1-866-404-5415 US and Canada
1-414-299-3896 Outside the U.S.

1-866-404-5415 US and Canada
1-414-299-3896 Outside the U.S.

Positive Displacement Flow Meters

Our Positive Displacement Flow Meters are an Excellent Choice for a Wide Range of Industries.  Lorem Ipsum from Smartmeasurement!

Positive displacement meters provide high accuracies up to ±0.1% of actual reading (RD), good repeatability, as high as 0.05% RD. Moreover, accuracies are not affected by pulsating flow unless it entrains air or gas in the fluid. Therefore, for liquids that are not clean or have gas bubbles, PD metes must have filters/strainers to filter particles such as ALPDFT and/or gas eliminators or ALPDGE to knock out gases before the liquid can be measured. For application where both filter and air/gas eliminators combined system such as ALPDFTG is the best option.  PD meters can function without power and do not require straight upstream and downstream pipe runs to maintain their excellent accuracies. PD meters are available in sizes from 1/8” to 12 inches or more. Operation turndowns can be as high as 100:1, although ranges of 15:1 or lower are much more common. Slippage between the flowmeter components is reduced and metering accuracy is therefore increased as the viscosity of the process fluid increases.  Therefore, PD metes are excellent for very good for high viscous fluids such as oils, fats etc.

As mentioned above, the liquid must be clean. Particles greater than 100 microns in size must be removed by filtering. PD meters operate with small clearances between their precision-machined parts; wear rapidly destroys their accuracy. For this reason, PD meters are generally not recommended for measuring slurries or abrasive fluids. In clean fluid services, however, their precision and wide rangeability make them ideal for custody transfer and batch charging. In industrial and petrochemical applications, PD meters are commonly used for batch charging of both liquids.

Although slippage through the PD meter decreases (that is, accuracy increases) as fluid viscosity increases pressure drop through the meter also rises. Consequently, the maximum (and minimum) flow capacity of the flowmeter is decreased as viscosity increases. The higher the viscosity, the less slippage and the lower the measurable flow rate becomes. As viscosity decreases, the low flow performance of the meter deteriorates. The maximum allowable pressure drop across the meter constrains the maximum operating flow in high viscosity services.

PD meters being one of the oldest if not the oldest flow metering techniques have many different styles.  The most popular ones are described on our “Positive Displacement Measuring principle. https://www.smartmeasurement.com/positive-displacement-measuring-principle

However, Smartmeasurement have the most popular PD flowmeters in the industry which are oval/lobe gears and bi-rotor or helix gear types. 

 

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ALBRPD

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ALOVPD

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ALGPD

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ALMGPD

Gear and Lobe Meters

The oval gear PD meter uses two fine-toothed gears, one mounted horizontally, the other vertically, with gears meshing at the tip of the vertical gear and the center of the horizontal gear, see figure below.  The two rotors rotate opposite to each other, creating an entrapment in the crescent-shaped gap between the housing and the gear. Oval gears can be very accurate if slippage between the housing and the gears is kept small. If the process fluid viscosity is greater than 10 centipoise and the flowrate is above 20% of rated capacity, accuracy of 0.1% AR can be reached.  At lower flows and at lower viscosity, slippage increases and accuracy decreases to 0.5% AR or less.

The lubricating characteristics of the process fluid also affect the turn- down of an oval gear meter. With liquids that do not lubricate well, maximum rotor speed must be derated to limit wear. Another way to limit wear is to keep the pressure drop across the meter below 15 psi (< 0.1bar). Therefore, the pressure drop across the meter limits the allowable maximum flow in high viscosity service.

A variation of an oval gear PD is a rotating lobe and impeller type PD meters however they do not share its precise gearing. In the rotating lobe design, two impellers rotate in opposite directions within the ovoid housing.  As they rotate, a fixed volume of liquid is entrapped and then transported toward the outlet. Because the lobe gears remain in a fixed relative position, it is only necessary to measure the rotational velocity of one of them. The impeller is either geared to a register or is magnetically coupled to a transmitter. Lobe meters can be furnished in 2-in to 24-in line sizes. Flow capacity is 8-10 gpm to 18,000 gpm in the larger sizes. However, the bi-rotor or Helix gear PD such as Smartmeasurement’ s ALBRPD are far better both in terms of accuracy, flow range and accuracy for bigger sizes, while for sizes the Oval gear PD or ALOVPD is the best options.

The lobe gear meter is available in a wide range of materials of construction, from thermoplastics to highly corrosion-resistant metals. Disadvantages of this design include a loss of accuracy at low flows. Also, the maximum flow through this meter is less than for the same size oscillatory piston or nutating disc meter. See https://www.smartmeasurement.com/positive-displacement-measuring-principle/

Another type of PD meter is the rotating impeller meter, gears entrap the fluid and pass a fixed volume of fluid with each rotation.  These meters are accurate to 0.5% of rate if the viscosity of the process fluid is both high and constant, or varies only within a narrow band. These meters can be made out of a variety of metals, including stainless steel, and corrosion-resistant plastics such as PVDF (Kynar). These meters are used to meter paints and, because they are available in 3A or sanitary designs, also milk, juices, and chocolate.

In these units, the passage of magnets embedded in the lobes of the rotating impellers is sensed by proximity switches (usually Hall-effect detectors) mounted external to the flow chamber. The sensor transmits a pulse train to a counter or flow con- troller. These meters are available in 1/10-in to 6-in sizes but as mentioned above, in both lobe gears are being replaced by oval gear PDs as its rangeability, accuracies are better.

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TYPICAL OVAL GEARS
Bi-rotor or Helix Meters

The helix meter is a positive displacement device that uses two radially pitched helical gears (see diagram below) to continuously entrap the process fluid as it flows. The flow forces the helical gears to rotate in the plane of the pipeline. Optical or magnetic sensors are used to encode a pulse train proportional to the rotational speed of the helical gears. The forces required to make the helices rotate are relatively small and therefore, in comparison to other PD meters, the pressure drop is relatively low. The best attainable accuracy is about ±0.1% or rate.

As shown in below, measurement error rises as either the operating flowrate or the viscosity of the process fluid drops. Bi-rotor meters such as Smartmeasurement’ s ALBRPD can measure the flow of highly viscous fluids (from 3 to 300,000 cP), making them ideal for extremely thick fluids such as glues and very viscous polymers. Because at maximum flow the pressure drop through the meter should not exceed 30 psid, the maximum rated flow through the meter is reduced as the fluid viscosity increases. If the process fluid has good lubricating characteristics, the meter turndown can be as high as 100:1, but lower (10:1) turndowns are more typical. 

Bi-rotor meters are the PD meter of choice, for fuel oils, fuel trucks, etc. The high price and volatility of fuel prices makes bi-rotor PD meters the instrument of choice because not only it’s very accurate, can operate without external power and is excellent in confined spaces where no straight run is required.   https://www.smartmeasurement.com/wp-content/uploads/2021/02/Straight-Run-Requirements-of-Various-Flow-Technologies.pdf  Shipping vessels are often times months out of port where fuel prices can change, therefore excess fuel when coming to port is sold in a custody transfer situation.

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BI-ROTOR METERS

Have questions about our PD Meters? Contact the experts at SmartMeasurement today!

Installation Methods

Primary Application

Special Features

Main Markets

Installation Methods

Primary Application

Special Features

Main Markets

Installation Methods

Primary Application

Special Features

Main Markets

Installation Methods

Primary Application

Special Features

Main Markets