HT/LT Cooling Water Pump
HT and LT pumps move fresh water, not seawater, around the two closed cooling loops that protect the main engine: the high-temperature circuit that jackets the cylinders, and the low-temperature circuit that cools charge air and lube oil, each running against different pressure and temperature targets.
Read more — HT/LT Cooling Water Pump explained ▾
What sets HT/LT pumps apart from other engine room pumps
The high-temperature (HT) circuit carries fresh water through the cylinder jackets and cylinder head cooling spaces of the main and auxiliary engines, running at 70-90°C on most medium and slow-speed diesels, and rejects its heat to the LT circuit or directly to the fresh water generator. The low-temperature (LT) circuit runs cooler, typically 30-40°C after seawater cooling, and serves the charge air cooler, lube oil cooler and sometimes the HT circuit's own heat exchanger. Both are centrifugal pumps running on closed, treated fresh water rather than raw seawater, which is what separates them from the seawater cooling pumps upstream - closed circuit water is chemically dosed against corrosion and scale, so these pumps see a far more benign fluid than a seawater pump does, but the consequence of a leak is loss of treated inventory rather than just lost suction.
Main components
- Pump casing and impeller - centrifugal, single or multi-stage depending on required head, usually bronze or cast iron wetted parts since the fluid is non-corrosive fresh water.
- Mechanical seal or gland packing - the most common source of visible leakage, particularly on older packed-gland designs.
- Electric motor drive - most HT/LT pumps are motor-driven rather than engine-driven, allowing independent operation and standby duty.
- Non-return and isolating valves - allow a standby pump to be brought online without draining the circuit.
- Expansion tank connection - each circuit has a header/expansion tank that accommodates thermal expansion and provides the static head the pump suction needs.
Selection / Sizing
Pump capacity is set by the engine manufacturer's specified cooling water flow rate at rated output, expressed in m³/h, with head determined by the circuit's total resistance including the heat exchangers, engine water jackets and piping runs. HT and LT pumps are normally sized identically to their duty pump as a 100% standby, arranged so either pump can be started manually or automatically on a pressure-drop signal. Motor rating follows directly from the flow/head duty point on the pump curve; oversizing wastes electrical power continuously since these pumps run for the entire time the engine runs.
Regulations / Class
Class rules require engine cooling water systems to include standby pumping capacity capable of maintaining cooling if the duty pump fails, consistent with SOLAS Ch. II-1 requirements for essential machinery redundancy. Automatic standby pump start on low pressure is commonly a class notation requirement for unmanned machinery space (UMS) operation. There is no dedicated international convention specific to HT/LT pumps beyond these general machinery redundancy and periodic survey requirements.
Typical faults
| Fault | Consequence |
|---|---|
| Mechanical seal wear or gland packing leakage | Gradual loss of circuit water, falling expansion tank level, air ingress |
| Impeller wear or erosion | Falling flow and pressure, engine temperatures creeping up under load |
| Bearing failure from inadequate lubrication or misalignment | Vibration, noise, eventual seizure and unplanned pump stoppage |
| Air lock in the circuit after maintenance | Pump loses prime or cavitates, poor circulation despite pump running |
| Standby pump not tested regularly | Fails to start automatically when needed, defeating the redundancy it exists for |
What to look for in a supplier
- Pump curves matched precisely to the engine maker's specified duty point, not a generic nearest-fit selection.
- Seal type compatible with the treated water chemistry already in use on board.
- Spare parts commonality with other cooling pumps already carried, to reduce the spare parts inventory.
- Documented bearing life and lubrication interval so maintenance planning can be scheduled realistically.
A slowly falling expansion tank level with no visible leak is almost always a gland or mechanical seal weeping into the bilge - trace it before it becomes a running loss that masks a real cooling water shortage under load.
Typical Manufacturers
2 manufacturers · 2 models
Azcue
1
- Seal face damage from scale
- Cavitation at HT side
- Coupling failure
- Designed for reliable operation at 80‑90 °C with a high‑temp mechanical seal that resists leakage
- Compact footprint suitable for confined engine‑room spaces
- Stainless‑steel wetted parts provide good corrosion resistance in seawater service
- Standard marine coupling dimensions simplify integration with existing drive systems
- Azcue’s reputation for robust, low‑maintenance pump designs
- Seal face can be damaged by scale or mineral deposits if water quality is poor
- Cavitation may occur on the high‑temperature side if suction conditions are inadequate
- Coupling failure reported when alignment tolerances are not strictly met
- Requires regular mechanical‑seal inspection and maintenance to avoid premature wear
- Higher initial purchase price compared with generic low‑temp cooling pumps
Shinko
1
- Scale buildup on impeller
- Seal failure
- Bearing overheating
- Integrated HT and LT loops reduce piping complexity and save engine room space
- Robust bearing design suitable for continuous operation on large vessels
- Standardized seal arrangement allows relatively quick replacement
- Compatible with typical marine control systems that monitor differential pressure across coolers
- Proven reliability in a wide range of vessel classes
- Impeller prone to scale buildup when operating in high‑hardness seawater; requires regular cleaning
- Seal wear can lead to premature failure if not inspected on schedule
- Bearing overheating reported under excessive load or inadequate lubrication
- Unit weight may be higher than single‑circuit alternatives
- Spare parts availability can be limited in remote ports