### Transcription of A Guide to Sizing Orifice Plate Flow Meters - IDC …

1 A **Guide** to **Sizing** **Orifice** **Plate** Flow **Meters** The **Orifice** **Plate** is a very robust **flow measurement** device. It is one device that is very easy to use and can easily be adaptable to many **flow measurement** applications. Its cost of operation is minimal and familiarity with the device is near universal. All these pluses make the **Orifice** **Plate** the first choice **measurement** device in almost every flow application. It does however have some limitations which makes the **Sizing** process a little tricky. Two key limitations include: (a) Limited Turn Down (b) Non-linear loss of accuracy at low flow rates as shown by the graph below: As can be observed from the above graph, large errors are introduced into the **measurement** system by **Orifice** plates at low flow rates.

2 This is because flow is proportional to the square root of differential pressure as shown by the graph below: ISO 5167-2:2003, **measurement** of fluid flow by means of pressure differential devices: **Orifice** plates This square root relationship between flow and differential pressure for the **Orifice** plates makes compromise in either flow or differential pressure inevitable in the **Sizing** process. Mind you, there are other sources of errors in **Orifice** flow meter measurements. These include variations in pipe diameter, **Orifice** **Plate** machining tolerances and errors introduced due to flange taps. The errors introduced by these sources are however minimal. Significant errors can be introduced in the **measurement** system due to variation in temperature in liquid applications but are usually compensated for in the **measurement** setup.

3 In gas **measurement** systems with the **Orifice** **Plate** , errors introduced by variation in temperature, static pressure and specific gravity are even more pronounced such that online compensation for these variables have proven to be the best strategy that has yielded the best results. **Orifice** **Plate** **Sizing** Factors: Some **Sizing** factors greatly impact the accuracy of the **Orifice** **Plate** are: (a) Differential Pressure across the **Orifice** **Plate** (b) Maximum Flow Rate for the given application (c) Beta Ratio Differential Pressure (DP). The differential pressure(DP) across the **Orifice** **Plate** meter is proportional to flow. However the relationship is not linear. At low flow rates, the error in DP **measurement** becomes large making the device less accurate at low flow.

4 Consequently, a low full-scale DP corresponding to maximum flow introduces errors at low flow rates while a high full-scale DP may achieve good accuracy but may come at a greater energy cost (pumping cost) and a permanent pressure loss in the system which may far outweigh the gain in accuracy. Standard ranges of DP in use are 50 inches, 100 inches and rarely 200 inches of water column. When **Sizing** , consider using 100 inches instead of 50 inches; 200inches instead of 100 inches but be careful! Maximum Flow The maximum flow is the highest flow that the **flow measurement** system can handle and still maintain the right accuracy. Selecting an unnecessarily high maximum flow will definitely introduce errors at low flow rates. The right maximum flow should be selected based on actual conditions (arrived at through a flow study of the system that requires the **flow measurement** ).

5 When done, this will yield the right result in terms of accuracy. Beta RatioAs discussed in flow instrumentation principles the beta ratio ( =. d/D) is the ratio of the **Orifice** **Plate** bore diameter (d) and the pipe internal diameter (D). During **Sizing** , keep your beta ratio in the range to This is the range of beta ratio that gives the best performance from the **Orifice** **Plate** . Anything outside this range introduces more errors and other flow problems. Keep in mind that these days, **Orifice** **Plate** **Sizing** is done by software provided in most cases by the **Orifice** **Plate** manufacturers themselves. So when **Sizing** in addition to other factors, ensure you select the right DP. range, maximum flow and an optimum beta ratio for best performance of the **Orifice** **Plate** .

6 Note that Beta ratio and DP for the **Orifice** **Plate** are related. Altering one alters the other. Source.