To obtain the lowest overall metering uncertainty, Custody Transfer flow metering stations are often supplied with prover installations or master meter (transfer proving) solutions. The major advantage of a properly designed proving system is that the flow meter can be proved at any time whilst in service, without interfering with its normal operation. ODS Metering Systems design and manufacture our own bi-directional sphere provers and can offer integrated solutions with compact piston provers and master meter (transfer proving) installations.
Make sure your prover is calibrated regularly to check the accuracy and conditions over its operating lifetime!
A sphere prover, also called a ball prover or pipe prover, operates on the principle of the repeatable displacement of a known volume of liquid from a calibrated section of pipe between two detectors. Displacement of the volume of liquid is achieved by an oversized sphere/ball traveling through the pipe. A corresponding volume of liquid is simultaneously measured by a meter installed in series with the sphere prover. A meter that is being proved on a continuous-flow basis must be connected at the time of proof to a proving counter. The counter is started and stopped when the displacing device actuates the two detectors at the ends of the calibrated section. A sphere prover or ball prover is custom designed and its size is based on type and size of the flow meter, in accordance with API chapter 4. The pipe provers are designed and tested for a volume repeatability (over 3 calibration runs) of better than ±0.01% of mean volume (0.02% peak to peak).
The two types of continuous-flow displacement provers are unidirectional and bidirectional. The unidirectional prover allows the ball to travel in only one direction through the proving section and has an arrangement for returning the ball to its starting position. The bidirectional prover allows the ball to travel first in one direction and then in the other by reversing the flow through the displacement prover. This is done with a four way valve (4-way valve).
A pipe prover can be integrated on the metering skid or delivered on a separate skid.
Small Volume Piston (SVP) Prover
A compact ‘small volume piston’ prover can be an alternative to pipe provers because of its portability and space- and weight-saving advantages. The principle is the same as a sphere prover however displacement of the volume of liquid with a small volume prover is achieved by a piston which provides more accurate readings. The small volume prover (SVP) meets the same stringent uncertainty requirements for meter proving according API. For bigger flow rates the SVP shall use multiple passes per prover run to achieve the required repeatability. Our metering engineers can help you to select and size your prover.
Master Meter Prover
The primary flow element can be calibrated using a ‘master’ meter, which may or may not be of the same type. Master meter proving is used when proving by the direct method can not be accomplished because of meter characteristics, logistics, time, space, safety, and cost considerations. Direct master meter proving is the method in which the proving of a line meter is performed indirectly by means of a prover in series with the master meter and the line meter. In indirect master meter proving scenarios, the ‘master’ meter is periodically removed and calibrated at a traceable facility on a different flow stream and/or conditions and returned to service. In principle proving duty meters with a master meter has greater uncertainty than direct proving with a prover.