Measurement: proving methods - technical background:
Static proving tanks
A static proving tank is usually fixed on firm foundations and consists of a main chamber calibrated to as fine a degree of accuracy as possible. It narrows at the top to a sight glass or a subsidiary compartment of small cross-section, enabling extremely accurate measurements to be made of the over-delivery or under-delivery quantity of oil shown by the meter under test.
The tank may be sited near the meters to be tested and connected with the pipeline system to enable tests to be carried out without the meter. At large installations, however the excessive length of pipeline connections to the tank might necessitate removal of meters to a separate proving station. This would not reproduce the normal operating conditions of the meter and would not therefore be acceptable for proving meters to be used for raising revenue accounts.
Mobile proving tank
A mobile proving tank is similar in principle to a static proving tank, but is usually of a small capacity, such as 1,500 litres. The tank is mounted on a wheeled bogey and before it can be used, it must be jacked up to raise the wheels and then levelled. Meters fitted at road and rail tank wagon loading bays can then be tested on site under operational conditions.
A reference meter, sometimes called a “master” meter, is normally a high-grade instrument of a type similar to the meters which it is to test. It is installed ‘in series’ (as opposed to “in parallel”) with the meter to be tested and the readings of the two meters are compared. It may be mounted on a wheeled bogey and equipped with controls to form a mobile test rig.
When a meter is proved using a product, temperature and/or flow rate different from those used to calibrate the reference meter, the appropriate corrections obtained from the reference meter correction graph are to be applied to the reference readings. A correction is also necessary when the difference between the pressures exceeds 1.5 bars(27 p.s.i.). The correction graph must have been dated and authenticated at the time of the last proving of the reference meter.
A reference meter must not be used to prove meters which operate at a higher rate of flow than that at which it has been tested.
Reference meters must be proved and calibrated against a recognised National Standard and a certificate to that effect issued by the testing authority must be available for examination.
Being a mechanical device, the reference meter may develop faults which are not immediately apparent and it may be damaged during handling or transit. Proving by reference meter is, however widely practised and is probably the most convenient method for most oils installations.
Pipe provers and prover loops
In-line proving by a permanently installed prover loop is the most satisfactory method from the revenue aspect, as a test can be carried out without delay whenever required. The content of the prover pipe or loop between two detectors is determined as accurately as possible.
“Unidirectional” provers have the calibrated section of pipe in the form of a loop to facilitate the installation of a device for the launching and recovery of a rubber or plastic liquid-filled sphere. Solid spheres are sometimes used with the smaller provers.
The launcher is operated by electrical means from a control near the meter read-out indicator. When a test is conducted, a sphere which completely fills the cross-section of the pipe is launched into it and is carried along at the same speed as the oil. The sphere is detected as it enters the calibrated section or “loop” and this starts a counting device incorporated in a computer or similar ancillary equipment. As the sphere leaves the calibrated section it is detected again, causing the count to cease. The sphere is recovered mechanically and returned to the launching position.
The reading of the counting device showing the pulses generated by the meter during the passage of the sphere through the calibrated section can then be related to the known content of that section, corrected if necessary for temperature and pressure. The number of pulses generated for the corrected volume is then used to determine the pulse/unit volume factor, which is applied to produce the unit of measurement for the quantity register.
“Bi-directional” provers may have a straight calibrated section of pipe or they may be in the form of a loop. They incorporate either a sphere or a piston which, after passing through the calibrated section is reversed by the operation of valves to pass back through the calibrated section to its original starting position. The proving run consists of the ‘round trip’ of the two movements through the calibrated section.
Pipe provers test meters under working conditions without interference with the operation in progress. They are particularly suitable for testing meters controlling cross-country pipelines or long jetty lines.
Increasingly, use is being made of the “small volume” or “compact” prover. Such provers utilise a system of pulse interpolation which enables pulses from a meter to be identified to a fraction of a pulse thus reducing the rounding-off errors which arise when pulses are counted to the nearest whole number. They also perform best with meters, such as turbine meters, whose pulses are emitted at regular intervals. Their advantage is their greatly reduced size compared with conventional provers and the fact that the proving operation can be carried out much more quickly. Such provers may be mobile or in-line, and uni-directional or bi-directional.