Single suction pumps and double suction pumps are two types of centrifugal pumps. The main difference between them is the number of suction inlets they have. Single suction pumps have only one suction inlet, while double suction pumps have two.

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The double suction design allows for a more balanced hydraulic load, which can result in reduced axial thrust and longer bearing life. As a result, double suction pumps are often used in high-flow applications, such as in water supply and circulation systems. In contrast, single suction pumps are commonly used in low to medium-flow applications, such as HVAC and industrial processes.

Types Of Pumps

Are you looking to purchase a new concrete pump but need clarification about the difference between a single and double suction pump? Look no further! In this article, we&#;ll provide an in-depth analysis of both types of pumps, including their working principles, advantages, disadvantages, and pricing.

In this article, you&#;ll better understand which pump type best suits your needs.

Double Suction Pump

The double-suction pump utilizes a pair of impellers arranged in a back-to-back configuration, and the water discharged from the impeller is directed into a spiral casing. Owing to its superior design, this pump is extensively employed in various engineering applications due to its exceptional flow rate and high head.

With two suction chambers, it is commonly believed that one of the chambers serves as the inlet while the other functions as the outlet. The pump&#;s suction and inlet are perpendicular to the axis&#;s lower section, primarily determined by the impeller&#;s orientation.

One significant benefit of the double-suction centrifugal pump is its reduced susceptibility to cavitation at the same rate and flow due to a reduction in the inlet flow.

Single Suction Pump

The suspended single suction pump employs horizontal and axial suction and upward and radial discharge. As a result, the rotor components can be removed for maintenance without requiring dismantling the inlet and outlet pipelines.

The pump is linked to the motor using a standard or extended elastic coupling, and the pump shaft seal is secured using soft packing. A single row of radial ball bearings lubricated with oil is used in the bearing. The motor rotates in a normal motion when viewed from the motor end.

Functional Principle

Upon starting a single suction pump, the shaft drives the impeller to spin at a high velocity, inducing the liquid that had already filled the spaces between the blades to rotate. The centrifugal force causes the liquid to move radially from the impeller&#;s center to its periphery, where it gains energy and experiences an increase in flow rate due to the accompanying increase in static pressure energy.

After exiting the impeller and entering the pump casing, the liquid slows down due to the gradual widening of the flow channel. This converts kinetic energy into static pressure energy before it flows tangentially into the discharge pipeline. The spiral pump casing collects the liquid that exits the impeller while also functioning as an energy conversion device.

A low-pressure zone develops in the center as the liquid is ejected from the impeller&#;s center to the exterior. The real difference in potential energy between the liquid level in the storage tank and the impeller&#;s center draws the liquid into the impeller&#;s center.

The impeller&#;s constant operation results in a continuous inflow and outflow of liquid, with the mechanical energy gained by the liquid in the industrial centrifugal pump manifesting as an improvement in static pressure energy.

Distinction Between Single Suction Pump And Double Suction Pump

Diverse Inlet

The diverse inlet design of double suction pumps allows for the smooth conveyance of large flows and ensures stable operation. In contrast, a single suction pump with a high flow requirement should have a large inlet and outlet diameter.

Additionally, double suction pumps have superior resistance to cavitation due to the symmetrical impeller action, which creates a more stable hydraulic balance. However, this enhanced performance comes at a higher price than single suction pumps. Therefore, selecting a suitable pump type depends on the user&#;s specific operational needs and conditions.

Variation In Flow

When it comes to conveying large amounts of fluid, industrial double suction pumps are a better choice as they require a stable operation. However, if the flow requirements are high, single suction pumps should have a large inlet and outlet diameter.

Resistance To Cavitation

A double-suction pump is more resistant to cavitation than a single-suction pump. An axial single suction pump produces less stable hydraulic balance than a double suction pump due to the symmetrical impeller movement on both ends of the latter. Additionally, since the medium&#;s low flow rate, the impeller is less susceptible to cavitation.

Differentiation In Price

In terms of cost, double-suction pumps are generally more pricey than single-suction pumps. Even though single-suction pumps are less expensive and can be used in multiple applications, double-suction pumps have many advantages. Users should consider their operating conditions to determine the most suitable option.

Conclusion

Single and double suction pumps differ in inflow, flow rate, cavitation resistance, and price. In addition, single-suction pumps have an impeller filled with water at one end, while double-suction pumps have two inlets. As a result, double suction pumps are better for large flows, while single suction pumps need a large inlet and outlet diameter and stable operation.

Double suction pumps are more resistant to cavitation due to their symmetrical impeller action but are also more expensive. The choice between the two types of pumps depends on the user&#;s specific needs and operating conditions.

Suction Pipeline Design

The root cause of many pump problems and failures can be traced to poor upstream, suction-side, pipeline design. Common problems to avoid are:

• Insufficient fluid pressure leading to cavitation within the pump.

• Narrow pipes and constrictions producing noise, turbulence and friction losses.

• Air or vapour entrainment causing noise, friction and loss of performance.

• Suspended solids resulting in increased erosion.

• Poor installation of pipework and other components.

Cavitation

A liquid&#;s boiling point corresponds to the temperature at which its vapour pressure is the same as the pressure of its environment. If water, for example, is subjected to a sufficient drop in pressure at room temperature, it will boil.

Across any pumping system there is a complex pressure profile. This arises from many properties of the system: the throughput rate, head pressure, friction losses both inside the pump and across the system as a whole. In a centrifugal pump, for example, there is a large drop in pressure at the impeller and an increase again within its vanes (see diagram). In a positive displacement pump, the fluid&#;s pressure drops when it is drawn, essentially from rest, into the cylinder. The fluid&#;s pressure increases again when it is expelled.

