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Product category: Level and tank contents instrumentation
News Release from: Emerson Process Management - Mobrey Measurement | Subject: Level control article
Edited by the Processingtalk Editorial Team on 26 January 2005

Selecting the right cost effective level
control

Liquid level control is integral to the site safety and environmental strategies of a very wide range of processes: selecting the right technology for cost effective level measurement

Liquid level control is integral to the site safety and environmental strategies of a very wide range of processes From a simple sump or buffer tank, where overflow or 'run-dry' events can have expensive consequences, to a sophisticated reactor vessel with a level transmitter providing dynamic measurements to the control system, reliable level measurement is key to process efficiency and safety

Significant market drivers in the level measurement market at this time are the continued need to cut production costs and legislation relating to plant safety and environmental issues.

New regulations are emerging such as the European IEC 61511 functional safety guidelines for the process industry sector, and in UK legislation, the Environment Act (1995) which requires instrumentation to be installed to measure discharges in open channels.

In a complex, mature market such as this one, crowded with proven technologies, changes tend to take the form of a slow trend rather than any dramatic shifts.

Current analysis shows a continuation of the move towards increased automation of processes using the whole suite of level measurement technologies and continued popularity of intelligent non-contact, and increasingly, non-invasive liquid level measurement.

In addition, there remain niche markets which cling stubbornly to traditional mechanical float switches.

A recent study of the US market [US process level measurement and inventory tank gauging industry, March 2003, Venture Development Corporation] predicts that radar and ultrasonic level measurement technologies will experience growth rates between now and 2007 of 9% and 7% respectively, at the expense of more traditional technologies such as hydrostatic, load cell, electromechanical and capacitance sensors.

The reasons for the growing popularity of these technologies are straightforward.

A non-contact measurement made by a sensor with no moving parts is generally easier and less expensive to install and maintain, and, for most applications, the measurement is more reliable.

However, looking more closely at the balance between radar and ultrasonic within this market, recent developments paint an interesting picture.

Traditionally ultrasonic technology is seen as being at one end of the cost/performance scale, and radar at the other.

Users could select high cost, high accuracy radar instruments for applications such as measurement of high value product or, for less demanding applications such as storage or process vessels, ultrasonic technology would provide a reliable and cost-effective solution.

Now, manufacturers are developing both of these technologies to fill the gap between the two.

Recent developments in ultrasonics and new radar products give users a very wide choice of cost, accuracy and functionality allowing them to select exactly the right instrument for any non-contact level application, including difficult liquids and even dry products.

There is now a clear trend in development of ultrasonic technology to provide higher performance instruments - more powerful microprocessors, better echo processing algorithms and improved interfaces making them easier to use - all built into a single 4-20mA transmitter and available at very competitive prices.

This point has been made by a recent study by ARC [Continuous Ultrasonic Level Worldwide Outlook, 2003, ARC] which cites the traditionally high cost of radar and the introduction of new lower cost and more functional ultrasonic devices as creating renewed interest in ultrasound.

This 'swing back' towards ultrasonics proves wrong many in the industry who sounded the death knell for the technology when non-contact radar was heavily promoted to the market in the early nineties.

In this competitive market, radar technology is also being forced to become increasingly affordable.

Lower cost plastic rods are now available on many models as an alternative to expensive metal horns.

Pulsed systems, which deliver less power to the target, offer a solution in less demanding applications at a lower cost than 'top of the range' frequency modulated continuous wave (FMCW) instruments.

Although users are becoming aware that these cheaper options do not always offer similar accuracy and reliability over the range and application base of the more expensive instruments, they may provide an appropriate solution for some simple level measurement applications.

Essentially, ultrasonic and radar transmitters achieve the same objective, but radar waves are more penetrative and less attenuated by air and other gases, so can travel further.

Radar can therefore be used in larger tanks and with liquids which create more difficult ullage conditions.

On the other hand, ultrasonic devices are ideally suited to simpler applications and are always much less expensive - about 50% of the cost.

For most applications it makes sense to consider both options, but if the application conditions allow the use of ultrasonics, then this will usually be the more cost-effective solution.

For simple buffer and storage tank applications, ultrasonic technology usually provides the most cost-effective option.

