Impeller – The Soul Of The Centrifugal Pump

Impeller Clearance

When speaking of hydraulic pumps, the most commonly used and ideal pump is the centrifugal pumps. It finds its application in the domestic and industrial facilities. You may ask why are centrifugal pumps so popular? The reason is that they can induce flow or even raise pressure of a fluid. It has a simple working mechanism.

Construction of the impeller:

However, a centrifugal pump cannot work without this thing called the impeller. It lies at the center of the system. An impeller is basically a rotating structure with a series of curved vanes/blades. These blades are fitted inside the shroud plates and immersed in water. The engine or the motor shaft that the impeller is connected to produces centrifugal force within the pump casing.

Working of the impeller:

When the impeller rotates, the fluid surrounding it also rotates. The water flows out radially through the centrifugal force imparted by the impeller. The pressure and kinetic energy of the water rises at the discharge side of the impeller because the rotational mechanical energy is transferred to the liquid. On the other hand, a negative pressure is induced at the eye on the suction side of the impeller where water is getting displaced. The centrifugal pump impeller when works in tandem with the volute helps to create a partial vacuum and a low pressure. When this vacuum is is maintained it helps to the fresh water stream to move into the system.

The centrifugal pump impeller combined with the volute decides the following:

  1. flow of the pump,
  2. pressure inside the pump and the liquid
  3. the solid handling capacity of the centrifugal pump.

Open Impellers:

One of the most frequently used and popular type of centrifugal pump impellers is the open impeller. Their specialty is that can even pump liquids containing suspended solids. These materials usually include twigs, slurries etc. Open impellers do not clog easily since they have an external adjustment that helps in their maintenance and clearance when the internal parts begin to wear.

How can you achieve a higher efficiency in centrifugal pumps?

For a commendable centrifugal pump performance, the front side of the vanes must be set with a clog clearance. However, with time, the impeller and suction plate will start wearing out. With that, the performance of the pump will also start deteriorating. But still if we regularly carry out clearance checks and make sure that the periodical adjustments are done, we can maintain the good performance of the pump.

We can also increase the clearance behind the impeller by adjusting it towards the suction wear plate. The vanes are located on the non-pumping side of the impeller are called back pump-out vanes. These are smaller in size and can act like another centrifugal pump. They also help in maintaining a low pressure. So that is the reason why it is necessary to keep the impeller in a good shape. The performance of the centrifugal pump is largely based on the impeller!

Basic Differences Between Open and Closed Impellers in Centrifugal Pumps

When buying a centrifugal pump, we always look for pumps with high efficiency, minimal maintenance and good reliability. One must always consider all these factors, however, one thing that people forget to look at is the selecting the right impeller style for your centrifugal pump. Earlier, impellers generally were divided into two categories open and closed. But these days, there is also a third type available. It is called a semi open impeller. Here are a few differences between the open and closed impeller:

  1. The open impeller has a series of vanes attached to a central unit. This is done for the mounting on the shaft. However, this design is more sensitive to wear and tear of the blades. The closed impeller has a side wall on the either sides (top and bottom) of the vanes.
  2. Closed impellers are the most commonly used impellers in the industry since they can deal with volatile and explosive fluids.
  3. The closed impeller is initially really efficient, but with time, loses its efficiency as the clearance of the wear ring increases, whereas the efficiency of an open impeller can be maintained through clearance adjustment.
  4. The pump had to be disassembled when it has a closed impeller to check the status of the wear rings. In an open impeller, no pump disassembling is necessary.
  5. An open impeller is less likely to get clogged and even if it does, it is easier to clean whereas in a closed impeller, if stringy material or solids are pumped, the impeller can clog and it becomes really difficult to clean them.
  6. The internal parts of a closed impeller are hidden hence it is difficult to cast and inspect for flaws whereas in an open impeller, all the parts are visible. It thus becomes easy to inspect for damage.
  7. The design of a closed impeller is more complicated and expensive since the additional wear rings are needed while the open impeller is less costly to build.
  8. You cannot easily modify a closed impeller so as to improve its performance. While the vanes can be easily cut to improve the capacity in an open impeller.
  9. In a closed impeller, speed choices are limited. But you have a wide range of specific speed, choice is an open impeller.

