Page 60 - Know and Understand Centrifugal Pumps

The Affinity Laws

treatment of potable water, sterile pharmaceuticals, and petroleum
refining.
Let’s consider sterile water, used in the preparation of medications for
injection. A typical process to sterilize water would require boiling the
water at 35 psi, and pumping the water at 40 gpm to 70 gpm,
according to consumption. The 35 psi is a constant for the water to
pass through the heat exchanger, and a bank of filters. To compensate
for the change in demand for sterile water, the affinity laws are used,
varying the diameter of the impeller, so that the pump can pump 40
gpm at 35 psi, or 50 gpm at 35 psi, or 70 gpm at 35 psi. This allows the
operator to use the same pump and motor, and only change the
impeller diameter depending on the needs of production. This precise
manipulation of pumping parameters could not be obtained by opening
and closing valves, or by simply controlling the pump speed with a
VFD.

Most people change their wardrobe and clothes as the weather changes. Most people
would change their cars if their transportation needs change. As the authors of this
book, it has always seemed strange to us that most pumps are sold with only one
impeller. There is absolutely nothing wrong in selling (or buying) a pump with various
impellers of different diameters, ready to be changed when the needs of production
change seasonally, or with an advertising campaign. This is the reason that back
pullout pumps exist.
Many clients specify and buy pumps with the back pullout option, and they never
take advantage of the option. This is like buying a car with an air conditioner and
never turning it on. Many engineers, operators, and even pump salesmen believe that
the back pullout feature is designed to facilitate maintenance. This is wrong. The back
pullout pump exists to facilitate the rapid and frequent impeller change, adapting the
pump to the ever-changing needs of production. The back pullout pump exists to
facilitate production.

Manipulating flow and controlling pressure by varying the impeller
diameter conserves kilowatts of energy, and this is the third affinity law
in this group. A pump consuming 10 BHP with a 10 inch impeller,
would only consume 7.3 horses with a 9 inch impeller.
This means a 10% reduction in the impeller diameter, would bring
about almost 30% reduction in energy. These energy savings will easily
cover the cost of multiple impellers and the manpower to change them
frequently.