The Calibrated Pipe Prover-An Improved Method for Calibrating Meters in LACT Units
Authors: Pfrehm R.H. 
Published: December 2007

Meters in LACT units traditionally have been calibrated with either prover tanks or master meters. After a brief discussion of the mechanical operation, advantages and disadvantages of these methods, this paper introduces a third tool-the hi-directional calibrated-pipe meter prover. Details of the these mechanical operation, application, and design and construction features are given. Compared to the prover-tank system, the completely-automatic calibrated pipe prover is faster, more accurate, less expensive, operates continuously, is unaffected by fluid properties, needs no pump-down, is adaptable to proving meters at various rates and pressures, and is suitable for multimeter installations. The potential accuracy of the system is somewhat better than 0.02 per cent, since errors due to starting and stopping the meters are eliminated.

Introduction

The outstanding function of any successful lease automatic custody transfer unit is its ability to accurately measure and test the liquid passing through it. A large percentage of the LACT units now in service use positive displacement meters as measuring media. It is with these meters that this paper is primarily concerned. Meters have proved to be reliable and accurate devices capable of acceptably measuring petroleum within the specifications of the oil industry. However, a meter is a mechanical device subject to wear and to variations in operation due to characteristics of the liquid. It must be calibrated periodically to assure its accuracy. A meter is only as accurate as its calibration.

Calibration by Prover Tanks The most widely accepted method of calibrating meters involves a prover tank. Prover-tank volume is established by withdrawing water from the tank and measuring it in containers certified by the U. S. Bureau of Standards. The prover-tank method compares a known volume of liquid to the volume of the same liquid as indicated by the meter register. The comparison establishes the "meter factor". Volumes of liquid subsequently passing through the meter are adjusted by this factor. The use of prover tanks is a satisfactory method of calibrating meters and, to a large extent, has been responsible for the acceptance of meters within the oil industry. However, there are certain recognized inaccuracies and limitations in prover tanks. The meter's rotor (or turbine blade in the case of turbine meters) is accelerated at the start of each run and decelerated at the end. This causes variation in the meter performance. The prover tank must be large enough to make this error less than the maximum tolerance. Encrustations of deposits on the prover tank will cause a calibration error. These conditions are more pronounced when heavy-paraffin and asphalt-base crude oils, and low ambient temperatures are encountered. Liquid temperatures within the prover tank will vary, as well as the temperature between meter and prover tank, due to the effects of ambient temperature, sun and wind. Other problems include throttling to maintain proper flow rate and meter proving pressure when going into a prover tank, and time-consuming "pump-down" at the end of the run.

Calibration by Master Meters

Another method of proving meters commonly used on LACT units is the master meter. This method involves the use of a frequently-checked portable meter which is moved from location to location. Master meters are usually of the positive displacement type with all unnecessary equipment removed, leaving just the meter movement and registration equipment. This is done to eliminate as many sources of error as possible. Master meters, if they are to give accurate results, must be proved frequently against a calibrating device at the rate and in the liquid used when proving LACT or other meters. To prove a meter using a portable master meter, the two meters are connected in series and their registrations compared for the prover run. Connections are such that the entire stream passing through the meter being tested also passes through the master meter.

JPT

P. 1087^