Due to their relative simplicity of design, centrifugal pumps are the most common pump type in the market. Centrifugal pumps are categorised into either single or multistage pumps, both of which have their uses. What is important to understand is where which should be selected, which ultimately comes down to their working principle and the application in question. So, let’s understand the differences between the two…
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The primary difference between single and multistage pumps is their impellers and how this affects operation. Whereas a single stage pump has only one impeller rotating around the shaft, multistage pumps have two or more. This characteristic has several implications for the performance of the pump, which we will go onto now.
1. Pressure capabilities
The multiple impellers of a multistage centrifugal pump are all installed and rotated on the same shaft, and essentially act like separate pumps. This mean that as the flow progresses from one stage to the next, the head increases approximately by the same amount, resulting in the multistage pump design having much higher pressure capabilities than a single impeller is able to alone.
With the above in mind, multistage centrifugal pumps are often selected when the pressure requirements of the application exceed the capabilities of a single stage pump. For example, perhaps the fluid must be moved over a very large distance with considerable friction loss, which in most scenarios is achieved via multiple impellers. A typical example of this may be providing water supply to the top of tall buildings or towers. Single stage centrifugal pumps are on the other hand the better suited solution in higher flow, lower pressure installations.
2. Efficiency
As multistage pumps reply on multiple impellers used to impart energy into the fluid being handed, each impeller can be smaller in diameter than it would be alone and can operate with tighter clearances between the impeller and volute. These smaller tolerances mean that each impeller operates closely to its best hydraulic efficiency. The result of this is that multistage centrifugal pumps are able to achieve higher performance with a smaller motor size and using less energy.
Due to the tight impeller clearance with the pump volute, multistage pumps are unable to handle solids and abrasive content unless they are oversized, which is why they are generally used with water and other low viscosity fluids. This combined with their high pressure range, makes multistage water pumps popular in applications including:
Multistage pumps are defined as pumps in which the fluid flows through several impellers fitted in series.
The head of a single-stage centrifugal pump is largely governed by the type of impeller and the circumferential speed. If the rotational speed cannot be increased due to other operating conditions and a larger impeller diameter would lead to very low specific speeds resulting in uneconomical efficiencies, fitting several stages in series (also see Series operation) can be an economic option of increasing the head. If the number of stages is altered at unchanged dimensions and speeds, the flow rate of such a multistage pump remains constant while the power input and head increase proportionally to the number of stages.
An example of a pump with several stage casings of the same type fitted in tandem arrangement is the ring-section pump. This type of pump is often used in power station applications, e.g. as a boiler feed pump and in industrial applications requiring high pressures.
The individual stages of a multistage pump do not necessarily have to be arranged in tandem. The balancing of axial thrust can be enhanced by arranging the stages back to back in pairs or groups (see Back-to-back impeller pump). A typical example would be the pipeline pump. Multistage pumps are an economic means of covering the higher pressure ranges of pump series selection charts Further advantages are that multistage pumps can easily be tapped downstream of a stage or that dummy stages can be fitted for future pressure increases.
A disadvantage of very large numbers of stages is the increasing sensitivity of the pump rotor to external or natural vibrations.
Each stage consists of an impeller, a diffuser and return guide vanes), (usually combined with the diffuser), which are all located within one and the same stage casing.
Irrespective of the number of stages an inlet casing with radial or axial inlet nozzle is arranged upstream of the first stage, and the last stage is fitted in the discharge casing containing the balancing device and a shaft seal Only the common pump shaft, tie bolts and baseplate have to be adjusted to accommodate the required number of stages. See Fig. 1 Multistage pump
When is it appropriate to use a multistage pump?
A single-stage pump is defined as having one impeller and its related discharge collector that together produce fluid flow when powered by a driver, but due to having just one impeller, there are limits to the performance of a single-stage pump. On the other hand, a multistage pump is one where fluid flows through multiple impellers that are in series, which increases the total head (pressure) generated by the pump. Single volutes, double volutes and diffuser casing designs can all be configured into multistage pumps.
As the fluid flows through each stage, pressure becomes higher than it was in the previous stage. In multistage pumps, the impellers sometimes differ in design but should always be designed to have nearly identical flow characteristics. An example where the impellers will differ is to avoid significant cavitation in low net positive suction head (NPSH) systems. In this case, or others requiring a pressure boost prior to the subsequent stages, the first stage will be designed for low NPSH operation (such as double suction impeller) and effectively operate while increasing the pressure, prior to entering the subsequent stages.
IMAGE 3: General selection chart for rotodynamic pumps that shows multistage pumps are used for higher head.
The main purpose for selecting a multistage pump is to efficiently operate in systems that require a high total head. There is no clear-cut delineation when to move to a multistage pump, but Image 3 can serve as a guide, which shows that multistage pumps are generally selected above 1,000 feet of total head.
While the fact that multistage pumps produce higher head is the most important application consideration, there are other reasons why a multistage pump would be used. All rotodynamic pumps produce noise, and this noise is contributed to by each of the individual components of the pump. Generally, the noise generated by these pumps is by hydraulic effects that are transmitted to the pump case.
Data has shown that multistage pumps will exhibit lower noise levels when compared to single-stage pumps of the same power levels. This is due to the energy being spread out over multiple stages rather than a single stage. Because of this, multistage pumps may be a better fit when high noise level is
a concern.
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 M 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?
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Such designs are utilised for fluids
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.