Master Meter Proving
In some situations the metering system has limitations where prover can not be used to get acceptable results. Whether it is on a separator system, or on a platform in the middle of the swamp, Master Meters are used to provide a source of certainty in an unproven system.
A Master “Meter” is still a meter just the same as the meter under test, however the master meter is known to have a reliable history and repeatability under various operating conditions. Usual master meters are positive displacement meters because of their good repeatability in most conditions and they are extremely easy to use. The limitations of using a positive displacement meter for a master is their size, weight, and high initial cost. That is why most master positive displacement meters are limited to around 1500 Bbl/hr.
Popular master meters for flow rates higher than 1500 Bbl/hr are Turbine meters. Unlike PD meters, the size and weight are similar to the pipe as the sizes go up. Turbines also have a slightly lower up front cost than PD meters. The drawback of using a turbine instead of a PD meter are the pre/post run requirements for flow conditioning in order to get acceptable repeatability between proving runs. API recommends 10 pipe diameters upstream and 5 pipe diameters down stream with a profile conditioner upstream of the rerun. This become difficult with larger Turbine meters as the pipe length requirements don’t allow it to be easily used as a master meter.
Using MPMS API chapter 4.5, we determine our master meter repeatability.
“To establish a MMF, a proving shall be performed with a repeatability that results in a demonstrated random uncertainty of 0.029 % or better at a 95 % confidence level. Any combination of consecutive runs (minimum of 3 runs to be statistically significant) and repeatability requirements that results in an uncertainty of 0.029 % or lower will meet the requirements of this standard. Increasing the number of proving runs, while maintaining the same repeatability requirements will decrease the uncertainty of the MMF.”
Once the master meter has an MMF under the same conditions as the line meter, the master is now a proven meter, and it can be used to compare to the line meter. We do with by starting a timer set for 45 seconds; within that time we collect pulses from both meters. We then correct and compare each run, looking for a target repeatability of 5 runs that repeat within .05% of each other. Once acceptable results are obtained from the line meter, the prover is used again to prove the master meter to verify the MMF hasn’t changed. If it has changed we take the two MMF’s average and use that in the calculation of the line meter factor.