Simon Hooton 13 September 2019

What Are Single Stage Centrifugal Pumps?

Centrifugal pumps are the most common type of pump available and account for approximately 70% of pump types used and refers to one of the main three families of pumps.

A single-stage centrifugal pump consists of one impeller rotating on a shaft within a pump casing which is designed to produce fluid flow when driven by a motor. It is one of the simplest designs of pumps available and many variations in design exist to satisfy the duty requirements of applications. Impeller design is changed to accommodate clean, dirty and solid laden liquids, with several mechanical seal types to accommodate variations in applications.

The different types of centrifugal pump include:

Close-coupled - where the pump head and motor are directly connected without a coupling.

Long-coupled – where the pump and motor are mounted on a base plate and connected via a coupling.

Vertical Inline - Pump is mounted Vertically for space saving. Vertical inline pumps can be close coupled of long coupled when fitted with a spacer bracket.

Different standards exist in centrifugal pump design to define the conditions in which the pumps will operate in and also the expected design down to individual components:

DIN24255 / EN733 - A design standard incorporating set dimensions between the centre line of the inlet and outlet flanges.

ISO2858 / ISO5199 / ASME B73.1 Chemical Standard - These standards detail the dimensions between the centre line of discharge and inlet and outlet flanges, levels of vibrations, shaft deflection, and permissible forces within the pump.

API610 - A pump design typically used within the oil industry (American Petroleum Industry) containing the most stringent design standards available ensuring pump has an overhung bearing design, maximum permissible forces, service intervals, casing thickness and life span of 26 years.

There are a range of Centrifugal P&ID Symbols which can be used in Process Drawings - learn more.

View our Range of Single Stage Centrifugal Pumps

Download our Guide to the Parts of a Centrifugal Pump below:

FAQS

How are they Primed?

Typically the casing is filled with liquid either via the priming port or by the opening of valves allowing product to flow into the pump head. Opening the priming port not only allows liquid to be poured in to the pump casing but allows air to escape preventing air locking.

What is the correct procedure for stopping a centrifugal pump?

Any pumping system needs to be protected from the sudden starting or stopping of high pressure or high flow pumping. When equipment is suddenly started or stopped it can cause water hammer which can damage the system, components or accessories.

Centrifugal pumps should be stopped by slowly closing the outlet valve until the pump reaches its Minimum Safe Continuous Flow (MSCF) before slowly being stopped via a variable frequency drive. If the pump does not have a vfd then the pump would have to switched off.

The pump should then be left to cool, before inlet and outlet valves are completely closed, and the pump drained. If the pump has been used on hazardous liquids, it should be flushed and decontaminated before being maintained.

Why are centrifugal pumps not used on oils?

Such designs can be used on oils but only light oils up to 200-300cst. Centrifugal pumps allow fluid to recirculate back towards the inlet meaning they are unsuitable for viscous oils, and if back pressure or fluid viscosity changes with temperature, the flow rate produced by the pump will vary.

Can this pump type run dry?

No, definitely not! Centrifugal pumps will incur damage even after short periods of dry running. The mechanical seal requires lubrication and cooling while the pump is operating. Without the presence of fluid, the mechanical seal will overheat and crack, and this may cause the pump to leak and fluid to enter the motor. There is also the possibility that the motor will burn out. Our advice is to ensure that the pump has a flooded suction or always make sure that the pump casing and inlet pipe are filled with water; one way of ensuring this is to fit a check valve on the inlet line to stop water escaping when the pump is inactive. Another way of protecting the pump is to fit a dry running device, this will turn the pump off if it detects that no fluid is entering the pump. If you think that dry running is inevitable, then please speak to a member of our sales team and we will select a more suitable pump for your application.

How do I know what pump materials are correct for my application?

Firstly, always check the compatibility of the materials available against the fluid being pumped. The main materials to check are the pump casing, impeller, o-ring and mechanical seal.

It may be that more than one material is suitable for your fluid and selection could be based on the application type. For instance; cast iron, bronze and stainless steel are all suitable for fresh water. If it is a simple transfer application, then the most cost-effective material cast iron will be best. However, if it is a sanitary application, then stainless steel or bronze are better choices.

What is NPSH and why is it so important?

NPSH is an acronym for Net Positive Suction Head. NPSH measures the absolute pressure present in a fluid.

There are two main ways that NPSH is expressed in a pump system

NPSHa - This is the amount of Net Positive Suction Head available at the pump inlet. NPSHa demonstrates the amount of pressure acting on a fluid as it enters the pump. This measures the amount of pressure between the liquid staying in its current state and forming vapour bubbles (beginning to boil).

NPSHr - This is the amount of Net Positive Suction Head that the pump requires to operate without experiencing the damaging effect of cavitation, thus causing a dramatic reduction in pump performance.

It is very important to pay attention to these values when making a pump selection. Selecting a pump that requires more NPSH than is available in your system will cause fast and long-lasting damage to the pump and thus you will incur large repair costs and downtime.

What does the term best efficiency point (BEP) mean and why is it so important?

The best efficiency point or BEP is a point along the pump performance curve that indicates where efficiency for the pump peaks. When selecting a pump, you must try and get as close to the BEP as possible to ensure that the pump is at maximum efficiency when operating. The closer to the BEP the pump is when operating, the lower the energy costs will be, thus saving significant amounts of money during the pump’s lifetime.

Also, vibrations will be at their lowest meaning maintenance costs are lower and the lifespan of the pump is maximised. It is very important to pay attention to the BEP when your pump is selected, as an oversized or undersized pump could cost you significant amounts of money.

How do I know what size pump and motor speed is correct for my system?

A clear picture of the pump system is required to make an accurate selection. The main pieces of information required include; a description of the application, bore of pipework, the fluid, flow rate and pressure/head. With these pieces of information, a pump can be sized correctly to ensure it delivers the required flow rate and pressure and that is also operates at its best efficiency point to lower lifetime costs.

Knowing if the pump is running intermittently or continuously also allows the correct motor speed to be selected. For instance, a pump running continuously 24/7 will require a 4 pole motor rather than a 2 pole motor. Running the motor slower and oversizing the pump will reduce wear of the motor and the pump, therefore lowering maintenance costs during their lifetime.