Positive displacement meters: pros, cons and selection

Positive displacement flowmeters, sometimes known as PD meters, have been around for more than 100 years. They are commonly used in a wide range of applications from domestic water measurement to measuring ultra flow rates of chemical at high pressures subsea.

First off – what is a “positive displacement” meter? Well, as the name suggests it involves the positive displacement of a volume of fluid – this is usually a liquid but there are some units suitable for gas. There is a chamber and inside the chamber, obstructing the flow, is a rotor.
The shape of the rotor and chamber vary greatly with each meter type but they all provide an output for each rotation. Most meter designs therefore lend themselves to being totalisers. Most can have the flow rate calculated from this primary data.
Advantages
An accurate PD meter will have minimal ‘leakage’ across the rotor seal. This is generally minimised with the use of more viscous liquids and accuracies of ±0.1per cent are sometimes quoted. On the other hand rotary piston flowmeters are used by the water industry in the UK for measurement of water over a normal flow range to accuracies of ±2 per cent.
Because they measure a volume precisely it does not matter if the flow is pulsing. They will follow the increase and decrease of flow found in reciprocating pumps of all types. With higher viscosities the turndown ratio can be high. Even with water 100:1 is not uncommon and 3000:1 is possible at 250cSt. Few applications require this but it does enable measurement of ultra low flow rates without miniature parts or normal flow measurement at minimal pressure drop.
Most meters are simple to maintain as they have only one or two moving parts and are coupled with simple readouts that are easily understood in the field. There is no requirement for straight pipe lengths like that might be needed for electromagnetic or turbine devices. They can be connected directly to elbows or valves and in most cases in a variety of orientations.
Designs are relatively easy to adapt for high pressure applications eg over 100 bar.
 
Disadvantages
All PD meters require clean fluid so a filtration level of 100 micron is usual. Some meters can actually block the flow if a larger particle is trapped in the wrong place. Many meters are not made in high specification materials and therefore corrosion can be a concern. An all plastic or all 316SS meter is the exception rather than the rule. As the application flow rate increases the size of the PD meter seems to increase by a square law! It is rare to find meters over 12-in in size although they exist at these elevated sizes for the prime reason of accuracy – frequently being utilised for custody transfer reasons.
In the author’s opinion, the most common PD meters are as follows:
  • Rotary Piston: As mentioned above these form the basis of domestic water measurement but the design of the rotary piston that oscillates in a circular chamber with a fixed web has been modified and extended to ultra low flows and high flows, as well as high pressures and for food applications. A good all-rounder.
  • Spur gear: The fluid rotates two gears and is forced around the outside of the gears and the inside of the chamber. Depending on the location of the sensor these can yield very high pulses per litre values useful in batching and fast acting processes.
  • Diaphragm (or bellows meter): These are common in many people’s home as their domestic gas meters. When the gas flows through it alternately fills and empties bellows causing levers to crank a shaft providing an output. Very useful for wide-ranging gas totalisation.
  • Oval Gear: Quite similar to the spur gear where two oval gears mesh together and sweep the chamber. The volume displaced is much larger than the round gear. Fairly low cost and some designs available in plastic.
  • Nutating Disc: This meter is the hardest to understand but is effective. The rotor is a circular disc attached to a ball. The shaft on the ball is inclined. As the disc rotates in a spherically sided chamber the disc and therefore the shaft wobble creating an output.
  • Helical Screw: Possibly the most accurate PD: meter two intersecting cylindrical bores are fitted with 2 interlocking helical screws. As the fluid passes through they rotate. On standard applications the author has observed differences of just ±0.37 per cent of reading over 50:1 turndown over annual recalibrations over 10 years – quite an achievement. Also common nowadays fitted on petrol pumps.
  • Slide Vane: Historically the most accurate of PD meters with the rotating element having a number of moving blades that rotate about a fixed cam. Linearities have been claimed of ±0.02 per cent.
  • Others: If we go back to Felix Wankel’s seminal work on rotary machines we see that there are as many designs for PD meters as there are pumps. He explored in a rational way the various shapes of rotor and chamber. While the majority don’t see the light of day in the marketplace this brief essay illustrates the variety in general use, and this is without discussing the Roots meter, wet gas meter and multi rotor designs.
Two decades ago the PD meter was considered to be old technology and likely to be overtaken by more modern electromagnetic and ultrasonic devices. Nowadays the PD meter still represents good value and can provide excellent measurement in a wide variety of duties.

