Previous in Forum: Are These Displays Still Made?   Next in Forum: Using I.S Barriers With a Non-I.S Sensor?
Close
Close
Close
15 comments
Commentator

Join Date: Sep 2009
Location: Adjacent to the brick fields in Beds
Posts: 57

Differential Pressure For Orifice Plates

05/29/2018 6:10 AM

I occasionally do o/p calculations. I have several programs to achieve these results. One of the standard items of given data is differential pressure to establish the o/p bore. Can anyone tell me how this dp is calculated? I know it is based on Bernoulli's equation, but, despite the information available via the internet, I have yet to find a simple and easily understood explanation. Can anyone help me please?

Login to Reply
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29966
Good Answers: 809
#1

Re: Differential Pressure For Orifice Plates

05/29/2018 8:15 AM
  • Perry: "The Chemical Engineer's Handbook", any edition.
  • "Kempe's Engineer's Yearbook", any edition.
  • Etc.
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Commentator

Join Date: Sep 2009
Location: Adjacent to the brick fields in Beds
Posts: 57
#3
In reply to #1

Re: Differential Pressure For Orifice Plates

05/29/2018 8:46 AM

I have a copy of Perry and Miller's Flow handbook. Alas, both have confused me. I'm just looking for a VERY simple explanation.

Ian Purdie

Login to Reply
Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 4281
Good Answers: 128
#5
In reply to #3

Re: Differential Pressure For Orifice Plates

05/29/2018 3:50 PM

Assuming it's liquid flow

Volume flow q = C*Ao*√(2*(p1 – p2)/ρ)(1 – β4)

where

C = coefficient of discharge

Ao = orifice area

p1 = upstream pressure, p2 = downstream pressure

ρ = fluid density

β = orifice dia/pipe dia.

all in SI or other consistent units of course.

C is usually taken as 0.62

p1 at tapping 1 dia upstream, p2 = at ½ dia downstream pressure

The velocity-of-approach factor (1 – β4) is needed because the fluid is already moving before it reaches the orifice, but if the orifice is small relative to the pipe dia it is close to 1.

That should get you started, but there are various other possible locations for the pressure tappings, and that, along with the Reynolds number, can affect the coefficient of discharge. It’s many years since I looked into this in any detail, but if you need something more precise I suggest you study Perry or other reference work

__________________
Give masochists a fair crack of the whip
Login to Reply
Commentator

Join Date: Sep 2009
Location: Adjacent to the brick fields in Beds
Posts: 57
#2

Re: Differential Pressure For Orifice Plates

05/29/2018 8:43 AM

Thank you for the info. I have Perry as well as Miller's Handbook which have confused me somewhat. I'm looking for a VERY simple explanation.

Ian Purdie

Login to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29966
Good Answers: 809
#4
In reply to #2

Re: Differential Pressure For Orifice Plates

05/29/2018 10:31 AM

'Differential pressure across an orifice plate' is a very 1950s technique for measuring flow. Modern instrumentation uses different techniques and can give an easily-scalable output. For water other than purified, consider using electromagnetic flow-metering instead.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply Score 2 for Off Topic
Guru

Join Date: Jul 2010
Posts: 664
Good Answers: 175
#14
In reply to #4

Re: Differential Pressure For Orifice Plates

11/21/2018 6:59 PM

1950's ? ?

Orifice flow measurement is 1920's technology.

published 1921, or

Login to Reply
Commentator

Join Date: Sep 2009
Location: Adjacent to the brick fields in Beds
Posts: 57
#15
In reply to #14

Re: Differential Pressure For Orifice Plates

11/22/2018 4:48 AM

Thank you for the information. I think it warrants some investigations.

Ian Purdie

Login to Reply
Guru

Join Date: Apr 2010
Location: About 4000 miles from the center of the earth (+/-100 mi)
Posts: 8762
Good Answers: 999
#6
In reply to #2

Re: Differential Pressure For Orifice Plates

05/29/2018 10:09 PM

Here's a stab at "simple" (if that's possible)

There's a derivation of the formula here. It looks formidable, but if you look at it one step at a time, it isn't too bad.

The derivation depends on two principles:

  • Conservation of energy: a parcel of liquid has kinetic energy 1/2 ρV2 . For example, when it moves to a region of higher pressure, it takes work and it slows down, energy comes from the kinetic energy (velocity). The relation is between velocity and pressure.
  • Continuity: The incompressible volume flow rate is constant so if the cross-section reduces, the velocity increases and vice versa. The relation is between velocity and cross-sectional area.

Putting these together, you get the relationship of flow velocity, pressure difference and ratio of cross-section diameters.

https://en.wikipedia.org/wiki/Orifice_plate

Login to Reply Score 1 for Good Answer
Commentator

Join Date: Sep 2009
Location: Adjacent to the brick fields in Beds
Posts: 57
#8
In reply to #6

Re: Differential Pressure For Orifice Plates

05/30/2018 8:20 AM

I'm hoping that this will be copied to all respondents

Thank you to all who sent in answers to my recent inquiry regarding the ∆p for o/ps. Unfortunately, I did not explain my question correctly, for which I do apologise.

