MechChem Africa March 2017

⎪ Pump systems, pipes, valves and seals ⎪

Motor rating

110 kW

Pump discharge pressure 640 kPa Pressure downstream of the valve 380 kPa kWwasted

= (640-380)/640×110 = 44.7 kW

Energy wasted = 44.7×8 760×0.9×R0.90/kWh = R317 000 per year Table 1: A table showing how much energy and money is wasted by throttling a pump and how much money can be made available to fix the problem. A 90% utilisation factor is assumed.

capacity. We were able to achieve the same flowrate(actuallyslightlyhigher)withonlyfive pumps running. And shutting down one pump reduced the total power drawn by 258 kW. These pumps have since been running in this state for over a year with no adverse effects on the motors, resulting in saving of over R1.5-million/year in electricity costs and 2 050 000 kWh per year in energy savings. Identifying energy-saving opportunities fromthrottled pumps is actually easy. All that is needed is a stroll through the plant with your eyes open. • Look for control valves with pressure gauges upstream and downstream of the valve and use the formulas outlined here toestimate thewastage through the valve. • Look for ammeters showing pumps are drawing maximum amps – these pumps are, most likely, being throttled. • Question every answer you get and never assume the original reasons are still valid.

If you discover that any pump is being throt- tled, the first question is, does it need to be? If it is onlybeingdone to limit themotor current, then it is often unnecessary. Open the valve and see what happens. If there is a valid reason for throttling the pump, then two far better andmore economi- cal options are available to pump operators. The first is to trimthe pump impeller. Thiswill return the pump’s operating point to the duty point of system, saving significant amounts of energy and money. The second solution, which is slightlymore expensive but still highly cost-effective, is to reduce the pump speed by using a variable speed drive (VSD). There arenogood reasons to continuously throttle pumps. Not only are you throwing electrical energy and money away, but also the reliability of the pumps always suffers if they are operated too far away from their duty points. q

Pumps are often throttled to limit the current drawn by the pump motor. In many the original reason for doing this is forgotten and it is no longer necessary. have just read the maximum value from the technical spec sheet incorrectly. The solution? Open the valve and see if there are any adverse effects on the pump or motor. The energy savings can be quite stupendous. In a parallel pumping case study for a steel mill cooling water system, for example, six pumps were operating in parallel, providing cooling water into a steel mill. All the pumps were throttled to between 25 and 50% open to ensure the current neverwent above 73A, even though the rated current for themotors was found to be 85 A. When opening the valve on one pump to 100%, we observed that the current never went higher than 79 A, well within the rated

March 2017 • MechChem Africa ¦ 11