Author: Rossella Mimmi
Lease Automatic Custody Transfer (LACT) units provide a method to transfer the ownership of crude oil from the production facilities to pipelines or trucks. Their use is constantly increasing due to the many advantages they present compared to tank gauging and other systems: for instance, increased measurement accuracy, better use of labor, better scheduling of runs to pipelines, elimination of measurement errors due to tank botom build-up or encrustation, and reduction in investment and maintenance costs. Manual tank gauging also brings up concerns regarding safety.
A crucial component of a LACT unit is the flow meter itself, suitable for custody transfer measurement; meter types that are commonly used are positive displacement, or Coriolis meters. The meter will accurately and precisely measure the fluid stream volume or mass and accumulate the total throughput.
However, a periodic calibration of the meter is necessary, as its performance can be affected by changes in fluid physical properties (pressure, temperature, flow rate, density and viscosity), mechanical wear, obstructions in the pipe, or deposits from asphaltenes and paraffin. The performance of the meter must be regularly verified to make sure that repeatable results are consistent and traceable to an external reference. Meter calibration provides traceable evidence of meter performance through on-going trending of meter accuracy and repeatability.
A largely used method to check the performance of the LACT meter is Proving, that provides on-site meter calibration. It is based on the principle of comparing a known volume against the meter output; the ratio between the prover reference volume and the meter reading is the meter factor, which will be used to correct the meter reading. Provers can be uni- or bi-directional, and use a sphere (ball provers) or a piston (piston provers).
Proving should be performed under the same conditions as the meter is normally expected to operate. During meter calibration, the prover is connected in series to the meter to compare the volume registered by the meter and the prover whilst liquid is flowing steadily from one to the other.
Provers are available also in compact configuration; compact provers were developed to answer the need for a smaller and lighter proving device where space is limited and mobility is desirable. A single prover may be used in multiple locations for proving various sizes of meters and large volumes.
Another way of calibrating a flow meter on-site is to put it in series with another flow meter called master meter and compare their readings. The meters that can be used as master meters must be compatible with the fluid being tested, be very reliable and repeatable, and give reproducible results. Although this method adds one additional step to the calibration chain, so it is considered to have a higher uncertainty than direct volume proving, it has the distinct advantage of speeding up the whole process and giving satisfactory results. Other advantages of master meters are lower initial investment compared to provers and short payback time, reduced maintenance and no need of planned activities; they will also produce better repeatability because the prove run-time can be as long as desired, or chosen by the batch size rather than defined by the size/volume of the prover.
Master meters can be calibrated on-site with a volumetric prover (direct method) under the same operating conditions; that is the preferred method as it reduces the uncertainty. Another acceptable solution is to send the meter to external laboratories that provide calibration services (indirect method); the 3rd party calibration facility will normally provide detail of the offset from the reference standard for the meter over the calibrated flow range. This known offset can then be applied as a meter factor correction within the flow computer.
Proving and validation of LACT meters can be a costly operation, but necessary to ensure that the agreed system uncertainty can be achieved. Many challenges are involved in this particular application: frequent changing of operating conditions and fluid quality, compliance to increased standards and regulation requirements, necessity of ensuring measurement system lifecycle sustainability and reliability. Emerson can provide a range of efficient and cost effective solutions for proving and validations systems to help ensure on-going commercial and standards accountability and avoid fiscal risk.