There may be no liquid underneath the heat exchanger and no visible leakage on the outside. Yet oil appears in the cooling water, the liquid level in one circuit rises or the composition of the process medium changes.
In that case, the heat exchanger may be leaking internally.
With an internal leak, one medium flows into the other circuit through a hole, crack or damaged connection. The outside of the heat exchanger may remain completely dry. This makes internal leakage more difficult to identify than a leaking connection or gasket.
Nevertheless, the consequences can be serious. Cooling water may enter lubricating oil, seawater may contaminate a closed cooling circuit and process media may mix. It is therefore important to look beyond visible leakage and take changes in pressure, liquid level and composition seriously.
What is the difference between internal and external leakage?
Inside a heat exchanger, two media are separated by plates, tubes or other heat-transfer surfaces. Heat passes from one medium to the other without the liquids or gases normally mixing.
With an external leak, one of the media escapes from the heat exchanger into the surrounding environment. This may occur through:
- a worn gasket;
- a loose connection;
- a damaged flange;
- a weld;
- a crack in the housing.
External leakage is usually relatively easy to detect.
With an internal leak, a connection forms between the two separate circuits. One medium then flows into the other circuit. Nothing needs to escape from the heat exchanger, and there may be no visible abnormality on the outside.
Other terms used for this include internal cross-leakage, mixing of media, cross-contamination and intermixing.
What causes internal leakage?
Internal leakage is usually caused by damage to the material that separates the two media.
Possible causes include:
- pitting corrosion;
- erosion-corrosion;
- cracking;
- material fatigue;
- vibration;
- thermal stress;
- pressure or temperature shocks;
- excessive flow velocity;
- mechanical damage;
- incorrect assembly;
- unsuitable material selection;
- damage around a seal or connection.
The most likely cause depends on the type of heat exchanger, the media used and the operating conditions.
Even a small defect may be enough to connect the two circuits. Particularly with thin heat-exchanger plates or cooling tubes, the visibly affected area does not need to be large before complete perforation occurs.
Internal leakage in a plate heat exchanger
In a plate heat exchanger, the two media flow through alternating channels. Thin, profiled plates keep the circuits separated.
Internal leakage may be caused by:
- a hole resulting from pitting corrosion;
- a crack in a plate;
- material fatigue;
- a deformed or mechanically damaged plate;
- damage around a port opening;
- an incorrectly assembled plate pack;
- a defective seal around a port;
- an incorrect plate sequence.
When a gasketed plate heat exchanger leaks, worn gaskets are often suspected first. However, a damaged outer gasket usually causes visible external leakage.
When the two media mix internally, the plates, port seals, plate sequence and assembly must also be investigated.
A single small hole in one plate can create a direct connection between the hot and cold circuits.
Internal leakage in a shell-and-tube heat exchanger
In a shell-and-tube heat exchanger, one medium flows through the tubes while the other flows around them on the shell side.
Possible leak paths include:
- a perforated tube;
- a crack in a tube;
- leakage at the connection between a tube and the tube sheet;
- damage to the tube sheet;
- erosion near a tube inlet;
- vibration damage;
- thermal loading;
- corrosion on the inside or outside of a tube.
Blocked or unevenly flowing tubes can create large temperature differences. This may cause local stress that places additional strain on tube connections and other components.
Excessive flow velocity may also cause problems. When the protective surface layer is repeatedly damaged, erosion-corrosion can eventually lead to perforation.
Internal leakage in a brazed or welded heat exchanger
In a brazed or fully welded plate heat exchanger, internal leaks may be caused by:
- corrosion of a plate;
- deterioration of a brazed or welded connection;
- fatigue cracking;
- unsuitable media;
- poor water quality;
- thermal or mechanical overloading.
These heat exchangers generally cannot be opened in the same way as a gasketed plate heat exchanger. This makes locating and repairing an internal leak more difficult.
The repair options depend on the design, the location of the damage and its accessibility. Replacement may be necessary when a reliable repair cannot be carried out.
In which direction does the leaking medium flow?
Through an internal opening, the medium generally flows from the circuit with the highest pressure to the circuit with the lowest pressure.
For example:
- when the oil pressure is higher, oil may enter the cooling water;
- when the cooling-water pressure is higher, water or glycol may enter the oil circuit;
- seawater may enter a closed freshwater circuit;
- process medium may leak into a utility circuit;
- utility water may contaminate a product stream.
The direction is not always fixed.
During start-up, shutdown, standstill or pump changeover, the pressures in the two circuits may change. A medium that leaks to the other side during normal operation may leak less during shutdown or even flow in the opposite direction.
