Applications for various types of PD meters are as follows.
Rotary vane. The rotary vane PD meter is the most common design. It is widely used for crude oil, mid-weight and heavy distillates, lube oil and additives, and chemicals of similar viscosity. The rotary vane meter is also used for asphalt (iron trim required). Many custody transfer measurements use rotary vane PD meters.
Bi-rotary lobed. The applications for rotary lobed (also called bi-rotor) meters are similar to those for rotary vane meters. They are both considered to be the most accurate of all PD meter designs (see the discussion of “Performance Characteristics” above).
Piston. Oscillating piston and rotary piston PD meters are used for chemicals, LPG, and batching processes with ±0.5% to ±1% accuracy. They are available in small sizes (1-inch to 3-inch sizes are typical) and are used as dispensing meters at service stations.
Oval gear. Oval gear PD meters can be used for heavy oils with higher viscosity and for corrosive liquids but not for asphalt service. Typical designs allow maximum viscosity of 200 to 300 CP (1,000 to 1,599 SSU), at maximum flow rate.
Higher viscosity, up to 1,000 to 1,500 CP (4,800 to 7,00 SSU), may be possible but the maximum flow rate has to decrease proportionally and special clearance must be used.
Nutating disc. The nutating disc PD meter is primarily used in water meters and sometimes for chemical batchings. Accuracy is approximately ±1%.
Rotating paddle. Rotating paddle PD meters often come in small sizes (1 to 2 inches). They are used in oil fields for water flooding. Accuracy is usually ± 2% or less.
In summary, liquid PD meters can be used for measuring the flow of a wide range of fluids from LPG to asphalt. They are used instead of inferential type (e.g., orifice, turbine) meters where one or more of the following circumstances obtains.
• Requirement for higher accuracy
• Requirement for wider rangeability (10:1)
• Fluid viscosity too high for turbine meters
• Lack of space for long meter runs as required by differential pressure meters and turbine meters (PD meters do not need flow straightening devices)
• Specification by parties involved in custody transfer
Rotary vane and bi-rotor PD meters generally provide better accuracy than inferential flow meters, which include differential pressure (e.g., orifice) and velocity (e.g., turbine) meters.
Minimum accuracy requirements depend upon applications or contractual agreements. Typical minimum accuracy requirements for PD meters are shown in Figure 500-26.
With the exception of the nutating disc meter, PD meters are ideal for viscous liquids not suitable for turbine meters. Generally speaking, PD meters are ideal for fluid viscosities greater than about 4 CP (typical for No. 2 fuel oil or 40° API gravity crude oil) because at these viscosities PD meters are relatively insensitive to flow rate and viscosity variations that may occur during operation. In other words, the accuracy and rangeability of a PD meter increase at higher viscosities (opposite to that of a turbine meter) because the meter factor shift due to bypass decreases with higher viscosities. Figure 500-27 shows graphically the effects of viscosity on PD meter accuracy.
Heavier high viscosity oils will tend to cling to the internals of meters. Oils which are not effectively wiped off the blades and other moving parts of a PD meter will form a coating on the meter’s internal surfaces and reduce the volume of the measuring chamber.
The maximum viscosity for a typical 4-inch rotary vane PD meter is 400 CP (2,000 SSU), considered standard by the manufacturers. Higher viscosity is obtainable by using wider blade clearance.
Until now, many pipeline meters have been designed and operated at 30% to 80% of the manufacturer’s specified maximum. This figure is too conservative, even for custody transfer applications. PD meters usually stay near or within the linearity specification at 70% to 100% of the maximum flow rate.
At low flow rates (i.e., less than 20% of maximum), however, the linearity of a PD meter becomes unpredictable. Therefore, a more practical view is to allow the meters to operate between 20% to 90% of their maximum flow rates.
A meter should operate at the flow rate at which it was calibrated (“proved”). It should be recalibrated whenever the flow rate changes more than ±10% of the maximum flow rate (full scale). In some places, the practice of recalibration is ±10% of the operating flow rate—a conservative and sometimes impractical practice.
Lubricity has a significant effect on wear and meter life. Low lubricity usually causes increased friction between moving parts and accelerates wear. PD meters should run at lower rates with low lubricity fluids (note: turbine meters may perform better at higher rates with low lubricity fluids because the rotor tends to float better at higher rates).
LPG, condensates, and water generally exhibit low lubricity. Most crude oils and diesel oils usually exhibit good lubricity. Gasolines and jet fuels are intermediate. Crude oils containing corrosive water and/or sand should flow through PD meters at fairly high rates and as continuously as possible to keep those materials from settling out in the bottom bearing.
Cyclic operation with short run periods and long dormant periods tends to distort the meter factor and permit abrasive materials to build up in the bottom of the meter. If an automatic temperature compensator (ATC) is used, it may not be able to adequately compensate in short cycle operations because of lack of response time.