Example 3-3 Suction Lift

Total Suction Lift as water = 8.4 — 33 = —24.6 feet. The pump curves show that 21 feet suction lift of water is all the pump can do, hence the 24.6 feet is too great. A different pump must be used which can handle this high a suction lift. Such a pump may become expensive, and it may be preferable to use a positive displacement pump for this high lift. Normally lifts are not considered reasonable if over 20 feet. 

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Be aware that in some cases, you ll have to live with cavitation. Many pumps suffer cavitation for reasons of inadequate design, hor example, when operating only one pump in a parallel system, this pump tends to go into cavitation. Pumps that perform more than one dut through a valve manifold tend to suffer cavitation. Pumps that fill and drain tanks from the bottom tend to suffer cavitation. The last pump drawing on a suction header tends to cavitate. And of course vacuum pumps and pumps in a high suction lift arc candidates for cavitation. 

Centrifugal pumps, 181 Discharge systems, 187 Example calculation, 186 Flow friction losses, 185. 186 Friction losses, pipe, see Chapter 2 Friction, 188 Pressure head, 184—186 Static head, 184-186 Suction head, 184, 185 Suction lift, 184, 185 Suction systems, 186 Hvdroclones, 265—267 Application system, 267 Ignition, flammable mixtures, 493 Impellers, centrifugal, reducing diameter, 203 Impellers,... 

The suction pressure Ps is determined by applying the Bernoulli equation to the suction line upstream of the pump. For example, if the pressure at the entrance to the upstream suction line is P1 the maximum distance above this point that the pump can be located without cavitating (i.e., the maximum suction lift) is determined by Bernoulli s equation from Px to Ps ... 

Suppose that we repeat the pump-starting experiment shown in Fig. 9.4 for a centrifugal pump. As in Example 9.3, we let the pump turn at 1800 rpm and have an impeller of 10-in diameter. If the pump is full of water, the pump has a pressure rise of 42 psi so there is no problem with the suction lift other than those discussed before in connection with PD pumps. However, when we start the pump, the fluid around the impeller is air. Therefore, from Eq. 9.9 we know that the pressure rise across the pump is... 

Example 13-6. Suction Lift A rotary pump is handling a 0.8 specific gravity liquid which hcts a bubble ]x>int of 9.3 psia at the pumping temperature. The maximum possible lift will be... 

Air can be drawn into the piping and pump from diverse forms and different points. Air can enter into the piping when the pump is in vacuum. An example of this is a lift pump. Lift pumps tend to lo.se their prime and a.spirate air into the suction piping and pump. 

The safe behavior checklist should be developed through thorough analysis, using input from many different company personnel. The checklist should contain identified safe behaviors for given tasks. For example, a safe behavior checklist might contain items such as Tut on gloves, Set-up ladder, Use suction device to pick up part, Bend at the knees when lifting, and so on. 

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