Maximizing Flow Meter Accuracy in Upstream Piping Systems

INTRODUCTION:

One of the most common questions when designing a flow metering system with a venturi meter– typical of the PFS-HVT model, is how much straight pipe is required upstream and downstream of the meter location?

SHORT, PRACTICAL RESPONSE:

1. With the PFS-HVT venturi meter design, the meter’s accuracy is unaffected by any piping configuration downstream of the discharge end of the meter.  So, if you have a disturber upstream of the meter location, you can consider moving the meter downstream up to an including directly connected to a disturber (elbow, tee, valve or whatever) without any concern for impaired accuracy.  By doing this, you may be able to secure enough upstream straight pipe between the first upstream disturber and the venturi meter to eliminate any impairment to the meter’s accuracy (see the chart in item 3 below).
2. There are 4 elements that control what the right amount of straight pipe is for an HVT installation:
   • What type of HVT is being considered: flange x flange pressure vessel (HVT-CI OR FV). Insert design (HVT-PIF or HVT-FIF) with standard, corner high pressure tap. Insert design (HVT-PIFS or HVT-FIFS) with upstream static high pressure tap.
   • What is the beta ratio of the meter that is being used (Beta ratio is d/D where “d” is the throat size and “D” is the line size.
   • What is the first upstream disturber (tee, elbow, reducer, etc)?
   • How far away is that first disturber from the desired location of the meter?
3. From a pure “length of straight pipe” point of view, the following are the guidelines for the required upstream straight pipe for the PFS-HVT assuming your application needs the standard, non-lab calibrated accuracy of the HVT which is +/-0.5% of actual rate of flow.  This data assumes a static tapped, pressure vessel type HVT with a 0.5 beta (example would be a 12.0” HVT-CI with a 6.0” throat:

4. Note that the above is the maximum upstream straight pipe required – consult PFS for the specific requirement for your application.

5. PFS can provide a design flow calculation for your specific application as well as an installed statement of accuracy based on whatever your piping configuration is.

6. Remember that since the upstream straight pipe requirements are a function of the meter’s beta ratio (throat/line size), in some cases a change to a smaller throat may result in no accuracy impact for a given upstream piping condition.

Overall Conclusions:

Among a number of considerations when you are designing a flow metering system where accuracy and repeatability are the key elements to a successful installation, certainly the impact of the piping into which the flow meter is installed is a key concern. While some flow meter types require both upstream and downstream straight pipe for a certain distance from a given disturber, the PFS HVT is concerned with only the upstream piping up to and including the first disturber.  If there is a disturber which has an impact on either the high or low pressure taps being able to sense static pressure, the discharge coefficient may be affected and the accuracy would be impaired.

GLOSSARY OF TERMS USED IN THIS DOCUMENT:

Beta ratio:  ratio of the throat size (d) to the line size (D) of the meter (ergo: 6 inch meter with a 3” throat ((3/6)) = 0.5 Beta

Discharge coefficient:  the most important element of physics when using a venturi meter.  Sometimes referred to the “meter factor”, the designation in differential measurement terms is “discharge coefficient or C.  In simple terms, the discharge coefficient accounts for the difference between the actual indicated flow rate and the ideal flow rate as if there were no error(s) in the development of the signal from the meter that is used in the flow formula to determine the meaning of the differential in terms of a flow rate.

Disturber:  any pipe fitting which has a discharge flow profile which is other than if it were a straight pipe section (elbow,tee,valve,reducer,increaser etc)

Independent laboratory flow calibration: a calibration performed by an organization that is totally independent of any flow meter manufacturer. In the US, there are 2 that we recommend: Alden Research Laboratory (ARL) and Utah Water Research Laboratory (UWRL).  The result of a lab calibration tells us how the physical configuration into which the meter is installed has changed the discharge coefficient from its established and proven value to a different value which will be used in the flow formula to relate differential to a flow rate.  Typically the accuracy of the venturi meter reverts to +/- 0.25% of actual rate of flow with a lab calibration; if the calibration includes the immediate upstream piping up to and including the first disturber, then the discharge coefficient from the lab will reflect the impact of that piping on the meter’s coefficient – effectively, since we will now know what the piping affected coefficient is, the calibration nullifies whatever the piping effect would have been.

Pipe Reynolds number:  a key calculated number which is the ratio of forces pushing the flow through the line divided by all of the forces that try to prevent flow from occurring.  In this respect, the pipe Reynolds number expresses the “quality” of the flowing conditions as it relates to turbulent flow conditions versus laminar flow conditions where all DP devices require turbulent flow conditions, not laminar conditions.

Accuracy: defines the +/- accuracy requirement of the application typically across the full minimum to maximum expected flow rates.

Repeatability: the ability of the meter to provide the same reading when the same flow rate is repeated.

Bench calibration:  assigns an accuracy to the meter based on prior calibrations of the same meter design in the same general installation conditions.

Model testing:  the use of a scale model to extrapolate what the C is for a given installation.

True Static Pressure: true static pressure is defined as high and low readings in the venturi meter that report the pressure sensed perpendicular to the flow.