Benefits of Variable Frequency Drives on Pumping Systems in Enbridge Liquids Pipelines
Authors: Ferrari André-Michel 
Published: November 2016

An electrical Variable Frequency Drive (VFD) is a device that controls a motor by varying the frequency and voltage supplied to the motor. Frequency (or hertz) is directly related to the motor’s speed (RPMs). If an application does not require an electric motor to run at full speed, the VFD can be used to ramp down the frequency and voltage to meet the requirements of the electric motor’s load. As the application’s motor speed requirements change, the VFD can simply turn up or down the motor speed to meet the speed requirement which in turn controls the output of the connected equipment, in this case a pipeline pump (i.e. flow and pressure).

This device is important in pipeline applications as it provides the operator with improved control over the critical parameters of the pump and in doing so increases pumping efficiency while reducing energy costs.

Enbridge Liquids Pipelines has gradually introduced more VFD units on its mainline pumping systems since 1994. To date, 50% of the mainline pumps in the Enbridge Liquids Pipeline network operate under the control of a VFD. There is now sufficient historical operating data on those assets in order to quantify the benefits related to this particular system. This paper focuses on the operational reliability aspects of the VFDs and equipment controlled by the VFD. This includes failure probabilities and throughput performance over the life cycle of the system but excludes technical implications such as VFD selection, application, specification or design.

From a pure maintenance perspective, VFDs contribute to a marked improvement on pumps in terms of failure reduction. For general pump failures including components such as mechanical seals, bearings, shaft, wear rings or couplings, it is demonstrated that the probability of failure is lower on pumps combined with VFDs compared to pumps without VFDs. In terms of mean time between repairs (MTBR), this equates to an increase of 65% relative to pumps with VFDs.

All mainline pumps in Enbridge are driven by electric motors. With regards to drive motor failures, there is also a significant reduction in repairs; MTBR increases by approximately 25% on VFD driven electric motors.

Another factor which can enhance the benefits associated with VFDs is the sparing options. In Enbridge Liquids Pipelines, there are two types of sparing options related to VFDs:

• Dedicated VFDs (1 VFD controlling a single mainline pump)

• Shared VFD (1 VFD shared between a set of mainline pumps) with back-up across the line starting.

The throughput performance between the above mentioned existing options has been shown to differ substantially. On a specific pipeline built initially with shared VFDs then fitted with Dedicated VFDs, the number of pump failures decreased by 60% leading to a throughput loss reduction of 66%.

However, while the VFD helps preserve the asset it runs in conjunction with, the VFD itself introduces a high frequency of failures in the overall pumping system. For the systems studied in Enbridge pipelines, adding a VFD increases the frequency of downtime events by 118%. However, these failures are short in duration which in the long term add up to less downtime (276% less) on a pumping system with VFD.

Finally, VFD units have a high capital cost which can double the cost of ownership of a pumping system over the life cycle of the asset. However, this cost can be offset by the increase in availability provide by the VFD but this needs to be vetted through a Life Cycle Cost analysis.