How to Pick a Centrifugal Pump Part 5: Choosing a Single or ...

As a result, double suction pumps are also less vulnerable to the wear and tear caused by axial thrust, which occurs when water enters the impeller. Since water enters both sides of a double suction impeller, one side effectively cancels out the axial thrust of the other &#; assuming that flow is even on both sides. But since that is not always the case, double suction pumps have inboard and outboard bearings to absorb some of this load. These bearings can afford to be smaller than those on single suction pumps simply because there is better balance across the shaft of a double suction pump.

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Double or Single Suction?

We typically recommend a double suction pump design for HVAC applications around 2,000 GPM or an 8&#; pump size.  When it comes to applying a double suction pump we suggest:

(1) Selecting a pump with the shortest shaft span possible to minimize shaft deflections. 

(2) Specifying based-mounted pumps with flexible couplers whenever possible. These pumps are easier to service than double suction inline pumps, which require the motor to be removed to periodically replace worn out throttle bushings.  

Generally speaking, a double suction pump can be twice the cost of a comparable single suction design.  Obviously, pricing is positive when it comes to single suction pumps. They are also easier and less expensive to maintain.  On the other hand, double suction pumps last longer &#; as much as 30 years. The double suction design eliminates the axial forces on the impeller which allows for higher flows than single suction.

Different Types of Double Suction Pumps

When it comes to selecting a double suction pump, there are three types to choose from, including:

Horizontal Split Case &#; These pumps have a &#;clam shell&#; type volute that is split horizontally across. The top part of the volute is bolted to the bottom.  It has been around for close to 100 years. Because the pump is split horizontally, there is significantly more service time and rigging required to lift the heavy top of the pump. Watch for very old specifications. Many people called double suction pumps &#;split case,&#; all of these pumps have split casings. It just depends on whether the case is split horizontally or vertically. 

Structural Characteristics of Centrifugal pumps

Impeller

The impeller for the centrifugal pump is available in different shapes and sizes depending on the required performance and the characteristics of the liquids being pumped.

All types of impeller are fitted with special blading on the back of the hub disc to compensate axial thrusts and reduce the pressure in the seal chamber. The head generated by this blading contrasts the active pressure difference between the spiral and the seal chamber, which pushes the liquid being pumped towards the chamber itself.

The impellers are made from a range of materials depending on the chemical harshness and/or abrasive power of the liquid being pumped.

All impellers are dynamically balanced before they are fitted to the pumps.

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Casing

The casing for the centrifugal pumps is fitted with a single suction nozzle with a single spiral tube aerator available in two versions: with narrow spiral or wide spiral.

Casings are generally made from the same materials as impellers, however different materials can be used for specific requirements.

The seal between the casing and the cover is achieved through a built-in flat seal to better resist stresses caused by pressure and temperature.

The materials used for these seals are completely asbestos free.

The casing can be produced in a heated version (/RR), therefore featuring a chamber for steam heating up to a pressure of 7 bar and a temperature of 180 °C. Pumps from the RN/RNS and RKN/RKNS series are standardised based on UNI EN norms and in addition to ISO norms.

Cover

The cover is produced in such a way that enables the impeller to be extracted without having to remove the casing from the pipes. The materials used for the cover are the same as those used for the casing.

The external zone of the seal chamber can be produced in two versions:

a)  Cooled version /R
It is produced by a cooling chamber that circulates water up to 4 bar.
It is used when the extremely high temperature of the liquid being pumped may compromise the operation and durability of the shaft seal.

b) Heated version /RR
It is produced by a heating chamber that circulates steam up to 7 bar and 180 °C.
It is used when the liquid being pumped tends to solidify if it does not keep its temperature. Should this happen, it could compromise the operation of the shaft seal.

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