Meter Factor Variations and Factors Affecting the Accuracy of Positive Displacement Meters
Authors: Rhoades M.C. 
Published: December 2007

The purpose of this paper is to discuss normal meter factor variations which occur in positive displacement meters operating under normal field conditions and those factors which alter positive displacement metering accuracy. The discussion will be based on information derived from observations and tests concerning the measurement of crude oil by positive displacement meters installed in ACT units. The term PD meter refers to the rotary type only. Since all PD meters used in crude oil measuring are designed for the same purpose and have essentially the same characteristics, the term PD meter as used in this paper will have no meaning as to any specific brand name or manufacturer.

Introduction

The research and development programs of many companies interested in custody transfer of liquid hydrocarbons have contributed much to the development of PD metering units, proving methods and to the setting of basic standards and procedure. Manufacturers have published performance data indicating tolerance in repeatability and accuracy which could be expected of their meters. As in any development of a new machine, there is usually some difference between the performance of the system operating under controlled or laboratory conditions and the actual operating or commercial performance. In the case of the PD meter, no absolute specific rules could be expressed stating the exact allowable tolerance concerning repeatability and accuracy which would apply to all operating conditions. As a result, the stated tolerances are broad enough to include all general operating conditions. Changes in temperature, pressure, gravity, flow rate, wear of mechanical parts, torque load and variance in proving during field operations are some of the most common things which bring about changes in accuracy and meter factor variations.

In the event conditions are such that a meter will not perform within tolerances set forth, it then becomes a matter of agreement between the interested parties as to what variations should be allowed. This procedure is satisfactory and workable; however, there is a chance of some disagreement in the event either or both of the parties do not understand proper factor averaging. There are those who believe that 0.25 per cent factor variation between meter provings is too much, while others say it is not enough. Some think the meters should be proved monthly and others feel that six months or even longer intervals between provings is ample. The question is, are we expecting too much or too little accuracy, and if either is the case, what can be done about it.

With increased emphasis being placed on operational economy and automation, the installation of ACT units is increasing rapidly. Calibration and maintenance requirements are also on the increase. The problem is to develop methods and procedures concerning calibration and maintenance which are as economical as possible and result in quality measurements. During the past three years, the West Texas Div. of Phillips Pipe Line Co. has developed a program of calibration and maintenance procedure which is economically sound and is resulting in quality measurements.

Factors Affecting Accuracy

The PD meter is in reality a positive displacement pump operating in reverse, or it could be referred to as a hydraulic motor. The measuring chamber in a rotary meter is designed to consecutively seal off portions of oil each time the meter completes a revolution. By the use of a gearing arrangement, the volume which passes through the meter is recorded on a counter. Actually, there are some 30 or more factors which affect the accuracy of a PD meter. All things affecting the performance of the meter are important and should be given proper consideration; however, for the purpose of this discussion only those factors which have been observed most frequently and are of primary concern to the operator in the field will be discussed. When there is a significant change in the gravity of a crude oil, the temperature coefficient, viscosity and pressure coefficient will change and affect the accuracy of measurement. In most cases where two or more types or grades of crude oil of different gravity are to be metered by the same meter, the oil is segregated and batched through the meter. In other cases different gravity crudes are commingled at the lease by maintaining the required proportion of each gravity oil to the total volume in a mixing tank. The composite mixture is then measured through one meter.

JPT

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