F UNCIÓN SOCIAL DEL P ARQUE N ACIONAL E STEROS DE F ARRAPOS E

Tank provers have a capital cost advantage over pipe provers in fixed installations. They are best suited to low capacity or remote locations without electric power. Tank provers are not suitable for high vapor pressure liquids or for viscous liquids that may not completely drain from the tank prover inner surfaces.

Operation of static fill type provers, such as tank provers, requires that the flow be started and stopped and thus they do not provide for a meter correction factor under actual measurement conditions of flow, pressure and temperature.

7.4.7.2 Conventional Pipe Provers

Conventional pipe provers operate by displacing a known volume of liquid within a calibrated section of pipe. Repeatable displacement of fluids is achieved by an over- sized sphere or piston travelling through the pipe between detectors. The first conven- tional pipe provers standardized by API achieved the required measurement resolution of 0.01% pulse resolution by generating no less than 10,000 m pulses during a proving pass. The pipe prover design is such that the full flow through the metering stream be- ing proved will pass through the pipe prover.

Pipe provers are an important part of a custody transfer metering station and are used to calibrate the meter K-factor periodically when flow rates or conditions change. The pipe prover is used to prove the accuracy and repeatability of flow meters on actual operating fluids by measuring the volume of fluids passing through the meter in rela- tion to the number of pulses generated by the meter. The basic operating principle of the pipe prover is to meter the fluids swept by the displacer through a calibrated volume of pipe by counting the number of meter pulses between the start and stop detectors at either end of the calibrated volume. The displacer, either a piston or sphere, actuates the start and stop detectors and is designed to form a sliding seal which moves at the same rate as the flowing liquid. Temperature and pressure corrections are required to convert the calibrated volume at standard conditions to process conditions. The vol- ume indicated by the meter is compared to the calibrated volume to determine a meter

K-factor. Generally, meter K-factor calibration must achieve repeatability over five successive runs to within a band of 0.1% to meet the generally accepted requirement for overall measurement uncertainty of ±0.25% at the meter station.

API MPMS Chapter 4 sections 1 to 9 and ISO 7278 parts 1 to 4 are the current publications standardizing pipe prover design and operation.

The conventional positive displacement pipe prover generates no fewer than 10,000 pulses for each proving pass to achieve a measurement resolution of 0.01% as defined in the API MPMS and ISO standards. Conventional pipe provers can be constructed in a number of configurations such as unidirectional or bi-directional pipe provers with piston or sphere displacers.

7.4.7.2.1 Uni-directional Provers The uni-directional prover utilizes an oversized

sphere which is launched from a sphere handling valve during the proving cycle. The sphere enters the prover piping and passes the first detector switch actuating the meter pulse counter. The sphere travels with the flow through the calibrated section of pipe past the second detector which stops the pulse count and the sphere then drops into the receiver piping and back to the entrance of the sphere handling valve. The system is then ready to begin the proving cycle again (Figure 7-25).

The uni-directional provers are typically used for proving large meters with high flow rates. The unidirectional prover assembly does not need the 4-way diverter valve required for the bi-directional provers and this reduces the system cost. Unidirectional provers are more suitable for those applications where the product always flows through the prover such as products with high viscosity or high pour point at ambient temperatures.

Figure 7-25. Uni-directional sphere prover — MPMS 4.2 (reproduced courtesy of the Amer-

7.4.7.2.2 Bi-directional Provers With this type of prover, a sphere is moved by the

product flow from the launch piping into the proving loop. The sphere then continues past the first detector switch into the calibrated section of pipe, then past the second detector and out into the receiver chamber. When the sphere passes the first detector switch, the pulse counter is triggered to count meter pulses until the second detector switch is triggered. The number of pulses accumulated on the pulse counter while the sphere moves between detector switches is compared to the calibrated volume of the prover section to obtain a meter factor.

The proving cycle of a bi-directional prover is one round trip of the sphere. The travel of the sphere is reversed by activating the four-way flow reversal valve and the pulse coun- ter accumulates pulses for the two runs of the sphere. Bi-directional pipe provers require 20,000 pulses for each prover round trip 10,000 pulses for each pass (Figure 7-26).

A bi-directional piston prover (Figure 7-27) works on the same principle as the sphere prover with the only difference being the way that the displacement piston is stopped at the end of each proving run. In this case, the piston comes to rest once it is past the diversion piping at the end of the prover pipe section. When the four-way di- version valve is actuated, flow in the prover pipe is reversed and the check valve in the diversion piping closes and flow is directed to the end of the prover pipe. This actuates the piston in the opposite direction.

7.4.7.2.3 Small Volume Displacement Provers Compact or small volume provers

are similar to standard pipe provers with the noticeable difference that the displacer is not free; it is in fact a piston connected to a piston rod. The rod extends outside the barrel of the prover and is usually fitted with an indicator either of the micrometer type, or is fully electronic using a displacement transducer and flags/proximity switches (Figure 7-28).

Figure 7-26. Bi-directional sphere prover — source MPMS 4.2 (reproduced courtesy of the

Figure 7-27. Bi-directional piston prover — MPMS 4.2 (reproduced courtesy of the American

Petroleum Institue)

Figure 7-28. Uni-directional compact prover — reproduced courtesy of FMC Technologies,

Due to the relatively small volume of product displaced by the piston’s travel or stroke, a means had to be found to ensure compliance with the API’s recommended stan- dard minimum of 10,000 pulses per run. The means employed was pulse interpolation.

There are several methods of employing pulse interpolation, but by far the most used is the dual chronometry (double clock) method (see API MPMS Chapter 6. Prov- ing Systems — Pulse Interpolation). Essentially, this method involves the use of two timers (T1 & T2) both driven by the same high-speed clock.

The sequence employed is:

Start Timer 1 (T1) when first detector switch is activaed. ·

Start Timer 2 (T2) at the leading edge of the next flow meter pulse following T1 ·

start. (This is also where the pulse counter will start counting.) Stop Timer 1 (T1) when the final detector switch is activated. ·

Stop Timer 2 (T2) at the leading edge of the next flow meter pulse following T1 ·

stop. (This is also where the pulse counter stops counting.)

The formula to determine the number of full and partial pulses is then:

1 1 Vol 2 T n K ´ T + = where K = proving K factor

T1 = time elapsed for Timer 1 over duration of run T2 = time elapsed for Timer 2 over duration of run n+1 = number of pulses counted during T1 timing Vol = base volume of the prover

7.4.7.2.4 Master Meter Provers Master meter proving is typically used when other

prover methods are not practical. Master meters add another layer of calculations that provide additional uncertainty to the measurement. The master meter must first be proven to establish its meter factor and then this factor is used to determine the field operating meter’s meter factor. In the master meter method, a meter is first proved by a prover that has been calibrated by the water draw method.