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Trends and technology developments in level instrumentation

22 June 2009

The need for accurate measurement and reliable level alarms—while meeting ever more stringent health and safety regulations—continues to push equipment manufacturers to provide better performing instrumentation.

The first float switch to combine ATEX and marine approvals in one standard product.
The first float switch to combine ATEX and marine approvals in one standard product.

The market for microwave (radar) and ultrasonic level transmitters continues its rapid growth across an expanding application base, thanks to their proven reliability and low maintenance requirements.

Meanwhile, differential transmitter (DP) level systems benefit from better system construction, sizing and installation practices, giving increased installed reliability, while point level switches are widely used to meet safety standards for high/overfill and low level detection.

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RIGHT: This Mobrey magnetic float switch is the first to combine ATEX and marine approvals in one standard product.

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Further developments in enhanced instrument diagnostics enable remote interrogation of instrument condition and advance warning of potential problems.

With a wealth of information now available, what was once a simple level switch or transmitter for monitoring and control purposes may now be a powerful source of long-term process data.

Radar

Two-wire radar is a powerful and versatile solution in a range of difficult applications. It provides a reliable measurement method where other technologies fail, and can also provide interface measurement.

Radar technology is available in two forms, both top-mounted; a top down ‘through-air’ non-contacting transmitter and a contacting transmitter guided wave radar (GWR), which uses a guide wire or rod.

Both are capable of measuring almost any liquid and are unaffected by fluid properties such as density and viscosity. Non-contacting radar is ideal for applications with aggressive and corrosive liquids, while GWR is particularly suitable for low dielectric liquids such as hydrocarbons, often under high temperatures and pressures. GWR measures both the level and interface in tanks or bridles, including separators, settling and condensate tanks, and can also be used for liquefied gases such as LNG, LPG and anhydrous ammonia.

Through-air radar relies on the ‘echo’ signal being reflected back from the liquid surface to the transmitter. It provides a reliable and accurate level reading with no moving parts or contact with the liquid, thus eliminating costly maintenance requirements. In difficult applications, stray signals bouncing around the tank or in the ullage caused by reflections or interference can disrupt the measurement, and difficult surface conditions such as heavy foaming can reduce echo sizes considerably. Advances in signal processing available on the latest generation, top-of-the-range transmitters allow reliable, repeatable measurement in the face of challenges such as unusual-shaped tanks, obstructions such as stirrers or agitators in the vessel, high process temperatures or high levels of electromagnetic interference.

Similarly with GWR some applications, especially those involving hydrocarbons, significantly reduce the reflected signal. One of the latest developments from Emerson is a rapid (3ns) solid-state switch for transmitting and receiving the radar signal. Named direct switch technology (DST), the system provides a measurement signal many times stronger than traditional GWR devices to provide reliable measurement over long distances and with very low dielectric materials, even with a single lead probe.

Use of a smart galvanic interface (SGI) increases the ability of the transmitter to handle electromagnetic interference—a common phenomenon in open silos and plastic vessels.

Some of the most exciting developments in radar level measurement technology are in the use of tank graphs—effectively a ‘map’ of the tank echoes. By analysing and comparing such graphs over time, processors can identify trends, which may be helpful in planning maintenance or making long-term improvements.

The MSP900FH ultrasonic level transmitter features a pre-wired remote temperature sensor.
The MSP900FH ultrasonic level transmitter features a pre-wired remote temperature sensor.

Products such as the Rosemount 5600 can also measure low dielectric solids with a measuring range capability of as much as 50m, handling applications including powders and granules such as lime, cement, fly ash, corn, metals and ore.

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LEFT: The MSP900FH ultrasonic level transmitter features a pre-wired remote temperature sensor.

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Ultrasound

Ultrasonic level measurement technology is tried, trusted and reliable, particularly for aqueous liquids, non-volatile hydrocarbons and chemicals, suspended solids and sludge blanket monitoring.

Although the technology is seen as being at the other end of the cost/performance scale to radar, today’s ultrasonic instruments feature powerful microprocessors and advanced signal processing. They offer a simple, high reliability, cost-effective solution ideal for less demanding applications such as storage applications or uncomplicated process vessels.