It is therefore important not only to look at which medium has become contaminated. The pressure relationship during operation, start-up and shutdown must also be considered.
Which signs indicate internal leakage?
Because the outside of the heat exchanger may remain dry, internal leakage is usually identified through changes in the installation or the media.
Possible warning signs include:
- unexplained liquid loss from one circuit;
- a rising level in the other circuit;
- pressure loss on the high-pressure side;
- pressure build-up on the low-pressure side;
- oil or an oily film in the cooling water;
- water or glycol in the oil;
- discolouration or cloudiness;
- foaming;
- a change in conductivity;
- an abnormal chloride level;
- changes in pH or chemical composition;
- contamination of a product stream;
- recurring alarms without visible leakage;
- abnormal sample or laboratory results.
The most useful measurement depends on the media involved.
In a seawater and freshwater cooler, for example, the chloride level or conductivity may be checked. In an oil cooler, testing may be required for water in the oil or oil in the cooling water. In a process installation, a product-specific substance or tracer may be used.
Can a heat exchanger still cool properly while leaking internally?
Yes.
A small hole or fine crack does not necessarily have an immediate major effect on heat transfer. Most of the heat-transfer surface may continue to operate normally while the media are already mixing.
A heat exchanger may therefore still perform reasonably well from a thermal perspective while already being technically and operationally unreliable.
Conversely, reduced cooling capacity does not automatically indicate internal leakage. Fouling, trapped air, insufficient flow, pump problems or incorrect operating conditions may also reduce performance.
Three different aspects must therefore be considered:
- thermal performance: is enough heat still being transferred?
- hydraulic performance: are the flow rate and pressure drop correct?
- integrity: do the two media remain reliably separated?
A correct outlet temperature alone does not prove that the heat exchanger is internally leak-tight.
What are the consequences of media mixing?
The consequences of internal leakage depend greatly on the application and the media involved.
Possible problems include:
- loss of the lubricating properties of oil;
- corrosion of bearings and engine components;
- seawater or chlorides entering a closed cooling circuit;
- damage to pumps, pipework and other equipment;
- contamination of process media;
- product loss;
- rejection of food products or other sensitive products;
- microbiological growth;
- loss of cooling capacity;
- safety or environmental risks;
- unplanned downtime.
When cooling water enters lubricating oil, for example, it may affect the viscosity and lubricating properties of the oil. If seawater enters a closed cooling circuit, chlorides may also cause new corrosion elsewhere in the system.
The problem is therefore not necessarily limited to the heat exchanger itself.
How is an internal leak detected?
There is no single test method suitable for every type of heat exchanger and every defect. Several checks are usually combined.
Checking operating data
The first step is to compare:
- pressure on both sides;
- liquid levels;
- flow rates;
- inlet and outlet temperatures;
- trend data;
- sample results;
- operating and shutdown conditions.
This information helps determine whether the observed changes are consistent with internal leakage and in which direction the medium is likely to be flowing.
Taking samples of both media
Depending on the application, testing may be performed for:
- water in oil;
- oil in water;
- glycol;
- chlorides;
- conductivity;
- product-specific substances;
- chemical tracers.
Sample analysis may confirm that the media are mixing, but it does not automatically identify the location of the defect.
Performing separate pressure or leak tests
By pressurising the two circuits separately under controlled conditions, it can be determined whether pressure is lost or whether a medium leaks into the other circuit.
A pressure test can show whether the heat exchanger is leak-tight under the applied test conditions. However, it does not always identify the exact location of the defect. Very small or not yet fully penetrating defects may also require additional investigation.
Tracer-gas testing
During a tracer-gas test, a detectable gas is introduced into one circuit. Sensitive measuring equipment is then used to determine whether the gas leaks into the other circuit.
This method may be suitable for locating very small leaks and microcracks, particularly when the heat exchanger cannot easily be opened.
Inspecting the plates
In a dismantlable plate heat exchanger, the plates can be inspected individually after cleaning for:
- holes;
- cracks;
- corrosion;
- deformation;
- damaged port areas;
- damage to gasket grooves;
- errors in the plate sequence.
A contaminated plate cannot be inspected reliably. Limescale, sludge, oil and other deposits must therefore first be removed.
Inspecting the tubes
Various non-destructive testing methods can be used for shell-and-tube heat exchangers.
Depending on the material and suspected defect, possible methods include:
- wall-thickness measurements;
- eddy-current testing;
- ultrasonic testing;
- IRIS inspection;
- dye penetrant testing;
- visual inspection of the tubes and tube-sheet connections.
