How Can Motors & Drives Reduce Energy Use In Chemical Plants?

Louise Smyth

Upgrading the motors and drives in chemical plants could significantly reduce energy use in pumping systems while improving safety, reliability and profitability.

Chemical plants can have hundreds of pumping systems, and the motors and drives running these consume enormous amounts of electricity.

Pumps account for 10% of the world’s total electrical energy consumption. Pump systems are responsible for around 20% of industrial electricity use with the chemicals sector being one of the biggest pump users.

Pump Efficiency Can Be Less Than 40%

However, studies show that pumping efficiency in manufacturing and processing plants can be less than 40%. Process intensive sectors, such as the chemical industry, have a high base production flow that is generally managed by pumps – usually centrifugal pumps – operating at constant speed and there is always a need to control the process flow over time.

Operating pumps at a constant speed and using traditional methods of controlling the flow, such as throttling (forcing a bottleneck) wastes a lot of energy. Improved equipment or control system changes could deliver between 30% and 50% in energy savings.

The Benefits Of Drive & Motor Technologies

Modern drive and motor technologies offer far more efficient control and monitoring of pump operations.

Variable Speeds for Efficiency

Traditionally, controlling fluid flow rates in chemical plant pumping systems was a bit like driving a car with the accelerator pedal pushed flat on the floor, while simultaneously using the brake pedal to vary the speed.

A control valve is put on the outlet side of a centrifugal pump, which is powered by an electrical direct-on-line (DOL) motor running at full speed and using the valve to throttle or choke the liquid to achieve the desired flow. To be on the safe side, pumps and DOL motors selected are often oversized in the engineering phase, but in practice the maximum conceivable flow rates are rarely reached.

Simply put, the fluid comes out of an oversized pump run by an oversized motor working at full speed and hits a partially closed flow valve. The flow rate is controlled but in a very crude manner, causing severe wear and tear on the system components and wasting energy.

The excess mechanical energy that is expended in the piping system often shows up as unwanted side effects – such as vibration, heat and noise. Over time, these negative factors can weaken the integrity of the pumps, pipes, valves, joints and instrumentation, leading to expensive downtime and reduced process reliability. Pumping systems in industrial processes are frequently reported to have the highest overall maintenance costs compared to other motor-driven systems. In addition, pumps and valves are very common sources of process leaks, which can be both costly and dangerous.

Variable Speed Drives (VSD)

A far better alternative for adjusting flow rates and pumping capacity is to use a variable speed drive (VSD) on the motor. Flow control can be more precise and energy savings very large due to the ‘Cube Law’, which describes the relationship between flow output and energy input for centrifugal pumps or fans. For instance, a 20% drop in motor speed can reduce its energy consumption by 50%.

Recently, an ABB test compared data from a centrifugal pump with throttle valve control driven by a 37kW direct-on-line motor run at constant speed with an equivalent VSD-control pump system. The results highlight the advantages of VSD control at partial flow rates.

The curves with markers show the input powers required by an IE3-class induction motor operated at constant speed, and by the same motor with a VSD. Electricity (input power) usage is reduced by 80%, 61% and 35% at flow rates of 50%, 67% and 83%, respectively, compared to throttling valve control.

Synchronous SynRM and Reluctance Fa-SynRM Motors

Energy savings can be further increased by replacing induction motors with advanced synchronous motor technologies such as synchronous reluctance (SynRM) and ferrite-assisted synchronous reluctance (FA-SynRM). To compare the efficiencies of the different technologies, three motor drive systems were set up using an induction motor (as reference) and a SynRM and FA-SynRM motor. All three were rated 11kW with a nominal synchronous speed of 1,500rpm at 50Hz in IEC frame size 160.

Compared to the reference induction motor, which was a high performance IE3 efficiency class product, the SynRM and FA-SynRM motors provide additional annual energy cost savings of US$159 and US$281, respectively, for the constant torque duty profile.

The savings are US$122 and US$213, respectively, for the quadratic torque duty profile. These values correspond to payback times well below two years, which means these motors deliver significant cost savings over their life cycle.

Compared to less efficient induction motor drive systems, the energy savings could more than double.

  • Synchronous motors not only improve energy efficiency, but they also typically operate at lower temperatures, which contributes to improved safety and higher reliability.
  • Induction motors are relatively inexpensive and robust, and the lack of a commutator and brushes means they are reliable and fairly maintenance-free.

Their main drawback is their asynchronous speed, which causes losses in the rotor conductors. These losses reduce efficiency and increase heat production in the bearings, leading to shorter bearing lifetimes. Although VSD control is possible with induction motors, it introduces additional stator, rotor and VSD losses, resulting in even lower efficiency.

In synchronous motors, the rotor moves at the same speed as the driving magnetic field, eliminating most causes of rotor losses. The motor can therefore run at a lower temperature, which promotes longer lifetimes for components like the bearings and insulation system.

Total Cost Of Ownerships Savings

Although the numbers will vary according to the application, energy makes up 50-95% of the  total cost of ownership (TCO) of a motor-driven pumping system over its lifetime. Big savings are possible but are often overlooked.

For example, a customer recently told ABB that his plant had made progress on energy saving by eliminating 500W of light bulbs that had previously been continuously illuminated. This was indeed a saving but ABB’s experts told him how a 75kW motor-driven pump running continuously with a partly closed valve could cut energy use by 50% and save 37,000W – 75 times what the light bulbs saved.

Take a look around you and consider this: you may be able to save millions of Euros a year in your chemical plants if you modernise your drives and motors.

Predictive Maintenance

Besides cutting energy costs, technical developments with drives now include functionality that greatly increases their value on centrifugal pumping systems. Process control can be made tighter due to continuous data gathering, which provides digital insight and enables predictive maintenance.

For example, we can now sense what’s happening inside a pump, such as when it gets dirty or clogged, just from the drive data. This digital monitoring function tells the motor, and therefore the pump, to start ramping its speed up or down to clean the impeller before larger problems develop or even unexpected failure results. Other smart functions that modern drives can initiate and manage include procedures such as soft pipe filling, cavitation detection, sensor-less flow calculations, multi-pump control, level control, dry-run protection and pump cleaning.

Predictive maintenance and remote condition monitoring enable the pumping process to be fine-tuned and also allows for deeper checkups on the health of any pumping system without having to physically examine the pump at its operating location. This is a tremendous advantage in the chemical processing industry, where there are hundreds of pumps spread throughout a plant, often in difficult to reach locations.

Petteri Hyytiäinen is with ABB

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