Cooling 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, reliable measurements are critical in calculating cooling tower efficiency, and are they important for controlling blowdown 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 vapors. 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.
Integrating wireless sensor technologies such as WirelessHART instrumentation and asset-management software allows facilities to automate cooling tower operation by obtaining the information needed for more reliable and efficient processes.
Common Industrial Cooling Tower Problems
Large fans generating airflow are the principal 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.
|See the related web exclusive, "One Refinery’s Real-World Results with WirelessHART Asset-Monitoring Devices," that shows how wireless monitoring improved process efficiency of the industrial cooling towers between 10 and 15 percent.|
Refineries do not want cooling tower fans to fail, but they also do not want to overmaintain them. Each time maintenance is done on a fan, the entire cell in the cooling tower is shut down.
The most 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 to 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 behavior of each fan, or the pH and conductivity measurements. Instead, they can only apply chemicals in relation to water consumption.
Cooling towers are a tough environment that includes chemical vapors, which are highly corrosive to wiring. Wired instruments require frequent maintenance. As a result, operators spend a good deal of their time manually gathering process information. At one refinery, operators perform three rounds per day, or 1,095 rounds per year — which calculates to 8,760 hours annually. This is not only time-consuming and potentially unsafe for operators, it 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. When a cooling tower goes offline, it slows production. In some cases, it can even cause the refinery to shut down completely.
Automating Cooling Tower Operation
Online measurements are required to provide the data needed to tightly control cooling towers, maintain the assets and prevent overmaintenance. Automated monitoring also prevents failures, thus limiting downtime in each cooling tower section.
Automated monitoring provides better control to improve tower efficiency and minimize 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.
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 vapor 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. Also, it can take the necessary steps to control the system or instruct maintenance to solve a problem. Typical measurement points 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.
Wireless Control for Industrial Cooling Towers
While it is desirable to make such measurements, the cost of installing and maintaining conventional 4 to 20 mA wired instrumentation can be prohibitive. As noted, the process environment around cooling towers is corrosive to wiring, mainly due to chemical vapors. 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 from multiple sensors, and a means of transmitting the data back to the control system.
By contrast, wireless instrumentation such as WirelessHART instruments does 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 a WirelessHART 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 cooling tower but away from the corrosive environment. Wired integration from each gateway into the refinery’s distributed control system (DCS) can be done via Modbus or other digital data network. A backhaul network with 802.11 Wi-Fi radios can provide data integration into local HMIs via OPC.
Analyzing Cooling Toewr Performance Data
The next step in improving cooling tower control is analyzing the data gathered from the wireless transmitters. Various software packages provide early warning of limited cooling and also provide diagnostics to help operators spot bearing, lubrication or alignment problems in cooling tower pumps and fans. Automatic alerts flag personnel to cooling water conditions so they can adjust blowdown rates and minimize the use of water treatment chemicals. This helps refinery operators recognize and prevent cooling failures before they occur.
In addition, some companies offer cooling tower monitoring software runs as a part of their asset-management systems. The software can provide a pre-engineered, plug-and-play solution that analyzes process and asset data to determine faults. These solutions also apply statistical analysis to detect meaningful changes. One manufacturer’s software presents this data as overall asset health information via user-friendly operator displays with automatic alerts (figure 4). This allows refinery operators to:
- Take action based on real-time alerts.
- Diagnose root causes.
- Employ predictive maintenance in the cooling towers.
- Prevent failures by heeding early warnings.
Such programs provide early detection of asset health degradation to give a refinery time to perform preventive maintenance. The refinery can bring a spare online before the fan shuts down unexpectedly, avoiding downtime and costly repairs to the fan. Taking a fan out of service before it fails catastrophically saves repair costs. Instead of $1.6 million to overhaul a fan that was run to failure, repairs may only cost $40,000 per fan.
Some asset-monitoring systems with user-friendly operator displays provide alarms and alerts that otherwise would not be available without a cooling tower expert looking at instrumentation data. The software analyzes the data and alerts the operators, who then can call in a cooling tower expert to determine a plan of action. This online, continuous analysis is vital for early warning to avoid shutdowns.
And of course, it is not only cooling towers in refineries that can benefit. Any facility with large cooling towers such as power and chemical plants can benefit from asset-monitoring solutions for its evaporative cooling equipment.