Page 35 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

Bachus Custodio formula explained in Chapter 8. In most cases, the
pump is relatively close to the vessel being drained by the pump. In
this case the Hf is probably negligible. Hf is subtracted because
friction in the suction pipe robs energy from the fluid as it
approaches the pump.
5. The Hi, inlet head, is simply a safety factor of 2 feet. Some pumps
have an insignificant Hi. Other pumps have inlet losses approaching
2 feet. The Hi is losses to the fluid after it passes the suction
pressure gauge and goes into the impeller eye. In a maintenance
fimction, you can't be precise about what's happening to the fluid
in this part of the pump. Just call it 2 feet.

Now let's apply the hints and the formula to the following system
figures and we can determine the NPSHa within one or two points.
The important thing is that the NPSHa of the system is greater than
the NPSHr of the pump. If the NPSHa should be inadequate, the
pump is being starved, becomes unstable and cannot perform its duties.
The inadequate NPSHa may lead to cavitation.
Remember that NPSHa > NPSHr
This open system pumping water is at sea level (Figure 2-1). Therefore
the Ha is 33.9 feet. The level in the tank is 15 feet above the pump
centerline, so the Hsl is 15 feet. The friction losses in the suction piping
give us 2 feet. The water is 70" F so the Hvp is 0.839. The Hi is a safety
factor of 2 feet.

Open tank
Ha = 33.9
I n

Hs 15

I I I Hvp=0.839
70 ?F
I

Finiirp 3-1

n 18 1
..
.