Positive Displacement flow meters, or PD flow meters, provide an excellent solution for applications involving viscous liquids. This flow measurement technology provides excellent accuracy, delivers readings that are unaffected by high viscosities and changing viscosities, and does not have any requirement for upstream / downstream straight pipe runs. This combination of features makes the PD flowmeter an ideal for situations with tight space constraints and high accuracy requirements. PD meters also provide excellent range-abilty / turndown ratio, making them an excellent choice for low flow measurement applications.
Positive displacement flowmeters , or PD meters, feature two precisely machined intermeshing gears which rotate within a measuring chamber to determine the volumetric liquid flow rate by counting the number of the gears’ rotations. The PD meter functions in much the same manner as a positive displacement pump with the exception being that in the PD pump the gears drive the fluid media, whereas in the PD flowmeter the fluid media drives the gears. The positive displacement technique is one of the oldest flow measurement technologies in existence and has been in use for over one hundred years. During that time, many different varieties of the PD meter have been developed including versions that feature round gears, oval gears, lobe gears, helical gears and nutating disc rotating elements. These meters are all characterized by high accuracy (up to ±0.1% of reading), good repeatability (up to 0.05% of reading), wide turndown ratios of up to 100:1, and the ability to maintain this level of performance for both high viscosity and changing-viscosity fluid media.
Another advantage of the positive displacement meter is that accuracies are not affected by pulsating flow unless it entrains air or gas in the fluid. The PD meter can still accommodate pulsating flows in the event of entrained air occurring by making use of an air/gas eliminator such as the SmartMeasurement ALPDGE or the ALPDFTG which adds a filter element. These devices are installed upstream of the meter to knock out gases before the liquid can be measured. Due to the tight clearances inside of the PD meter’s measuring chamber and between the intermeshing gears, it is important for the media to be clean as high particle counts or large particle sizes can cause the meter to seize. For applications such as well output monitoring in the Oil & Gas Industry where there may be particles such as sand or rocks entrained in the fluid, the ALPDFTG is the best option.
Another common reason for using the PD meter is that it can function without power. By outfitting the meter with a mechanical register, totalized flows may be accurately monitored in virtually any engineering unit desired by the end-user without the need to run power to the meter. This is ideal for applications where the meter must be installed in a remote location where power is not available. PD meters are available in sizes from ⅛” 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 meters are excellent for highly viscous fluids such as oils and fats.
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. However, for clean fluid service, their precision and wide rangeability make them ideal for custody transfer and batching applications. In industrial and petrochemical settings, PD meters are commonly used for batching and mixing control.
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, come in many different styles. The most popular ones are described on our “Positive Displacement Measuring Principle” page found here .
SmartMeasurement offers a range of positive displacement meters including the most popular styles of PD flowmeters in service today which include oval/lobe gear type, bi-rotor or helix gear types, and oval gear.
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, as shown in the illustration 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 cP and the flowrate is above 20% of rated capacity, accuracy of 0.1% of reading can be reached. At lower flows and at lower viscosities, slippage increases and accuracy decreases to 0.5% of reading or less.
As with other styles of positive displacement meters, when an oval gear meter is being used, the lubricating properties of the fluid media can have an impact on the maximum turndown ratio that the meter is able to achieve. If a given liquid does not have good lubricating properties, the maximum rotor speed must be de-rated to prevent or limit wear. Alternatively, wear can be limited by maintaining pressure drops across the meter body that are less than 15 psi (< 0.1bar). If this method is used, the pressure drop across the meter will also limit the maximum possible flow for high viscosity liquids.
Variations of the oval gear PD meter include the rotating lobe and impeller type PD meters. One key distinction between these meters and the oval gear meter is that they do not share the precise gearing of the oval gear PD. A lobe-type PD meter contains two impellers that rotate in opposite directions of one another inside of the meter housing. As the lobes rotate, a known fixed volume of liquid is simultaneously allowed into and forced out of the meter housing. Because the lobe gears remain in a fixed relative position, it is only necessary to measure the rotational velocity of one gear. The lobe is either geared to a register or it is magnetically coupled to a pulse counting electronic device such as a reed switch or a Hall Effect. Lobe meters are available for line sizes ranging from 2″ to 24″. Effective flow measuring rages can be from 8-10 GPM up to 18,000 GPM for larger sized meters. However, bi-rotor or Helix gear positive displacement meters such as SmartMeasurement’ s ALBRPD are far better both in terms of accuracy and flow range.
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 that of the same sized oscillatory piston or nutating disc meters.
Another type of PD meter is the rotating impeller meter. For this type of PD meter, gears entrap the fluid media and pass a fixed volume of fluid with each rotation. These meters are typically accurate to 0.5% of reading 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 as well as corrosion-resistant plastics such as PVDF (Kynar). These meters are commonly 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 type) mounted external to the flow chamber. The sensor transmits a pulse train to a counter or flow controller. These meters are available in ¹⁄₁₀” to 6″ sizes but as mentioned above, they are declining in popularity. Today, oval gear meters are replacing lobe gear and impeller type PD meters due to their superior accuracy and range-ability.
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, and crude oil monitoring. The high price and volatility of fuel prices makes the bi-rotor PD meter the instrument of choice because its highly accurate, can operate without external power and is excellent in confined spaces where there is no straight piping run or very limited straight runs available. See our pdf for more information. This style of meter is very popular for custody transfer applications. 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 and a meter like this is used for the transaction.
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