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Level Measurement

How Wireless Vibrating Fork Frequency Monitoring Improves Process Safety

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Example 1: Monitoring of Decanting Process

In the first a US chemical company produces acid for a variety of industries. The acid is held in tanks with a layer of water on top to preserve stability. When the acid is decanted into other containers it is vital that none of the stabilising water is included so as not to contaminate the batch. The decanting process has to be stopped before water is introduced into the container. Top down radar was considered as a way of monitoring the interface of the two liquids, but for this application the microwave signal could not pass through the water layer, and magnetostrictive devices proved to be complex and unsuited for use with the different fluids.

Previously, the only available solution was for operators to visually monitor the phase change through a site glass in a pipe spool piece, radioing their control room to turn off a valve to stop decanting. This process obviously contained the potential for human error and consequent huge variations in batch consistency, and well as implications for operator safety and overall personnel productivity.

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The eventual solution was achieved with the installation of a Rosemount 2160 Wireless Vibrating Fork Level Switch from Emerson Process Management. The device’s ability to accurately and immediately distinguish between water and acid provided reliable information of the phase change. Rapid deployment was facilitated by the presence of an existing Smart Wireless network on site, minimising installation costs and complexity. Use of the appropriate wetted material option, here Alloy C-276, resolved any issues of acid corrosion of the device.

The customer’s product quality immediately improved with more consistent throughput, and with consequent cost savings thanks to the elimination of spoilt batches. More importantly, the solution removed the need for a human presence, improving the health and safety of personnel by keeping them out of hazardous areas. The chemical company is now planning to equip a second tank employing the same effective solution.

Example 2: Meeting Safety and Environmental Directives

In the second example the Rosemount wireless vibrating fork’s versatility was able to assist an international oil and gas company to meet new European Union (EU) safety and environmental directives with regard to floating roof tanks. The EU directives require enhanced monitoring to avoid overfill and vapour leakage, as well as automation of the monitoring process.

To meet these directives it is important to identify floating roof tilt, which can lead to vapour leakage and potential liquid spills. Floating roof tilt can be observed in its early stage by the build-up of rainwater or melted snow at a low point of the tilted roof. In more serious instances of tilt the hydrocarbon contents of the tank can leak on to the upper surface of the roof.

Additional Information
At a Glance: Rosemount 2160 Wireless Vibrating Fork Level Switch …

… offers continuous insight into process and instrument health so operators can respond to abnormalities sooner and prevent shutdowns, and alerts can be sounded to indicate changes based on user-defined configuration.

The Rosemount 2160 features four user-configurable alerts: Output State (PV), Sensor Frequency (SV), Device Temperature (TV), and Supply Voltage (QV). These may be configured for a rising or falling value and a limit or threshold can be set to trigger the alert. For example, by monitoring the sensor frequency, problems with vibrating forks can be detected early. There is also the provision to apply a dead band or hysteresis.

Detectable issues include external damage to the fork, internal damage to the sensor, media build-up, and excessive corrosion on the fork. In addition, the device provides fail-safe operation for both high and low level applications.

Mechanical methods previously used to identify roof tilt proved unreliable and are now prohibited, and the new regulations also prohibit visual inspection by personnel. An automated solution was required. Emerson’s wireless vibrating forks provided a versatile solution, able to detect both roof tilt and the type of fluid present. To achieve this, three wireless fork switches were simply installed on top of the floating roof, on support beams equidistant around its circumference.

Since they were wireless, the installation was a simple mounting connection off a support beam. A wireless repeater was added to the rim of the tank so that the wireless signal was available even when the tank level was low. One second updates were sent to the control room via a Smart Wireless Gateway connected to a PLC.

The visualization for each tank showed the three measurements and their frequencies. If the switch went from a dry to a wet state the control room operator would know liquid was present. By monitoring the frequency as well, it is possible to determine whether the liquid was water or oil.

* The author is Marketing Manager at Emerson Process Management.

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