As we just saw, there are very specific benefits and pitfalls in both the types of impellers. But it depends on the kind of application you are buying the pump for. Would you think about buying a car that has a poor mileage? No, right? Similarly, when buying a pump if you keep the above mentioned points in mind, you can surely make the best decision without any regrets in the future.

The Crucial Role of an Impeller in a Trash Pump

There are a hundred different types of pumps available in the market these days. From diaphragm pumps to positive displacement pumps, you name the application and you will find a pump suitable for it! The most commonly used pump type however is the centrifugal pump. Centrifugal pumps come with an impeller. This impeller when spins, transfers energy to the fluid with the help of centrifugal force. It directs the water to the discharge point.

A type of centrifugal pump that can pump solids is known as trash pump. It can pump materials like those in a typical sewage pipeline. What differentiates a trash pump from a normal centrifugal pump is the impeller and the pump casing. The impeller has the solid-handling capacity and is sized accordingly to let spherical solids pass through it.

Importance of Impeller in a Pump

Impellers have blades that help in adding velocity to the fluid. When the impeller moves, the length of the blades which are also known as vanes control the pressure produced by the pump, while the width of the blades control the volume.

Impeller Design

When you are buying your industrial pumps, you must always review the construction applications and your industry type so that you can buy a pump with specific impeller configurations. An open or semi-open impeller offers more space for non-compressible solids such as rocks up to 5 inches in diameter to pass through it. These types of impellers reduce the risk of clogging. Open impeller designed trash pumps can be used for a wide range of applications including wastewater and other industrial fluids.

Different Types Of Impellers

For open impellers, cast steel is the best material that can be used since it provides excellent impact resistance when non-compressible solids pass through them. An open impeller with a cutter vane can also prevent clogging as discussed earlier.

When dealing with smaller solids, one can use a closed impeller since it is also better equipped to handle abrasives and slurries. Fine rocks encountered during mining applications can pass through this type of impeller.

Installation Methods

While selecting a trash pump, you must know the maximum depth of the fluid and then consider the installation method. Trash pumps are normally available in two installation methods:

  1. submersible
  2. above-the-ground

Generally, above-the-ground trash pumps are driven by an engine or with electricity and submersible pumps are hydraulically or electrically driven. Any of the installation methods will have its own advantages and limitations.

Other Uses

Trash pumps were originally designed to pump solids. The impeller design is such that the cavity of the blades is large. Hence, when compared to other centrifugal pumps, trash pumps can take higher capacities of fluids. Therefore, if your application requires a high volume of fluid to be pumped, you must consider buying a trash pump. With the introduction of impellers of large diameters, liquids can be pumped at a longer range on high pressure. This makes the trash pumps more versatile. From an owner’s perspective, having one pump that can work on multiple application projects reduces the overall cost and need to service and replace different units if any problems arise.

Understanding the Net Positive Suction Head of the Centrifugal Pump

NPSH In Centrifugal Pumps

In the last article we discussed about the cavitation in centrifugal pumps and methods to prevent it. We understood that cavitation is situation that requires serious thought. To avoid this penetrating problem, one must ensure that the pressure of the fluid flowing through the centrifugal pump at all points inside the pump remains above the saturation pressure. The quantity that is used to determine whether the pressure of the fluid that is being pumped into the pump is adequate for avoiding cavitation is termed as NPSH: Net Positive Suction Head.

So before we discuss about NPSH of a centrifugal pump, it is important to know the fundamental working principles of this machine. A centrifugal pump is basically a machine responsible for imparting energy to a fluid for two purposes:

  1. for increasing the pressure of the fluid or
  2. moving it along the pipeline.

