Accounting for fuel is one of the most important aspects of operations in the field. If you are using meters for
accountability, then you must understand how they work in order to properly protect the meter and ensure
that they are properly calibrated.
PART A - TYPES OF METERS
Positive Displacement (PD) Meters. As the name implies, a positive displacement meter measures
the flow by separating the flowing stream into volumetric segments and counting them. There are typically
three basic subassemblies in a PD meter.
External housing. This is the pressure vessel with inlet and outlet connections. Meters may be of single
or double case construction. The single case acts as a pressure vessel and as the outer wall of the
measuring element. Small meters (less than 6 inches) are normally single case. Double case
construction is used on meters over 6 inches. The advantage of double case construction is that the
piping stress or pressure is not transmitted to the measuring element.
Internal measuring element. Measures the volumetric flow by continuously separating a flow stream into
discrete volumetric segments and counting them. It is also used to drive the counter.
Counter drive train (counter or totalizer). The gear ratio of the drive train is designed to convert the fixed
volume per revolution to the number of gallons or barrels and transmit it to the counter. The calibrator
(adjuster) is used to adjust the counter for slippage through the meter. It may be used if the meter is
outside the authorized tolerance.
Because of the design of the PD meter it must be protected from dirt and trash. Any solid material passing
through the meter will cause it to malfunction. The meter will also register any air passing through the
housing. The clearance between the housing and the van is usually .004 inches to .005 inches. Because of
their simplicity, positive displacement meters are the most commonly used in petroleum operations and are
used as master meters in the place of provers.
Turbine Meters. Turbine meters are precise measuring devices, even though they are classified as
inferred rate meters. They go from simple measuring devices to very complex instruments, and they are
much smaller than PD meters. There are three basic subassemblies in a conventional turbine meter.
Meter housing. The meter housing assembly is constructed of a flanged pipe spool and houses the
Internal parts. The heart of the internal parts subassembly is the rotor blade suspended in the flowing
stream on the platform bearing and rotor shaft.
Detector subassembly. In the more complicated meters you have the viscosity compensator and the
magnetic reed switch. In this kind of meter the flow is corrected to 60 degrees Fahrenheit.
When the meter is installed it must have straightening vanes upstream and downstream of the meter. The
pipes are usually 5 pipe diameters long downstream and 10 pipe diameters upstream. This prevents fluid
swirl and cavitation due to back pressure, which will cause the thermometer to lose accuracy. The meter is
accurate up to 1/10th of 1 percent and is used for custody transfer. However, the meter requires a lot of
maintenance and expertise. (They are generally not used by the military.)
Turbine meters, like the PD meters, will register air and any obstructions from trash which can effect the
flow (the meter must be protected while in operation). Protective devices consist of air eliminators and
basket type strainers.
Inferential Rate Meters. The inferential rate meter is an instrument whose primary element (orifice
plate), when placed in a flowing stream, infers the flow rate by known physical laws (pressure drop based on
the viscosity of the fuels). The orifice plate is machined to exact tolerances.
The secondary element is used to record the quantity of fuel passing through the meter. It consists of
piping, high- and low-pressure sealing pots, and a 24-hour recording chart. The 24-hour chart registers the
viscosity of the fuel (based on pressure drop) and the flow rate in barrels over a 24-hour period.
When the meter is installed it must have straightening vanes upstream and downstream. The length of
the pipe depends on the size and type of valves used. The meter also requires protection from trash in the
flowing stream (line strainers). The meter itself requires a great deal of maintenance and expertise to keep it