If the pressure of the fluid at any point in the pump is lower than its vapour pressure, it will literally boil, forming vapour bubbles within the pump. The formation of bubbles leads to a loss in throughput and increased vibration and noise but the big danger is when the bubbles pass on into a section of the pump at higher pressure. The vapour condenses and the bubbles implode, releasing, locally, huge amounts of energy. This can be very damaging, causing severe erosion of the pump&#;s components.

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To avoid cavitation, you need to match your pump to the fluid, system and application. This is a complex area and you are advised to discuss your application with the pump supplier.

Understanding NPSH

To avoid cavitation, the pressure of the fluid must be maintained above its vapour pressure at all points as it passes through the pump. Manufacturers specify a property referred to as the Net Positive Suction Head Required or NPSH-R &#; this is their minimum recommended fluid inlet pressure, expressed in metres. The documentation supplied with your pump may contain charts showing how NPSH-R varies with flow.

In fact, NPSH-R is defined as the suction-side pressure at which cavitation reduces the discharge pressure by 3%. So, in designing the suction-side pipework for your system, you must ensure that it exceeds the manufacturer&#;s NPSH-R rating for the operating conditions. Your calculated value is termed the NPSH-Available (NPSH-A).

Remember, a manufacturer&#;s NPSH-R rating is the minimum recommended inlet head pressure: a pump is already experiencing cavitation at this pressure. Consequently, it is important to build in a safety margin of 0.5 to 1m to take account of this and other factors such as:

• The pump&#;s operating environment &#; is the temperature constant?

• Changes in the weather (changes in temperature and atmospheric pressure).

• Any increases in friction losses that may occur occasionally or gradually during the lifetime of the system.

Turbulence and Friction

Pumps, and especially centrifugal pumps, work most efficiently when the fluid is delivered in a surge-free, smooth, laminar flow. Any form of turbulence reduces efficiency and increases wear and tear on the pump&#;s bearings, seals and other components.

There should be at least 5 pipe diameters&#; worth of straight piping connecting to the pump. Never connect an elbow, reducer, valve, or strainer within this final run of pipework. If you connect an elbow directly to the pump flange, the fluid is effectively centrifuged towards the outer curve of the elbow and not directed into the centre (the eye) of the impeller. This creates stress on the pump&#;s bearings and seals which often leads to wear and premature failure.

Sometimes, it&#;s just not possible to make provision for a sufficient settling distance in the pipework before the pump. In these cases, use an inline flow conditioner or straightener.

It&#;s standard practice to employ suction-side piping one or two sizes bigger than the pump inlet - you should certainly never use any piping that is smaller than the pump&#;s inlet nozzle.

Small pipes result in larger friction losses, which means it costs more to run your pumping system. On the other hand, larger diameter pipes are more expensive &#; so you need to weigh up the increased cost with the likely energy saving resulting from reduced friction losses.

It also makes sense to keep the run of pipework to a minimum by positioning the pump as close as possible to the fluid source.

Larger pipework means that you&#;ll need a reducer before the pump inlet. A reducer is a constriction and requires careful design to avoid both turbulence and the creation of pockets where air or vapour might collect. The best solution is to use an eccentric reducer orientated to eliminate the possibility of air pockets.

As a general rule of thumb, suction pipe velocities should be kept below 2 m/s. At higher velocities, the greater friction causes noise, higher energy costs and increasing erosion, particularly if the fluid contains suspended solids. If your system contains any narrow pipes or other constrictions, bear in mind that the pipe velocity will be a lot higher at these points.

Air or Vapour Entrainment

It&#;s best to keep air or vapour out of the pipework. Entrained gases cause a loss in pump performance, increase noise, vibration and component wear and tear. It&#;s therefore important to position the feed pipe correctly in the tank or vessel. It should be fully submerged. If it&#;s too close to the surface of the fluid, the suction creates a vortex, drawing air (or other vapours) into the liquid and through the pumping system. The feed pipe should also be clear of any other pipes, agitators or stirrer-paddles &#; anything that might drive air into the fluid. In shallow tanks or ponds, it may be advisable to use a baffle arrangement to protect the feed pipe from air entrainment.

Suspended Solids

You should also make sure that the feed pipe isn&#;t too close to the bottom of the tank or pond. If it is, the suction may draw up solids or sludge instead of air or vapour! The fluid may contain suspended solids in any case.

Some displacement pumps can cope with a mixed phase supply without any damage or major loss in performance. Centrifugal pumps are not so robust and must be protected from solids. In this situation you&#;ll need to install a filter or strainer. Filters can create a large pressure drop and be responsible for cavitation and friction-loss. The filter screen should have at least three times the free area of the pipe cross-section. Use a differential pressure gauge across the screen to look out for any increased pressure drop before clogging problems arise. This will also help in the accurate assessment of NPSH-A.

Installation

Obviously, pumps should be securely located - but so should the pipework. Don&#;t use one to support the other. All other components must be just as securely located and create no stresses or strains on any other parts of the system. Ensure that the pipe connecting to the pump&#;s inlet flange is aligned precisely with it. If you need to install non-return valves or flow control valves fit them on the discharge side of the pump, and never in suction-side pipework.

Summary

Problems in suction side pipework often have damaging consequences for the system pump and can be avoided by following these guidelines:

• Ensure that conditions do not favour cavitation, particularly if you are using a centrifugal pump. This requires careful selection of the pump, its positioning and the head pressure.

• Position the feed pipe to minimize entrainment of air/vapour and solids.

• Minimize friction and turbulence by choosing appropriate pipes and components:

• Use pipes with a diameter twice that of the pump&#;s suction side flange.

• Ensure that the pipework is aligned with the pump&#;s flange and straight for at least 5 pipe diameters.

• Use an eccentric reducer orientated to eliminate air pockets.

• Keep the pipe velocity below 2m/s.

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