Q&A: Resolving shaft currents and electrical discharges

Rui Fernandes, General Manager, Shaft Current Solutions (dba Sohre Turbomachinery), discusses the problems created by shaft current and electrical discharges, as well as how to resolve them.[/caption]

Shaft Current Solutions (dba Sohre Turbomachinery) manufactures equipment and provides consulting services, relating to the protection of rotating equipment from electrical discharges. They sell grounding brushes, design shaft grounding/earthing systems, and help diagnose the cause of shaft current problems.

What are some of the problems that can result from shaft currents and electrical discharge?

The most common problem is pitting and deterioration of the interface surfaces between components such as bearings, journals, seals, and gears. Depending on the intensity of the electrical discharges, the damage can be gradual, slowly eroding bearing surfaces and requiring the replacement/refurbishment of components during scheduled services; or it can cause extensive damage and force the machine offline in just a few hours of operation.

What misconceptions might operators have about shaft current?

The biggest misconceptions are that installing a grounding brush assembly will address the problem, and that one should not use more than one brush in a machine train. Although installing grounding brushes is a good first step, an effective system must take into consideration the grounding circuit, how all the components of a machine train are electrically connected, operational requirements, and safety of the grounding system. We have not found it to be true that installing more than one grounding brush on a machine train can lead to “recirculating currents.” However, we found on several occasions that grounding brushes can be too far from the components they are meant to protect.

What might happen if you ignore such issues?

Depending on the source of the electrical discharge, ignoring the issue can lead to reduced component life and unscheduled outages. In some cases, magnetized components can produce high enough current levels to force machines offline. There were some extreme cases in which the levels of magnetism were intensified by the current generated (known as self-excitation), which led to rapid and catastrophic machine failure.

What are the various ways to deal with this issue?

There are three primary approaches to address the problem: eliminate the source of the electrical current, change the properties of lubricating fluids to eliminate charge accumulation, or dissipate excess electric charges to ground (earth) using sliding contacts. The elimination of electrical sources is ideal, and we work with our customers to do so whenever this is feasible, as is the case with magnetized components. In our experience, that is the most dangerous source of electrical currents. However, the generation of electrical charges is inherent in the flow of fluids and those we cannot fully eliminate. There have been several attempts at changing the conductivity of lubricating fluids to prevent the accumulation of charges. However, our anecdotal experience is that this seems to change the discharge voltage threshold rather than eliminate electrical discharges. Lastly, the approach that is used by us and others, is the use of sliding contacts to dissipate accumulated charges.

Why is your approach better?

Unlike other applications of sliding electrical contacts, such as motors and generators, where emphasis is placed on carrying large and known current levels, shaft currents require materials that can carry a relatively low level of energy at unpredictable voltage levels for long periods of time with low levels of maintenance. Sohre brushes are designed from the ground up for this single purpose. They are part of a complete system focused on dissipating shaft currents. From bristle materials (silver, gold, etc.), to electrical grounding/earthing circuit, everything is specifically developed for the task of protecting machinery from stray shaft currents. Other shaft grounding/earthing brushes do not address the whole circuit from the brush to ground/earth and make use of repurposed (less expensive) materials such as carbon and copper. This can compromise long-term reliability, maintenance, and effectiveness.

Explain how it works.

We work with our customers to design shaft grounding/earthing solutions for the complete machine train. The solution addresses a variety of factors such as: operating environment, safety, run cycles, and maintenance intervals. This can range from providing the components to a comprehensive solution involving installation, maintenance procedures, and baseline measurements. Sohre brushes are available as installed options by most turbomachinery OEMs, and as retrofit.

What is involved in installation?

Our preferred approach to install a shaft grounding/earthing system on a machine train is to follow these steps:

1) Take baseline shaft voltage measurements of the running machine to understand the situation

2) Design the system to customer requirements

3) Work with the customer to install and review the system

4) Measure shaft voltage after the installation and develop a maintenance plan

Have you noticed any recent trends?

There has been an increase in the number of brushes we have installed in potentially explosive environments. This is challenging. But with the right steps, we have been able to install effective shaft grounding/earthing systems in problematic environments. ■