Vertical Multistage Pump: Working Principle and Applications

A vertical multistage pump is designed for applications that need relatively high pressure but do not have much floor space available. It is commonly used in building water supply, industrial pressure boosting, water treatment, boiler feed, irrigation, cooling systems, and selected data center liquid-cooling projects.

The main difference between this pump and a single-stage centrifugal pump is the number of impellers. A multistage pump has several impellers arranged in series. As the liquid passes from one stage to the next, its pressure increases.

This makes the pump suitable for duties where a single-stage pump cannot produce enough head. However, higher pressure does not automatically mean better performance. The pump still has to match the actual flow, system resistance, inlet conditions, and liquid properties.

A vertical multistage pump is a non-self-priming centrifugal pump with the motor, shaft, and hydraulic stages arranged vertically.

In a typical CDL or CDLF pump, the motor drives the pump shaft through a coupling. The impellers and diffusers are installed inside a pressure cylinder. The suction and discharge connections are located at the bottom of the pump, usually on the same horizontal level. This arrangement makes the pump relatively easy to connect to an existing pipeline.

The term vertical multistage pump is the standard technical description. Buyers may also search for vertical multi stage pump or vertical stage pump, although these forms are less common in technical documentation.

Because the pump is not self-priming, it must be filled with liquid before it starts. The inlet also needs to remain flooded or under sufficient positive pressure. Running the pump dry can damage the mechanical seal within a short time.

Liquid enters the center of the first impeller. When the motor rotates the shaft, the impeller transfers energy to the liquid and moves it outward by centrifugal force.

The liquid then passes through a diffuser. The diffuser slows and redirects the flow, converting part of its velocity into pressure before guiding it into the next impeller.

The same process is repeated at every stage. Flow remains broadly continuous, while the head generated by each impeller is added to the total pump head. After the final stage, the pressurized liquid enters the discharge chamber and flows into the system.

Increasing the number of stages normally increases the available head. It does not necessarily increase the rated flow. More stages also affect motor power, pump height, axial load, and maximum working pressure, so the number of impellers should be selected from the required duty point rather than treated as a general measure of pump quality.

Main Construction

A CDL/CDLF vertical multistage pump normally consists of a motor, coupling, pump shaft, mechanical seal, pressure cylinder, impellers, diffusers, bearings, and a combined suction and discharge chamber.

The catalog specifies a totally enclosed, fan-cooled, two-pole motor with IP55 protection and Class F insulation. The standard 50Hz performance curves are based on speeds of approximately 2,900 or 2,950rpm.

For an export project, frequency and voltage should be confirmed before selection. A pump selected from a 50Hz curve should not simply be connected to a 60Hz supply. The change in speed will affect flow, head, absorbed power, and NPSH.

The mechanical seal also deserves attention. For ordinary clean water, the standard seal may be sufficient. Hot water, glycol mixtures, saltwater, cleaning chemicals, and other special liquids may require a different seal or elastomer.

The main difference between CDL and CDLF pumps is the construction of the wetted section.

CDL models generally combine stainless steel impellers, diffusers, and pressure cylinders with cast-iron suction chambers, discharge chambers, pump heads, or bases, depending on the pump size. They are mainly intended for clean, non-corrosive liquids.

CDLF models use more stainless steel in the wetted section and are better suited to clean liquids with mild corrosive properties, provided the materials have been checked against the actual liquid.

Common catalog materials include AISI 304 stainless steel for the impellers, diffusers, and pressure cylinders. Depending on the model, the shaft may be AISI 304, AISI 316L, or another specified grade.

For more demanding applications, wetted components may be offered in SS304, SS316, or 2205 duplex stainless steel. The correct choice depends on temperature, pH, chloride content, chemical concentration, pressure, and operating time.

In practice, the words "stainless steel pump" do not tell us enough. We still need to know the material of the suction chamber, discharge chamber, shaft, mechanical seal, and elastomers. This is particularly important for pool saltwater, seawater, and chemically treated water.

• Building Water Supply and Pressure Boosting

Vertical multistage pumps are widely used in residential buildings, hotels, hospitals, office buildings, and other facilities where the available mains pressure is not sufficient.

They can be installed in constant-pressure booster sets, roof-tank supply systems, and pressure zones in high-rise buildings.

The required head should be calculated from the static height, pipe losses, equipment resistance, and pressure required at the final outlet. Selecting a pump only from the number of floors often results in too much or too little pressure.

If the system demand changes considerably during the day, two or more smaller pumps with variable-speed control are often a better choice than one large pump.

