Multistage Centrifugal Pump
What is a Multi-stage Centrifugal Pump?
A multi-stage centrifugal pump is a type that pressurizes the fluid in two or more stages. Each pumping stage consists of guide vanes, a diffuser, and an impeller, and these components are housed in a pump casing. A multi-stage centrifugal pump uses a series of impellers arranged in different chambers to pressurize water. The impellers are installed and rotated on the same shaft. Similar or different types of impellers may be used for each of the pump stages. Water is first drawn into the first chamber, the impeller then imparts some kinetic energy by accelerating its speed, and then the diffuser converts the kinetic energy into pressure. The water is then sent into the second stage where the pressure is increased further and the process proceeds to the third stage and so on until the required pressure to pump the water to the desired location is attained. This pump is also characterized by a pull rod. Pumps of this kind discharge water with a higher pressure than the pressure of water at the pump inlet.
A multi-stage centrifugal pump is also one of the seven types of centrifugal pumps and it classifies under the types of centrifugal pumps based on the number of impellers. Its counterparts are the dual and single-stage pumps. unlike the single and dual-stage pumps, the design of a multi-stage centrifugal pump is a bit complex. Generally, a multi-stage centrifugal pump is used with water and low viscosity fluid. Due to their high-pressure capability, they are used for high-pressure applications such as boiler water feed water
Figure: Multi-stage centrifugal pump
Working principle of multi-stage centrifugal pumps
As discussed earlier, a multi-stage centrifugal pump works in several stages. The breakdown of the working steps of a multistage centrifugal is as follows:
- First, fluid is drawn from the sump into the eye of the first stage impeller.
- once the fluid strikes the vanes of the impeller, some kinetic energy is imparted leading to an increase in velocity of the fluid.
- Due to the centrifugal force acting on the impeller, the fluid discharges from the vanes of the impeller and enters into the diffuser
- In the diffuser, the pressure of the fluid increases due to a reduction in its speed, and the fluid is then sent into the 2nd stage where fluid pressure further increases.
- This process continues until the last stage of the pump where the fluid attains the required pressure and pumps to the desired location.
Components of a multi-stage centrifugal pump
The main parts of a multistage centrifugal pump are pump impellers, pump casing, and shaft.
Pump casing
The casing is the major stationary component of a multistage centrifugal pump housing the impellers and diffusers. In other words, it contains various pump chambers or stages. The pump casing serves 3 main functions:
- It converts the kinetic energy or velocity head from the impeller into the pressure head
- It guides the flow to the outlet
- it creates a pressure boundary for the pumped liquid
The pump casing construction has two main designs: volute casing and diffuser casing. A multistage centrifugal pump commonly employs a diffuser type of casing. This type of casing consists of a series of vanes surrounding the impeller. The vanes take in the discharge from the impeller, reduce its velocity, and guide the lower velocity liquid into a vanned return channel which directs the liquid into the eye of the next stage channel. The diffuser casing offers several advantages including high pump performance efficiency, fully balanced radial loading, and long pump life.
Multiple impellers
In a multistage centrifugal pump, there are several impellers mounted and rotating on a single shaft but in different chambers. The purpose of an impeller is to directly transfer the mechanical energy from the electric motor to the fluid leading to an increase in fluid flow rate and fluid pressure. The design of an impeller features several vanes and blades to help achieve its functions. A multi-stage centrifugal pumps’ impellers are usually made from metallic materials such as steel, iron, or bronze. The pump performance is highly dependent on the design of the impeller and a perfect impeller is one that eliminates turbulence, optimizes flow, and maximizes efficiency. The impeller design has three main types including open, semi-open, and closed types. Each of these types has its advantages and limitations. In an open impeller, the vanes are free on both sides while in a Semi-open impeller the vanes are open on the suction side but closed on the delivery side. In a closed impeller, the vanes are covered on either side with front and rear cover plates. A multistage centrifugal pump may use one or a combination of the different impeller designs across its various chambers. this type of impeller. The closed impeller is highly efficient for conveying cleaning liquid without impurities.
Pump shaft
The pumps shaft is the central component of a multistage centrifugal pump and it carries the impellers. Its major function is to transmit the drive torque and mechanical power to the impellers of a multistage centrifugal pump. The drive power mostly comes from an electric motor that couples with the pump shaft. The pump shaft is designed to balance all the components of the rotor so that they do not touch the bore during rotation. The shaft is usually fitted with shaft seals for the following reasons:
- To prevent the liquid in the pump casing from leaking out along the shaft
- To prevent the outside air from leaking into the pump casing
Types of multi-stage centrifugal pump
Pump manufacturers avail two major types of multistage centrifugal pumps in the pump industry.
- Horizontal multistage centrifugal pump
- vertical multistage centrifugal pump
Horizontal multi-stage centrifugal pump
In this pump, the pump shaft and the impeller are horizontal to the ground surface This pump consists of a segmented pump housing and modular intermediate parts. the rotary parts are supported on bearing housing to ensure their balance under high fluid pressure. A horizontal centrifugal pump can manage higher flow rates compared to the vertical multistage centrifugal pump. it also has excellent construction material and productivity.
