MASTER METER USE AND REQUIREMENTS | CONTROL SYSTEMS
Performance
A master meter shall meet the manufacturer’s most stringent performance specification.
The master meter must not be affected by installation conditions relating to pipe stress,
flow profile distortions due to upstream piping configuration, and must have negligible
performance bias due to changes in fluid properties and ambient conditions.
Direct Drive
A master meter must have a direct drive from the measuring element to the pulse
generator or the mechanical register (e.g., no calibrators, etc.)
Size
One must carefully select the size of the master meter according to the operating
requirements of the working meter. A master meter should be capable of proving the
working meter at both its maximum and minimum normal operating flow rate, and able to
perform a slip test at 15-20% of the rated flow (maximum design flow rate) of the working
meter, if necessary.
Flow Rate
The flow rate through the master meter must be within the normal allowable range
specified by the manufacturer (20 to 80% of its maximum design flow rate), and
preferably having ±0.15% linearity over the normal flow rate.
Calibration under conditions similar to working meter operating conditions
A master meter may be used to calibrate a working meter only if the master meter can
itself be calibrated or proved under operating conditions similar to those at the location of
the working meter.
Repeatability
A master meter may be operated at any rate at which it has been calibrated to, or have a
meter factor repeatable, within 0.02 percent (%).
Temperature compensation
A master meter should be non-temperature compensated (that is, the raw or un-altered
pulse count should be available). The master meter should have a direct drive to the
counter without a calibrator.
Temperature and pressure measurement
An industrial mercury thermometer and dependable pressure gauge are required for use
with a master meter.
The pressure gauge should be located as close as possible to the inlet side of the master
meter. A portable master meter should be equipped with a strainer and may be equipped
with a check valve.
Electronic master meters – If the master meter is of an electronic type, and equipped with
a pulse transmitter and a temperature sensor (RTD), the master meter can be proved by a displacement-type prover (conventional type pipe prover or “small volume prover” – See note below).
Note: A prover that counts more than 10,000 unaltered pulses is commonly known as a “conventional type prover”. Whereas a prover that counts less than 10,000 unaltered pulses is known as a “small volume prover”. All small volume provers use double chronometry to compensate for the lack of accumulating 10,000 unaltered pulses. Small volume prover (a.k.a compact prover) is a vendor specific name.
Temperature sensors should be of suitable range and accuracy, and should be installed as close as possible to the master meter and at the inlet and outlet of displacement-type provers. Caution must be exercised to ensure that the temperature sensors are located where they will not be shut off from the liquid path. Refer to API MPMS, Chapter 7 for requirements.
Electronic master meter controller
By the use of an electronic counter and a master meter controller it is possible and
convenient to prove a working meter by the master meter method using continuous run
procedures. Continuous run procedures increase accuracy and eliminate the need to
stop the meters to read the meter counters.
Requirements for Master Prover
Volumetric tank prover as master prover
(1) Size of tank prover – The volume of the tank prover must not be less than the volume delivered in one minute through the master meter at its normal operating flow rate, and preferably should be one-and- a-half to two (1-1/2 to 2) times that volume. Acceptable master meter/tank prover combinations are:
Master Meter Tank Prover
4” 3000 – 5000 liter
6” 4000 – 5000 liter
2) Temperature – Temperature devices of suitable range should be graduated in fractional degrees and should be accurate within 0.1 °F (0.05 °C) or better. They should be verified/compared against a reference device (such as a thermometer) traceable to the national standard (for example, NIST).
The thermometers should have a minimum stem immersion of 12 inches and should be graduated in increments of 0.2 °F (0.1 °C) or less. The preferred thermometer stem immersion is 1/3 or more of the radius of the tank prover.
Thermometer accuracy should be verified at the time of installation and re-verified periodically, but not less frequently than every six (6) months.
The number of thermometers and their locations are shown in Table 2 below:
Table 2 Number and Locations of Thermometers on Tank Provers.
(3) Pressure gauge – Pressure gauges and transmitter are not needed if an open tank prover is used.
(4) Neck, bottom drain, sight glass, and insulation
Neck – The capacity of the upper neck falling within the gauge glass length should be at least 2 % of the prover volume. The capacity of the lower neck falling within the gauge glass length should be at least 1% of the prover volume. The diameter of the neck should allow the smallest readable graduation to represent no more than 0.02 % of the total volume of the prover. In all cases, the inside diameters of the necks should be at least 31/2 inches.
Bottom – On provers where the bottom level is determined by complete draining, the bottom drain valve should be installed in a vertical position or in a downward sloping length of pipe. There should be means for detecting leakage from the bottom drain valve.
Sight (gauge) glass – The tank prover consists of a relatively large center section joined by transition sections to reduced-area necks on both top and bottom.
The reduced-area necks are provided with sight glasses and scales to indicate the opening and closing volume of liquid in the tank. The sight glass scales should be secured and sealed.
Insulation – The prover should be insulated if the liquid temperature differs excessively from the ambient temperature, and should be coated on the inside to minimize corrosion (and paraffin encrustation if black oil is measured).
(5) The tank prover must have a valid calibration certificate, traceable to the national standard.
(6) Last calibration of the tank prover should be within five (5) years.
Displacement-type prover as master prover
(1) The displacement prover, either a piston-type small volume prover or a pipe prover shall meet the requirements in API standard (API MPMS, Chapter 4), which are not repeated here.
(2) If a displacement-type prover (pipe prover, piston-type small volume prover) is used as the master prover, the capacity of this master prover should allow it to be operated between the minimum and maximum velocity of the sphere or within the normal operating flow rate of a piston-type prover.
(3) The displacement prover must have a valid calibration certificate, traceable to the national standard.
(4) Last calibration of the portable pipe prover, or piston small volume prover should be within three (3) years.
(5) A discrimination of at least 1 in 10,000 must be provided, e.g., 1 liter if the minimum proof-run volume is 1,000 liter, or 0.01 barrel if the minimum proof-run volume is 100 barrel.