ITS 'Pump It' Series - Centrifugal Pumps

Centrifugal Pumps

Used across the food, beverage, dairy and pharmaceutical industries, centrifugal pumps are frequently requested as the pump of choice but often the reasons behind the selection are not properly considered. As first in the ITS ‘Pump IT’ series of guidance articles, we hope to give clarity as to what a centrifugal pump is, what is does, how it works, and for what applications it is suitable for, along with providing some basic insight into the maintenance and troubleshooting topics that can keep your centrifugal pump in optimum working order, should it be the right choice for your processing requirements.

What is a Centrifugal pump?

A centrifugal pump essentially transfers liquids from point A (suction side) to point B (discharge side) using centrifugal force (outward force moving away from a central point).

How does a Centrifugal Pump work?

The key component to a centrifugal pump is its impeller, that sits inside the static pump casing on a rotating shaft. The impeller spins at a speed of 1500-3600 rpm, drawing liquids into its centre from the suction / inlet pipe. The high speed and design of the impeller generates centrifugal force to push or throw liquids to the outer edge of the pump casing. The volute design of the casing, guides these fluids around the outer-edge and through to the discharge port / connection.  

What Applications are Centrifugal Pumps suitable for and why?

Centrifugal Pumps are excellent for low viscosity product between 1-100 centipoise (cP), e.g., milk, heated vegetable oil, juice or cleaning fluid, but are capable of transferring higher fluid viscosities of up to 800 (cP), although this can cause inefficiencies due to centrifugal force having less impact on higher viscosities and thus typically, a larger motor and pump is required.

The main processes where Centrifugal Pumps can be found are in high flow and relatively low pressure applications. Centrifugal Pumps are capable of reaching flow rates of between 1 m³/hr – 450 m³/hr and in some cases can achieve differential pressures in excess of 10 bar. Some of the most common applications of Centrifugal Pump technology can be found in cleaning-in-place (CIP) systems, where high flow rates and low pressures are required to generate turbulent flow to aid cleaning.

Centrifugal Pumps are best suited for none to low volume particulate products where particle size is minimal such as sugar solution or fruit juices. They are not however suitable for products with large particles, as these run the risk of being damaged by the impeller or even particles building up between the tight clearances of the impeller and casing, therefore reducing the performance of the pump, and potentially causing premature failure.

Due to the high sheer rate (the force / stress applied to a product) in Centrifugal Pumps, it is important to consider carefully the product going through the pump, including product viscosity, as it could be damaged and even risk the taste or texture being changed, for example with creams and yoghurts.

For applications of high viscosity products, sheer sensitive products and products with particles, ITS would suggest the use of Twin Screw, Rotary Lobe or Eccentric Disc type technologies which work on a positive displacement principle, where the pump effectively doesn't work as hard and is much more gentle at transferring fluids.

Types of Centrifugal Pumps available.

There are various centrifugal pumps available, but in the Hygienic sector, 3 are most commonly used.

1. Standard Single-Stage – These have one stage of impeller and offer high flow and low pressure capabilities.

2. Multi-Stage – With more than one pump head, these pumps are best suited to low flow, high pressure scenarios.

3. Liquid Ring – These dual-purpose pumps are ideal for processes where scavenging or CIP return duties are required.

Design Options - Impeller & Drive

All types of centrifugal pump come with an impeller, although the impeller itself is subject to three different types of design:

• Open Impeller
• Semi-open Impeller
• Closed Impeller

For hygienic applications, the most common design is a ‘Semi-open’ impeller, which is easily cleaned and highly efficient. Whilst closed impellers are more efficient, it is harder to reach shadowed areas, making cleaning more difficult and time consuming. With open impellers, cleanability is at its easiest, but this impacts the performance and efficiency of the pump.

Some impeller designs include balance holes which enhances circulation of fluid and cleaning liquids around the shaft seal and reduces axial forces. This reduces wear on the shaft seal and motor bearings, and aids a greater level of cleanability.

The impeller itself connects to the pump shaft (driver) via a thread, keyway or magnetic force. The Shaft is then driven by an electric motor to generate the rotating force. In all cases a 'seal' is required to prevent liquid passing from the static pump head, around the rotating shaft and into atmosphere. This 'seal' is achieved in one of two ways as shown below:

Mechanical Shaft Seal – There are various methods of mechanical sealing options available. Low cost solutions can be provided with Packed Gland, Triple Lip or O-Ring technologies where flexible elastomers seal the pump head. These low-cost solutions can experience high rates of wear and have over time become less favoured compared to the more commonly known solid faced mechanical seals. Solid faced mechanical seals comprise of a rotating face and a stationary face, which are in constant contact with each other and provide the barrier needed to prevent the fluid in the pump head from passing through the casing, where the rotating shaft penetrates.