Product labelling – QR codes, documentation access

We also unveiled a new electronic project documentation system. Details of each flowmeter supplied as part of a single project – for example, all the flow meters supplied for a particular chemical injection skid – will be held at a unique URL. The website will include items such as calibration certificates, PMI certificates and material certificates as well as specifications, manuals and instructions.

The address of the website will be printed on a chemical-resistant and wear-resistant label securely attached to each meter. The label will also carry a QR code linking to the website which will make it easy for service personnel on site to call up all the documentation on a smart phone by simply pointing it at the label. We believe most customers will note down the simple address and access it from a control room.

LM QR code

Litre Meter QR code allows remote access to documentation

How well do you need to know it?

What factors should you look for in flowmeter selection relating to the output or display?

  1. Precision. Often misunderstood, but in the most part, it’s what matters in measurement. It’s the degree to which repeated measurements under unchanged conditions show the same results.
  2. Accuracy. The degree of closeness of measurements of a quantity to that quantity’s actual (true) value.
  3. Linearity. For flowmeters it’s the curve of accuracy compared against flow rate.
  4. Resolution. If the digital display only has 3 digits then selection of the units has more effect than the accuracy (etc.) of the meter itself. For example, set up with a maximum of 110 US gallons/min the resolution of ±1 US gallon per minute is 1% at best and 5 or 10% or worse at lower flows. Changing over to litres improves the resolution by a factor of 4. More importantly this shows the value of having enough display digits to match the users requirements and, probably, the accuracy of the meter.
  5. Traceability. So the supplier gives you a set of data, a claim of performance. All meaningless without reference to something solid, something comparable like a National Standard.
  6. ISO 5725. According to ISO 5725-1, the terms trueness and precision are used to describe the accuracy of a measurement. Trueness refers to the closeness of the mean of the measurement results to the actual (true) value and precision refers to the closeness of agreement within individual results. Therefore, according to the ISO standard, the term “accuracy” refers to both trueness and precision.
  7. ISO17025. Simply a laboratory standard: General requirements for the competence of testing and calibration laboratories.
  8. Repeatability. Another word meaning precision but often taken as the closeness of one set of results to some more with conditions unchanged. Probably should include some reference to time and:
  9. Hysteresis. In some systems the precision varies according to whether the flow is increasing to the measurement point or decreasing. In particular near the start-up flow rate it may be found that, with the flow increasing from zero, the meter provides an output at ‘x’ whilst, when the flow decreases the meter may continue to provide an output at lower than ‘x’.
  10. Long term accuracy. This could be restated as: will it measure the same tomorrow as it does today and what about next year?
  11. Recalibration. The periodicity at which the meter should be recalibrated is not set in stone. Some meter types are less stable than others. Where the meter is used to calculate tax or fiscal amounts then a daily recalibration is sometimes necessary. In a benign fluid, with flow rates kept within bounds then others might need checking every 10 years. Litre Meter recommend a yearly check at first, analysis of the results, then an increased period depending on the customers needs.
  12. On site calibration. Whilst every flowmeter that Litre Meter manufactures is calibrated in laboratory conditions on a similar fluid and at a steady flow rate there are differences such as meter orientation and pressure pulsation.  Whilst pressure pulsation won’t affect positive displacement meters it can have a severe effect on turbines, for example. So Litre Meter recommend that each meter is calibrated in-situ. Various techniques are described in the flowmeter manual.

 

What flow units do you want to use?

  1. Gas measurement: what’s the difference between Standard and Normal? It’s simply the reference conditions. Standard and Normal are different though depending which country you’re in and what industry it’s applied to. However, if you know the reference conditions eg 0°C and 1 bar then everything can be calculated. Mass flow of gas is often expressed as standard Continue Reading →