Orifice flow meter sizing

In this article, I would like to share with you my personal experience of Orifice flow meter sizing.

Introduction

The orifice flow meter is one of the most commonly used flow-measuring devices in the industry.

They operate on the principle of placing a restriction in the line to cause a differential pressure head. The differential pressure, which is caused by the head, is measured and converted to a flow measurement.

There are two elements in the Orifice flow meter, the primary element (orifice plate) is the restriction in the line, and the secondary element is the differential pressure measuring device (DP transmitter).

Generally, the indicating instrument extracts the square root of the differential pressure and displays the flow rate on a linear indicator.

For Orifice flow measurement there are two world-renowned standards preferred as below.

1. ISO 5167 / ASME-MFC-3M (generic for liquids and gases, line size 2” up to 40”)
2. AGA 3 / API MPMS 14.3 (primarily for Natural gas and HC gas applications)
3. ASME MFC-14M (for integral orifice meters, line sizes less than 2”)

In general, most Client specifications state that calculations are done in accordance with ISO 5167-2, for orifice plates not covered in ISO 5167, sizing methods shall be ASME MFC-14M (latest Edition) or AGA Report No. 3 or ‘Flow Measurement-Engineering Handbook’ by RW Miller.

Straight run recommendation as per calculated Beta value is as per Table 3 in ISO 5167-2 wherein column ‘A’ specifies straight run recommendation for ‘zero uncertainty’ (normally considered only for ‘metering’ and ‘custody transfer’ applications) and column ‘B’ for ‘additional 0.5% uncertainty. Note: Ensure the required straight run requirement is communicated to the piping team for their layout considerations.

Due to uncertainties associated with the coefficients and square root determination of differential pressure, orifice flowmeters have a recommended flow turndown of 3:1 only. However, some client specifications allow turndown up to 5:1

Orifice flow meter sizing

Many of us may be unaware of the orifice sizing considerations and hence, through this article, I have tried to explain the steps to be followed for appropriate sizing of the ‘orifice’ plate (concentric type).

1. ISO5167 does not cover the use of orifice plates in pipe sizes less than 2” or more than 40”, or for pipe Reynolds numbers below 5000.
2. For line sizes less than 2”, a honed integral-type orifice flow meter (with corner tapping) as per ASME MFC-14M is used.
3. Flange taps are generally used for line sizes 2” to and including 14” line size. Above 14” line size D and D/2 taps shall be used. For ANSI CL600 rating and above, flange taps shall be used for line sizes up to 12 inches while D-D/2 taps shall be used for line sizes more than 12 inches.
4. Identifying the DP range is an iterative process with 2500mmWC as the preferred starting point since it tends to result in a good combination of instrumentation and process conditions. Let me brief you on the calculation steps I followed.

In orifice sizing our main goal is to find an acceptable Beta ratio and Bore dia. by adjusting the DP range and Meter max flow rate as explained below:

1. Beta Ratio (d/D)

Beta Ratio = Orifice Bore Diameter / Pipe Inner Diameters. As per ISO5167, the beta ratio has to be between 0.3 to 0.7 as the coefficients are defined as such in the standard.

Sometimes, client specification specifies a certain Beta ratio range as per company standards. The same is to be considered for the calculation.

As we know from the above list if we want to find the Beta ratio we must have a value of meter max Flow and ΔP.

2. Flow Range Selection

The process department will give you the Min, Nor, and Max flow values and other process parameters (based on liquid or gas)

And sometimes times Process department only gives Normal or Maximum flow.

So how will we decide the calibration range of our flow meter?

for that, we need to first see the client specs. and as per spec, we will follow the steps. I want to give you an example of my project.

the orifice’s normal flow will be 50 to 70% of the fully selected measuring range.

Minimum and Maximum flow is between 20 to 90% of the fully selected measuring range.ie if your flow range is as below then your measuring range is 0-10 m3/h.

You also need to keep in mind the turndown ratio of the orifice flow meter which is 3:1(sometimes client spec allows for higher turndown) and if you divide the Maximum flow by the Minimum flow value must be less than or equivalent to the turndown ratio.

turndown ratio limits the maximum and minimum measurement range if you want to achieve stated accuracy. If you need to measure higher turn-down flow ratios then an additional transmitter may be added to measure the lower range. However, this may be subject to client approval.

(You also need to keep in mind the turndown of the Differential pressure transmitter. Ensure that the DP at the minimum flowrate is within the turndown ratio of the DP transmitter)

Differential Pressure Range(ΔP)

Differential pressure range is specified standard ranges like (5, 10, 50, 100, 200 inH2O) is generally preferred.

Also do check the client spec document to see if they provide other ranges. Generally, we avoid the DP range above 200inH2O if you still want to go above then pls check API MPMS 14.3

if the DP range goes beyond 200inH2O after doing many trials and errors in sizing software for the selection of Beta Ratio within 0.7

Then the recommendation is not to increase the DP range above 200inH2O but you have to increase the line size up to meter length if it is feasible for the Piping Department.

Example of Orifice flow meter sizing

Flow range:

As we know from above as per the above data selected range is between 0-10 m3/h

Then as we know first, we have to select the standard DP range for the maximum selected flow range. and we already know the pipe size from P&ID.

Case 1. Beta Ratio within client specified range or (0.3 to 0.7)

Your sizing is correct.

(Note: One thing that needs to be checked after sizing is that permanent pressure loss is not more than the maximum allowable pressure loss considered by the process department during hydraulic calculations)

Case 2. Beta Ratio less than 0.3

Select a lower standard DP range (i.e. you can select 100inH2O then select 50inH20) and run the calculation again.

Case 3. Beta Ratio of more than 0.7

First, check if your like size is big like more than 10″ client specs will allow you an additional Beta Ratio Up to .75 or other pls check client specs.

If the above criteria are not satisfied and you still get a higher Beta ratio then select Higher DP Range

(i.e you can select 100inH2O and then select 200inH20) if you already selected 200inH2O and you got a High Beta ratio then you have two paths mentioned below and the first path is generally selected.

Path 1. Increase the line size up to meter run (current line size 4″ then take 6″)

Path 2. Select a higher DP range as per API MPMS 14.3 standard but you also need to check whether the DP range is available in Transmitters or not.

(If you are going for a higher range then of course you are making a compromise in measuring accuracy)

Note: Final Sizing is as per vendor recommendation and provided by the vendor because they are using their proprietary software for doing sizing.

EndNote

I would Like to thank Sreekanth Sir (Lead I&C engineer) at Técnicas Reunidas for providing such valuable input for this article.

feedback on Orifice flow meter sizing is highly appreciated if you have any confusion pls comment below.

It is my first try to explain this complicated process of Orifice flow meter sizing for the first time on the Internet. I think you can never find this type of practical information about orifice flow meters.

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