Mechanical Technology September 2016

⎪ Pump systems, pipes, valves and seals ⎪

We sequentially opened each of the valves and checked that the amps drawn were less than the maximum motor rat- ings. Unthrottled, all the motors were well within their ratings. In addition, when all the valves were 100% open, we were able to achieve the required flow rate with one pump shut down completely. Running five pumps instead of six led to savings of 258 kW, which translates into 2.1 GWh in reduced energy costs per year – and this return from a zero cost intervention. Why were the pumps being throttled? There may have been a reason as some point, but things change and if the con- sequences are not being measuring, how can pump operators know that throttling every pump still has any beneficial ef- fect? Even if there was a valid reason for throttling at some point, shouldn’t it have been investigated to determine whether that reason still applied? Or do we continue to throw away two-inches of prime fish because this is what we have always done? But what happens in cases where the motor is drawing more amps than its al-

Figure 2: Pumps in parallel often have their discharge valves throttled to ensure that the current drawn is kept below the maximum motor rating. The author has found numerous instance where the current drawn is far less than maximum motor rating and fully open valves do not overload the pump motors.

a topic for the next article, however, when we will discuss energy efficient ways to reduce pump output, such as trimming the impeller, reducing the pump speed or changing the pump. q

lowable maximum. Surely this is a case when throttling a pump can be justified? Wrong! There are other ways to fix such situations and none of them involve wasting energy through throttling. This is

Mechanical Technology — September 2016

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