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Positive displacement liquid meters
Positive displacement (PD) liquid meters have long been the standard of measurement for liquid hydrocarbons such as crude oil. Over the years, numerous design improvements have resulted in an expanded product line that now serves industrial as well as petroleum applications.
Theory of operation of PD meters
A liquid meter is, in essence, a hydraulic motor with high volumetric efficiency that absorbs a small amount of energy from the flowing stream. This energy is used to overcome internal friction in driving the meter and its accessories and is reflected as a pressure drop across the meter. Pressure drop is regarded as a necessary evil that must be minimized. Pressure drop across the internals of a PD meter actually creates a hydraulically unbalanced rotor, which causes rotation. A PD meter can be broken down into three basic components (See Fig. 1):
- These are the external housing
- The measuring unit
- The counter drive train
The external housing is the pressure vessel that contains the product being metered. It can be a single- or double-case construction. A single-case meter has the housing and the measuring chamber walls as one integral unit. In double-case construction, the external housing is separate from the measuring unit and serves only as a pressure vessel. This type of construction has two major advantages:
- The measuring chamber walls only sense the delta pressure across the meter inlet and outlet, which allows for thinner chamber walls with less distortion
- System piping stresses that are absorbed in the external housing are not transmitted to the precision measuring element
The measuring unit is a precision metering element and is made up of the measuring chamber and the displacement mechanism. The six PD designs most commonly used (See Fig. 2) are:
- The piston
- Sliding vane
The counter drive train is used to transmit the internal motion of the measuring unit into a usable output signal. Many PD meters use a mechanical gear train, which requires a rotary shaft seal or packing gland where the shaft penetrates the external housing. Other meters may use:
- Magnetic drive couplings
- Reed switch outputs (contact closure)
- Differential inductance (DL) pickoffs
These last three offer the advantages of lower driving torque and no seals that could leak the product.
Design considerations for PD meters
Most hydrocarbons are metered using a capillary seal PD meter. In this design meter, the capillary action of the metered product forms a liquid seal between moving and stationary parts. This type of meter requires very close clearance dimensions and is sensitive to differential pressure.
Product slippage is the most crucial problem affecting the accuracy of a capillary seal PD meter. All capillary seal PD meters have some clearance between moving and stationary parts and some differential pressure across these clearances. For this reason, there will always be some product that is allowed to bypass the measuring chamber by “slippage” through these clearances. If slippage were constant at all operating conditions, it could be corrected by the counter drive train gearing and would cause no inaccuracy. However, it is not constant and does vary with flow rate, pressure drop, temperature, viscosity, and clearance dimensions.
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