• Industrial Water and Pressure Boosting

Typical industrial duties include process-water supply, production-line boosting, equipment washing, filter-system feed, and high-pressure cleaning.

These systems often have more than one operating condition. A cleaning line, for example, may require high flow for short periods but much lower flow during normal production. In such cases, the pump should be checked at the normal operating point as well as the maximum demand.

If the required flow is high but the head is relatively low, we would normally compare the multistage pump with a single-stage inline or end-suction pump. A multistage pump is not automatically the most efficient choice for every industrial water system.

• Water Treatment and Membrane Systems

Vertical multistage pumps are commonly used in filtration, ultrafiltration, pure-water equipment, reverse-osmosis systems, and other water-treatment processes.

The pump may be used to feed filters, maintain pressure before a membrane section, or circulate water through part of the treatment system.

Its duty depends on the required treatment flow and the resistance of the filters, valves, membrane modules, and pipework.

Filter resistance does not remain constant. As a filter collects contamination, its pressure drop rises. The pump should therefore be checked for both clean and dirty filter conditions. Differential-pressure monitoring is useful because it shows when cleaning or replacement is required.

For seawater desalination, CDL/CDLF pumps may be considered for pretreatment supply, auxiliary circulation, or membrane feed where the pressure and materials are suitable. A standard model should not be assumed to be suitable for high-pressure seawater reverse osmosis. High salinity, chloride concentration, operating pressure, and continuous service require a separate material and pressure review.

• Pool Saltwater and Saline Water

A vertical multistage pump may handle pool saltwater if the materials, mechanical seal, and operating conditions are suitable.

The information we normally need includes salinity, chloride concentration, pH, water temperature, disinfectants, and operating hours. Hair, sand, fibers, and other debris should be removed before the water reaches the pump.

A filter or membrane can protect the pump from particles, but it does not prevent chloride corrosion. SS304 may be acceptable in some low-chloride applications, but it is not a universal choice for saltwater. SS316, 2205 duplex stainless steel, or another material may be required depending on the actual water analysis.

• Boiler Feed, Hot Water, and Condensate

The CDL/CDLF range can be used for selected boiler-feed, hot-water, and condensate duties.

The catalog gives a liquid temperature range of -15°C to 70°C for standard versions and -15°C to 120°C for hot-water versions. The exact limit still depends on the selected model, seal, pressure, and inlet conditions.

Hot water needs particular attention because its vapor pressure is higher. A pump may meet the required discharge pressure and still cavitate if the inlet pressure is too low. For boiler-feed and condensate applications, NPSH and minimum inlet pressure should always be checked.

• HVAC and Industrial Cooling

Vertical multistage pumps can be used in cooling-water systems, air-conditioning circuits, heat-exchanger loops, and other systems with relatively high resistance.

In a closed circuit, the pump mainly overcomes friction and equipment pressure losses. It does not usually need to lift the water through the full height of the building. Treating the building height as pump head in a closed loop can lead to serious oversizing.

For a large-flow, low-pressure cooling circuit, a single-stage pump may be the better option. The system curve should decide the pump type.

• AI Data Center Liquid Cooling

Some data center liquid-cooling systems have enough pressure loss to justify a vertical multistage pump. Possible duties include CDU circulation, plate heat-exchanger loops, long pipe runs, rack-branch pressure boosting, and redundant cooling pump sets.

The pump is normally part of a larger circuit containing CDUs, heat exchangers, cold plates, filters, control valves, hoses, and quick connectors. The pressure loss through all these components must be included in the calculation. For parallel rack branches, the most hydraulically unfavorable branch also needs to be checked.

Coolant properties are just as important as flow and head. Water-glycol mixtures have a higher viscosity than water, especially at low temperature. This increases pipe resistance and can change both pump performance and motor power demand.

Deionized water and dielectric coolants introduce different questions about corrosion, conductivity, seals, and elastomers. A standard clean-water pump should therefore not be specified for a liquid-cooling project without reviewing the actual coolant.

Reliability is another major consideration. Critical facilities may require N+1 or 2N redundancy, automatic pump changeover, variable-speed control, leak detection, and communication with the building or data center management system.

• Irrigation and General Industrial Service

The range can also be used for farmland irrigation, sprinkler systems, drip irrigation, equipment water supply, and other clean-liquid duties.

Water containing sand, fibers, or significant suspended solids should be filtered before it enters the pump. Standard CDL/CDLF pumps are not sewage pumps and should not be used for abrasive or heavily contaminated liquids.

