Chemical Technology April 2015

WirelessHART automates cooling tower operations by Nikki Bishop and Jason Sprayberry, Emerson Process Management

Wireless instrumentation and an asset management system reduce chemical, maintenance, lost production and repair costs.

C ooling towers are heat removal devices used to transfer process waste heat to the atmosphere. They vary in size and in the amount of instrumentation to monitor process variables. Accurate, reliablemeasurements are critical in calculating cooling tower efficiency, and are im- portant for controlling blow-down and makeup flows, as well as the pH of the water to minimize fouling of the equipment. Cooling tower instrumentation in many refineries is often old, with many measuring devices out of service (Figure 1). Measurements are difficult because the process environment is corrosive to wiring, mainly due to chemical vapours. As a result, these areas can be poorly instrumented and poorly controlled. Consequently, control and monitoring are poor, operations are inefficient, and the towers require a great deal of maintenance and manual operator interaction. This article discusses how to use WirelessHART instru- mentation and asset management software to automate cooling towers by obtaining the information needed for more Large fans generating air flow are the principle heat removal devices in cooling towers. Typically, each process area has a cooling tower, and each tower has six to 12 cells with one or two cooling fans in each cell. These fans are expensive and monitoring is critical to prevent failure. At one refinery, it costs an average of $1,6 million per fan in maintenance and repair fees when a fan runs to failure. reliable and efficient operation. Cooling tower problems

Refineries naturally do not want the fans to fail, but they also do not want to over-maintain them, as each time maintenance is done on a fan, the entire cell in the cooling tower is shut down. Themost common leading indicator of failure in a cooling tower fan is high vibration of the motor (Figure 2). Fan failure decreases the cooling capacity and efficiency of the tower, and emergency shutdowns due to cooling tower damage can last 4-8 hours, causing a significant loss in revenue. These fan failures also cause an increase in water consumption, which leads to an increase in the quantity of chemical needed by the cooling tower. Chemical dosing is often provided by an external company, but they frequently do not have the necessary data about the behaviour of each fan or the pH and conductivity measurements, so they can only apply chemicals in relation to water consumption. Cooling towers are a very tough environment with chemi- cal vapours highly corrosive to wiring, and wired instruments require frequent maintenance. As a result, operators spend a good deal of their time manually gathering process informa- tion. At one refinery, operators perform three rounds per day, or 1 095 rounds per year, which calculates to 8 760 hours annually, as rounds take a long time. This is not only time- consuming and unsafe for operators, but also often results in poor readings. Although excessive maintenance, fan replacements and chemical costs are significant, the biggest problems most refineries face are shutdowns because of equipment failures.

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Chemical Technology • April 2015