Vertical multi-stage centrifugal pump
In this pump, all the rotor components (shaft, impeller) are perpendicular to the ground surface. A vertical multistage centrifugal pump mostly consists of 4 runners mounted on a vertical shaft and it is powered from an external electric motor. The vertical design minimizes their overall space requirement
Figure: vertical multistage centrifugal pump (image credits: mechanicalboost.com)
Applications of multi-stage centrifugal pump
Multistage centrifugal pumps are most suitable for high-pressure applications. The following are some of the major applications of this pump.
- Water supply boosting e.g., in agriculture and domestic uses
- Boiler feedwater
- Desuperheater
- Reverse osmosis
- Irrigation
- Water circulation
- Heating/cooling conditioning
- high-pressure shower system
- Fire fighting
- commercial/high rise building
- medical systems
Advantages of multistage centrifugal pump
- High hydraulic efficiency
- high-pressure capabilities
- can pump water for long distances
- can achieve higher performance with small motor size using less energy
- low running cost
- reduced leakage loss because the head per stage is less
- smaller impeller size because head per stage is small
- It can operate at a lower speed
- very stable in performance
- Available in a variety of designs: horizontal and vertical designs
- high reliability
Disadvantages of multistage centrifugal pump
- Hard to maintain. The maintenance operation is quite difficult because the pump contains many parts inside the casing that need maintenance and service.
- Initial installation and maintenance cost is too high
- complex and costly
- parts inside the casing are difficult to access
Basic maintenance of a multistage centrifugal pump
Ensure good lubrication: The bearing should be well lubricated. Select lubricating oil according to the manufacturer’s guide. Do not add excess oil to the rolling bearing during overhaul because it will affect the heat dissipation process and eventually damage the bearing. keep the lubricating oil safe and free of contaminants such as water.
Ensure regular maintenance of wearing parts: wearing in a multistage centrifugal pump is experienced various parts including sealing ring, oil cup, and gaskets. The components should be inspected and timely replaced to prevent pump leakage. Also, ensure flanges and coupling are well tightened every time the pump is running
Maintenance of electrical components: ensure that the pump wiring system is free of loose connections and the motor windings free of dust. Also, inspect the stator arching and overheating.
Troubleshooting multistage centrifugal pump
Pump fails to prime
- Air leaks on the suction side of the system (check suction hoses for leaks)
- Discharge valve open
- Suction lift too high (lower pump)
- worn-out pump packing or venturi (replace packing or venturi)
Pump shut down
- Engine run out of fuels (refill with fuel)
- clogged fuel filter (change filter pr breed of fuel)
- low engine oil pressure or high temperature
Pumps give out little discharge
- Leakage in the pipe (replace damaged pipes)
- The suction lift is too high (ensure there is no obstruction at the pump inlet)
- Impeller strainer or check valve is clogged
- Wrong direction of rotation (reverse polarities)
- The riser pipe is partly chocked by impurities (unclog the pipes)
- The impeller lodged with impurities (clean
- Too many depositions in discharge pipe (clean)
- Air in pump or suction pipework (ensure pump and piping system is filled with water)
Excessive vibration
- Air in the fuel line or clogged filter
- misalignment of pump and engine (realign)
- Worn out bearing, impeller (replace)
- suction or discharge pipe not supported
- Pump working in cavitation (Too low Net Positive suction head, inspect suction losses, fully open the inlet valve, correct suction design)
- Unbalanced parts (rebalance pump parts)
- Pump working outside duty range (check duty point)
Pump overheating
- cavitation due to improper pump design
- Airlock (loosen vent plug to purge air)
- Poor coupling alignment (realign pump and coupling, check coupling rubber)
- Excess grease (drain excess grease)
- Contaminated grease (clean the bearings and housing)
Summary
This article discusses the multistage centrifugal pump, its working principle, and its applications. It also provides some detail on maintenance and troubleshooting techniques as well as the pros and cons of the pump. A multistage centrifugal pump is a type of centrifugal pump that pressurizes the liquid in two or more stages. The pump is characterized by several guide vanes, diffusers, and impellers installed on a shaft and housed in diffusing type of pump casing. A multistage pump has two main designs including a horizontal centrifugal pump and a vertical centrifugal pump. The two differ in terms of the orientation of the rotor component (shaft and impellor) whereby the horizontal centrifugal pump has its shaft and impellers placed horizontally while the vertical centrifugal pump has its shaft and impellers placed vertically. A centrifugal pump applies for pumping of low viscosity fluids and high liquid pressure application in both homes and industries. Common industrial applications of the pump include boiler feed water, irrigation, pressure boosting, desuperheater, cooling conditioning, and water circulation. Some of the advantages of this type of pump include high hydraulic efficiency, high-pressure head, minimal leakage losses, and the ability to pump water long distances. However, the pump is complex in design and costly, hard to maintain, and the initial installation cost is too high. Maintenance is needed for some mechanical components including bearings, seals, impellers, gaskets. Bearing lubrication is one of the most important maintenance activities for the pump since lack of lubrication can cause bearing overheating, bearing damage, and excessive pump vibration. Like with any other pump, troubleshooting is at times required for this pump to uncover and solve some pump performance challenges for example when the pump fails to prime, gives little discharge, excessive vibration. This article points out some of the causes of such challenges and the corresponding cause of action.