There are limitations to the pressure which can be generated by a single stage design of centrifugal pump. In order to generate higher pressures, a multistage design of pump must be used.
A multistage pump is similar in design to that of a centrifugal pump in that several impellers rotate around a shaft to enable the pump to reach higher pressures than that if only one impeller was used.
There are limitations in the pressure which can be generated by a single impeller, as once an impeller reaches a particular diameter, the impeller side friction increases causing a reduction in pump efficiency. This means it is necessary to add several impellers along a single shaft to generate high pressures, whilst maintaining a high level of efficiency.
Impellers within such designs are mounted in Ring Sections along the shaft with each ring section consisting of an Impeller, with a suction casing on one side, and discharge casing / diffuser on the other.
The impeller draws fluid in through the suction casing, to the outside of the impeller before being discharged through the discharge casing, which will then enter another ring section repeatedly until the fluid discharges through the outlet.
The ring sections are held together with tie bolts which run along the casing. Internal designs varying for shaft seals and bearings being on one end of the pump, or both dependent on duty and application.
When stages of impellers are added the flow rate is not altered but the total head and shaft power increases proportionally to the number of stages. Each stages consists of an impeller and diffuser.
Models can be constructed either vertically or horizontally depending on whether designs are required which are space saving, or which can be maintained in situ without removal of the motor.
Multistage designs of pumps are also used within submersible designs of pumps known as borehole pumps where water is required to be pumped at high pressure either to extract water from deepwells, or to feed offshore platforms from sea level. Immersion pumps can also be built with multiple impellers to deliver high pressures from deepwells.
Double suction pumps or pipeline pumps also use a similar concept with a back to back double impeller producing higher flows at pressures almost double of a single impeller pump mounted on a single shaft.
Why and where are multistage pumps used?
Such equipment is used for a variety of reasons:
1. When a centrifugal pump for high head discharge is required but is outside the duty range of a single stage centrifugal pump which typically have a maximum of discharge pressure of 150M vs 1000M from designs with multiple impellers.
A high discharge pressure requirement could be dependent on the application such as providing high floors in skyscrapers with domestic water, meaning the fluid path is long, with large friction losses as is common in pressure booster set applications, or perhaps a filtration process requires a low viscosity liquid to be pumped through a fine filter as in reverse osmosis.
2. Where an economic and efficient solution is required of a clean fluid at high pressure, a multistage design can be used which is more efficient, as the impellers are not only smaller but at full impeller size meaning they are efficient even when ran at lower RPM.
Single stage designs are trimmed to duty point, losing efficiency as the clearances are larger between the edge of the impeller and casing.
Which applications use multistage pumps?
Such designs are utilised for fluids <300cst in viscosity, and unless designs are oversized or large they can only be used fluids free from solids. Fluids can be fuels, or other fluids such as chemicals so long as they are chemically compatible with the casing and elastomer materials.
They will be typically used for:
· Mine Dewatering
· Boiler Feed
· Reverse Osmosis
· Pump as Turbine
· Spraying in Sprinkler Systems for skyscrapers or Water Deluge systems
· Irrigation
· Pressure Boosting of mains water supply for high rise buildings and in applications involving long runs of pipework.
Multistage pumps can be utilised in Pump as Turbine applications where pumps are used as an alternative to generate electricity to turbines. Their design means higher pressured water which runs through the impellers generates more power through the shaft, generating larger amounts of electricity than that of a standard centrifugal Pump as Turbine.
There can be numerous considerations when selecting a unit for your process:
Pumps can have their inlet and outlet flanges configured at manufacturing stage to match existing pipework, or for ease of connecting to other pipework. The flanges can rotate across the pump body at 90° angles with no effect on the required outlet flow or pressure. An Example of the combinations of flanges is below:
Multi Outlet Multistage pumps have a single inlet, but dual outlets which deliver 2 different discharge pressures simultaneously. This design of pumps has various benefits over a standard design of pump being:
1. They eliminate the requirement for water storage tanks at intermediate levels within a building
2. Eliminate the requirement for pressure reducing valves meaning equipment is more efficient
3. Over pressurisation of sprinkler heads is prevented.
4. Two duties from one pump saves on equipment cost
Multistage Pump vs Centrifugal
Multistage Pump vs Jet Pump
Multistage Pump vs Side Channel
Multistage Pump can achieve higher pressures
Jet Centrifugal pumps are Self-Priming by design, Multistage pumps require a foot valve for manometric suction lifts
Side Channel Pumps are Self-Priming by design, Multistage pumps require a foot valve for manometric suction lifts
Multistage pumps are designed with full impeller sizes in series meaning they are more efficient than centrifugal pumps
Jet pumps are typically a domestic / light industrial pump
Side Channel pumps cannot accept any solid passage, whereas multistage pumps can be manufactured in large sizes or oversized to accommodate fibrous solids
Multi Port configurations are available with multistage pumps as well as ports orientated to match pipework
Jet pumps are usually close coupled, meaning should seal fail water enters the motor.
Side Channels produce lower flows and pressures than multistage pumps
Centrifugal Pumps can be fitted with different impellers to accommodate solids or large particles, where as multistage pumps are not designed for solids unless pumps are large or oversized.
Liquid ring pumps can handle entrained gas, have lower NPSH and can completely empty a container
Shaft on Multistage pump is typically supported at both ends with bearings and seals meaning radial thrust is balanced more equally
If you have an application which requires high pressure contact us to discuss your application, or view our range here.
If you are having issues with your multistage pump view our troubleshooting guide.