A submersible pump and a dry-installed centrifugal pump share the same basic hydraulic principle, an impeller spinning inside a casing to add kinetic energy to the fluid, but the installation philosophy, the sealing arrangement, and the way the motor is cooled are completely different. Submersible units are designed to operate fully flooded, with a hermetically sealed wet motor sitting in the same fluid they pump. [S1][S2]
Dry-installed centrifugal pumps keep the motor out of the fluid, mount above the suction reservoir, and rely on a primed suction line to move liquid. That single geometric difference drives downstream choices on noise, maintenance access, NPSH margin, and the need (or absence) of a dry well. [S2][S4]
Where the motor sits changes everything
A submersible pump combines pump end and motor into one sealed assembly that is lowered directly into the fluid, which is why it is used in boreholes, sumps, and flooded wet wells. In a KSB UPA wet-installed design, the motor is filled with water and uses hydrodynamic bearings, so the unit is described as completely maintenance-free in the field. [S2]
A dry-installed centrifugal pump keeps the motor and coupling in free air inside a pump house, with the suction pipe reaching down to the wet well or reservoir. KSB's Multitec high-pressure pump, used as the reference dry-installed design, allows vertical, horizontal, or angled mounting and is air-cooled, which makes every part of the pump and its connections freely accessible for service. [S2]
This physical split is the root cause of almost every other operational difference. For a wider primer on the centrifugal pump family and how impeller geometry sets head and flow, the encyclopedia entry is a useful baseline.
Suction lift versus flooded suction
A dry-installed centrifugal pump must be located above the liquid level and primed, so it depends on suction lift, atmospheric pressure on the sump, and adequate net positive suction head available (NPSHa) to avoid cavitation. Eddy Pump notes that most centrifugal pumps can run with a suction lift as long as they are primed first, while a submersible pump simply has to be underwater to work. [S5]
Submersible pumps eliminate the suction-lift problem by sitting below the liquid surface, which is why they are the default for deep well, borehole, and sump applications. Zoeller's product line groups sump, sewage, effluent, and grinder pumps under the same submersible architecture, with discharge heads typically rated for tens of feet of lift once the unit is submerged. [S3]
For a deeper look at the borehole-specific configuration, the submersible pump encyclopedia page covers stator, seal stack, and cable ratings for deep installations.
Cooling, sealing, and the wet-motor concept
Cooling is one of the clearest dividing lines. In a dry-installed pump the motor is air-cooled by the surrounding room, which is simple and reliable but also the main source of noise. In a submersible pump the motor is water-cooled by the process fluid itself, which keeps winding temperatures stable but requires every cable entry, seal, and bearing to be designed for permanent immersion. [S2]
KSB's UPA submersible motor pump uses a wet motor filled with the handled fluid, hydrodynamic bearings, and a pressure-shroud option that lets the same wet-end be deployed where a dry-installed unit would normally be specified. A flooded installation also means the motor will not be harmed if the station is submerged, an important resilience factor for stormwater and flood-prone sites. [S2]
This is also where dry-installed pumps gain an automation edge: with the motor in free air it is straightforward to fit vibration, temperature, and current sensors for condition monitoring and predictive maintenance, something KSB explicitly lists as an advantage of the dry-installed Multitec architecture. [S2]
Noise, maintenance, and access
Dry-installed centrifugal pumps are noisier because the motor and coupling radiate sound into the pump room rather than into a fluid that dampens it. Submersible units, with the motor fully surrounded by water, are markedly quieter in operation, which is one reason they are preferred in residential sump, sewage ejector, and dewatering applications. [S1][S2][S3]
Maintenance access goes the other way. A dry-installed pump exposes the mechanical seal, coupling, bearings, and impeller for visual inspection and replacement without lifting the unit out of a well, while a submersible pump normally has to be hoisted to the surface for any meaningful service work. Enviro-Line's wastewater treatment comparison (2020) flags this trade-off directly: submersible units avoid the cost of a dry pit, but they are not as easy to service in place. [S4]
In practice, plant operators often pair a submersible pump with a rail-system or guiderail lift so the unit can be pulled without entering the wet well, a configuration that closes much of the service-access gap for hazardous-duty installations. [S3]
Application fit: who is each type for?
