orifice plate design 1

[Plasmech]

We are designing an orifice plate to measure what we think will be about 50 SCFM of air at 70 degrees F. The pipe ID is 1.875", the orifice diameter is 1.25". I have a few questions regarding the plate:

1. Do we go with straight edges in the actual orifice or do we taper the edges out by say 45 degrees? Is there any industry standard orifice profile/taper?

2. How does temperature come into play...if I was putting 350F air through the same orifice plate, how do I account for that? I guess we get into an ACFM versus SCFM discussiion?

3. I have read that pipe taps are no good for this (???). What is the best way to put the pressure taps in the device?

Thanks a lot for any help!

 

[zdas04]

If you want to do gas measurement, then the thickness of the plate and shape of the edges is very well documented in API 14.3.

David

 

[danw2]

If your air temp is 350°F then you need to size the orifice plate for that temperature (which is then the design temperature), otherwise you have enormous errors. The chart below shows the %error for a 60°F sized DP flow, at air temperatures other than 60°F

For applications where the air flow covers a range of temperatures, a 'multivariable DP transmitter' is used. A multivariable uses a temperature signal from a thermcouple or RTD inserted into the flow stream to do dynamic temperature compensation. Multivariables also compensate for working line pressure variations, which also cause signficant errors when uncompensated. Multivariable transmitters provide an inferred mass measurement, as opposed to a volumetric measurement.

Unless you can guarantee that the flowing media is always at design temperature and pressure, you don't have SCFM, you only have ACFM.

I've used Honeywell's multivariable,which is at

http://hpsweb.honeywell.com/Cultures/en-US/Products/Instrumentation/flow/smv3000/default.htm

Dan

 

[Plasmech]

Dan,

What does "DP" stand for? I usually take it to mean Dew Point in my field.

Also, confused as to why I have to change the orifice diameter as the temperature changes...isn't the orifice diameter part of the orifice plate equation? Also note that I am using the orifice plate as a measuring instrument, not as a means to intentionally cause a known pressure drop in the pipe.

 

[danw2]

DP stands for differential pressure.

>Also, confused as to why I have to change the orifice diameter as the temperature changes

The flow is the square root of the DP at design temp and design pressure. Air is compressible and measurement of its flow by DP, changes dramatically with temperature and pressure, as the chart shows.

You don't have to change your orifice size, but you do have to calculate what the DP for 350° air will be at your max flow rate and base your measurements on that, not on 60° air.

> I have read that pipe taps are no good for this (???). What is the best way to put the pressure taps in the device?

"orifice flange unions" are commercially available in various mounting styles: pipe threads, socket welds, butt weld

The flange unions have threaded ports into which the impulse piping/tubing connects for connecting a DP transmitter or gauge.

Screw or weld the flange unions onto the pipe, then the orifice plate fits between the flanges, which bolt together.

Since flow is square root function a gauge should reflect that with square root (non-linear) markings. Industrial grade electronic transmitters will have square root extraction and if you pay for it, a digital indicator read-out.

Gauges and electronic transmitters have temperature limits. For gases, the meter is typically mounted above the line, so any condensation will drip back into the process pipe, and not fill the impulse tubing. At 350°F, the transmitter should be a couple feet from the pipe to minimize the conduction of heat to the meter body through the impulse piping.

I'm lazy, I don't even do my own orifice plate(OP) sizing calculations, don't own sizing software. For $30, the OP supplier runs the calculation from the provided design info: working temp, working pressure, medium, max flow rate, desired DP.
I get back a sizing sheet with the actual DP at max flow, which is entered into the transmitter so the output signal is ranged properly.

Dan

 

[dcasto]

sorry Danw2, having a vendor doing engineering is not being a responsible engineer. If you can't afford software, build it yourself. The equations are public domain (latest API must be purchased).

There even shareware programs, but its up to you to assure its used properly and is up to latest standards.

To top it off, you need to do the physical design of the pipe and any flow conditioners.

If accuracy is not a concern, pipe taps are just fine and they can be put installed just about anywher. Corner taps, venturi, and loads of other devices can be used.

 

[rmccallum]

First off, why design an orifice plate when you can buy one for 50 bucks? Use the OFU (orifice flange union) and buy a 2" x 1.250" paddle type orifice plate. Also, make sure of the actual/nominal ID of your pipe. A 2" pipe (sch80) typically has a nominal ID of 1.939". Ok, lets talk measurement. Flowrate (Q) is calculated by determining your PE (pressure extension) by multiplying the sq. rt. of your DP by the sq. rt. of your absolute pressure. You then multiply your PE by your C' which is calculated by Fb x Fpb x Ftf x Fg x Fpv x (6 more factors of which the y expansion is one).

Since all orifice plates are required to be based at 68deg F, the y expansion is calculated for the expansion/contraction experienced by the orifice plate/meter tube when placed in operating temperature conditions. (All of this is listed in the API 14.3 (1985) and has since been replaced by 14.3 (2000) which changed the flow equation drastically. 14.3 (2000) does not even recognize pipe taps anymore.

The calcs above use all linear numbers such as 14"H2O (DP) and 100 psig. A sq. root gauge (meter?)is not necessary. A little Sonney orifice calculator is all that is really needed to calculate non-critical flow. Also remember to use a density of 1 when calculating the flow rate of air.

 

[bosse1950]

Just one other reminder - don't forget to assure that the interior welds at your orifice flanges are ground smooth. this is normally called out by a note on the piping isometric.

 

[toothless]

Buy it, don't build it. The people who sell these know all the details and have the proper tooling to do it right. Especially for a plate this small, buying it is more economical and accurate.

 

[imran95109]

Hiya
can any tell me how to size the Orifice plate. I fond on formula

To calculate the correct restriction size to use in a choked flow system try the following equation:

d = SQRT(Q/(.01749(P1/29.7)SQRT((29/MWx)SQRT(528/T)))

Where:

d = Orifice diameter / mm
Q = Gas flow / mL / min
P1 = Inlet pressure / psia
MWx = Molecular weight of gas x
T = Temperature / °R

Alternatively the coefficient of flow of a critical gas can be used to determine the restriction size with the folowing equation:

Cv = Q((SQRT SGx x T)/(816 x P1))

Where:

Cv = Coefficient of flow
Q = Gas flow rate / ft3 / hr
SGx = Specific gravity of gas x
T = Temperature / °R
P1 = Inlet pressure / psia

Can anyone tell me the reference book for this. I fond this one from one of the forum.

Thanks

 

[dcasto]

l. k. spinks was the master of the head meter or differential head meter. You can google orifice meter design and find all you need. here is one one of the first hits. You should be able to write your own excel program.

http://www.efunda.com/formulae/fluids/calc_orifice_flowmeter.cfm

Many a control room had the DP transmitted back and the strip paper or gauges were marked in square roots ie at 50%, the value was about 7. Then the orifice plate was cut to an exact dimension so that at 100% DP, the rate would be a round number ie 300 CFM, so at 7 roots, the flow was 7/10 *300 = 210 cfm and the plate was 1.2345 inches.