×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

control vavle flow rate calculation

control vavle flow rate calculation

control vavle flow rate calculation

(OP)
I am calculating the flow rate through a control valve:

steam condensate,liquid, temperature difference between inlet and outlet is about 40 F.

Cv=127
Sg=0.924
Delta P=23.8

The equation I am using:Q=Cv*Sqrt(DeltaP/Sg),I got some number real big. I am wondering: am I using the right equation for this?


Thanks. Appreciate it.

RE: control vavle flow rate calculation

Units would help as well as the value of the flow you are getting.

The equation you are using is correct for non-flashing/cavitating liquid.  Q is in gpm.  dP is psi. SG is unitless.

What is the Cv you are using?  Is it the installed Cv of the valve or the Cv of the valve at the % it is open?

Is this a single phase non-cavitating liquid?

RE: control vavle flow rate calculation

(OP)
TD2k:

yeah, Q is in gpm. Cv is assumed normal Cv. I think it's Is this a single phase non-cavitating liquid.

RE: control vavle flow rate calculation


Without flashing how do you explain the 40 deg F cooling effect ?

RE: control vavle flow rate calculation

Your problem is clearly flashing. We think you are using a Cv based on the valve travel, but applying as if is only a liquid flow. The valve has to open farther than you think is neccessary because it is handling the flashing liquid.

best wishes,
sshep

RE: control vavle flow rate calculation

Yep, you have the equation for noncritical liquid flow.
As identified earlier, you probably have critical conditions and the real flow deviates from the noncritical flow markedly.  I ran it 3 different ways:

Temp(F)      352     265     60
P1 (PSIG)    125     24.8    100
P2 (PSIG)    101.2    1.0    76.2
Q  (GPM)     457.2   218.8   618

These all meet your criteria of Cv of 127, and DP of 23.8
The first 2 conditions are slightly subcooled condensate flashing through a valve, and the third condition is noncritical cold water.  

I ran this through the Flowserve program.  The Fisher, Masoneilan, and Leslie programs should give similar results.  You really need to state your temp, Inlet pressure, outlet pressure, and either Cv or Flowrate for the programs to calculate properly.  The referenced sizing programs have steam tables built-in to account for critical conditions.  

RE: control vavle flow rate calculation

(OP)
thanks for you guys help.I appreciate it.

JimCasey:

the inlet is at 53.2 pisa,280F and outlet is at 29.4psia 240F. inlet fluid is liquid(water)only. Could you help me to run it at flowserve? what are the equations for this case?

Thanks lot!

RE: control vavle flow rate calculation

At those conditions I get 319.14 GPM, flashing.  

Some of this will depend upon the specific parameters of your valve.  The valve I used was the default unbalanced contoured globe valve.  The flow is choked, and choking is dependent upon several geometric parameters such as Fl.  

The flash is 3.3 percent.  specific volume is 0.482 ft3/lbm downstream, or about a 30:1 expansion over liquid water.

Also, the T2 is 249.17, or 30.83 degrees F cooling.  

I can't paste a document into this forum, or a graphic, and the equations for critical flow of liquids cover 2 pages.  

RE: control vavle flow rate calculation

(OP)
Jim casey:

Thanks so much! I really appraciate it.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources