Heat exchanger leaks
What causes leaks in shell and tube heat exchangers?
The causes of leaks in a shell and tube heat exchanger can be sorted into two categories:
- manufacturing issues with the tubes, and
- improper use of the heat exchanger by the operator.
Design that avoids leaks is one of the considerations of good design of shell and tube heat exchangers.
In most of cases (99%+), heat exchanger tubes are seamless with the tube sheet. They are not welded but are made as a single piece. This greatly reduces the chance of any leakage.
However, heat exchangers are sometimes manufactured with materials or techniques unsuitable for the application to, most of the time, reduce the budget. This is when problems arise.
Here are some examples.
In a heat exchanger exposed to thermal stresses or corrosion, seamless or welded tubes might fail. Note that the percentage of welded tubes that fail is significantly greater.
In a heat exchanger handling high-temperature fluids (e.g. steam superheater), the tubes can simply be burned to the point of permanent damage.
When the heat exchanger is handling chloride and water, high temperatures cause the water to boil off, leaving a higher concentration of chloride than water. This corrodes the pipes and causes a leak. Furthermore, when parts used to build heat exchangers have residual stresses generated during the manufacturing process (U bend tubes), stress corrosion cracking might occur.
Finally, let’s consider not just high heats, but a high temperature distribution across a tube sheet. When one side of the tube sheet is at a vastly different temperature from the other, the entire tube sheet warps. This is fatal for the functioning of the gaskets.
In all cases, the result is a mixing of the hot and the cold materials.
The lesson is to always design the heat exchanger with the correct materials for the application, and build it with modern, high quality manufacturing techniques.
Heat exchangers are generally over-designed. They must operate after years of use and in environments that cause fouling to the tubes. If a heat exchanger was needed in an oil refinery that produces 1,000 barrels of oil per minute, the heat exchanger would likely be designed to handle about 1,500 barrels.
Building heat exchangers this way is great for longevity and reliability, but what happens when that oil refinery wants to produce 50% more oil? They might push the system to its limits. The heat exchanger can handle it, but at a cost. Fluid is going through the same cross-section at a greater velocity, and when velocity goes up, vibrations occur. Then you get erosion of the nozzles.
Pushing a heat exchanger past its design parameters will wear out the equipment much faster, leading to leaks and failure.
How to avoid shell and tube heat exchanger leaks
Heat exchanger leaks can be avoided by sticking within their design parameters. If you want to change the operation of the heat exchanger, you should talk to your heat exchanger manufacturer for advice, so the potential impact of any changes can be investigated.
Work with a trusted manufacturer who will use suitable designs and materials. At Sterling TT, we have over 100 years of experience in heat exchanger design, and always put the relationship with our customers first. Find out more on this page: Why partner with us?
What to do if your heat exchanger leaks?
The following steps should be carried out either with your internal engineers or with help from your manufacturer.
The first thing is to inspect the leak. See how many tubes are leaking and the severity of the leak.
For fixing a leak, there are a few options.
You may be able to plug the tubes causing the issue. Because shell tube heat exchangers are overdesigned, if you have only a small percentage of tubes leaking (less than 5%, for example), and you block them, the exchanger still will meet its duty.
Because of fouling factors, some heat exchangers are 50% bigger than strictly needed. So few percent here and there doesn’t make any impact on operation.
Instead of plugging tubes, re-tubing might be the preferred option. This is applicable when the tubes are not welded to the tube sheet. The problem tubes are taken out, replaced, and the heat exchanger can run as normal.
In cases in which the tube sheet is warped, it might be possible to machine work it back to usable condition. In more severe cases, the entire tube sheet might have to be replaced, though this is an expensive solution.
Typically, substantial repair work means sending the heat exchanger back to the manufacturer, though occasionally the work can be carried out on-site. We explain how this works at Sterling TT on our Aftersales and Services page.