Page 352 - Centrifugal Pumps 2E

Chemical Pumps Metallic and Nonmetallic 317

levels for the suction nozzle, discharge nozzle, volute, and the wall of the
casing. This also applies to the tensile, compression, and hoop stress.
Likewise, the modulus of elasticity that is used for the design will change
from one process to another. Experience has shown that design values of
the actual piece or structural part may be Vs to Vio.that of the test bar.
The modulus of elasticity is between 1 to 2 million.

Pressure vs. Temperature

Unlike metallic pumps, there are no standards for pressure-tempera-
ture ratings of the flanges. Presently, the ratings change from manufac-
turer to manufacturer and material to material. Good design practices
have demonstrated that the pressure reinforced vinyl ester capability at
ambient temperature of the flanges can be equivalent to that of the metal
flanges using the same dimensions as the metal flanges. The pressure-
temperature gradient is a linear factor and basically degrades above
1QO°E
Because heavy wall vinyl ester material is a good insulator, the temper-
ature gradient from the liquid side of the pump case to atmosphere is bas-
ically 100°F. This is based on tests of heat soaking the vessel for 24
hours. This allows the manufacturer to pump higher temperatures with-
out excess bearing temperatures. This is also good for the user since
there will be little loss of heat from the fluid while passing through the
pump,

NPSHR
As with metallic pumps, NPSHR is established by using 3% head loss
as a criterion. However, it should be recognized that pumps operating at
3 % drop in head or relatively close to this mode of operation in incipient
cavnation. Where damage might not be apparent on metallic pumps, it
will be observed after a period of time on nonmetallic pumps. It is sug-
gested that the NPSHR offered by the manufacturer be 3 to 5 feet higher
than metallic pumps in order to give equivalent life to the nonmetallic
counterparts.

General Construction of Nonmetallic Pumps
Most nonmetallic chemical pumps presently being offered for the same
hydraulic range as the ANSI pumps are being built to the ANSI dimen-
sional standards envelope. Consequently, most manufacturers are using