A firm hand on the gauge

1st February 2013

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Manual gauging is not usually associated with advanced technology but recent developments by one manufacturer working in this sector have taken the concept onto a different plane. Andre Odermatt reports.

When they hear the word 'manual' in relation to tank gauging, most people would picture a traditional tape and bob assembly. 'Automatic' equipment would suggest more sophisticated technology linked via remote control system to a computer. The reality is more complex, however.

State-of-the-art manual electronic gauges are highly accurate portable measuring instruments incorporating microprocessor control and reliable electronic sensors.

The latest examples in the field, such as those recently introduced by Enraf Tanksystem, use special tapes with wires on either side for the power and signal transmission between the sensing head and the unit. An insulated coating covers the entire tape, which must be capable of surviving immersion in aggressive chemicals.

Moreover, unlike automatic systems, portable gauging units are designed to be used on many different tanks and can be calibrated in a laboratory independent of the tank geometry. Portable electronic tapes, such as Enraf Tanksystem's Hermetic UTImeter Otex model, are designed to offer fast, accurate and reliable gauging.

A single penetration of the tank can establish ullage, oil/water interface level and product temperature. The accuracy of these measurements is important not only to ensure proper inventory control and guard against short delivery of expensive product, but also in respect of quality assurance standards such as ISO 9000 or legal metrology requirements. It must be possible to verify and recalibrate the gauging instruments when used during custody transfer operations. The unit must be certified by the relevant authorities and fulfils standards such as those laid down by the American Petroleum Institute (API) and the UK's Institute of Petroleum (IP).

Reliable and accurate

Portable electronic gauges are often used to measure hazardous liquid cargoes, not only on tank ships but also on tank barges and land based storage tanks. They can be exposed to an extremely harsh environment and design considerations have to include a number of safety factors. If the instrument is to be used in classified areas it will have to be intrinsically safe and approved by the relevant testing authorities. It must be easy to carry and install on deck or tank roof and, therefore, a small, compact and light instrument is to be preferred. When used to gauge corrosive chemicals the sensing head and gaskets need to be designed with this in mind and FKM, for example, would not be a suitable gasket material in many cases.

A closed gas tight version would in this case also be preferred as our UTImeter Gtex 2000. In terms of cost it is not only the price of the unit which needs to be considered but also any ongoing maintenance and repair costs.

According to research carried out by Enraf Tanksystem, the most frequently undertaken repairs involve the tape assembly. The earliest manual electronic gauges were designed to be operated under open conditions and their materials of construction limited their use to non-corrosive liquids. Today, though, gas-tight instruments with gaskets manufactured from chemically resistant polymers, notably PTFE or FFKM can be employed with a wider range of products while avoiding any vapour release.

How do they do that?

On a storage tank the upper reference point for all level measurements is located on the roof. Whether manual or automatic, all level gauges measure the ullage, the vacant space between the roof and the level of the liquid inside the tank. However, measurements taken by this method can be rendered inaccurate by deformations in the tank roof. There are many technologies that can be applied to liquid level detection.

Optical and capacitive sensors have been shown to offer good accuracy, but are not suitable for all applications. Enraf Tanksystem has, therefore, developed a broad range sensor which has provided high repeatability in all types of liquids, ranging from highly viscous to corrosive chemicals.

Developments such as the new range of portable electronic level gauging devices, Enraf Tanksystem believes, allow the company to offer safe, portable equipment at a low cost with no compromise in terms of accuracy. Indeed, they can be used to calibrate or verify installed automatic systems. Inspection companies, storage tank operators and shipowners alike need to be able to carry out gauging operations to high levels of accuracy and reliability. Equipment such as the UTImeter Otex allows them to do just that.

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Andre Odermatt is with Enraf Tanksystem SA, Bulle, Switzerland.

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