Page 119 - Centrifugal Pumps Design and Application

100 Centrifugal Pumps: Design and Application

Figure 8-18. Estimating K,.

The extent of cavitation damage depends on the proportion of vapor re-
leased, the rapidity of liberation, and the vapor specific volume, lairing
this into consideration the cavitation face calculation can be corrected by
applying a gas-to-liquid ratio factor C b (Figure 8-20). On this basis, cold
water must be considered the most damaging of the commonly pumped
liquids. Similarly, this difference in behavior applies to water at different
temperatures. A review of the properties of water and its vapor at several
temperatures shows the specirlc volume of vapor decreases rapidly as
pressure and temperature increase. This difference in behavior under
cavitating conditions makes cold water more damaging than hot.
The problems associated with cold water are substantiated by operating
experience in the field, where pumps handling certain hydrocarbon flu-
ids or water at temperatures significantly higher than room temperature
will operate satisfactorily with a lower NPSHA than would be required
for cold water.