Advances in temperature compensation methods now mean that ultrasonic transmitters can be reliably used on open channel flow applications where their on-board software converts level in the channel to flow, meeting the requirements of environmental agencies. For example, the Rosemount ultrasonic system features pre-configured flow curves for Parshall flumes and weirs as well as user-defined curves for non-standard structures. An onboard logger records up to 7,200 flow readings and displays both daily and cumulative flows.

Differential pressure

DP level transmitters remain a favourite method of level measurement in all types of applications. These pressure transmitter based instruments have two connections to the process vessel, one at the base and one at the top, such that any ullage pressure can be measured and subtracted from the pressure measured at the base of the vessel, giving a liquid level measurement.

Continuous improvements in DP level system construction and direct-mount configurations enhance reliability. Emerson recently announced the availability of a system sizing and performance function for its DP transmitter range, which 'tunes' the system according to the characteristics of the individual application for optimum results.

For use in vented or open tanks, a hydrostatic level transmitter is available. Made of rugged stainless steel or aluminium bronze, the most robust products have a tough flush mounted ceramic sensor for long life and are simple and low cost to install.

Point level measurement

While today’s continuous level transmitters can provide so much functionality for sophisticated monitoring and control, point level detection is still widely required to meet legislative safety standards for critical high / overfill and low level. Environmental, equipment and plant personnel protection needs continues to drive the need for high reliability switches.

The Rosemount 5300 Series premium two-wire guided wave radar transmitter features direct switch technology for a stronger signal and better measurement reliability in challenging level and interface measurements on liquids, slurries and solids.
The Rosemount 5300 Series premium two-wire guided wave radar transmitter features direct switch technology for a stronger signal and better measurement reliability in challenging level and interface measurements on liquids, slurries and solids.

Point level switching technology includes magnetic float switches and vibrating fork devices. While these technologies have not changed fundamentally in many years, manufacturers continue to drive for ever-better long-term reliability. Examples include a new ATEX-approved version of the Mobrey magnetic float switch, and a new high temperature version of the Rosemount 2100 vibrating fork liquid level switch, designed for use in extremes of temperature from -70 to + 260 degrees C.

—Tim Chettle and Peter Lagerlof, Emerson Process Management

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RIGHT : The Rosemount 5300 Series two-wire guided wave radar transmitter features direct switch technology for a stronger signal and better measurement reliability in challenging level and interface measurements on liquids, slurries and solids.

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SIDEBAR 1: Monitoring a turbulent liquid propane surface level using GWR

Liquid propane is used as a refrigerant in many industrial chemical plants. A standard process problem is monitoring the level of liquid propane in the accumulator tanks to control the recirculation process and ensure the propane supply is not exhausted. Because of surface turbulence, low dielectric constant and temperature of the liquid, level measurement has historically been unreliable.

To address this problem, engineers applied a GWR level measurement system, the Rosemount 3300. They chose a coaxial probe design, which concentrates the measurement in a single defined area, almost like a stilling well. This increases the signal to noise ratio of the surface signal in a low or variable dielectric fluid.

The test site had a probe length of 1.5m, and was installed through a flange on an existing process connection on the tank. The propane level was detected immediately and the measurement system made what were reported to be accurate measurements. The plant management team concluded that this was a persistent problem that had been eliminated through the use of the GWR probe.

SIDEBAR 2: Handling open channel flow measurements

The MSP900FH (see second photo in the above article) features a pre-wired remote temperature sensor and is ideal for helping water and industrial processing sites comply with MCERTS legislation.

Typically, such systems consist of an ultrasonic level transmitter mounted over a flume or weir. Used in the open air, conventional ultrasonic level transmitters are prone to suffer from inaccuracies caused by ‘solar gain’ when the sun shines directly on them and heats the air between the transmitter and the liquid surface. The MSP900FH transmitter has been designed to overcome this problem by using a remote temperature sensor wired directly into the transmitter.

Measuring just 10mm diameter x 25mm length, the compact sensor can easily be located in an appropriately shady spot between the liquid surface and the transmitter. In this way, the system ensures that the air temperature measurement used in the speed of sound calculation is truly representative. The sensor is hard-wired into the transmitter with a two-metre cable, so there is no additional cabling or set-up required, and only a single connection to the controller, keeping installation costs to a minimum. The signal from the transmitter is fed back to the MCU90F controller, which converts the level reading into a flow reading.


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