Page 91 - Root Cause Failure Analysis

82 Root Cause Failure Analysis

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- Flow

Figure 7-3 Pumps in series must be properly matched.

One of the most common problems with pumps in parallel is suction starvation. This
is caused by improper inlet piping, which permits more flow and pressure to reach one
or more pumps but supplies insufficient quantities to the remaining pumps. In most
cases, the condition results from poor piping or manifold design and may be expen-
sive to correct.

Always remember that, when evaluating flow and pressure in pumping systems, they
always will take the path of least resistance. For example, given a choice of flowing
through a 6-in. pipe or a 2-in. pipe, most of the flow will go to the 6-in. pipe. Why?
Simply because there is less resistance.

In parallel pump applications, there are two ways to balance the flow and pressure to
the suction inlet of each pump. The first way is to design the piping so that the friction
loss and flow path to each pump is equal. Although theoretically possible, this is
extremely difficult to accomplish. The second method is to install a balancing valve in
each suction line. By throttling or partially closing these valves, the system can be
tuned to ensure proper flow and pressure to each pump.

Entrained Air or Gas Most pumps are designed to handle single-phase liquids
within a limited range of specific gravities or viscosities. Entrainment of gases, such