In industry, custody transfer flowmeters are found in large numbers in the oil and gas industries. For example, flowmeters are used at the discharge of oil and gas wells to determine royalty payments and taxes thereon. Custody transfer flow measurement systems can also be used to determine the amount of petroleum products transferred between companies. Payments associated with some of these applications can involve the payment millions of dollars, so the ability to verify flowmeter performance is important. To put this in perspective, a calibration error of one percent of a $5,000,000 transfer is $50,000, so attention to detail and in-situ calibration can be important.
According to Zaki Husain, Senior Engineering Specialist, Chevron-Texaco, Houston, Tex., “Many custody transfer installations in the oil and gas industry are calibrated in-situ using a flowmeter proving system.” A prover is a device that derives the flow rate by measuring the amount of time that an object, such as a ball or piston, travels through a known volume. Piping provisions are made to allow the prover to be installed in series with the flowmeter to measure the same flowing stream. Valves in the prover allow the flow to bypass the prover during the time that the prover is not measuring the flow rate. Due to the limited volume available in most in-situ provers, test times are relatively short and can be completed in just a few seconds.
“In North America, the equipment and methodology used to determine payment is usually dependent upon the agreement between the parties,” Zaki adds. “Depending upon the agreement, the custody transfer measurement system could be implemented using such varied measurement systems as a 30-year old orifice plate with a circular chart recorder, or a precision positive displacement flowmeter with a microprocessor-based flow computer.”
Depending on the agreement, these systems may or may not be calibrated using a prover.
Paolo Fiorletta, General Manager, Metroval, Nova Odessa, Brazil, works for a company that makes flowmeters for custody transfer applications. Fiorletta says, “Governmental constraints have been placed on installations in Brazil, primarily to ensure that the proper amount of royalties and taxes are paid and to ensure proper development of the reservoir. Compliance with Brazilian regulations requires that the flow measurement system be designed to meet certain performance requirements and be calibrated on a periodic basis by qualified personnel.” As a result, in-situ calibration of the flowmeter system is virtually mandated in order to maintain performance during operation. Many other countries follow the same or similar regulations.
Non-Custody Transfer Applications
Most flow measurement systems installed outside the oil and gas industry do not involve custody transfer, but rather monitoring and controlling flow. Therefore, the in-situ testing procedures described above are generally relaxed for the majority of these installations. The degree to which they are relaxed is dependent upon the nature of the application and the sophistication of the end user.
In-situ flow calibrations of the entire flow measurement system (primary and secondary) are typically not performed for non-custody flowmeter applications. In many cases, only the secondary is calibrated because it is relatively expensive to perform an in-situ calibration of the primary. As a result, few flowmeter primaries are calibrated in-situ and it can be years until many of the primaries are even inspected.
Matt Otto, Engineer, Bayer, New Martinsville, West Va., takes exception to this, stating that Bayer’s flowmeters are calibrated using a water source and a portable master flowmeter. “Piping connections to implement this type of calibration are routinely provided in new installations,” he says. For high viscosity applications, material is allowed to flow to a portable tote bin that is weighed. The weight of the tote bin is then compared to the flowmeter measurement. Environmental audits generally incorporate in-situ testing of applicable flowmeters that is performed by outside contractors.
Roy Gregor, Component Engineer, Vermont Yankee Nuclear, Vernon, Vt., has mostly orifice plates and differential pressure flow transmitters to measure water flow rates. “The orifice plates are not routinely pulled for inspection because orifice damage would show up symptomatically as a low flow rate discrepancy when compared to other parameters monitored, such as differential pressure and temperature. The transmitters are calibrated using a ‘water pot’ onto which air pressure is applied.” This procedure is an in-situ calibration of the secondary. However, it does not constitute an in-situ flowmeter calibration because calibration of the primary is not performed. It tells you whether the pressure sensor and transmitter are working, but gives you no information about the state of the orifice plate or venturi.
Juergen Amann, Instrument Supervisor, City of Tampa, Tampa, Fla., says, “Most of our magnetic flowmeters in sewage service are so large that wet testing them in-situ is impractical, so technicians can only calibrate the transmitters. In addition, the cost of returning primaries to the factory for calibration is excessive.” Juergen added that most flowmeters cannot be removed from service so it is difficult to gain access to even inspect the primaries. In some installations, material balances and historical information are used to verify the reasonableness of the flow measurements.
Juergen and other flowmeter users in the water and wastewater industry have elected to purchase magnetic flowmeters with expanded diagnostic coverage. These flowmeters can monitor selected parameters in the secondary and primary, such as its associated electrical parameters. Variation from factory specifications would indicate a potential problem. The terminology vendors are using to describe these systems can be misleading. They provide in-situ verification of the parameters, the measurement of which may be traceable to a national laboratory such as NIST. However, these systems do not perform in-situ flowmeter calibration.