Page 85 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Best Practice 2 .12

Fig 2.12.2 Centrifugal pump component damage and causes as a function of operating point

Fig 2.12.3 Head required by lower density fluid

Fig 2.12.4 Fluid head
the product since the maximum head produced by the pump
will be much less than the head now required. The discharge
pressure gauge will drop in pressure. This is commonly known as As can be seen from this figure, either a reduction in pressure
‘vapor lock’ or ‘loss of suction’. (holding fluid temperature constant) or an increase in temper-
ature (holding pressure constant) will cause a phase change.
Vapor pressure Another way of stating these facts is presented in Figure 2.12.6.
How can a liquid change phase inside a pump? The vapor pressure Before we leave this subject, let’s examine how water can
for any fluid is that pressure at which the fluid changes from its change phase (refer to Figure 2.12.7).
liquid to vapor phase. Vapor pressure changes with fluid tem- The most common cause of phase change for water is an
perature. The vapor pressure for any fluid can be obtained from increase in temperature at constant atmospheric pressure.
its Mollier diagram. For a given fluid temperature, the vapor When the vapor pressure exceeds the pressure surrounding the
pressure is the intersection of that temperature and the saturated liquid, 101.3 kpa (14.7 psia), the liquid will change phase or
liquid line. Figure 2.12.5 is the Mollier diagram for ethylene. ‘boil’. However, water can also change phase if its pressure is

59