Maintenance for shell and tube heat exchangers
Seth Price | October 05, 2024Shell and tube heat exchangers are commonly used in the chemical processing industry. They can be used to heat up chemicals to speed up reactions or cool down products to reduce waste heat or to remove heat from water that is going to be returned to natural bodies of water. Because of their widespread and continuous use, heat exchangers must be routinely maintained, just like vehicles or any other piece of industrial hardware.
The effects of not maintaining heat exchangers can lead to catastrophic results. Some of the most dangerous failures are gasket failures, where the gasket between the shell and tube sides fails either internally or externally. Both tubes and shell components can corrode causing leaks as well. Also, scale and debris can deposit, and biological agents can grow on the inner surfaces of the heat exchanger, reducing the heat transfer between the surfaces.
Scale and impurity buildup
Any impurities in the process fluid streams can fall out of the flow and deposit on the insides of the heat exchanger. This is particularly problematic in the bends of the tubes, where material collects and can limit heat transfer. If the deposition of impurities is allowed to continue for long enough, some of the tubes can become clogged, limiting heat transfer and flow through the system. In extreme circumstances, these clogs can lead to overheating and perhaps explosive conditions if a flammable liquid (and the vapor space near it) are not allowed to flow and exposed to high temperature for longer than expected.
Water is often used as one of the process fluids, particularly in cooling applications. It is tempting to think that water will keep the heat exchanger clean. Unless this water is ultrapure, it will also lead to scale buildup in the system. Even dissolved gases in the water can lead to oxidation of the heat exchanger material; while not a thick layer that will cause a clog, the oxide layer can limit heat transfer. A new heat exchanger, with bright, shiny copper tubes and only a thin layer of oxide will conduct more heat than an old one with dull, heavily oxidized tubes.
At the very minimum, water that is used for a process fluid should be first run through a set of particulate filters to remove the worst offenders. The particulate filters can be quickly replaced, rather than allowing the particulates to build up in the heat exchanger.
Scale removal
Scale and impurity deposition can be removed by a number of techniques, all of which have distinct advantages and disadvantages. Disassembly and manual brushing, reverse flow, solvent flow and ultrasonic cleaning are the most common techniques.
Disassembly
The most labor-intensive technique for cleaning heat exchangers is to completely disassemble them and manually brush away any impurities. This is even more difficult than it sounds. It requires shutting down the heat exchanger and removing the shell to scrub between tubes for shell-side cleaning.
For tube-side cleaning, it is even more complicated, depending on the number of passes the tubes make; the more passes, the more bends. Also, the more internal baffles, the more bends. A single-pass heat exchanger may require special brushes that can snake around through the baffles. At each bend, the brush becomes harder to move and rotate, meaning its chances of getting stuck are increased. Furthermore, each bend makes it more difficult to get a thorough cleaning. For multi-pass tubes, this might not even be possible without further disassembly of the tubes or access ports into the tubes, all of which become potential leak points during operation.
Besides brushes, pressure washing can help remove scale and verify flow through the tubes. The nozzle of the pressure washer can be extended deep into the tubes, and when the wastewater drains from the opposite side of each tube, flow is verified and clogs are not present.
High pressure nozzle for pressure washing heat exchanger tubes. Source: Serck Global
Reverse flow
One method of cleaning that can remove some scale is to simply reverse flow through the heat exchanger. If a heat exchanger is oriented from north to south, and typically fluid flows from north to south through the tubes, the plumbing connections can be reversed periodically. This is often enough to loosen some of the scale and flush it from the system.
The disadvantages to backflushing the heat exchanger is that both connections should be reversed (both the shell side and the tube side) to ensure that the heat transfer properties remain similar. Also, it does not fully remove the scale, it only removes some of it. Backflush or reverse flow is one method to reduce the amount of time required between more thorough cleanings.
Solvent flow
Another technique is to plumb another solvent or solvents through the system. If the heat exchanger uses water as a process fluid, it might be possible to run an alcohol or mild detergent through it to remove what the water has left behind.
Solvents must be chosen carefully to avoid reactions with gaskets, contaminants and other components that might be exposed. A gasket designed for water may react to a different solvent. Simple immersion tests, where a spare gasket is immersed in the solvent can indicate how a proprietary gasket, whose material composition is unknown, will react.
There also needs to be a method for verifying that the solvent has been removed. This is particularly challenging with tubes that may be clogged. If a tube is clogged and a solvent gets trapped behind the clog, it can increase exposure time to the tubes, or form a pocket of potentially explosive vapor under the right (or wrong!) circumstances.
Ultrasonic cleaning
Ultrasonic cleaning can be performed by vibrating the tubes ultrasonically. This tends to loosen scale, breaking the Van der Waals bonds that keep it stuck to the inside of the tubes. It works similar to an ultrasonic toothbrush; the vibrations break the scale free, much like plaque on teeth.
Depending on the specific configuration, ultrasonic cleaning can be performed either dry or while the heat exchanger is full of process fluid. Some are even targeted to trouble areas and can be embedded in the heat exchanger for continuous use. This prevents scale from building up at all.
Corrosion prevention
Corrosion can never be fully prevented, especially when running acidic materials through metallic heat exchanger components. However, corrosion rates can be slowed through the use of sacrificial anodes that are selectively attacked instead of the heat exchanger. Most commercial heat exchangers have sacrificial anodes that must be replaced on a regular basis; once the anode is reacted away, corrosion will begin on the heat exchanger components. Sacrificial anodes are often mounted in such a way as to facilitate ease of replacement, without removing the head pieces or gaskets between shell and tube bundles. Depending on the specific design, there may be multiple anodes, some for the shell side and some for the tube side.
In some cases, corrosion inhibitors can be added to the process fluids to slow the corrosion process. However, these will need to be removed from products and return water streams before they are used, so it does create additional complexity to the process. Furthermore, it simply slows corrosion, and so it must be routinely monitored.
Sacrificial anode. Source: Beta Marine USA.
Final thoughts
Unfortunately, heat exchanger maintenance is a time-consuming and difficult process. Pre-filtering of water and embedded ultrasonic transducers can limit the amount of scale that forms inside the tubes but does not prevent it entirely. Sacrificial anodes can help reduce corrosion, but it still occurs.
The best plan is to build heat exchanger maintenance routines into the larger maintenance plan, such that when other process equipment is down for maintenance, the heat exchanger can be maintained. Furthermore, redundant heat exchangers can keep the process semi-continuous without having to shut down the process entirely. In this case, be sure that both heat exchangers have access to all pressure relief hardware and that blocking off either heat exchanger does not cause a safety or process hazard.