Page 141 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 119

suction recirculation damages the pump, in-
creases failure rates and raises pumping system
costs.
➢ A rule of thumb is that reliable operation requires
using pumps with N SS under 8500. As N SS in-
creases, the reliable operating range of the pump
usually decreases. Some low N SS pumps continue
to have problems. Some high N SS pumps appear to
operate with widely varying flow rates without
problems.
FIGURE 5.14 Pump suction through a sump.
➢ Experience has shown two moves can improve
reliability, namely, running with lower rpm pumps
and raising the NPSH available, NPSH A , at the
& Suction lift is too high or insufficient margin between pump suction. Reducing rpm increases the diam-
suction pressure and vapor pressure. eter of the pump needed or mandates more stages
& Air/vapor present or entrained in suction liquid. Air in a multistage pump.
entrained in the inflow should not pass into the ➢ Boosting NPSH A at the pump suction usually
suction opening. This can usually be prevented by requires elevating vessels higher above the
providing sufficient submergence of liquid over the pump. Both steps can be expensive. No clear
suction opening. guidelines detail the trade-offs. One solution is
& An alternative sump design is to provide a baffle in to always operate close to the BEP by using
the sump as illustrated in Figure 5.14. pump recirculation.
& Wrong direction of rotation. . What are the effects of high-viscosity liquids and en-
& Too low or too high speed. trained gases on centrifugal pump performance?
& Total system head is higher/lower than pump design & As viscosity increases, head and capacity deterio-
head. rates and markedly when viscosity reaches 65 cP.
& Specific gravity/viscosity of the liquid is far from BHP increases.
design value. & Above a viscosity of 65 cP, it would be better to use a
& Very low capacity operation. positive displacement pump.
& Entrained gases decrease performance in terms of
& Foreign matter in the impeller. Installing a filter on
the suction side, such as a wire mesh strainer with a efficiency, head, and power leading to high levels of
mesh size of about 120 mm, is advisable. This pro- severe effects when air/gases increase beyond 6% by
vision becomes all the more a necessity at the time of volume, measured under suction conditions. 0.5% is
plant start-up to avoid debris entering the pump tolerable limit.
casing. . What is the effect of high pump speed while pumping
& Damaged impeller/wrong size impeller installed. highly viscous liquids?
& While handling viscous liquids, selecting too high a
& Misalignment of shaft or bent shaft.
pump speed results in insufficient inflows, causing
& Lack or loss of lubrication. Contamination of
cavitation problems.
lubricant.
. How can reliability in the operation of a centrifugal
& Abnormal noise and vibration levels.
pump be improved?
& Driver HP not adequate.
& Once a unit is built, often the only option to improve
& Seal-related problems such as leakages, loss of flush-
pump reliability is to replace the pump with a lower
ing, cooling, and quenching systems.
speed one. It is found that dropping from 3600 to
& Pump problems can be summed up into three cate-
1800 rpm nearly solves most N SS -related problems.
gories, namely, design errors, operational errors,
& It has been useful to use the concept of suction
and maintenance errors.
energy, E Suction, in predicting pump reliability as
& Pump failures generally are due to three reasons,
related to suction conditions by using a suction
namely, mechanical problems within the pump, recirculation factor that includes peripheral velocity
driver problems and hydraulic limits in the system. in the impeller eye at the vane edge maximum radius
➢ Suction recirculation imposes mechanical limits
in its calculation:
and hydraulic requirements. Operation with