Page 48 - Know and Understand Centrifugal Pumps

Cavitation

feed water pump where the pump drains the deaerator (d-a) tank.
Because this pump generates high discharge pressure, it must also
generate a strong vacuum in the eye of the impeller. Typically, this
pump can generate 200 psi (460 ft of head) and require 30 ft of NPSHr
at its duty point.
The d-a tank is normally one of two varieties:
w Vented and exposed to atmosphere.
Closed and pressurized.

Once again, the formula and elements of NPSHa are:
NPSHa = Ha + Hs - Hvp - Hf - Hi, where:
Ha Atmospheric Head. It is 33.9 ft at sea level.
Hs Static Head. It is the level in the d-a tank above the pump
centerline. This is normally about 12 to 15 ft.
Hvp Vapor Head. It is based on the feed water temperature. See
Chapter 2, Properties of Water I and 11.

Hf Friction Head, or the friction losses in the suction piping. We
could assign this a value of 1 ft.
Hi Inlet Head. The losses in the pump suction throat to the
impeller eye. These losses could be insignificant up to 2 ft,
depending on design.
The feed water in the d-a tank normally runs about 190°F in an open
tank. Then we have:
NPSHa = 33.9 + 15 - 22 - 1 - 2
NPSHa = 23.9 ft
if the feed water temperature should be 205°F:
NPSHa = 33.9 + 15 - 30- 1-2
NPSHa = 15.9 ft
if the d-a tank should be closed, then the Ha = Hvp. Therefore they
cancel:

NPSHa=15-1-2
NPSHa = 12 ft
If the d-a tank should be sealed and artificially pressurized with steam
gas (sometimes they are, or should be), then each 10 psi adds 23.1 ft of
artificial head to the system’s NPSHa. If the boiler feed water pump has
a NPSHr of 30 ft at the duty point, now you can see why boiler feed
water pumps are sometimes considered problematic regarding
cavitation. This also demonstrates the need to seal and artificially
pressurize d-a tanks to get the NPSHa above the NPSHr of the pump.

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