Where radar does look to be the best option, FMCW instruments which use a continuous signal provide a reliable and accurate measurement in difficult ullage conditions.

Pulsed systems cost less but are typically best suited to less demanding applications.

In making the choice, the ability to get a good return signal to the instrument is the critical factor.

As a rule of thumb, radar performs better in more difficult applications; radar penetrates vaporous or volatile ullage spaces better than ultrasound does, allowing a larger return signal to reach the transmitter but there are important exceptions.

The size of the return radar signal is proportional to the dielectric constant of the liquid, and low dielectric liquids such as hydrocarbons reflect very little.

If there also happen to be reflections from the bottom of the tank, then radar may struggle.

In such applications an ultrasonic device gives better results because of the larger return signal.

Alongside the growth in non-contact electronic devices is a growing market for non-invasive technologies for use with toxic or aggressive chemicals where vapour recovery is an issue, or in hygienic pharmaceutical or food applications.

Here, radar technology excels.

The 'process seal' radar is mounted on the tank lid with a PTFE window through which the measurement is made.

True non-invasive instruments are designed to bolt on to the window so that they can be removed for maintenance without breaking the process seal.

As well as the clear growth in non-contact level measurement technologies, there remains a significant market for point level detection, with electromechanical float switches accounting for the largest share of this market.

Apart from the use of exotic materials for chemical compatibility and improvements to switching mechanisms, there has been essentially no change in the basic design of these instruments for over 50 years.

Time proven in literally millions of applications worldwide, float switches remain the simplest 'fit and forget' solution.

In addition to its vast range of more sophisticated continuous level measurement and point level detection solutions, Solartron Mobrey still sells tens of thousands of float switches every year.

The VDC study referred to above indicates that it is the humble mechanical float switch which has the largest share - more than one quarter - of the point level market in America.

In the process industries, float switch use is predominantly in the petrochemical, marine and power markets in aggressive applications and in remote or difficult to access locations.

Float switches are known to be highly reliable even in harsh high temperature or high pressure conditions, or where there may be excessive vibration, which would not be suitable for instruments that rely on electronics for operation or control.

They are often chosen for tank monitoring application on ships, where their long term reliability and ease of maintenance has advantages in terms of spares inventory and skills carried on board.

In some European countries (for example Germany and Belgium), overfill safety legislation is in place that insists on the use of high level warning devices to prevent over-spill of hazardous substances.

Typically, such legislation insists on the use of fail-safe devices and this has led to an increase in the market for electronic point level devices, particularly the popular tuning fork type.

As a result, such practices are becoming the norm world-wide as international and multinational companies standardise on instrumentation practices.

In 1998, the first parts of the 'functional safety' standards [IEC61508 Functional safety of electrical/electronic programmable safety-related systems] were published.

Although it is not mandatory, responsible manufacturers and processors are committing to ensuring safety systems compliance in order to ensure they are following best practice.

The effect that the legislation is having on the level measurement market is not yet clear.

IEC61508 uses a risk-based approach to determine the safety integrity requirements of the entire safety-related system.

For example this might be a complete inventory control system including tanks, level instrumentation, pumps and controllers, or a complete set of process tanks with associated mixing, dosing and control equipment.

It looks at overall system safety from concept right through to implementation, operation and maintenance.

For new installations, compliance with the standard will usually be achieved most easily by the use of the latest technology solutions including non-contact or non-invasive level measurement techniques, fail-safe devices and high integrity control systems.

For existing installations being upgraded to the standard, analysis of the entire system may conclude that compliance with the standard can be achieved more cost-effectively by upgrading other parts of the system rather than by replacement of the level measurement and control technology with more expensive options.

The level measurement market is large and complex.

Developments by manufacturers continue to give users an ever wider choice of cost, accuracy and functionality allowing them to select exactly the right instrument for any application, including difficult liquids and harsh environmental conditions.

The range of technologies available and the healthy competition which exists between them means that customers who are prepared to research the market can find highly cost-effective solutions to meet their requirements.

For advice selecting a level measurement technology, talk to manufacturers such as Solartron Mobrey who offer a wide range of technologies and can demonstrate good applications experience.

Article provided by Tim Chettle of Solartron Mobrey.

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