially, you need to check for chocked flow and if it occurs, adjust the dP).
rho is the inlet density, lb/ft3
K is the resistance coefficient. If you have the wide open Cv, you can approximate it using Cv = 29.9*d1^2/K^0.5

Post the above information and I'll run both sets of numbers for you. Include your email address and I'll forward a copy of some of the key pages out of Crane that I've scanned.

 

[poetix99]

With a lot more flow going through your steam line, it might also be useful to know if bypassing the reducing valve is going to increase the downstream pressure because you're overwhelming the destination for the steam, or conversely, will it decrease the upstream pressure because you're running down the source.

Calculate the flow (with TD2K's help), then consider the rest of your (friend's) entire system.

 

[Whe]

Yes, you need to consider the effect of the system (at least rule it out as not significant if this is indeed the case). The pressure drop across your globe valve is probably going to be less than 90psi due to an increase in line losses.

Some other questions for you to answer for yourself:
1) Will the flow damage the globe valve? (refer valve supplier)
2) What is the pressure/temperature rating of the downstream piping? Therefore what is the effect of putting superheated, higher pressure steam in the downstream piping?
3) What is the safety valve setpoint of the downstream piping? Will changing the configuration result in non-compliance with legislative requirements for pressure protection? Or a continuous loss of steam?
4) Are there any OH&S noise issues from the valve with the higher flow?

Regards,

 

[dwyer]

TD2K,

Thanks for the information. The following is a copy of the question that was sent to me.

"I am looking for information on a crane 382P valve. It is installed as a bypass around a pressure reducing valve, and I need to know the maximum capacity of steam it could pass. The valve is either 1.5" or 2" (I cannot tell because it is under insulation), so I need to know how much steam each size would pass in each of these two cases: upstream pressure 100psig and downstream pressure 12psig, upstream pressure 125psig and downstream pressure 12psig. Thanks for your help."

The K value for both sizes according to Crane is:
1-1/2" valve = 9.1
2" valve = 8.5

I assume the piping is schedule 40 pipe and that the steam is saturated but I am waiting on a reply for a confirmation.

My e-mail address is dwyercraig@hotmail.com.

Thanks again for your help.

dwyer

 

[comet1000]

I have a question regarding natural gas pipe sizing for 2500 - 5000 psi range.

 

[sailoday28]

What is the question?

 

[Gac1]

Hi dwyer

The simple solution is to go directly to the manufacturer of the Control valve they often have tools or can provide you with the calcs you require to calculate your flow.

If you don't know the manufacturer there are many Control valve manufacturesrs who can help you I.e Cla-Val,Bermad Baker etc

Regards GAC1