The pressure that can be generated by a single stage lower centrifugal pump is limited. A multistage design of pump is required to create larger pressures.
What is a multistage pump and how does it work?
Multistage pumps are similar to centrifugal pumps because it has many impellers that rotate around a shaft to allow the pump to reach higher pressures than if only one impeller were employed.
Because once an impeller exceeds a certain diameter, the impeller side friction increases, reducing pump efficiency, there are limits to the pressure that can be generated by a single impeller. To generate high pressures while retaining a high level of efficiency, numerous impellers must be added along a single shaft.
Multistage pump working principle
Impellers are positioned in Ring Sections along the shaft in such designs, with each ring section consisting of an impeller on one side, a suction casing on the other, and a discharge casing / diffuser on the other.
The impeller sucks fluid in through the suction casing and discharges it through the discharge casing, which will then enter another ring section periodically until the fluid is discharged through the outlet.
The ring pieces are connected by tie bolts that run the length of the casing. Internal designs for shaft seals and bearings vary depending on duty and application, with shaft seals and bearings on one end of the pump or both.
The flow rate is not affected by adding stages of impellers, but the total head and shaft power increases according to the number of stages. An impeller and a diffuser are included in each stage.
Models can be built vertically or horizontally, depending on whether space-saving designs are required or models that can be kept in place without the motor being removed.
Multistage Submersible Pump
When water must be pumped at high pressure, multistage designs of pumps are employed in submersible designs of pumps known as borehole pumps, either to retrieve water from deep wells or to supply offshore platforms from sea level. Immersion pumps with numerous impellers can also be used to deliver high pressures from deep wells.
Double Suction Pumps
A back-to-back double impeller produces larger flows at pressures nearly double those of a single impeller pump installed on a single shaft in double suction pumps or pipeline pumps.
Why and where are multistage pumps used?
For a variety of purposes, such equipment is used:
When a centrifugal pump with multiple impellers is required for high head discharge but is outside the duty range of a single stage centrifugal pump, which typically has a maximum discharge pressure of 150M vs 1000M from single impeller designs.
A high discharge pressure requirement could be determined by the application, such as providing domestic water to high floors in skyscrapers, which requires a long fluid path with high friction losses, as is common in pressure booster set applications, or a filtration process, such as reverse osmosis, which requires a low viscosity liquid to be pumped through a fine filter.
Where a cost-effective and efficient solution for a clean fluid at high pressure is required, a multistage design can be employed, which is more efficient because the impellers are not only smaller but also at full impeller size, making them efficient even at lower RPM.
Single stage designs are trimmed to duty point, resulting in lower efficiency due to increased clearances between the impeller’s edge and the casing.
