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Difference between revisions of "Liquid flow meter proving and LACT units"
Revision as of 12:14, 17 September 2013
Meter proving is the physical testing of the performance of a liquid meter in a liquid service. The main purpose of the test is to assure accuracy. The basic principles of proving a liquid meter are the same whether it is a Coriolis meter, turbine meter, or a positive displacement meter. Each type of meter has its own characteristics when being proved, but the basic principles are the same:
Meter factor prover known volume/meter reading.
When proving a meter, the process-fluid conditions must be as stable as possible throughout the proving process. This includes:
- Flow rate
Before starting a meter proving, let the liquid flow through the meter and prover long enough so that the conditions stabilize. Check for leaks or fluid bypassing around the prover or meter. The only way to obtain a reliable meter factor is to have all the liquid that is measured by the meter also measured by the prover.
When in the field, a meter’s performance may change because of:
- Installation effects from piping
- Mechanical wear of the meter
- Changes in the physical properties of the metered fluid
Therefore, the meter is proved to adjust for these changes, and the meter factor is applied when calculating the total net volume.
Meters are proved on a periodic basis determined contractually by the buyer and seller or by company policy. Some meters are proved for every batch transaction, which could be several times a day, while other meters may be proved as little as once a quarter. Regular proving ensures that the metering system is providing accurate flow data and confirms the integrity of the metering system.
Lease automated custody transfer (LACT) units
LACT units are designed for unattended custody transfer of crude oil from a seller to a buyer. The LACT design is determined by:
- Flow rate
- Operating pressure
- Temperature of the oil
Minimum pressure drop through the piping and components is desirable.
LACT units have traditionally been fitted with positive-displacement meters, but a turbine meter can be used with certain types of fluid. New units being built today utilize Coriolis meters because they have no moving parts and can offer a lower cost. See Fig. 1.
Positive displacement meters require certain accessories to read throughput. Large numeral counters equipped with a switch to operate a sample solenoid or provide a meter-failure circuit are common. Right-angle drives and photoelectric transmitters provide a pulse output for proving the meter. Positive-displacement meters must be equipped with some type of mechanical temperature-compensating device or an electronic temperature averager.
Coriolis and turbine meters are available with electronic transmitters that provide a local display, temperature averaging, a sample solenoid switch, and a high-frequency pulse for proving. Coriolis meters also provide an online measurement of observed gravity and calculate corrected gravity.
LACT design considerations
Centrifugal pumps are commonly used as charge pumps for LACT units, which typically operate at low enough pressures to allow the use of 150 series American Natl. Standards Inst. (ANSI) flanges. This type of pump provides a smooth flow without pulsation and does not require pressure relief protection. LACT units need a strainer before the pump to trap sediments. Failure to remove these sediments can cause damage to the pump and/or internal parts of the meter. The LACT unit should also have an air eliminator on a rise between the pump and meter to eliminate air or vapors from pumped liquids. Often, the strainer and air eliminator are contained in a single unit. (See Fig. 2.)
LACT units are equipped with sediment and water (S&W) monitors that test the oil for the presence of water. The S&W probes are typically internally coated capacitance type for continuous monitoring. The monitor is used to detect unmerchantable oil, generally 0.5% water or more. The monitor sends a signal to an alarm panel that actuates a three-way divert valve that diverts the flow back to the tank to be treated again before it passes through the meter.
The sampling system is very important to the operation of an LACT. Sampler and S&W monitor locations are critical. They are normally installed in a vertical run of pipe downstream of an ell, where the flow is thoroughly mixed so that the probe and sampler "see" a representative sample. The sample probe can be placed downstream of a static mixer. Samplers should be paced by the meter and inject a common sample of 1.5 cm3 per stroke into the sample container. The size of a sample container is determined by the throughput of the LACT. The sample line from sampler to the sample container must be sloped downward toward the sample container with no high or low points in the line.
Downstream of the meter is a proving manifold. This manifold consists of three valves. The block and bleed is the inline valve and must be a double block and bleed type so it can be checked for leakage. Proving requires no leakage through this valve. The two bypass valves divert the flow through the prover and back into the line for full flow.
A backpressure valve should be installed downstream of the proving manifold to maintain a constant backpressure on the centrifugal charge pump, meter, and prover and assure constant flow rate through the LACT. A check valve is also needed downstream of the backpressure valve on an LACT unit so fluid cannot flow back from the pipeline and be metered twice.
Pressure gauges are needed on the pump discharge, at the air eliminator/strainer, and at the meter to check for normal operation. These gauges indicate if the strainer needs cleaning or if meter problems exist.
An electrical panel on the LACT controls the function of the unit. The control panel can be mounted on the skid in an explosion-proof enclosure or placed off skid. Newer units are being built with programmable controllers, thus eliminating relays and allowing better control of flow rate, pressure, and sampling.
LACT operation and maintenance considerations
- The meter and valve drains and all flanges must be checked for leaks.
- The strainer must be cleaned periodically to maintain normal flow rate.
- The S&W monitor should be recalibrated monthly or when a delivery is completed.
- PD meters require periodic maintenance of the gear train, packing gland, the counter, or right-angle drive.
- The block-and-bleed valve should be inspected at each proving for leakage. If a leak is detected, a proving should not be performed until the valve is repaired.
- The charge pump must be checked for excessive vibration or leaking seals. A drop in flow rate may occur if the impeller is partially plugged.
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