×
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 valve sizing question

control valve sizing question

control valve sizing question

(OP)
I was wondering what the best practice is for this...

If I have a pump distributing a liquid reagent, I can determine P1 (upstream) via the pump curve.  However, what does one normally do for P2?

I was recently talking to a vendor about a couple valve and had only the upstream pressure.  One valve was essentially discharging directly into an open tank, one had a couple hundred feet of pipe with about 30 feat of static head.  

I provided max/min/normal flow, temperature, SG, pump discharge pressure, pressure estimate at upstream face of valve (unchoked), line size.... I was asked for downstream pressure.  Do you suppose he meant final discharge pressure on the line?  I was at a bit of a loss here.  It is the 1st time i have ever really had issues sizing out a valve.

Thanks

RE: control valve sizing question

Assume the vender need the design pressure drop accross the valve.
usually, pressure drop will be at 1 to 100 psi,  when for liquid service.  if the pump deliver more then 100 psi,  and the liquid is flowing into an atmospheric tank (no backpressure,  you might need to add an Orifice to take part of the pressure, limit the flow and keep the pressure at the line when the valve is open.
    When you have few valves in paralel,  you need to check your system and decude what pressure drop is required for each valve, taking into consideration what will happned to the system (pump) when each (or all) the valves are fully open.
ilan
 

RE: control valve sizing question

(OP)
Ahh, I see...he is worried about choked flow causing critical pressure due to the increased velocity via the fixed u/s pressure and a the potential for a higher dp at points.  This reagent must have a lower critical pressure or something....I dont really ever run into the issue, but it makes sense.  Thanks a bunch.

RE: control valve sizing question

Trying to answer your original questions and assuming:

P1: pressure at inlet flange of the valve
P2: pressure at outlet flange of the valve

Then knowing:

Pup: pump discharge´s pressure
Pdown: destination pressure (tank)
DPup: pressure drop between the pump´s discharge and the valve (static and dynamic)
DPdown: pressure drop between the valve and the destination (static and dynamic)

P1 = Pup - DPup
P2 = Pdown + DPdown

And, of course:

DP = P1 - P2 = Pup - Pdown - (DPup + DPdown)
DP: pressure drop across the valve

P.S: DP is also equal to the difference between the pump curve and the system curve.

Having said that, the vendor normally only requires either P1 and P2, or P1 and DP.  

"We don't believe things because they are true, things are true because we believe them."

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