THE CORRECT SELECTION OF A
CENTRIFUGAL PUMP
GENERAL
The following guide relates to conventional shrouded or open impeller pumps of the radial vane type and must not be used in connection with types such as mixed flow, axial flow, recessed or regenerative impellers.
The performance graphs provided by us will relate largely to actual works performance testing and will provide sufficient data for normal selection purposes.
Some companies will provide ‘typical’ or ‘estimated’ performance graphs which must be treated with caution.
For a given duty flow rate and pressure requirement, the steps given below should be followed.
For corrosive chemical services, particularly where the pumped fluid contains solids in suspension, consideration of fluid velocity in the suction and discharge branches is of particular importance.
SUCTION BRANCH SIZING
Fluid velocity should be kept to a minimum of 2m/sec. (6ft./sec.) to avoid the possibility of solids settling out in the suction line, and a maximum of 3.5m/sec. (12ft./sec) to avoid excessive suction line pressure losses.
OUTLET BRANCH SIZING
This is traditionally one standard branch size below suction branch size. This results in a higher outlet flow velocity for a given flow rate, which is not detrimental providing a maximum velocity of 4.5m/sec. (l5ft./sec.) is not exceeded. Any higher velocity would result in increased erosive/corrosive effects and unacceptable discharge line frictional losses.
PRESSURE REQUIREMENTS
A centrifugal pump delivers fluid, whether it be water or mercury, to the same height (termed ‘head’) irrespective of the specific gravity of the fluid. However, the fluid pressure as measured at the pump outlet branch on a pressure gauge, will vary in direct relation to the specific gravity of the fluid. (A higher or lower specific gravity results in a higher or lower pressure at the discharge branch). Therefore it is normal for pump performance data and graphs to be expressed in terms of ‘head’ of fluid.
For the purposes of pump selection where the duty pressure requirement is given as a pressure (kg/cm2 , lbs/sq.inch etc.) of a given fluid at the discharge branch, this should be converted to the equivalent ‘head’ of water by applying the appropriate conversion factor and dividing by the specific gravity e.g.
Pressure kg/sp.cm x l0 = Metres Head
Specific gravity
OR
Pressure lbs/sp.inch x 0.704 = Metres Head
Specific gravity
The selection data and performance graphs provided by us, refer to net generated head which is the differential pressure between the suction branch and the outlet branch. Therefore the value for ’head’ to be used for a pump selection, must be arrived at by taking account of the pressure at the suction branch , as well as the pressure at the outlet branch.
The head generated by a centrifugal pump at varying flow rates is dependent on the diameter and speed (rpm) of the impeller and that relationship is expressed in the following ‘Affinity Laws’.
Varying the impeller diameter or speed will:-
Vary the flowrate in direct ratio to that change.
Vary the generated head by the square of the ratio of that change.
Vary the absorbed power by the cube of the ratio of that change.
These are a good guide for calculation but may vary by some degree in practice.
POWER REQUIREMENT/ MOTOR SIZING
The total power absorbed by a centrifugal pump, is the sum of the hydraulic power (work done achieving the duty flowrate and head) and the frictional losses (power taken by the pump bearings, seals etc. and fluid friction within the pump).
The pump efficiency is the ratio of the hydraulic power to the total power absorbed.
Although the ‘head’ generated by a centrifugal pump is not affected by the differences in fluid specific gravity it is important to remember that total power absorbed is dependent on fluid specific gravity.
The performance graphs supplied by us indicate power absorbed for water (specific gravity 1.0). Although the specific gravity only affects the hydraulic power element, it is usual to arrive at a duty power by multiplying total power for water by the fluid specific gravity. This is ‘on the safe side’ for specific gravity values above 1.0 but not for values below 1.0 where a lesser adjustment should be allowed.
When determining the motor size for a specific application, these are some of the considerations to be taken into account;-
Total absorbed power (values from performance graphs are for water s.g.1.0)
Fluid specific gravity
Fluid viscosity (for values above 50 c stokes, refer to Pumping & Technical Services for advice)
Slurry concentration
Safety factor over maximum duty requirements
Site electrical specifications and ambient temperature rating
Motor control requirements, (a de-rate may apply for Inverter operation).
The above information relates specifically to the actual pump selection but account must also be taken of the effects of the associated system and installation.