The correct method depends on factors including the tube material, diameter, accessibility and type of damage.
What does a successful pressure test prove?
A successful pressure test is important, but the result must be interpreted correctly.
The test shows that during the test:
- no detectable leakage occurred;
- the tested seals functioned;
- the unit withstood the applied test pressure.
A successful pressure test does not automatically prove:
- that no early-stage defect is present;
- that corrosion will not continue to develop;
- that the heat exchanger will remain leak-tight at every temperature;
- that the cause of an earlier leak has been removed;
- that the heat exchanger still delivers its original cooling capacity.
The test result must therefore be considered together with the visible condition, material specifications, operating data and any additional inspections.
Can an internally leaking heat exchanger be repaired?
This depends on the type of heat exchanger, the location of the defect and the extent of the damage.
In many dismantlable plate heat exchangers, an individual damaged plate can be replaced. When several plates have been affected by the same corrosion or fatigue mechanism, the rest of the plate pack must also be inspected carefully.
Depending on the design, a damaged shell and tube heat exchanger may sometimes be: plugged; fitted with a sleeve; re-secured in the tube sheet or replaced completely. Plugging tubes reduces the available heat-transfer surface. When several tubes are damaged, it must therefore be verified that sufficient cooling capacity remains.
Repair options are more limited for non-dismantlable units.The choice between repair and replacement depends on:
- the location of the defect;
- the construction type;
- accessibility;
- the possibility of reliable welding or repair;
- the required operational reliability;
- the cost of repair compared with replacement.
What should you do if internal leakage is suspected?
A systematic approach is important when internal leakage may be present:
- Check the pressure in both circuits.
- Review liquid levels and trend data.
- Take samples from both media.
- Consider changing pressure relationships.
- Limit further mixing and consequential damage.
- Isolate the heat exchanger in accordance with the applicable procedures.
- Have both circuits tested separately.
- Clean and open the unit when the design allows this.
- Inspect the plates, tubes, connections and sealing areas.
- Repair or replace damaged components.
- Perform a suitable pressure or leak test before returning the unit to service.
Replacing only the contaminated medium is not enough. As long as the defect remains present, the media may begin mixing again.
How can we help YOU?
Briefly describe your situation, and we will contact you soon.
Have internal leakage investigated before consequential damage occurs
A heat exchanger may leak internally while the outside remains completely dry and the cooling capacity still appears acceptable.
Changes in pressure, liquid level or composition may therefore be important warning signs. Oil in cooling water, water in oil, a rising level in one circuit or abnormal chloride readings should always be investigated further.
At Blue Orange, we follow a clear process:
assess operating data → analyse the media → test → clean and inspect → locate the defect → repair or replace → retest
Where possible, we dismantle and clean the heat exchanger so that the plates, tubes, connections and sealing areas can be inspected carefully. Damaged components are repaired or replaced when this is technically responsible. The heat exchanger is then tested again.
Do you suspect that two media are mixing inside your heat exchanger? Have the unit inspected before a small internal leak causes corrosion, product contamination or damage elsewhere in the installation.
Yes. A hole or crack in a plate or tube can connect the two internal circuits while the outside remains completely dry.
No. Fatigue, vibration, erosion, thermal stress, mechanical damage and assembly errors may also cause internal leakage.
Not always. The sensitivity depends on the test method, the defect and the applied test conditions. Very small or not yet fully penetrating defects may require further investigation.
In many dismantlable plate heat exchangers, an individual plate can be replaced. The complete plate pack must then be assembled correctly and tested again.
During shutdown, pressure, temperature and liquid levels may differ from normal operating conditions. A small defect may therefore leak at a different time or in a different direction.
No. In dismantlable heat exchangers, damaged plates, gaskets or tubes may sometimes be replaced or repaired. Replacement becomes necessary when a reliable repair is not possible or the damage is too extensive.
Need Help with Heat Exchanger Refurbishment?
Not sure whether your heat exchanger requires refurbishment? Or would you rather address a developing issue before it leads to unplanned downtime?
At Blue Orange, we combine practical experience with technical expertise. We provide inspections, technical advice and refurbishment solutions tailored to marine heat exchangers, your installation and your maintenance schedule. No one-size-fits-all approach—just clear recommendations based on the actual condition of your equipment.
Feel free to contact us to discuss the condition of your heat exchanger. Together, we’ll determine the most suitable solution to keep your cooling system operating reliably—both now and in the long term.