ISO 3171 Petroleum liquids - Automatic pipeline sampling
10 Flow measurement
10.1 Accuracy and rangeability
A flow-proportional sampler requires a flow signal to pace the sampler. The flow transmitter should have enough rangeability (turndown) to produce a reasonably accurate signal over the expected flow range of the pipeline. The accuracy of the flow signal should not degrade the representativity of the sample beyond the tolerante of the analytical technique (see clause 16). An accuracy of better than +/- 10 % of actual flow over the operating flow range should be achieved. When the water concentration figure is higher at low flows it is particularly important that the flowmeter produces a reasonably accurate signal at the minimum flow.

10.2 Use of custody transfer meters for samplers If the location uses meters for custody transfer, the signals from the custody transfer meters should be used to pace the sampler. If the pipeline flow is measured by several meters, the sampler maybe paced by an electronically combined total flow signal or a separate sampler may be provided for each meter.

10.3 Special flowmeters for samplers
If the measurement for custody transfer is by shore tank gauging, a separate flowmeter should be provided for flow-proportional sampling.
10.3.1 Insertion turbine meters are often used in large-diameter pipelines. The meter signal is a pulse train which simplifies the electronie design of the controller. Rangeability may be limited and accuracy tan be seriously affected by changes in the viscosity of the metered liquid. Insertion turbine meters may be fouled by fibrous matter or other debris in the oil, and therefore consideration should be given to the possibility of installing a second flowmeter.

10.3.2 Ultrasonic flowmeters are often used. The flowmeter signal is a pulse train and current which simplifies the electronie design of the controller. Ultrasonic flowmeters are not affected by viscosity. Minimum flow rate is typically 0.09 m/s.

10.3.3 Orifice plate flowmeters may be used; alternatively, either venturi meters or flow pitot tubes may be used to minimize the permanent pressure loss. The pressure drop across the flow element is measured by a differential pressure transmitter which provides an output that is proportional to the square root of the measurement. A single transmitter installation has a range in the ratio of about 4 to 1. Two transmitters, one with a high and the other with a low differential pressure range, may be used to extend the range. However, the controller design for a two-transmitter installation is complicated and problems can arise.

10.3.4 For small flow rates, a line-size positive displacement meter or turbine meter may be used. The meter installation can be simple since it does not have to provide custody transfer accuracy. Proving and calibration facilities are not required.

10.3.5 Other flowmeters, such as vortex shedding meters, may be used.

10.3.6 It may be possible to use a signal derived from a tank level gauge provided that the precision is within the limits recommended for flowmeters in 10.1.

10.4 Precautions
Any meter should be capable of operating both in the presence of a high concentration of water and in the presence of particulate matter or gas bubbles.

The final step of a cargo off-loading operation is usually the stripping. Consideration should be given to the possibility that during this operation quantities of undefined gas might be fed into the unloading line, simulating a liquid flow at the flowmeter output and also damaging mechanical meters by operating them too quickly.