This article is about Positive Displacement Flowmeters, application, suitability, advantages, and the precautions to be observed in their use are discussed here. Main keywords for this article are Positive Displacement Flowmeters Design Requirement. Capillary or Film Seal. Positive Displacement Flowmeters Accuracy. Capacity Ratings.

Positive Displacement Flowmeters Design Requirement in Process Industry

Positive Displacement Flowmeters are mechanical meters with one or more moving parts located in the flow stream. The distinctive feature of these meters is that all of the fluid passes through the primary element in completely or almost completely isolated quantities by repeatedly filling and emptying a chamber or chambers. The total quantity of fluid flowing through a meter in a given time is the product of the volumes of the chambers and the number of fillings. In the simplest arrangement, the number of fillings is obtained by a counter or register, mounted on and operated by the meter.
Sealing the measuring chambers to confine the fluid as it passes through the meter under a differential pressure is accomplished by either of two methods.

Positive or Mechanical Seal

With this method, chambers are closed off tightly to prevent, momentarily, any fluid from entering or leaving the chambers. Examples are the wet test gas and bellows meters discussed in section 5. In the case of liquid meters, an absolute seal is difficult to achieve since it depends upon the degree of contact between metallic or metallic and nonmetallic surfaces. Increasing the amount of contact increases the operating force or differential pressure to an extent which is impractical.

Capillary or Film Seal

a. With this method, a practical degree of measuring chamber tightness is attained by virtue of the strength of the surface tension of a film of the fluid between confining surfaces which do not touch. The clearance between these surfaces must be very small compared to the length of the shortest path the fluid would travel between them to escape. As an example, the distance between the tip of the impeller and the case in the lobed impeller gas meter, (Figure 9), may be as little as 0.002 inch compared to an arc length at the impeller tip of about 0.125 inch. Capillary or film scaled meters have a relatively low pressure drop which, however, is affected to a marked degree by the fluid viscosity and rate of flow.

b. Where meters rely upon capillary or film seals for tightness of the measuring compartments, slip or flow through the clearances may be a factor to be considered for maximum accuracy. Most meters have a mechanical adjustment which permits varying the relationship between the primary element and the counter by approximately plus or minus 2 to 5 percent. This adjustment is made by the manufacturer on the basis of the characteristics of the fluid to be metered but can be made by the user, if required, because of a change of liquid or liquid viscosity.

In addition to the accuracy adjuster , accessories generally available include: Totalizer with or without indicator dial and reset Printing counter Tachometer generator and remote rate indicator Temperature compensator Pulse transmitter and remote totalizer or rate indicator Pressure-temperature compensator Separate or combination air eliminators and strainers.

Materials of Construction

Meters for liquids are available in a wide variety of materials. However, standard materials provided by individual manufacturers generally are limited. In many cases, low-density materials are used for pistons, rotors, and vanes in order to reduce pressure drop and increase sensitivity. Available materials include Teflon® fluorocarbon resin, nylon, carbon, hard rubber, Kel-F, Penton, Lexan, cast iron, steel, aluminum, bronze, stainless steel, titanium, or “Teflon®,” Viton fluoroelastomer resin, and Buna N coated stainless steel. Case or housing materials include cast and ductile iron, steel, aluminum, bronze, stainless steel, and hard rubber or epoxy liners. Standard all-stainless meters are available up to and including meters with 2-inch connections (up to 150 gpm) but some manufacturers will supply larger sizes on special order. Gas meters are available with aluminum and iron rotors, and iron, steel, and aluminum housings. One manufacturer provides meters with all internals “Teflon®” coated. Most manufacturers will supply meters of special materials on request.

Pressure Temperature Ratings

Most standard meters have maximum pressure ratings in the range of about 150 to 300 psig, depending upon type, size, materials of construction, and manufacturer. A few standard meters are available for higher pressures, up to 5,000 psig, but most manufacturers can provide higher pressure meters on special order. Maximum temperatures for standard meters usually range between 65 and 120 °C (150 and 250 °F), limited by the internal materials of construction, internal clearances, or by the close-coupled secondary element. To increase maximum operating temperatures, alternate materials may be used, internal clearances may be increased, or isolators or spacers may be installed to increase the distance between the primary and secondary element.

Capacity Ratings

Maximum rated flow rates published by manufacturers for liquid meters generally are based upon intermittent service with low viscosity liquids (0.5 to 5 centipoises). For continuous service, maximum flow rates should be reduced about 25 to 35 percent, or as recommended by the manufacturer, to extend service life and maintain accuracy. Most meters may be operated up to 125 percent of the maximum rated intermittent flow rates for short periods as specified by the manufacturers. If the liquids are hot, or in some cases nonlubricating, maximum rated intermittent flow rates should be reduced about 20 percent for intermittent service and about 40 percent for continuous service. Published maximum flow rates for gas meters are based upon continuous service.

Positive Displacement Flowmeters Accuracy

The errors indicated are expressed as a percent of maximum flow rate and are average values. Some manufacturers flow test individual meters at two or several points with a “standard” liquid. This can reduce the error appreciably if the liquid to be metered has similar characteristics. If the metered liquid is not similar, the error reduction can be insignificant compared to the overall error. Where low viscosities are involved, slight differences in viscosity can shift a calibration curve because of changes in the leakage through the capillary seals. For example, a change from 1 to 5 centipoises can shift a calibration curve 1 percent in the 20 to 100 percent flow range and up to 5 percent in the 10 to 20 percent flow range.

If an average error curve is used to determine the metering error, the overall error could be 3 to 5 times greater than expected. Also, depending upon the type of secondary element installed, the load it imposes can result in as much as a two-fold increase in the metering error.

The various types of meters are affected differently as the liquid viscosity increases. Liquid build-up on the walls of the measuring chambers has the effect of reducing chamber volumes and, unless the meter is calibrated properly, can result in serious measurement errors. Where surfaces are “wiped,” as in the reciprocating piston, oscillating piston, and rotating vane meters, meter capacity and accuracy is less affected by changes in viscosity. However, as the viscosity increases, the pressure drop for a given flow rate increases rapidly. Manufacturers’ stated limitations must be observed to avoid overstressing internal components or exceeding the maximum working pressure of the case. If high accuracy is required, the meter should be calibrated by the manufacturer with a liquid having physical characteristics similar to those of the liquid to be metered and, if possible, at a comparable pressure and temperature (manufacturers’ calibration capabilities vary and should be investigated). The alternative is field calibration.

Positive Displacement Flowmeters Design Requirement in Process Industry