A centrifugal pump normally accomplishes this task through the actions of blades. As fluid enters the vane of the centrifugal pump, energy is supplied in the form of velocity. When the velocity is gradually reduced, the energy gets converted into pressure. It is clear that a pump will not be able to render velocity to a liquid that is outside the pump. Therefore the liquid must enter from the opening of the impeller and pass through the vanes before the process of transfer of energy begins.

While the conditions outside the centrifugal pump to force the liquid into the eye of the impeller, there must be sufficient energy available to the liquid at the eye to perform this energy transformation process and assure that the liquid remains in a liquid state. This energy is known as the NPSH(A): Net Positive Suction Head Available.

The formula of Net Positive Suction Head Available is the difference between:
the pressure at the suction of the pump and the saturation pressure of the fluid that is being pumped.

NPSH(A) = P(suction) – P(saturation)

When these pumps are designed, they have inbuilt physical and hydraulic characteristics that determine:

  1. the amount of energy that will be required to push the liquid into the impeller,
  2. energy needed to ensure that the liquid remains a liquid throughout its path through the impeller.

Some of the factors that also help determine the amount of energy needed by the pump are:

  1. the nature of the pump eye,
  2. the structure of the blades of the impeller,
  3. their diameter,
  4. the speed at which they operate

The amount of energy needed by a centrifugal pump is called NPSH (R): Net Positive Suction Head Required.

The pump can only perform its functions properly when the energy available is equal to or greater than the energy required. Therefore, one basic rule that every centrifugal pump must follow is:


And if we consider a margin of safety, then this rule can be best stated as:


To avoid cavitation, the minimum net positive suction head necessary is known as the Net Positive Suction Head Required.

You don’t want to install a noisy, slow and damaged centrifugal pump for your industry. Hence it is important to get the NPSH(R) value from your pump manufacturer and insure that your NPSH(A) will be enough to cover those needs.

Maintenance Regime of Trash Pumps To Ensure A Longer Pump Life

Self Priming Trash PumpsHow is trash pump different from centrifugal pump?

A trash pump, as the name suggests, allows larger debris to flow through as compared to semi-trash and centrifugal pumps. It allows stones, small sticks and other debris ranging from 0.75 to 1.25 inches to pass through it successfully. This property makes the trash pumps ideal for handling thick, sandy and dirty water.

Applications of trash pumps:

  1. Trash pumps are really a good choice for situations wherein high volumes of water containing trash need to be moved.
  2. With this type of pumps you can rest assured that even the large debris will be handled. When you wish to remove excess water from a land or a reservoir after heavy floods, only a trash pump will be able to solve your purpose. Such applications strictly require the use of a trash pump.
  3. They have the ability to keep the water moving. In spite of the water containing tiny bits of rocks and other fairly large sized sediments, these pumps can still move the water.

Maintenance of trash pumps:

Trash pumps are robust pumps that are designed for long life. Minimal servicing is required in them. However to ensure a longer life of your trash pumps there are some basic steps you must take. If you follow these guidelines to protect your pumps they will remain in good working condition for as long as possible.

  1. Place the pump near the liquid so that the suction lift is minimized. When the pump has to move a liquid at a shorter and closer distance, it will deliver more volume. More friction is produced in longer pipes and less volume of liquid is delivered.
  2. Check the suction pipe for any leaks and make sure all the fittings are tight. A lot of times, pump failures occur due to a faulty connection or a lose suction hose.
  3. Use reinforced hose on the suction side and flexible hose on the discharge side to prevent collapse.
  4. When using wet-prime pumps, you must always remember to prime the pump before you start its engine because even the tiniest leakage in the pipe could cause problems in priming. Air will not be able to escape and the priming of the pump will cease when the discharge pipe is clogged.
  5. Use trash pumps only for those liquids they are designed to handle.
  6. To prevent clogging, you must use proper strainer. Also check the suction strainer on a regular basis.
  7. Remember to drain the pump before it gets too cold outside in winters.
  8. Continuous monitoring of the pumps is required. You must make sure that they are running at their specified RPM (revolutions per minute).
  9. One of the best ways to ensure a longer pump life is to invest in quality trash pumps.
  10. Choose a pump that has a good resale value down the line and is provided by a reputable manufacturer.
  11. Last but not the least check the lubrication system for the mechanical seal.

