Multistage Self-Priming Pump

Multistage Self-Priming Pump
USM, UBM, MEF types

High head type with high performing self-priming mechanism

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Multistage Self-Priming Pump



Total head




USM type (Driven by 4-pole motor):

Multistage centrifugal pump with self-priming mechanism

UBM type (Driven by 4-pole motor):

High head multistage centrifugal pump with self-priming mechanism

MEF type (Driven by 2-pole motor):

High head multistage centrifugal pump with self-priming mechanism

Outstanding self-priming performance

This pump is equipped with our patented self-priming (water-air separating) mechanism, which displays outstanding self-priming power.

A large amount of incoming air can be separated and discharged during pumping. Even if pumping becomes subject to suction or mixture of air due to fluctuations of suction conditions, it continues pumping, constantly discharging air, and restores normal pumping operation as soon as suction conditions return to normal. Even continuous suction of air-containing water (i.e., gas-liquid two-phase pumping) is possible.

Even if cavitation develops due to fluctuations of suction conditions such as water level, temperature, or vacuum level on the suction side, this pump can still continue its pumping operation. Therefore, this pump does not require allowance for NPSH, and enables stable pumping operation even under fluctuating suction conditions.

Due to its simple structure without any inlet check valve, it is robust and easy to maintain. Moreover, there is no need for installing a foot valve or an intermediate valve on the intake side under ordinary operating conditions.
Operation is also easy with no need for priming.

Self-balancing mechanism

With the crossover balancing mechanism for UBM types, and with the floating disk type balancing mechanism originally developed by YOKOTA for the MEF type, the axial thrust of its main shaft balances without axial movement of the shaft. Therefore ordinary ball bearings can be adopted, and the pump can be disassembled and reassembled very easily.

Large selection of materials

A wide variety of materials are available, including FC, SCS and YOKOTA's corrosion and wear resistant special stainless steel casting (YST), to meet the needs of various kinds of liquid.

Principle of self-priming

USM, UBM types

The self-priming mechanism consists of a 1st-stage casing with a smaller passage and a larger passage forming a semi-double volute, and a 2nd-stage suction casing with a 2nd-stage suction port.

Multistage Self-Priming Pump / Principle of self-priming (PAT.)

1. Flow of circulation

During self-priming, the water discharged from passage A in the semi-double volute returns to the impeller through passage B-C, and is discharged again into passage A.

2. Removal of bubbles

This recirculation flow mixes the water and the air in the center with intense vortex D within the impeller and discharges it into passage A.

3. Water-air separation and exhaust

The water-air mixture is led from passage A to chamber B and separated. The separated air is discharged via an exhaust valve. The separated water, not entering the multistage side through the 2nd-stage suction port, returns to the 1st-stage casing to repeat the self-priming operation until separation and exhausting are completed.

MEF type

The self-priming mechanism consists of a 1st-stage impeller with a main passage on the suction side and a self-priming passage on the back side, guide vanes, and an exhaust valve.

Multistage Self-Priming Pump / Principle of self-priming (PAT.)

1. Flow of circulation

When the pump starts to rotate, water in the pump circulates from D to B, then to C.

2. Removal of bubbles

This circulating flow becomes an intermittent jet flow when it flows from passage B, on the back side of the 1st-stage impeller, to passage C, mixing the air with the water and carrying it away to passage C.

3. Water-air separation and exhaust

The water-air mixture is led from passage C to chamber D and is separated by centrifugal force. The separated air goes through passage E and is discharged via exhaust valve F. The separated water recirculates through D, B, and then C, returning to the 1st-stage impeller to repeat the self-priming operation.

4. Normal state

When self-priming is completed, water is pumped through main passage A of the 1st-stage impeller, and fed to the multistage impellers through passages C-D-G and C-E-G.


With the self-priming and pressurized discharge capabilities, these pumps can be used even under extreme conditions. They can be used, for example, as chemical pumps for liquids containing air, pumps for mixing air bubbles in an aeration system (i.e., gas-liquid two-phase flow pumps), condensate pumps, drain pumps, and pumps for extraction from sealed tanks.

Boiler feed

Transferring gas-containing liquid (gas-liquid two-phase flow)

Sealed (Vacuum) tank extraction

Water supply to high elevations

Pressurized water discharge

Seawater suction/transfer


Example applications and installations

Multistage Self-Priming Pump / Seawater intake pump

Seawater intake pump for the marine biological laboratory (MEF type)

Multistage Self-Priming Pump / Seawater drain pumps under operation in Kanmon Tunnel (USM type)

Seawater drain pumps under operation in Kanmon Tunnel (USM type)

There is detailed information about applications to seawater-related facilities. Please refer to "Current Topics: Undersea Tunnel".

These are self-priming type pumps for suction pumping.
For non-self-priming types (for force pumping) with similar specifications, please refer to
Multistage Centrifugal Pump MCF, MCH types.

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