Hycontrol, UK, has successfully measured the interface between crude oil and water in storage tanks to 20 m in Scandinavia.

Hycontrol, a UK based manufacturer and supplier of an extensive range of instrumentation to a wide and varied customer base throughout the world, has recently supplied Reflex VF7 Series TDR to a Scandinavian company involved in the tank cleaning business of crude oil vessels to measure the interface between crude oil and water in their storage tanks.

Since production of the VF04 ceased in 2005 the interface function has been integrated within the VF7 devices. As well as using the now obsolete VF04 this kind of measuring mode was restricted in so far, that it was absolutely mandatory to equip the new VF7 with a twin 4 mm cable or coaxial probe to achieve this function. For interface measurement two reflections have to be evaluated. On the one hand the level pulse and on the other hand the interface pulse has to be analysed. But even the interface pulse will be naturally absorbed by the product.

These above mentioned probe types provide a stronger focused pulse and thus larger signal amplitudes are therefore intended for interface measurement. To draw the comparison with VF04 the VF7 features a 2 GHz bandwidth, which is approximately twice as high as the old VF04. The high bandwidth produces considerably slimmer pulses and allows among other things a detection of interfaces to a minimum layer thickness of 50 mm.

Beside many advantages, which a higher bandwidth of the transmitted signal provides, another problem occurred which compelled Hycontrol to restrict the maximum length of the twin cable to 8 m. That’s the reason why interface measurement was restricted to 8 m. This was not really satisfactory to the company and it searched for a solution for detecting interfaces reliably in larger tanks. As VF7 offers a higher dynamic than VF04, the opportunity arose to use a mono cable for interface measurement.

A potential customer in Scandinavia allowed Hycontrol to carry out trials on one of their tanks – a tank, which is used for the separation of crude oil and water. Both products will be filled into the customer’s tanks while still mixed. This is the remnants after cleaning out the tanks of crude oil vessels, and remains there since the oil mostly has separated from the water and further cleaning processes will be carried out. This tank with a height of 14 m and a diameter of 9 m was perfect for the trial and evaluation. Just before the beginning of the trial the tank was filled with an oil and water together which formed an emulsion.

In diagram 1 one can see very well the reflection of the top layer of oil, the position is marked with a blue cross and the crossover to the water, position marked with a yellow cross. Within the signal progression in front of the interface pulse a plateau is generated. The reason for this effect is the emulsion, a non homogeneous phase in which water and oil are still mixed. In this layer various reflections will be generated, which finally leads to such a signal progression.

During the test an interface of an emulsion of 1 m could be tracked without any problem. Approximately 12 hours later the mixture was almost completely separated and two clear reflections obtained – one of the surfaces of the oil and the second from the interface of the oil and water, as shown in diagram 2.

In diagram 3 one can see the same situation, only with a twin cable. The signal progression is almost identical. One can say that interface measurement with a mono cable is possible without any problems. In case of a clearly constructed interface caused by the fact that containers of such sizes are typically used as storage tanks and therefore no strong emulsions have to be expected, there are no problems experienced.

In case of a clearly constructed interface caused by the fact that containers of such sizes are typically used as storage tanks and therefore no strong emulsions have to be expected, the company has seen no problems in applying VF7 with 4 mm mono cable up to a range of 20 m for measuring interface. If it temporarily might appear that the products will be mixed, the customer should be informed about the fact that a measurement can be guaranteed only after a complete product separation.

Courtesy: Pune Techtrol Pvt Ltd, exclusive distributors of Hycontrol Ltd for India, Bangladesh & SriLanka. Tel: 020-66342900, 27121052. Fax: 91-20-66342998. Email: ho@punetechtrol.com