The given data for o/p calcs will normally comprise the pipe size, flow rate [volumetric or mass] ab pressure, temperature, density, viscosity and sometimes a specific heat ratio [k] for gases, and of course ∆p. OK, I know from past experience that not all the data is given, but often it can be found or calculated. That will enable me to calculate the orifice bore, but my question is how is the ∆p arrived at?

Login to Reply
Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 4281
Good Answers: 128
#9
In reply to #8

Re: Differential Pressure For Orifice Plates

05/30/2018 9:55 AM

I think we all assumed (me included) that you want to measure flow, but reading your post again you didn't say that.

Either the ΔP is given, eg because your system design requires it, or the other parameters, flow, pipe and orifice dia etc are given and you need to calculate the ΔP. In either case the equation holds (once you've decided which one - appropriate coefficient of discharge, tapping location if it's for flow measurement, nature of fluid etc). Just need to rearrange it for what you're looking for. If everything else is fixed and you want orifice dia, which affects β, it can be solved explicitly (if I remember right) but could be easier to do it by trial.

Also if you're looking for loss for the overall system you might, (depending on the layout) need fully recovered loss. The information is in Perry and the Wiki link.

__________________
Give masochists a fair crack of the whip
Login to Reply
Guru

Join Date: May 2016
Posts: 1746
Good Answers: 87
#10
In reply to #8

Re: Differential Pressure For Orifice Plates

05/30/2018 11:26 AM

Not sure if this answers your question, but in test arrangements I have set up in the past, you use orifice flow calculations (https://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm) to generally size the orifice, but when things are all said and done, the one thing you can't measure ahead of time is the actual orifice coefficient to plug in. Regardless of the geometry there are a lot of small things that affect the final value.

In the end, you need to calibrate the orifice installed in the system for a given volumetric or mass flow rate to establish the final dP. In one installation, the orifice and section of pipe it was installed in was sent to a test lab for calibration where they put a known, weighed, volume of water through the orifice over a measured period of time. The dP readings were recorded and placed on a calibration chart and lookup table.

Two other times when measuring air flow rates, I constructed bags of known volume to inflate over a timed period and the dPs recorded for the calibration chart and lookup table. When constructing a test setup to calibrate an orifice, upstream and downstream conditions can drastically effect the orifice performance and you need to replicate the system for roughly 20-30 pipe diameters upstream and downstream of the orifice.

Login to Reply
Guru

Join Date: Apr 2010
Location: About 4000 miles from the center of the earth (+/-100 mi)
Posts: 8762
Good Answers: 999
#11
In reply to #8

Re: Differential Pressure For Orifice Plates

05/30/2018 11:30 AM

If you're working with gases, it gets a bit more complicated. If a fluid is compressible, then ρ varies with pressure so mass flow (which is constant) and volume flow are no longer proportional. The formulas have to be modified for compressible flow and this is described in the link.

https://en.wikipedia.org/wiki/Orifice_plate

Login to Reply
Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 4281
Good Answers: 128
#12
In reply to #8

Re: Differential Pressure For Orifice Plates

05/31/2018 6:05 PM

I'm still not clear what your problem is. If you want to find Δp and have all the other inputs it's quite straightforward, whether it's just the orifice plate Δp or fully recovered Δp.

If you want to find orifice dia and have the other inputs it's a bit more complicated, but if you're doing it from scratch you could eg use the Find function in Excel.

If you have a program (in your original post you say you have several) with the inputs you list (more than the basic flow, pipe dia etc) that will do it for you and presumably include a calculation for discharge coefficient, so should give a more accurate figure.

__________________
Give masochists a fair crack of the whip
Login to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29966
Good Answers: 809
#7

Re: Differential Pressure For Orifice Plates

05/30/2018 3:10 AM
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Guru

Join Date: Oct 2006
Location: 44.56024"N 15.307971E
Posts: 6111
Good Answers: 240
#13

Re: Differential Pressure For Orifice Plates

11/20/2018 12:41 PM

I have a Taylor Sybron manual around here somewhere that explains it very well,I will see if I can find it.Been around 30+ years since I have needed it.

This may not be the answer you seek,but the differential pressure is the square root of the flow i.g.: 50 percent DP = 70.07..% flow.

__________________
"A man never stands so tall as when he stoops to help a child." "Ignorance does not consist of what a person does not know,but of things he knows that just aint so"
Login to Reply
Login to Reply 15 comments
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Codemaster (3); HiTekRedNek (1); Ian Purdie (4); Iris (1); Jpfalt (1); PWSlack (3); Rixter (2)

Previous in Forum: Are These Displays Still Made?   Next in Forum: Using I.S Barriers With a Non-I.S Sensor?

Advertisement