Page 37 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

NPSHa = Ha + Hs2 -Hvp - Hf - Hi
NPSHa = 26.2 + (-14.0) - 0.411 - 1.0 - 2.0
NPSHa = 8.8 feet
To avoid problems with this pump during the process, be sure the
pump curve indicates NPSHr less than 8 ft at the duty point.
Many processes use sealed tanks and reactor vessels. For example, in a
milk processing plant or a pharmaceutical plant, it’s necessary to
prevent outside air from contaminating the sterile product. In a beer
brewery, you can’t let the gas and carbonization escape from the
process. In a closed un-pressurized vessel, the Ha is equal to the Hvp.
And because the Ha adds energy and the Hvp subtracts energy, they
cancel themselves. The formula is simpler:
NPSHa = Hs - Hf - Hi

The level in this sealed tank is 12 feet above the pump (Figure 2-3).
The Hsl is 12 feet. The purpose of this pump is to drain this tank to a
level 6 feet above the pump, so the Hsz is 6 feet. The Hf is 1.5 feet and
the Hi is 2 feet.
NPSHa = Hsl - Hf - Hi
NPSHa = 12.0 - 1.5 - 2.0
NPSHa = 8.5

The curve of the pump that drains this tank should register an NPSHr

Figure 2-3