Select the Right Liquid Level Sensor

It's important to consider a variety of factors when choosing the type of technology.

By John E. Edwards, P & I Design

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Level measurement, which is the detection of the phase split between vapor/liquid, liquid/liquid, vapor/solid and even liquid/solid, is a key parameter in the operation and control of modern industrial processes. A reliable outcome depends on the phase conditions being relatively consistent under all process conditions. Unfortunately, the importance of level control isn't always appreciated (see "Don't Underestimate Overfilling's Risks.") Failure to measure level reliably has resulted in some of the most serious industrial accidents, including those at the Buncefield, U.K., fuel storage depot and BP's Texas City refinery.

The technologies to measure and transmit process level have evolved significantly since the 1960s. Impulse lines, used to connect instruments to the process, appear less frequently on new installations and are being replaced on existing ones. (Where used, they require specialist knowledge during design, installation and maintenance for reliable measurement.)

Today, sensor developments coupled with data transmission innovation offer reduced installation costs, simplified maintenance and enhanced plant performance.

Transmission technology development has allowed universal application of self-powered two-wire 4–20 mA dc signals. In addition, SMART transmitters provide bidirectional digital communication and diagnostics capability via the HART (Highway Addressable Remote Transducer) protocol. The 4–20 mA and HART digital signals share the same wiring, offering a centralized capability to configure, calibrate, characterize and diagnose devices in real time, together with reporting capability. Data can be captured from multi-parameter devices without additional hardware, providing predictive maintenance capability.

Meanwhile, development in fieldbus digital communication has enabled field devices to be connected using a single cable bus structure, reducing cabling, installation time and cost. Fieldbus is a device-level network that sacrifices speed for security. Several protocols are available, with Modbus, Profibus PA and Foundation being the most common. (See "Take Advantage of Fieldbus.")

Fieldbus technology is more complex and costly, requiring suppliers to provide sensor options to meet the different standards. Plant layout, sensor interface capabilities and data management infrastructure guide fieldbus selection.

Here, we'll focus on liquid level measurement because it's usually the key to reliable and safe plant operation. Normally processors hold flows steady and let levels change within limits — this requires reproducibility. Accuracy is important for tanks used for stock and custody control.

A variety of mechanical and electronic technologies for level measurement are available:

Hydrostatic. This continuous indirect method measures the pressure due to liquid level and density plus over-pressure. The sensor measures the difference between this pressure and a reference one, normally atmospheric; so, it's not well suited for vacuum and pressure service. Instruments come in flanged-mounted or rod-insertion styles, the latter not being recommended for turbulent conditions. Typical accuracies claimed are ±0.2% of reading but this depends on process fluid properties and conditions.

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