Orifice plate liquid flow calculation 1

[sirdle]

I am a PLC programmer, not an engineer, so any help will be appreciated.

The hot oil loop in our plant uses an orifice plate to create a pressure drop. This drop is measured with a dP transmitter and sent to a PLC where the flow rate in gpm is calculated. The orifice plate has been changed and I need to modify the calculation to yield the new flow rate.

Can anyone supply a simple formula?

The existing formula is: gpm = 89.5 * SQRT (dP). This seems overly simplistic.

I know the differential pressure (inches of water), the specific gravity, the line size, and the bore size. I can get the temperature and pressure if I need to.

Thank You!

 

[d23]

sirdle

I found this formula at home. I’m not sure where I got it, but:

Q = (D+0.23)^2 * SQRT(dp)

Where:

Q = Flow in GPM
D = Orifice diameter in inches
dp = Differential pressure across the orifice

This assumes a spec grav of 1.00 and an orifice coefficient of 0.63.

If your fluid gravity is different or you are using something different than a sharp edge orifice you will need a different formula. If your needs are different let me know and I’ll look it up at my office.

D23

 

[pennpoint]

sirdle
Contact the person (group) who changed the orifice. Find out from them what the B ratio is (read as, Beta Ratio) for that orifice, you should be able to factor in that data and come up with a real number.
Do not be timid about asking answers of people when its you who has the critical question.

Good luck
pennpoint

 

[sirdle]

Thank you both for your responses.

pennpoint: It seems that the plant operators changed out the orifice plate, but I haven't been able to find out why it was changed or who decided what size plate to install. I am constantly encouraging operators, foremen, and supervisors to seek engineering design before starting projects. They claim they don't have money in the budget, but then spend much more fixing the problems they created.

D23: I see the similarity between your formula and the existing one. I do have a sharp-edged orifice. I don't have the specific gravity with me at home, but it is about 0.8.

By "orifice coefficient" do you mean the B-ratio? The internal pipe diameter is 7.625" (8" schedule 80) and the plate dr = 5.9996", so the B = 0.7868?

Do you suppose that the specific gravity and the B-ratio are included in the "0.23" in your equition?

I found a formula for fluid flow some time ago in a field office, but I don't know if it is appropriate.

gpm = 19.636 x 0.61 x (1/SQRT(1-B^4)) x (dr)^2 x SQRT (h)

where: B = orifice diameter / internal pipe diameter
dr = orifice diameter
h = differential pressure in feet

Does anyone recognize this formula? Is 0.61 the specific gravity?

Thanks again for all your help!

 

[MortenA]

Usually (in most companies) you have a datasheet specifillay for the orifice plate (often referred to as the "flow element". This sheet should tell you everything. There is an iso standard for calculating flow based on dP from an orfice flowmeter its called ISO 5167.

Best regards

Morten

 

[hacksaw]

Re. second formula,

You have one of the common forms used to estimate orifice plate flows. It is not particularly known for its accuracy.

The discharge coefficient for your plate is Cd~0.60 or so depending on the meter correlations used and the flow tap arrangement. The differential appears to be expressed in feet of liquid referenced to the process fluid. It would be less confusing if you included fluid gravity explicity, in which case include Gfluid under the square-root.


It appears that you may be measuring the flow in imperial gallons, that is something you need to check. It is also more conventional to use inches water column instead of feet in the calculation, but if you do you have to modify your coefficient to 19.636/sqrt(12)

Your d/D (beta) is pretty large for accurate flow measurement. Depending on the upstream condition you have a flow uncertainty as much as +/- 20%.

It is evident that you pipe is grossly undersized, if you intend to upgrade the meter but stay with a d/p flow measurement. You might consider a vortex meter.

You first flow eqn, was "dummied in" by the programmer to fit a given flow range. It can work only for a specific meter calibration; the accuracy of it is questionable.

good luck

 

[pmover]

sirdle,

i concur with mortena response. there are several resources for determining fluid flow through an orifice and he mentions a industry std. another is the aga3 report. another is the efunda website (engineering fundamentals). also, try conducting a search within this forum as this matter has been addressed in other postings.

the formula you provided was MOST LIKELY simplified for programming purposes (i.e. all the constants associated with the orifice and flow units are combined into 1 constant). this is something i have done in the past. rather than have the plc conduct all the calcs for the constants, all unchanging orifice constants/factors are combined into one constant. however, once the orifice is changed, so does that constant. hence, i highly recommend searching the original documentation for the orifice flow meter calcs (i.e. the data sheet mention by mortena) and request the mfg/supplier to conduct another computation based on current orifice plate size.

verify the original constant with original data to supplement your confidence factor.

if no original documentation is found, exercise good judgement and conduct the calcs based on what good information you know/have with a qualified individual.

simply changing the orifice plate does have other implications to the process (i.e. control elements) and responsible individuals need to be involved with the matter. simply making changes to help operations without qualified consultation may be hazardous to equipment/personnel.
good luck!
-pmover

 

[dquin]

Hello
If you know what the diameters were before and after your technicians changed the plate you can just modify your old formula.
Unfortunately you didn't say what the original orifice diameter was, it's a useful figure to have. Anyway, here's a look up table.

Original
Diameter New Formula
5.5 gpm=115.81*SQRT(dP)
5.6 gpm=110.15*SQRT(dP)
5.7 gpm=104.70*SQRT(dP)
5.8 gpm=99.46*SQRT(dP)
5.9 gpm=94.39*SQRT(dP)
6 gpm=89.48*SQRT(dP)
6.1 gpm=84.71*SQRT(dP)
6.2 gpm=80.06*SQRT(dP)
6.3 gpm=75.52*SQRT(dP)
6.4 gpm=71.07*SQRT(dP)
6.5 gpm=66.70*SQRT(dP)

The method used was
new formula=old formula*(dn/do)^2*sqrt((D^4-do^4)/(D^4-dn^4))
where
do=old orifice diameter dn=new orifice diameter
D=pipe internal diameter

Wish you the best
dquin

 

[Cockroach]

When it comes to orifice fitting measuement calculations, there is no easy, rule-of-thumb equation that necessarily applies. It depends on what level of accuracy is required. That's why engineering software is on the market for the purposes of orifice measurement computation(s).

I've done it by hand following the AGA Report Number 3 specification. Not fun, probably about a good two (2) hours worth of work assuming you can correctly list the physical fluid properties. I suggest getting a program off several websites listed in the latter pages of this website.

About beta ratio, a value of 0.65 is used in the industry to size an orifice plate to the flow, typically resulting in the desired differential pressure of 100 inches of water. When I'm asked to size an orifice meter run, this would be a very typical computation.

On the other hand, if you are asking on information which will enable you to custom write your own software, AGA Report Number 2 is the standard, although I prefer the indepth discussion from Handbook of Flow Measurement, Miller, which is the bible of the industry.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[hacksaw]

with a beta ratio approaching 0.8 it is unlikely that the meter calibration can be checked with the AGA correlations or even the more recent ones. typical to high beta installations, the approach velocity is excessive and an orifice plate meter would not normally be specified.