Rethinking the ROI of Modern Impeller Selection

A seawater pump is only as good as the health of its impeller.

Robin Blank | SPX FLOW, an ITT Company

In the world of marine engineering, harsh environments are the norm. Applications with extreme conditions often struggle with the same central question of how to manage the maintenance and longevity of the system’s pumps.

When it comes to engines, heating and cooling play a crucial role in maintaining the long-term prognosis of the systems. While factors like horsepower, torque and fuel efficiency affect the day-to-day performance of a system, the impeller is a major consideration for the long-term reliability and longevity of the pump and, in turn, the engine.

For example, a seawater pump is only as good as the health of its impeller. A defective impeller can result in reduced cooling capacity, pressure loss, risk of hub separation, increased vibration, excessive friction or slip around seals. Each carries its own issues and can impact a boat’s engine efficiency.

Heating & Cooling

There is a delicate balance between heating and cooling in engines. While heat is a result of energy, cooling is necessary to prevent overheating and serious damage.

In an engine, the energy stored in the fuel is converted into heat. Approximately 30% of the energy stored in fuel is converted into useful mechanical work, while about 40% of the supplied energy is lost as heat and kinetic energy through exhaust gases. The remaining approximately 30% is heat energy that the cooling system must remove. This article will explore marine engines specifically.

Impellers play a crucial role in cooling. In marine environments, seawater pumps use the surrounding water outside the vessel as a cooling medium—also known as raw-water cooling, which is by far the most common solution in modern inboard engines. This requires the boat pump to be self-priming, enabling it to draw in water and then pump it through the engine’s cooling system. The majority of seawater pumps are of the impeller type, meaning they use a flexible rubber impeller with vanes that rotate inside a pump housing.

For an engine to perform at its highest level on the water, an impeller needs to do the same, with adequate flow at a given inlet and head pressure that aligns with the engine’s rotations per minute (rpm). To give an idea of the amount of water that is involved, one of the larger marine inboard impellers can move approximately 211 gallons (800 liters) of water per minute. That is enough to fill five bathtubs every 60 seconds. Even smaller units perform a staggering amount of mechanical work. At 4,000 rpm, the impeller completes 240,000 rotations per hour. Over a mere 100-hour service window, that amounts to 24 million rotations. Yet, in a corrosive environment, impellers must maintain their shape and resist deformation to ensure consistent flow.

| IMAGE 1: Marine engine seasonal service with a rubber impeller (Image courtesy of SPX Flow)

Impeller Design

An impeller’s design can dictate everything from speed to maintenance intervals to durability and longevity. Depending on the unique factors for the system—such as application, environment, engine type, etc.—the impeller’s material can make a big difference.

Rubber offers several advantages, including:

Flexibility: As the impeller rotates, the rubber blades can bend while maintaining a mostly consistent shape.
Durability: Solid particles can pass a rubber impeller without damaging the pump.
Corrosion: Rubber can withstand degradation from saltwater.
Cost-effectiveness: Rubber is relatively inexpensive to produce and replace.
Self-priming: The pump can draw in water from outside of the vessel and then pump it through the engine’s cooling system. They are self-priming up to 3 meters (m)/10 feet (ft) (=0.3 bar) when lubricated but not filled with liquid, and up to approximately 5 m when filled with liquid.

Traditionally, polychloroprene has been a top choice for flexible boat impellers, but it is not without its trade-offs. When a polychloroprene impeller is inactive for extended periods, it will swell as it absorbs water. That swelling can lead to blade deformation, higher friction and sometimes even hub separation. In worst-case scenarios, it can cause pump failure right when the system is needed most, such as in the middle of a lake or ocean.

Maintenance

Impellers are often compared to a vehicle’s brakes and tires. They wear down when used and must be replaced before they fail. In an ideal world, operators should swap out an impeller every season or after 500 hours of operation, whichever comes first.

When servicing at these intervals, it is vital to inspect the entire pump ecosystem. Research indicates that in a new pump, up to 25% of the head pressure reaches the seal chamber. As components such as the wear plate, cam and end cover begin to deteriorate, the pressure can rise to approximately 40%. This increased pressure accelerates the wear on the mechanical and lip seals, which should generally be changed every 1,000 to 1,500 hours.

Key service benchmarks:

Impeller: Change the impeller every season or after 500 hours of operation, whichever comes first. Apply specialized lubricant on the impeller, and in the housing, before assembly. Do not use petroleum-based lubricants.
Wear plate: Check for visible wear and flatness, and inspect the pin for signs of fatigue. Replace after 1,000-1,500 hours. If there is damage, pitting or wear, both the plate and pin should be replaced immediately.
Cam: Replace after 1,000-1,500 hours. If pitting or wear is visible, change immediately. Apply a thin layer of flange sealant on the back of the cam before assembly.
End cover: Inspect for wear. Replace after 1,000-1,500 hours or if wear is significant and can compromise flow or suction performance.
Mechanical seal: Replace after 1,000-1,500 hours, or immediately if leakage is detected.
Lip seal: Replace after 1,000-1,500 hours, or if leakage occurs.
Shaft: Inspect for wear in the areas contacted by the lip seal and rubber impeller spline drive. Replace after 1,500-3,000 hours or if there is grooving in the lip seal area or significant fretting on the impeller spline drive.
Bearing: Inspect for signs of grease loss, corrosion or rough rotation. Grease loss often leads to increased noise, so listen for abnormal sounds during operation. Replace after 1,500-3,000 operating hours.

Additionally, when keeping inventory on hand, keep in mind that rubber is an aging material sensitive to temperature, light and airflow. Ideally, these components should be stored in cool, dark places in sealed bags. Temperature affects shelf life: an impeller stored at 77 F remains viable for five years, but that window shrinks to just eight months at 113 F, making protective packaging and storage conditions essential.

When selecting aftermarket parts, the lower price point of a “knock-off” can be tempting, but laboratory testing reveals there can be a stark performance gap compared to OEM parts. In recent evaluations of common copies of one pump, the units failed to meet the specified flow requirements, even at startup. This means even from the start, the engine did not receive adequate cooling.

More concerning is the degradation curve. A high-quality impeller maintains consistent pressure over its service life, while low-quality alternatives show a rapid loss of pressure over time. In testing, the non-OEM impeller began losing sections of its vanes during the initial 96-hour test. Furthermore, material analysis of the brass/bronze pump body and impeller hub showed that the lead content in the aftermarket copy exceeded the European Restriction of Hazardous Substances (RoHS) directive’s 4% limit, raising concerns about environmental impact and health risks

Robin Blank is the global product manager for Johnson Pump’s marine division, an SPX FLOW brand within ITT, Inc. With more than 13 years of experience, he focuses on marine pump and impeller technologies for engine cooling and fluid handling applications. For more information, visit spxflow.com/johnson-pump-marine.

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