Advances in wireless technology have made remote monitoring of process equipment and systems much easier and less expensive than previously thought possible. As a result, many segments in process plants, power stations and production fields are monitored continuously, reducing shutdowns and the possibility for accidents while increasing efficiency and production. However, not all systems are monitored equally.
This article, based in part on a presentation by Mark Murphy of Fluor and Melissa Toten and Ed Sanders from Emerson at the 2016 Emerson Exchange*, relates how cooling towers and air-cooled heat exchangers can benefit from remote monitoring, which reduces life cycle cost and increases safety.
Cooling towers are direct contact heat exchangers. In this type of system, fluids (generally water) mix with a gas (generally air). They work by rejecting waste heat to the atmosphere through the cooling of water to a lower temperature. Whether it is to cool liquid or humidify air, liquid partially evaporates and carries away heat. Much of the energy transfer is due to the evaporation of the liquid.
Air-cooled heat exchangers are often called “air fin coolers” or “fin fan coolers” and work somewhat like an automobile radiator. Multiple rows of finned tubes are arranged in a series and fans are used to move low temperature atmospheric ambient air over the finned tube coil surface in order to cool the hotter fluid media. This type of heat exchanger is used in oil, gas, petrochemical, refinery, power and condensation plants, and many type of fluids in process industries are cooled this way.
According to an article in InTech, much of the annual loss in production is due to inefficiently operating exchangers, pumps, valves, piping, compressors and vessels. Steam traps, valves and pumps are generally well monitored, but cooling towers and air-cooled heat exchangers are typically not monitored automatically, even though they are often remote from process operations, and there can be serious economic and safety ramifications if they fail or work inefficiently.
Cooling towers are typically repaired annually, or on some other routine basis, and measurements are taken manually. However, manual measuring is time consuming and can be inaccurate, and generally cannot be used for displaying trends, which are more accurate measurements of the health of the tower. By monitoring trends, degrading performance can be caught before it becomes a problem. Hydrocarbon leakage, scaling, corrosion, improper blowdown and make-up, biological growth and high vibration and/or bearing temperatures, or restriction in water flow can all threaten cooling towers. With only occasional monitoring, it would be easy to miss an intermittent malfunction in a valve or a vibration that could indicate corrosion.
Where Instrumentation Could Help
A more efficient and reliable method of monitoring the health of cooling towers would involve instrumentation to measure vibration, temperature and pressure, conductivity, flow rate and pH balance.
When the water temperature is consistently known, if it is too high adjustments can be made to save energy. Monitoring the water supply and disposal can also help save money because chemicals can be adjusted downward or upward, depending on the amount of scale, bacteria or corrosion occurring in the towers. Constant online monitoring will also let operators know, by measuring vibration, if there are problems with bearings, lubrication or alignment in valves, pumps and fans.
Air-cooled Heat Exchangers
There can be hundreds of air cooled exchangers in the average refinery, chemical or petrochemical plant. Typically, none of the fans in these systems are monitored remotely and manual monitoring is not always enough to identify degrading performance. According to Murphy, Toten and Sanders, 62% of the time, maintenance staff finds no problem during routine checks. But losing fans in these systems could result in sudden over-pressure and a lift of the relief valve, resulting in flaring or something much worse.
Constant online monitoring can detect any abnormalities including bearing, lubrication or alignment problems, limited cooling or operational issues with fans. All these issues can result in lower production and higher maintenance costs in a refinery. In an example illustrated in the presentation, one refinery’s profit could be improved by nearly $1 million annually simply by monitoring the air-cooled heat exchangers.
Capital costs of installing monitoring systems should not be the only consideration when determining how much information is needed, and how often, to improve efficiencies and safety in a cooling system. Consider the total life cycle cost of the equipment being monitored to determine how much will be saved if optimal performance can be attained with correct monitoring.
*The 2017 Emerson Exchange takes place in Minneapolis, MN, Oct. 1-6.