Reliability in a process plant commonly focuses on rotating equipment rather than fixed equipment. Through vibration monitoring and lube oil analysis a predictive maintenance program is realized. However, there is quite a bit of fixedequipment, like a heat exchanger, that can decrease process reliability and availability. If a heat exchanger is fouled it can reduce the throughput of a process. If it is fouled enough, it can even take the process offline. While it is common to measure differential pressure, a temperature measurement is equally important.
As the name implies a heat exchanger transfers energy from one fluid to another. This equipment can either heat or cool the process fluid, depending on what is required. Over time, contaminiation in the process fluid will lead to buildup in the exchanger, making the heat transfer less efficient. The process fluid flow rate will need to decrease to ensure proper heat transfer. The contimination also increases the pressure drop across the exchanger requiring more energy to pump the fluid.
Traditionally, a differential pressure measurement was made to detect fouling, as a reduction in heat transfer was of little concern. When the process fluid did not maintain the desired temperature, more energy was added to the exchanger. Energy was inexpensive so increased consumption was not an issue. Moreoever, other equipment in the process would require more frequent maintenance. As a result, heat exchanger cleaning was not viewed as a problem. With the improvement of process equipment and the need to control energy consumption, a differential pressure measurement is not as valuable.
A temperature measurement is becoming more common as it monitors heat transfer rates through the exchanger. It can also be used to determine if there is any fouling in the exchanger. A reduction in heat transfer efficiency generally indicates this is the case. In order to ensure a complete measurement a temperature reading should be taken at the inlet and outlet of both the process and transfer fluids.
Historically, the only way to make four temperature measurements was with four temperature transmitters, which was not desirable. This added significant cost to the instrumentation of a heat exchanger and control system. With the advent of high density temperature transmitters, this solution can be realized. Rather than four transmitters, there is one transmitter making multiple measurements. Not only can the pressure loss acroess a heat exchanger be realized, the heat transfer rate can be realized as well.
If you do not have thermowells on your heat exchanger a surface temperature can be made. There are new transmitters than can read surface temperature and correlate them to a process temperature. You get the benefit of a complete solution with minimal investment.
The best way to ensure the heat exchanger does not foul is with the use of filtration or sepration equipment. Filters are inexpensive and easier to change. A block and bypass piping solutions allows you to keep process equipment online during a filter change. Regardless of how it is done, a fully monitored heat exchanger will help you maintain a process that is realible, available and profitable.
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