Chemical Technology April 2015

PETROCHEMICALS

Figure 1: Cooling towers at many refineries have old instrumentation, some of it out of service.

include fan vibration, wet bulb temperature, level of the water supply, pressure and temperature on the water supply and return, and pH of the water supply. WirelessHART makes it possible While it is desirable to make such measurements, the cost of installing and maintaining conventional 4-20 mA wired instrumentation can be prohibitive. As noted above, the process environment around cooling towers is corrosive to wiring, mainly due to chemical vapours. This means extra care must be taken in installing wiring, conduit, cable and intrinsic safety devices in the presence of flammable gases. In addition, wired instrumentation requires a power supply, I/O cards to accept the signals, a data acquisition system to collect data frommultiple sensors, and ameans of transmit- ting the data back to the control system. WirelessHART instruments, on the other hand, do not require much of this infrastructure. Because WirelessHART devices can be battery-powered, they do not require a power supply, intrinsic safety equipment or any kind of wiring. The transmitters send data wirelessly to aWirelessHART gateway, either directly or through a network of other WirelessHART devices. To protect the devices from the corrosive atmosphere, pump vibration, flow, temperature, level and pH transmitters can be installed in enclosures mounted near the cooling tower (Figure 3). Gateways can be located close to the cool- ing tower, but away from the corrosive environment. Wired

When a cooling tower goes offline it slows production. In some cases it can even cause the refinery to shut down completely. Automating cooling towers On-line measurements are required to provide the data needed to tightly control cooling towers, maintain the assets and prevent over-maintenance. Automated monitoring also prevents failures, thus limiting downtime in each cooling tower section. Automated monitoring provides better control to improve tower efficiency andminimize water consumption. Having the right data enables proper dosing of the cooling tower water to prevent excess chemical use, and to provide the right water chemistry to avoid fouling and damage to the pipes. Proper dosing also improves cooling tower efficiency as well. Cooling towers are evaporative coolers and thus are limited by the wet bulb temperature of the cooling air. Wet bulb temperature measures how much water vapour the atmosphere can hold at current weather conditions. A lower wet bulb temperature means drier air and lower cooling tower temperatures. Tower performance is calculated as a function of supply and return water temperature, and wet bulb temperature. With proper data, the control system can calculate and monitor the cooling tower performance index, saturation index, conductivity, pH, and fan and supply pump health, and take the necessary steps to control the system or instruct maintenance to solve a problem. Typical measurement points

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Chemical Technology • April 2015