If you follow this routine for your industrial or commercial trash pumps, they will surely provide an uninterrupted operation for your business. You can also have a look at the wide assortment of high-quality trash pumps offered by Rotech Pumps.

What Factors Affect The Efficiency of a Centrifugal Pump ?

When we talk about the efficiency of a machine, we are basically referring to its capability to convert one form of energy to another. How well the machine does it becomes its efficiency. Let’s say we are supplying one unit of energy to a machine and its output is half unit, we say that the efficiency of the machine is 50 percent. The formula for calculating efficiency is (output/input) *100.

Coming to our central topic of discussion, centrifugal pumps are highly effective and efficient and are thus in great demand. Digging deep into it, we shall understand more about the efficiency element of these pumps. The centrifugal pump primarily converts mechanical energy into hydraulic energy. Here the flow, velocity, and the pressure are also considered. Many medium and large centrifugal pumps offer efficiency of 75% up to 90%. And the smaller ones usually offer 50% to 70% on an average.

How Is Centrifugal Pump Efficiency Attained ?

The overall efficiency of a centrifugal pump can be measured by multiplying three individual efficiency:

  1. Mechanical Efficiency: It includes the losses in the mechanical seals, stuffing box and bearing frame
  2. Volumetric Efficiency: It includes losses due to balancing holes and wear rings
  3. Hydraulic Efficiency: It includes losses in the impeller and the volute plus the losses due to liquid friction. Hydraulic efficiency is the largest factor that decides the overall efficiency of the pump.

Efficiency of a Centrifugal Pump

When you want to achieve optimum performance and reliability in a centrifugal pump, you must operate the pump close to its BEP (best efficiency point). The BEP is the most stable and suitable operating point for a centrifugal pump. At best efficiency point, the hydrodynamic unbalanced load of the centrifugal pump is at its minimum. Basically, when a pump operates at a point that is far away from the actual BEP, it results in an overall increase in hydrodynamic unbalanced load. This in turn affects the performance, reliability and efficiency of the centrifugal pump. So the main question is when do we experience maximum unbalanced load? Through experience and experiments we found at shutoff point, unbalanced load is at its peak, its advice to increase the efficiency of the pump, you must avoid long-term operation of the pump when it is running near to the shutoff point.

There are various conditions that decrease the efficiency of your centrifugal pump that you should be aware of. They include:

  1. Heat generated due to packing.
  2. Rubbing between wear rings and maintaining impeller clearances.
  3. Recirculation using a bypass line from the discharge of the pump to the suction.
  4. Double volute design.
  5. Throttled discharge valve.
  6. Corroded internal pump passages that cause fluid turbulence.
  7. Obstacles, hindrances or any sort of restrictions inside the piping passages which might include a foreign particles, or dirt.
  8. Over lubricated bearings.

If you are searching for centrifugal pumps with maximum efficiency, you might want to browse through a wide range of pumps offered by Rotech Pumps. Excellent quality equipment and after sales service are the key elements owing to the huge success of Rotech Pumps.

Horizontal or Vertical Inline Pump Design – Which One To Choose ?

We all know that centrifugal pumps are the most commonly used pumps on the market right now because they are highly efficient at transferring liquids over a wide range of flows and pressures. Normally, for industrial uses there are two basic types of inline centrifugal pumps.

  1. Horizontal
  2. Vertical

The primary difference between the both types is the shape and the position of the shaft. In a horizontal centrifugal pump, the shaft is normally in a horizontal position. It is sometimes overhung or placed between bearing design. While in case of a vertical pump is one whose shaft is in vertical position; it is always an overhang and of radial-split case type design. When it comes to making a choice between horizontal and vertical inline centrifugal pump, convenience is one factor that you need to keep in mind.

The following are the advantages and disadvantages of a horizontal and vertical design which will help you make your selection:

Horizontal Or Vertical Centrifugal Pump Design - Which One To Choose ?

