Custody transfer instruments and systems used for trade purposes in the petroleum industry require type approval according to the Trade Metrology Act before they can be certified for use in trade transactions .The legislation concerning the requirements for measuring instruments used in trade was originally based on onternational requirements .However ,with the rapid advances made in measuring technology legislation has not kept pace with the approval requirements of the new technology.
In 1991, the Trade Metrology function moved to the SABS, which operated its own extensive test facilities. The decision was made to upgrade the regulations to reflect OIML (International Organisation for Legal Metrology) wherever possible. The mechanical engineering section of the SABS was in the process of writing a standard for water meters at the time of the move in 1991 and this became the first instrument for which the OIML requirements were adopted.
While this move saw a greater standardisation for custody transfer systems, the biggest disadvantage for industry was the cost and time involved in initiating the laboratory tests necessary for obtaining verification and certification for each new instrument introduced onto the market.
A major change regarding custody transfer requirements for the petroleum industry, namely mass and volume measurement, was introduced in 1991 with the privatisation of the verification function. This was facilitated through the SABS 0259 standard which is based on ISO Guide 25. Although this took longer to implement than planned, today there are about 110 accredited companies capable of providing mass and volume verification.
The OIML certificate system for measuring instruments used for verification and certification facilitates the administrative procedures and lowers the costs involved in the international trade of measuring instruments subject to the legal requirements. The system allows a manufacturer to obtain an OIML certificate and test report indicating that a given instrument pattern complies with the requirements of the relevant OIML recommendations. These certificates and test reports are accepted by the SABS and laboratory tests need only be done for tests which have not been performed or which do not comply with SABS requirements.
Technology makes it even cheaper
The advances made in custody transfer technology have made it even less expensive in recent years. Traditionally, flow meters had been fitted with mechanical registers, which not only had limited accuracy but were also vulnerable to tampering. Part of the testing process of registers involved checking their resistance to tampering and fraud.
The introduction of electronic flow computers and software has changed this, not only making the meters far more tamper-proof but also more accurate. The software provides advanced supervisory control and management functions such as inlet and outlet valve control, pump start and stop, leak detection, flow rate calculations and temperature compensation according to API, ASTM and ISO requirements. The software integrates easily with off-the-shelf personal or industrial computers.
New developments in the petroleum industry focus on the delivery of fuel by means of bulk delivery vehicles without the use of vehicle-mounted flowmeters. The reason for this shift away from tradition is the need to further reduce costs. Control equipment installed on a delivery vehicle can cost anywhere up to 25% of the cost of the vehicle itself. By eliminating the control system, the operator not only reduces the system's cost, but also the cost of maintenance and calibration. In addition, by eliminating the control system, a saving in weight is gained which enables the operator to load more product onto the vehicle.
This trend presented a new problem for the industry, namely how to prove that the contents of the tanker has been delivered in full to the purchaser of the product. To solve this, the concept of 'sealed parcel delivery' was introduced whereby the tanker is filled with the exact amount required by the customers. Each tank compartment is then sealed either mechanically (such as a lock) or electronically to prevent tampering, and the product is delivered to the customer's site. Sensors installed at the top and bottom of each tank compartment provide an indication of full and empty status.
Such a system eliminates the need to equip each vehicle with a custody transfer system. Instead, custody transfer takes place at the depot, reducing system costs by confining the custody transfer to a single point.
Locally, the industry sees customers requiring partial deliveries of product. To solve this, companies such as Liquid Automation Systems are busy testing truck mounted tank gauging systems designed to continuously monitor product levels in each compartment, allowing the customers to purchase the amounts they require instead of having to take the entire compartment contents. The system, called DipCheck, is considerably lighter and smaller than conventional custody transfer systems and provides a level of flexibility to sealed parcel delivery. The system has received PTB approval for custody transfer in Germany and is presently being tested locally for South African conditions.
Custody transfer systems have come a long way from the days where approval and use made them prohibitively expensive. The advantages now far outweigh the costs involved and it is likely that a greater acceptance of new technologies and methodologies will be seen.
Liquid Automation Systems
(011) 397 3780
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