Magnetic Driven Shaft – This is a seal-less option and provides a lower maintenance alternative. Unlike mechanically sealed shafts, magnetic driven pumps do not have any physically connected parts between the impeller and shaft. Instead, rotation occurs via magnetic forces which are generated between two sets of magnet assemblies on the impeller and on the motor shaft.

The advantage of having a magnetic drive shaft is its long lifespan which leads to low maintenance intervals, but does however come at a much higher initial cost  

Centrifugal Pump shafts are driven primarily by Full-Speed (2-pole) or Half-Speed (4-pole), electric motors. These motors can be inverter driven to vary the output speed, making Centrifugal Pumps flexible for a wider range of duties.

Optimising Efficiency and Performance

Known for their efficiency, it is important to spec a Centrifugal Pump and tailor the design to reach the pumps ‘Best Efficiency Point’ (BEP) at a mid-point duty, to minimise energy input and achieve maximum output. Best Efficiency Points can be achieved by running pumps with frequency inverters to adjust the pumps speed, or by machining the impeller size to change the output of the pump to suit the systems conditions. Both options allow for flexibility and control of flow-rates, whilst maintaining a highly efficient pump design.

By designing or selecting a less efficient Centrifugal Pump, the excess energy that is input, is transferred into other unwanted forces, such as heat transfer to product, excessive vibrations and increased noise, all of which can impact both the product and lifetime of the pump. Designing the pump to its Best Efficiency Point ensures the energy that a user inputs is used effectively and maximises the lifetime of the equipment.

NPSH & Cavitation

NPSH (Net Positive Suction Head) is a measure of pressure on a fluid, based on suction conditions of a pump which change depending on the flow rate, viscosity of product, pumping speeds and the design of process pipework. NPSH is calculated under two criteria :

1. NPSHa (Net Positive Suction Head available) - The available NPSH conditions within a system.
2. NPSHr (Net Positive Suction Head required). - The required NPSH conditions of a piece of equipment to prevent cavitation.

The NPSHr will be calculated by a manufacturer and provide as part of their pump curves. However, the NPHSa should be calculated by the user with the following formula:

NPSHa = Pa ± hs – hfs – Pvp

Pa = pressure absolute above fluid level in tank. ( metres, 1 bar = 10 metres) hs = static suction head (metres) hfs = pressure drop in suction line (metres) Pvp = vapour pressure (metres)

Once calculated users can then compare NPSHa against NPSHr and determine if they are likely to experience cavitation or not.

When NPSHa is greater than NPSHr, the pressure on the liquid at the suction side of the pump exceeds that of the vapour pressure of the liquid and thus avoids cavitation. However, when NPSHa is lower than NPSHr, the pressure on the liquid at the suction side of the pump is now lower than the vapour pressure and consequently, small bubbles are formed. As the pressure on the liquid increases from suction side to discharge side, these bubbles collapse or implode, generating shockwaves or excessive pressures that damage the impeller and casing, ultimately leading to breakdowns. This implosion and excess pressure on the pump internal parts is known as cavitation

Key indications of cavitation can be seen in extreme noise and vibrations from the suction side of the pump.

A good pump design can minimise the NPSH requirements, which in turn prevents cavitation occurring in the pump head.

Problems with cavitation can be resolved by:

1.  Designing a pump with low NPSHr.
2. Change process pipework design to maximise NPSHa.
3. Lowering the temperature.
4. Increasing the diameter of the suction pipework.
5. Use of an impeller inducer.
6. Use of two parallel pumps with lower capacity.
7. Use of a booster pump to feed the principal pump.

Servicing and Maintenance

 The key to ensuring your centrifugal pump continues to run effectively is to have a regular maintenance schedule. A physical inspection and testing of the electric motors is crucial as are checks of the connections to make sure they are secured correctly. Pipe lines should be examined for visible wear and leaks and vents looked over for blockages and signs of overheating. Greasing the bearings and replacing the mechanical seal should also be priority as part of the preventative maintenance regime.

There are numerous reasons that a centrifugal pump might fail, most of which can be avoided with regular maintenance and working knowledge of the system and processes specific to the use of the pump.

Don’t run dry… Centrifugal pumps cannot be used to self-prime and must have flooded suction (gravity feeding) conditions to work. If run -dry, mechanical seals can overheat and wear excessively due to lack of lubrication. To prevent against this, programming can be put in place to stop the pump when the liquid supply ceases.

Overall Centrifugal Pumps are one of the most common technologies found within a hygienic process system. They are highly efficient, relatively low total cost of ownership and flexible solutions for a users fluid transfer needs.

Industrial Trading Solutions Ltd offer engineering capabilities to spec and size the right pump for your plant requirements and have a vast range of solutions available from our highly reputable brands.

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