Submersible pumps are the right call for deep well extraction, borehole supply, mine dewatering, sewage and effluent lift stations, stormwater sumps, and any site where the fluid is too far below grade for a practical suction lift. They are also the default for flooded wet wells and confined-space installations where dry-pit construction would be expensive or impossible. [S1][S2][S3][S4]
Dry-installed centrifugal pumps fit municipal water supply, HVAC circulation, chemical transfer, industrial process loops, and large irrigation stations where a pump house already exists, NPSH margins are good, and operators want direct access to the rotating assembly. Ken's Distributing lists chemical processing, water supply, and HVAC as core dry-installed centrifugal applications. [S1]
Fluid properties also push the choice. Submersibles handle clean water, raw sewage, slurry with solids, and corrosive drainage when the wetted parts are specified in cast iron, bronze, or stainless; dry-installed centrifugals are commonly used for low-viscosity, non-abrasive liquids where high flow at moderate head is required. [S1][S3][S5]
Efficiency: closing the old gap
The old plant rule of thumb that submersible borehole pumps cannot match the efficiency of dry-installed pumps is no longer a hard ceiling. KSB, in its July 2025 guidance, states outright that many plant operators are of the opinion that submersible borehole pumps cannot achieve the same efficiencies as dry-installed pumps, then notes that modern wet-motor designs with hydrodynamic bearings and optimized hydraulics have closed most of that gap. [S2]
For a synchronous-reluctance-motor dry-installed pump package, very high wire-to-water efficiency is achievable because the variable-frequency drive and motor efficiency stack cleanly. Submersible efficiency is now driven mainly by hydraulic design and motor fill-fluid management, with the trade-off being lower noise and zero suction-lift losses. [S2]
Where head and flow are stable, both architectures can land in the same efficiency band, and the deciding factor is usually civil cost (dry pit versus no dry pit) and serviceability rather than a percentage-point hydraulic gap. [S2][S4]
Selection criteria at a glance
Four decision criteria typically separate the two options in a real specification: installation depth, NPSH margin, service access, and noise/safety class. A flooded deep well with limited surface access and a residential noise limit points to a submersible; a process loop with a primed suction, a pump house, and a desire for condition monitoring points to a dry-installed centrifugal. [S1][S2][S4]
For systems where the choice overlaps, the diaphragm pump encyclopedia entry is a useful contrast, because positive-displacement units are sometimes paired with either architecture for dosing or solids handling.
Engineers sizing a submersible pump also have to verify cable voltage drop, motor cooling at low flow (for variable-speed operation), and the solids-passage rating of the impeller, while dry-installed selections must confirm suction piping air-tightness, priming strategy, and mechanical seal flush plan. [S2][S3][S5]
Selection also interacts with coupling choice on the dry-installed side; for drives between motor and pump end, the comparison in jaw vs tyre coupling for pump drives lines up torque, misalignment tolerance, and service interval with the maintenance philosophy of each architecture.
Failure modes and constraints
Submersible pump failures cluster around cable entry moisture ingress, seal-face wear, sand abrasion in borehole service, and overheating when the unit runs dry or partially submerged without thermal protection. A wet-motor design tolerates flooding but will not tolerate sustained operation in air, which is why most submersibles include thermal cutouts and moisture sensors. [S2][S3]
Dry-installed centrifugal failures tend to be loss of prime, cavitation from inadequate NPSHa, mechanical seal leakage, and bearing failure from misalignment or contamination. Because the motor is in free air, overheating from blocked ventilation or coupling misalignment is also a recurring failure mode, and the dry-installed pump is far more sensitive to suction-side air leaks than its submersible counterpart. [S1][S2][S5]
Solids handling is another constraint. Submersible grinder and sewage pumps are built to pass 2-inch to 3-inch spheres, while standard dry-installed end-suction centrifugals are usually limited to clean or lightly contaminated liquids unless a special vortex or chopper impeller is specified. [S3][S5]
Closing notes for specifying engineers
For a new water utility or process plant, the decision is rarely about which pump type is "better" in the abstract; it is about the civil work each one avoids or requires, the maintenance philosophy the operations team can support, and the noise and safety class of the site. Dry-installed units win on serviceability and automation; submersibles win on civil cost, noise, and flood resilience. [S2][S4]
Trackable signals for future updates: KSB's continued rollout of synchronous-reluctance-motor dry-installed packages, expansion of wet-motor submersible lines into higher head ratings, and the slow replacement of dry-pit sewage lift stations by rail-mounted submersible duplex packages in municipal work. Related process-side decisions, such as steam-trap sizing for plant steam loops covered in how to size a steam trap for condensate load, often sit in the same capital project as a pump-room rebuild and follow similar cost-versus-access trade-offs.