Orifice Plates - Turndown

Orifice plates normally only have a turn down of 10:1 and even then its a square law characteristic I think... I haven't any data to hand.

If its gas / air you are measuring why not use a pitot tube?

John.

The minimum flow measurement is more a function of the DP Transmitter than the orifice. Here is a link to an online orifice calculator that has been helpful to me.

http://www.clabberhead.com

At least it will make clear that orifice plate size is only one of several important factors to be considered.

If you require improved accuracy over a wide range of flow rates you might consider different sensing technologies.

Thanks. Found the page very helpful.

Orifice plates are best thought of as having a 3:1 or 4:1 turn down. The flow rate is interpolated from the measured differential pressure across the orifice. High pressure lines will typically have an orifice designed to produce 120 to 200 inches water column (in/w.c) differential at the maximum flow.

Example: An orifice plate in a 6" pipe with a .735 diameter orifice. A 200 scfm air flow rate produces a 197 in/w.c. differential. If the flow rate drops to 20 scfm ( 10:1 reduction), the measured differential will be 1.88 in/w.c. A 50 scfm flow rate (4:1) produces a 11.50 differential.

The problem boils down to measurement error. A transmitter with a stated accuracy of 1 % of scale would not accurately measure the 20 scfm rate. The newer digital transmitters have stated accuracies of .25 or even .10. But there are so many other variables that alter the equation that it's really not practicable except in lab work.

In large diameter ducts, averaging Pito tubes that span the duct are the most economical and accurate differential pressure method. But the turn down is still 4:1 at best unless you use multiple transmitters with different measuring spans.

If you really need high turn down ratios, hot wire anemometers (aka Thermal Mass Meters) are probably your best bet. They are economical and can do 100:1 ratios if selected and installed correctly. They measure mass flow and display in volume. Since they auto compensate for pressure and temperature changes, the low end readings tend to be accurate compared to pressure methods.

But for any large diameter duct you need to measure the flow at several points across the diameter if your flows have wide variations and you require accurate data.The flow is not consistent across the profile. The thermal mass meters can also be had with long probes that have multiple sensors to average the flow rate. If you really need high turn downs and accurate data I suggest you google "Thermal Mass Flow Meters" and speak to a manufacture.

Note that my examples are simplified. Flow measurement is endless in complexity and you need to determine what your minimum requirements are as a starting point.

I agree that Thermal Mass Flow can be advantageous here. First, a clear picture of all process parameters is needed:

• Flow
• Pressure
• Temperature
• Gas type
• Pollution, humidity
• Duct size
• Location

And, often overlooked: Operation time and power requirements to generate the desired flow should be ltaken into the equation as well. (Orifices cause permanent pressure loss, which can be a considerable waste of energy).

For large ducts, there are only a few specialists that offer multi point insertion Thermal Mass Flow Meters. As this forum is not intended to be an advertisement area, you can contact me if you need the name.

Three points of clarification might be of benefit.

1) While it is most commonly termed "accuracy" the 1% Accuracy, or any other number, is actually the Inaccuracy value. Seems pretty obvious when it is used in a sentence, "My flowmeter (thermometer, speedometer, etc.) is 1% accurate".

2) The stated inaccuracy, in most cases, is in relation to Full Scale. This means that if there is 1% at 200 inches then there is potential for up to a 2 inch error. This error is applied across the entire range, so at 10% or 20 inches, the potential error is now 10%. A magnetic flow meter (not usable for this application of air) is one techonolgy for which the inaccuracy is typically 0.1% of Reading not Full Scale.

3) The inaccuracy is a sum of all the error inducing components in the loop. In the case of an differential pressure system there would be the orifice plate (pitot, etc.), the sensing device, the conversion to an electronic signal, (the previous two are typically one unit in the form of a DP cell and transmitter), the receiving point conversion to 1) a digital signal and then the digital display or 2) the direct display on an analog meter of some sort.

When you stated 59.09" ID for the orifice plate, what is the duct size and what is fluid (air, gas, water, etc.) being measured? Is the ID for the plate or the actual orifice? What is being referred to as a manifold?

Agreed. Trying to decipher the transmitter manufacturer's claims and literature and applying them is perhaps the most difficult task of all when measuring high turn down flows. And the sum of all errors, as you pointed out, will limit differential pressure flow measurement to a 3 or 4 to 1 turn down. There are many methods to improve this but to me, they aren't worth the cost and complexity.

The beauty of an orifice plate is that you can make the hole bigger or smaller depending on the characteristics and sensitivity of the flow to the head added by the oriface. There is also one that is comprised of many small holes that yeilds the best span of turndown ratio for the widest range of flow with the best level of accuracy.

A point that has not been bought up that I think requires thought is boundry layer development. If the pipe diameter is large compared to the flow velocity below a certain Reynold number the flow will become laminar. As the boundry layer builds from the wall to the orifice diameter the pressure drop will go to zero and you wont measure anything.

I had this happen to me as a young engineer in the oil patch on older gas wells. Originally designed for high flow rates they had large diameter pipe. As the wells delpeted the flow regime wentinto this laminar regime and we were giving the gas away. The orifice meter was the cash register.