Advantages of a horizontal centrifugal pump design:

  1. Easier to install and maintain because the internal parts like rotor are easily accessible.
  2. Can be directly coupled to electric motor, engine or turbine.
  3. For low suction pressure: you can opt for an overhung design.
  4. For high suction pressure: you can opt for in between-bearing design.
  5. You can get various nozzle configurations that match your external site piping like:
    1. end suction top discharge
    2. top suction top discharge
    3. side suction side discharge.
  6. It is suitable for almost all indoor applications due to its low headroom requirements

Disadvantages of horizontal design:

  1. Its applications become limited where the net pump suction head (NPSH) required exceeds the NPSH available.
  2. It requires an auxiliary booster pump.
  3. The maximum allowable operating temperature as well as working pressure are both generally much lower than that in a vertical inline pump.
  4. A bigger footprint is required for horizontal designs.

Advantages of a vertical centrifugal pump design:

  1. It requires a smaller footprint than that of horizontal pumps
  2. It is suitable for spaces where the ground surface area is restricted.
  3. With a vertical centrifugal pump, the NPSH available can be increased.
  4. Such pumps are suitable for higher temperature and high pressure fluids.

Disadvantages of vertical design:

  1. They require large headroom for installation and maintenance especially when it comes to multistage units or turbine pumps.
  2. Only when direct coupling with an electric motor is available, such pumps can be used. With types of drivers like engine or turbine are used, vertical centrifugal pumps will not be the perfect choice for your industry.
  3. Due to the overhang design, it becomes difficult to balance the hydraulic axial thrust especially while dealing with high suction pressures.
  4. A multistage vertical inline pump would normally requires expensive barrel and pit.
  5. It will be prone various issues in its mechanical seals when it has to pump liquids with high dissolved gases since they will accumulate at the top of the seal chamber or the stuffing box. Here venting is difficult and less effective.

All the above given points will surely help you choose the right pump based on your specific requirements. Thus, it becomes very important to make sure you understand the specifications and limitations of each type.

Hope you found this article useful!

All You Need To Know About Cavitation In Centrifugal Pumps

What is cavitation in centrifugal pumps ?


The formation and bursting of vapour bubbles in a centrifugal pump due to change in pressure is called cavitation. This hydrodynamic phenomena have a significant impact on the overall performance of the pump. When sudden reduction in pressure occurs, liquids that have temperatures close to their boiling points often undergo this formation of bubbles.

Cavitation In Centrifugal Pumps Cavitation In Centrifugal Pumps


How does it affect the pump?

Effect Of Cavitation In Centrifugal Pumps

  1. Cavitation is actually responsible for the degradation of the pump which in turn results in fluctuations of the flow rate and pressure at discharge.
  2. Material damage: It can also damage the internal components of the pump.
  3. The bubbles cause a physical shock on the edge of the impeller vane which creates small pits extremely minute in size. However the cumulative effect of these pits formed over time can destroy the impeller.
  4. Cavitation in centrifugal pumps could also cause excessive vibration in the pump. Vibrations can damage rings, seals and pump bearings.

How to identify if a pump is cavitating ?

One of the major indicators of cavitation in centrifugal pumps is the noise. The pump will sound like shaking can full of marbles. Other indications include:

  1. Fluctuation in discharge pressure,
  2. Fluctuations in flow rate,
  3. Distinctive crackling noise,
  4. Fluctuations pump motor current,
  5. Erratic power consumptions,
  6. Reduction in pump efficiency and output

How can you avoid cavitation in centrifugal pumps?

