The accurate measurement of liquid volumes is essential to the proper functioning of the oil industry. The Swedish National Testing and Research Institute (SP) has developed on-site calibration rigs to reduc metering errors to minimal levels.
If the metering error on the three billion tonnes of crude oil which is refined each year is 0.2% - a good standard for petroleum products - the error is equivalent to six million tonnes, which at $18 a bbl, is worth more than $800 million, weights and measures director Harald Lundahl points out.
The mobile calibration units developed by SP have a measurement uncertainty better than 0.1%, thus providing greater accuracy than many operators' metering equipment, including that used on Norwegian offshore platforms, Lundahl says. Customers can have confidence in the calibration measurements as SP is the holder of Swedish standards for most physical quantities.
The institute is also a leading authority in the field of flow-meter calibration, and recently coordinated a calibration. Its mobile system uses a volume calibrator, which is essentially an accurate hydraulic cylinder; the volume flow is measured by the movement of the piston over a given period of time. The design is such that the result is accurate even if the flow rate varies.
Two rigs are currently used to provide SP's mobile calibration service; the larger one is mounted on a truck and the smaller on a trailer. Potential offshore customers have shown interest in the accuracy and reliability of the system. Taking it offshore would be straightforward - even the larger rig weighs only five tonnes and could be installed in a 20-ft container, Lundahl says.
On-site calibration offers a number of benefits, not least that of not having to move the metering equipment to the test place, in the process suffering an expensive shut-down. For the offshore customer the advantage could be that of replacing permanently installed calibration equipment with an occasional visit of a mobile rig.
On-site calibration also means that the meter is tested under normal operating conditions, using the normal liquid. Sources of error resulting from such causes as the system itself, piping, signal processing and the ancillary equipment, can be taken into account. The computer programme which controls the calibration process permits variations in temperature and pressure to be compensated for, and measurements to be accurately varied or repeated. The calibrator is connected in series with the meter, either upstream or downstream of it. If the piping system has been suitably designed, connection and disconnection can take place without interrupting the metering process.
The calibration equipment is suitable for testing most liquids, including crude oil, petrol and LPG. It can handle flows up to 26,000 l/min and media temperatures up to +120 deg C. It is approved for use in Class 1 hazardous areas.
Calibration should be carried out at regular intervals to enure that the required accuracy is maintained, Lundahl says. Meters drift over time, depending on factors such as the operating conditions, the medium, the type of meter and how often it is used. If the medium is oil which contains sand, for example, wear on the meter parts will affect its performance.
SP can also assist customers to develop their own calibration systems, another possibility which could be of interest to the offshore industry.
For more information contact Harald Lundahl, SP: +46 33 16 5000 or fax +46 33.
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