Riesgo de Liquidez

You saw, from Test Yourself 9, that small inaccuracies in measurement of oil can result in considerable revenue losses. In order to minimise any errors the meters are _{proved at regular intervals.} The term _{proving is used in the oil industry to refer} to the calibration of oil meters.

The procedure involves comparing the indicated (recorded) volume of oil passing through the meter with the actual (true) volume as measured by a very accurate device known as a _{prover. From this} comparison a correction factor can be obtained which is then used to convert the observed flow readings to true values.

This correction factor is known as the _meter

factor.

There are various types of meter prover, but the most common one is the _{pipe prover.}

The basic principle on which a pipe prover works is as follows:

A slightly oversize, elastic _{sphere is installed in a} special length of pipe. It is free to move within the pipe as it is pushed by oil flowing through. As it moves it forms a travelling seal against the inside of the pipe.

The prover is connected in series with the meter to be proved. So, the volume swept out by the sphere in a given time is identical to the volume passing through the meter.

Two detectors are installed in the special pipe near each end. These emit a signal when the sphere passes them, which is transmitted to the pulse counter of the meter. When the sphere reaches the first detector it starts the counter. When the sphere reaches the second detector it stops the counter. The pulses, and therefore the volume, recorded by the meter should be the same as the true volume displaced by the sphere as it travels between the detectors. If it is not, the recorded volume and the true volume are compared, to arrive at the meter factor.

The meter factor then is

accurately calibrated volume of prover volume registered by meter A _{flow control valve, which controls the flow of}

liquid through the metering run. When there are two or more metering runs, a central _metering

controller will apportion flow between the

different flow control valves to ensure that each meter run is operating within its limits.

A motor operated _{outlet block and bleed}

valve (MOV), which allows the metering run

to be positively isolated from the rest of the process downstream. This isolation is required when the meter run is out of service, or when it is being _{proved by the meter proving system.} A second, motor operated _{block and bleed}

valve (MOV), which is opened when the meter

run is being proved. When this occurs the flow is diverted through the second MOV to the meter proving system.

In practice, the pressure, temperature and density of the oil may change while the flowrate is being measured. To compensate for these changes, readings of the temperature, pressure and density are taken. This information is then fed, together with data from the flow measurement device, into the flow computer. Corrected values for volume flow rate, mass flow rate, etc., can then be computed and recorded.

Pipe provers usually consist of a U-shaped or W-shaped length of pipe. _{Figure 22 is an} illustration of a _{bi-directional U-shaped meter}

A _{bi-directional U-shaped meter prover loop,} operates as follows :

The flow enters the meter prover through the meter under test.

In the position shown, the oil flow is holding the calibration sphere against the buffer. If the 4-way diverter valve is now turned through 90 degrees, the flow through the prover loop is reversed. This reversed flow picks up the sphere and carries it round the prover loop for the _{first pass. Two sphere} detectors are mounted in the prover loop, and the internal pipe volume between these detectors is already known.

As the sphere passes _{sphere detector 'A', a} signal to the flow computer records the flowmeter reading at that point.

When the sphere passes _{sphere detector 'B', a} new flowmeter reading is recorded.

The difference between these two meter readings, representing the metered volume of the prover loop, is now computed and stored.

The calibration sphere, at the end of the first pass, is now held against the other buffer.

The flow computer now turns the 4-way diverter valve through another 90 degrees to start the

second pass.

The second pass is now completed as above, but with the oil flow reversed.

The flow computer will then average the two metered volumes from the first and second passes and compare this average with the known volume. If the volume recorded by the meter under test is the same as the known volume then the meter has been _proved.

If there is a discrepancy between the measured volume and the known volume, the flow computer will calculate a correction factor and then apply this to the meter under test. Another meter proving run will then take place.

When the flowmeter reading (including any correction factor) falls within 0.5% of the known volume, without adjustment, for at least five consecutive proving runs, it is classed as being accurate.