by Brian Dickson, Invensys Process Systems (IPS)

Pumps & Systems, February 2009

In the oil and gas industry, custody transfer transactions involve transporting physical substance from one operator to another, including transferring raw and refined petroleum between tanks and tankers, tankers and ships and other transactions. An accurate account of the amount of material transferred is of great value to both the company delivering the material and the eventual recipient. This is especially true in bunker fuel oil delivery since a ship's bunker contributes to the ship's operating cost. 

Digital Coriolis MeterA digital Coriolis meter in use for custody transfer.

The difficulty in measuring bunker fuel oil flow stems principally from air retention in the lines, which is caused by several factors. First, bunker fuel oil is highly viscous, which can cause the pump to draw a heavy vacuum on the suction side. In some circumstances, this can draw air into the pump's gland. Also, fuel oil remaining in the bottom of the tank may draw air as the tank is pumped dry or stripped. In addition, compressed air is introduced into the transfer hose when it is blown clear just prior to disconnection to prevent spillage of bunker fuel oil on the deck. Air entrainment can even be caused by the way in which the barge tank is filled. For example, drop loading and splashing can add air, which becomes entrained. Once air is entrained within the bunker fuel oil, it can take several hours for it to come out of suspension. Temperature and the viscosity of the product are the key variables.

For such reasons, ship owners have resorted to the practice of manually measuring or "dipping" the amount of bunker fuel oil on a fuel barge, but this method does not always result in measurements that all parties agree upon. Two years ago, BP Marine Fuels embarked on a research project to investigate the feasibility of using metering technology to produce irrefutable measurements in the delivery of bunker fuel oil to ships.

BP appointed an independent consultant to evaluate all available technologies. Initial conclusions suggested that ultrasonic meters were both economical and reliable, but the industry view was that Coriolis meters, the alternative metering technology, were unreliable and expensive. Some industry experts felt that the vibration present on bunker fuel oil barges would adversely affect Coriolis meters. However, after detailed discussions with BP's exploration, production, refining and chemical divisions, they suggested, based on their experience, that Coriolis meters were more economical and likely to be more reliable. To arrive at a definitive solution, BP developed a basic statement of requirements, which defined the ideal metering technology in the company's view. 

The requirements stated that an accuracy of 0.1 percent was required for single-phase flow, and an accuracy of 0.5 percent was required for two-phase flow. An invitation was then sent to potential partners and vendors to submit details of their metering products that complied with the basic statement of requirements.

Coriolis Technology In Depth

Coriolis technology offers accuracy and reliability in measuring material flow, and is often hailed as among the best flow measurement technologies. Conventional Coriolis meters, however, have had a significant limitation for custody transfer: They have not performed well in measuring two-phase flow conditions, which involve a combination of gas and liquid mass.

Coriolis meters measure flow by analyzing changes in the Coriolis force of a flowing substance. Coriolis force is generated in a mass moving within a rotating frame of reference. That rotation produces an angular, outward acceleration, which is factored with linear velocity to define the Coriolis force. With a fluid mass, the Coriolis force is proportional to the mass flow rate of that fluid.

While custody transfer operations do not typically feature rotating pipes, oscillating a section of pipe is required to exploit the Coriolis principle for flow measurement. Oscillation produces the Coriolis force, which can be sensed and analyzed to determine the rate of flow. To use Coriolis force for measurement, a Coriolis meter has two main components: an oscillating flowtube equipped with sensors and drivers, and an electronic transmitter that controls the oscillations, analyzes the results and transmits the information.

Reliable Coriolis measurement depends on consistent, reliable oscillation, which is determined by four factors: the density of the liquid, the balance of the tubes, the dampening caused by the flow stream itself and the physical isolation of the tubes from the environment.

Compromising even one of these factors will degrade Coriolis meter performance, and two-phase flow compromises every one of them. Consequently, applications involving negligible amounts of entrained gas-even as little as 2 percent volume-have been poor candidates for Coriolis measurement. This has been particularly troubling for those running batch operations, where reliable, highly accurate flow measurement can confer considerable bottom-line advantage, but where it is also necessary to begin with an empty or partially filled flowtube. 

Advanced digital Coriolis meters have changed all that. Microprocessors in their transmitters run advanced digital signal processing techniques that provide useful measurements of both mass flow and density, ensuring stable operation in either single-phase or two-phase flow conditions, such as those found in bunker fuel oil transfer.