According to the CDL/CDLF catalog, the standard pump is intended for clean, thin, non-flammable, and non-explosive liquids without solid particles or fibers.

The general limits are:

• Standard liquid temperature: -15°C to 70°C;
• Hot-water version: -15°C to 120°C;
• Maximum ambient temperature: 40°C;
• Standard installation altitude: up to 1,000m.
• Above 40°C ambient temperature or 1,000m elevation, reduced air density affects motor cooling. The motor may need to be derated or replaced by a higher-rated unit.

Maximum Working Pressure

There is no single pressure rating for the complete CDL/CDLF range.

Depending on the model and connection arrangement, the catalog lists maximum working pressures of 16, 20, 25, or 30 bar. Some higher values shown in brackets require a special configuration.

When checking system pressure, inlet pressure and pump-generated pressure must be considered together. Shut-off head and temporary pressure surges should also be included.

This matters in membrane and liquid-cooling systems because filters, membrane housings, CDUs, heat exchangers, cold plates, hoses, and quick connectors may have lower pressure ratings than the pump itself.

NPSH and Inlet Conditions

Cavitation occurs when the pressure at the impeller inlet becomes too low for the liquid temperature. It may cause noise, vibration, unstable flow, reduced head, and damage to the impeller or mechanical seal.

The risk is higher when:

• The liquid is hot;
• The pump draws from a low-level tank;
• The inlet pipe is long or too small;
• The pump runs near the high-flow end of its curve;
• The installation is at high altitude;
• A filter is blocked;
• The liquid has a higher viscosity than water.

A short, adequately sized inlet pipe with as few restrictions as practical usually gives better inlet conditions. Filters should be monitored for pressure drop so that progressive blockage does not starve the pump.

A reliable selection starts with the required flow and total head. Where demand changes, it is better to provide minimum, normal, and maximum flow instead of one peak figure.

For an open system, total head normally includes static lift, pipe friction, valve and fitting losses, equipment resistance, and the required outlet pressure. In a closed loop, the calculation is mainly based on pipe and equipment resistance.

Avoid adding excessive head "for safety." An oversized pump often runs with a throttled valve, consumes more energy, and operates away from its best-efficiency region.

The liquid should then be defined properly. Besides its name, we need temperature, density, viscosity, pH, chemical concentration, and solids content. For saltwater, chloride concentration is essential. For liquid cooling, the type and concentration of glycol or other coolant must be provided.

Finally, the proposed duty point should be checked on the actual pump curve. Flow and head alone are not enough. Efficiency, absorbed power, NPSH, shut-off head, and the allowable operating range should also be reviewed.

Common Selection Problems

Several mistakes appear repeatedly in pump enquiries:

• Selecting by motor power instead of flow and head;
• Assuming that more stages always mean a better pump;
• Ignoring inlet pressure and NPSH;
• Treating all stainless steel grades as equivalent;
• Applying a 50Hz curve to a 60Hz project;
• Assuming a filter will solve a corrosion problem;
• Ignoring the pressure rise across a dirty filter;
• Selecting only for peak demand;
• Forgetting the pressure produced at shut-off;
• Using a standard water-pump configuration for an unknown coolant.

Most of these problems can be avoided if the system duty and liquid data are provided before the pump model is chosen.

Information to Include in an Enquiry

For a normal water application, please provide:

• Required flow and head;
• Liquid and operating temperature;
• Inlet pressure or suction conditions;
• Maximum system pressure;
• Voltage and frequency;
• Preferred materials;
• Connection size and standard;
• Indoor or outdoor installation;
• Continuous or intermittent operation;
• Variable-speed or multi-pump requirements.

For saltwater, membrane, chemical, or liquid-cooling applications, also provide the liquid composition, chloride content, viscosity, filter pressure loss, operating hours, control requirements, and required redundancy.

A vertical multistage pump builds pressure by passing the liquid through several impellers in series. Its compact footprint and high-head capability make it a practical choice for building supply, industrial boosting, water treatment, hot-water service, irrigation, cooling, and selected desalination or data center projects.

CDL pumps are generally intended for clean, non-corrosive liquids. CDLF pumps use more stainless steel in the wetted section and can handle mildly corrosive liquids when the material compatibility has been confirmed. Depending on the application, SS304, SS316, or 2205 duplex stainless steel may be considered.

The final decision should be based on the actual duty point and operating conditions. Flow, head, temperature, inlet pressure, NPSH, materials, motor data, and maximum system pressure all need to agree with the selected pump curve.

That approach usually gives a better result than choosing a pump from motor power, pipe size, or the number of stages alone.