As cavitation happens on the suction side of the pump, all preventive measures must be directed towards this area. If the suction lift is too high, it is definitely going to cause cavitation. Hence, centrifugal pumps must be less than 4 meters above the water level to avoid this problem. Some of the other guidelines that you must follow are:

  1. Minimize the number of bends in the suction line
  2. Rather than using concentric reducers, make use of eccentric ones
  3. Also, make sure that the straight side of this reducer is properly fitted above the suction line
  4. The length of the suction must be minimal
  5. Use the less number of valves in the suction line
  6. Increase the size of the valves
  7. Suction pipe and pump inlet connection must be of the same diameter
  8. Suction pipe must be airtight

Manufacturers like Rotech Pumps design centrifugal pumps that can operate under conditions wherein cavitation is unavoidable. They design special centrifugal pumps that can withstand small amounts of cavitation occurring during their operation. Rotech Pumps continuously strives to make improvements in design and engineering of pumps, pump parts and mechanical seals. They are renowned internationally for their wide range of quality pumping products and accessories. You can log on to their website for further details.

Optimum Trash Pump Selection for Your Industrial Plant

As an industrialist, you must know the importance of making a well-informed choice when you are investing into any equipment for your business. When you make a decision based on a thorough research, it definitely pays-off in the longer run.

If your end goal is to select trash pump or sewage pump machinery that can accomplish your job most efficiently, here are a few important details you should care about.

Self Priming Pumps
– SCP/SFP Series
Self Priming Trash Pumps
– SPT/SPU Series
Self Priming Pumps - SCP/SFP Series Self Priming Trash Pumps - SPT/SPU Series
  1. Self-contained centrifugal pumps are popularly known as trash pumps in the market. They work on a power source which is normally electric motor. However also can be supplied with diesel engine. The pump end is mounted on a base frame.
  2. They are versatile and can handle liquids with debris such as small twigs, trash, slurry, rocks, up to 3″ sizes depending of % of solid and size of pump.
  3. Trash pumps are the most preferred choice of contractors.
  4. As an end-user, you must consult the specifications mentioned by the manufacturer for solid-handling diameters since majority of these pumps are capable of handling solids from 1 to 3 inches depending upon size.
  5. Sewage pumps are ideal for dewatering and construction applications.
  6. The main feature of this pump is to remove trash, waste just by opening front cover without disturbing piping. So you don’t need to remove pump from the line to clean waste.

Making the right purchase decision in matters of industrial pumps requires in-depth research on the part of the buyer. While looking for a trash pump/sewage pump for your industrial operations, there are a few factors that you need to take into account:

  1. Investigate: Start with getting to know the different options available in the market. As a buyer, you need to know for what exact application is the self-priming trash pump needed.
  2. Determining the proper pump for your project becomes easy when you have answers to a few questions. These are some specifications that will play an important role in choosing the correct pump from a specific vendor. Ask yourself the following questions: What is
    1. The required rate of flow?
    2. Total dynamic head and the pressure of the liquid?
    3. The size of solids?
    4. The altitude?
    5. The liquid being pumped
    6. The specific gravity?
    7. The viscosity of the liquid?
    8. The characteristics of the liquid?
    9. The temperature of the liquid?
    10. The suction lift?
    11. The type of liquid?
  3. The vendor experiences matter a lot. Check their reviews online. You need to know how reliable the seller is.
  4. The quality of manufacturing also matters when choosing a trash pump your business
  5. It must provide a trouble-free long-term operation
  6. The vendor should offer after sales services as and when required.

When you are shopping for industrial supplies, machinery and equipment, follow these easy steps before entering the industrial pump marketplace. When you take time to understand these details and purchase advice, it will ensure that the right sewage pump is purchased and installed at the first go. This means that your time and energy is saved, along with thousands of dollars.

At Rotech Pumps, you can find the best quality self-priming trash pumps with very little maintenance. Rotech Pumps is the number one choice amongst industrialists for their pumping applications. Rotech also offers various Material of construction for this pump like Cast iron, SS316, CD4MCU, Alloy 20 or any other duplex stainless steel. Please contact Rotech for further details.

Rotech also offers a special mechanical seal for the application like MEK (methyl Ethane Ketone) for which standard type 2, Viton would not work. Rotech has custom designed mechanical seal for this particular application and works absolutely fine. Rotech has the ability to design and supply mechanical seal for any custom application.