API MPMS 5.2:2005(R2010) pdf download.Manual of Petroleum Measurement Standards Chapter 5—Metering
5.2.5.2 CAUSES OF VARIATIONS IN METER FACTOR
There are many factors which can change the performance of a displacement meter. Some factors, such as the entrance of foreign matter into the meter, can be remedied only by eliminating the cause of the problem. Other factors depend on the properties of the liquid being measured; these must be overcome by properly designing and operating the metering system. The variables which have the greatest effect on the meter factor are flow rate, viscosity, temperature, and foreign matter (for example, paraffin in the liquid). If a meter is proved and operated on liquids with inherently identical properties, under the same conditions as in service, the highest level of accu- racy may be expected. If there are changes in one or more of the liquid properties or in the operating conditions between the proving and the operating cycles, then a change in meter factor may result, and a new meter factor must be determined.
5.2.5.2.1 Flow Rate Changes
Meter factor varies with flow rate. At the lower end of the range of flow rates, the meter-factor curve may become less reliable and less consistent than it is at the middle and higher rates. If a plot of meter factor versus flow rate has been devel- oped for a given set of operating conditions, it is possible to select a meter factor from the curve; however, if a proving system is permanently installed, it is preferable to reprove the meter and apply the value determined by the reproving. If a change in total flow rate occurs in a bank of two, three, or more displacement meters installed in parallel, the usual pro- cedure is to avoid overranging or underranging an individual meter by varying the number of meters in use, thereby distrib-uting the total flow among a suitable number of parallel dis- placement meters.
5.2.5.2.2 Viscosity Changes
The meter factor of a displacement meter is affected by changes in viscosity which results in variable “slippage”. Slippage is a term used to describe the small flow rate through the meter clearances which bypasses the measuring chamber. The meter factor accounts for the rate of slippage only if the slippage rate is constant. Viscosity may vary as a result of changes in the liquids to be measured or as a result of changes in temperature that occur without any change in the liquid. It is therefore important to take into account the parameters that have changed before a meter factor is selected from a plot of meter factor versus viscosity. It is preferable to reprove the meter if the liquid changes or if a significant viscosity change occurs.