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# Calculate Air Inflow Rate For Vacuum Break Valve

12 replies to this topic
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### #1 krishna106738

krishna106738

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Posted 18 April 2019 - 12:16 PM

Good Morning Team,

I have a high performance butterfly control valve in a vent line (50 DN)which connects to the vacuum line(150 DN) and  this vacuum line end as  suction line of vacuum pump through which vacuum produced for entire system. I have to size this control valve. Operating pressure of this vacuum line is  0.2-0.44 bar(a).To calculate the inflow rate of air for this valve,I was using a formula

q = 678 * Y * d2 * C * [DP * P1 / (T1 * Sg) ]Â½

where: q = Air Flow, SCFM

Y = Expansion Factor .79 (for vacuum sizing) .85 (for exhaust sizing at 5 psi) .93 (for exhaust sizing at 2 psi)

d = Valve Diameter, in

DP = Delta Pressure, psi The lower of 5 psi or pipe collapse pressure (for vacuum sizing) 2 or 5 psi (for exhaust sizing) P1 = Inlet Pressure, psia 14.7 (for vacuum sizing) 16.7 or 19.7 psia (for exhaust sizing at 2 or 5 psi)

T1 = Inlet Temperature = 520 Rankine

Sg = Specific Gravity = 1 for air

C = Discharge Coefficient = .7 for square edge orifice

from below reference.

http://www.valmatic....gAVs_4-7-15.pdf

Am i proceeding in right direction ? if not please help me out in this topic, how to size this valve i.e.how to calculate inflow rate of air ?

### #2 krishna106738

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Posted 19 April 2019 - 12:20 PM

Hi Team ,

### #3 krishna106738

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Posted 07 May 2019 - 11:23 PM

Good morning folks,

### #4 breizh

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Posted 08 May 2019 - 02:58 AM

Hi ,

To be frank I don't understand your scope . Is it an existing unit or a new project ? If there is a valve installed you should be able to find the data sheet and or purchase order with specification . Did you talk to the I&E engineer within your organization ?

Provide a sketch of the unit to save words and  please clarify the scope .

my view.

Breizh

Edited by breizh, 08 May 2019 - 03:12 AM.

### #5 krishna106738

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Posted 08 May 2019 - 12:15 PM

Hi Briezh,

Thanks for reply. This is for new evaporation unit, in which i have to prepare data sheet for this PCV(Pressure control valve) as shown in the attachment. I am providing sketch in below attachment , please have a look and give suggestion.

Thanks
Krishna

### #6 latexman

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Posted 08 May 2019 - 01:45 PM

It controls the vacuum by letting in "by-pass" air.  First you need the capacity of the vacuum pump while running at the vacuum setpoint (0.22 bar_a?).  I didn't see that in your data.  That sets the max. flow capacity (q) of the PCV.  Then you need to determine d.  Sonic flow will occur (1 bar_a inlet to 0.22 bar_a outlet) thru the valve.  There will be high velocities and it may be noisey.  If noise is a problem in this area, have you thought of a "drag valve" or an orifice in series with the PCV?  My \$0.02

### #7 Art Montemayor

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Posted 08 May 2019 - 01:51 PM

krishna:

Why have you chosen a butterfly type of control valve?  Are you expecting a highly erratic vacuum condition that has to be controlled immediately?   Normally, for this (2") size of control device (and process) I would expect a conventional globe type control valve - possibly a double-port, at most.

Also, where does the valve size equation originate?  Breizh's comments are precisely correct in preparing for a proper specification.  Process engineering should specify the capacity of air required to modulate the partial vacuum required and also state under what characteristic process conditions the required control valve should control.  A butterfly valve is usually much more difficult to maintain under modulating conditions.  It is basically a quick-acting, high-capacity valve.  Is that what is required to control the vacuum in the evaporator steady-state system?  An I&E engineer should be consulted after determining and explaining the operational characteristics of the process.

### #8 breizh

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Posted 08 May 2019 - 07:20 PM

Hi ,

http://www.hassmfg.com/vscform.pl/

A simple calculator to support your work .

As stated above , your work is to determine the  flow rate of vapor to size the pump ( normal , max) , to establish a pump spec .

Talk to the pump's vendor about the management of the vacuum set point and the size , the type of the auto valve .

note : You can download on Internet several apps for sizing  valves , one I like is NorriSize .

Another famous one is Pentair

Good luck.

Breizh

#### Attached Files

Edited by breizh, 09 May 2019 - 03:03 AM.

### #9 krishna106738

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Posted 08 May 2019 - 11:00 PM

Hi Art,

Agree with your point.  It should be a globe Valve only.  My Senior corrected me after discussing with him the same.  Thanks for correcting the concept.

### #10 krishna106738

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Posted 09 May 2019 - 11:45 AM

Thanks Breizh and Latexman for sharing the thoughts.

### #11 Art Montemayor

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Posted 10 May 2019 - 04:43 PM

Krishna:

You have your valve definitions all crossed-up.  You inserted a reference for a "valve" sizing after I requested identification of your equation source and you have submitted something that is totally out of the scope of what your flow diagram designates.

Your submitted reference regards the sizing of a 2-PHASE SEPARATION OF ENTRAINED AIR OR NON-CONDENSABLES.  It has no relationship to a simple, conventional control valve used to allow ambient air into a vacuum system in order to control the level of partial vacuum you desire or need.

You need to refer to the excellent references that Breizh gave you in his post.

Are you familiar with control valves and how they are applied to process controls?  If not let us know and our members - such as Breizh and Latexman - can certainly help you out.

### #12 breizh

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Posted 10 May 2019 - 08:03 PM

hi ,

To support your work , a few document about control valves .

Breizh

### #13 krishna106738

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Posted 11 May 2019 - 02:18 PM

Hi Art,

Thanks for correcting me, i understood now what mistake i made here.I am reading the reference attached by Mr. Breizh about liquid ring vacuum pumps and  it is really useful.I will go through the reference attached by you and Mr. Breizh.

Thanks team for correcting my mistake and sharing the references.