Case study 6: Variable speed drive on a circulating pump in a batch process
Case study summary
A reduction in seal failures and maintenance cost savings were achieved by using a variable speed drive (VSD) to control a circulating pump in a batch process for a major chemical manufacturer. Additional benefits were energy savings and increased productivity. This was a retrofit application where a constant speed pump previously controlled the process.
Other potential applications
Pump applications that have high maintenance costs due to frequent seal failures caused by dry running or pump cavitation. Pumps that experience bearing failures due to operation at very low flows for extended periods. Any application that utilizes a throttle valve to control a process.
Investment cost
The total installed cost was approximately €/$ 25 000.
Savings achieved
Total savings were €/$ 62 500 per year.
Payback period
Payback was achieved in less than 5 months.
Installation and operation details Original control method
This manufacturer produces fragrance-based chemicals that are used in various consumer products such as detergents and soaps. Since some of the chemicals being pumped were highly corrosive, the pump was constructed with expensive duplex stainless steel.
The process utilized an end suction centrifugal pump in a distillation tower for circulation. The pump recirculated process liquid through a heat exchanger and back through a reboiler to keep it hot. Running at a fixed speed of 1170 r/min the pump was emptying a suction tank to fulfill a process need in a batch application. As the level of liquid in a steel pot was distilled off, the suction level would drop from 100% to virtually 0%. As a result the pump was experiencing dramatically varying suction conditions. Under normal operating conditions the pump would function well; however, during the last hour of each process the pump experienced extreme cavitation due to the low suction tank level. With these severe operating conditions the pump was experiencing on average a mechanical seal failure every 17 days.
Intelligent VSD based solution
A VSD was installed which had custom pump protection and control software that resides directly on the VSD microprocessor. The custom pump software, with feedback from instrumentation, provided unattended control of the pumping system and protected the pump from operating under damaging conditions. The VSD automatically reacted to process system changes and protected against problems such as dry running, cavitation, operation below safe minimum flow or against a closed suction or discharge valve.
In this application the VSD reacts to a lowered suction level condition by gradually slowing the pump down to prevent cavitation and damage to the mechanical seal. Since installing the VSD, the mean time between failures (MTBF) of 17 days has increased to almost no failures. Another major benefit is that productivity has increased between 5 and 10%.
Energy savings
Monitoring was initially performed with the system operating under the original conditions and again with the VSD control. The original control method utilised a fixed speed 30 kW (40 hp) 1200 r/min motor. The VSD based solution currently uses between 5 kW (7 hp) and 7.5 kW (10 hp) while operating between 550 and 700 r/min. Based on a €/$ 0.04 per kWh, this results in an annual saving of €/$ 8500.
Economic analysis
Savings due to reliability improvements have been dramatic and resulted in a very short payback period. The virtual elimination of mechanical seal failures produced average annual savings of €/$ 54 000 per year and resulted in the elimination of downtime. The original fixed speed units proved to be oversized. VSD operation resulted in an additional saving of €/$ 8500. Total savings on this retrofit application were €/$ 62 500. The simple payback was under 5 months on an investment of €/$ 25 000.
Additional savings were achieved due to a 5-10% increase in productiv ity; however, the chemical manufacturer would not make these savings available for publication.