A crucial safety measure for installation of drives in the oilfield
by Derek Lizotte, Eaton’s Crouse-Hinds business
February 6, 2014

Using a variable frequency drive (VFD) provides many benefits. However, what happens when those benefits are outweighed by the major challenges and costs associated with hazardous environments? Historically, a VFD has not been used inside a classified enclosure in Class I, Division 1 or 2 locations, because the heat generated by the VFD inside the enclosure causes it to fail. Overcoming this problem often requires complex systems or impractical upfront installation costs. Some manufacturers have developed a better solution—explosion-proof enclosures designed specifically to house a VFD without the risk of overheating. These explosion-proof VFD enclosures can be installed within the hazardous area, significantly reducing construction and installation costs while eliminating the risk of explosions. This article provides an in-depth view of VFDs, from their purpose to installation types.

VFDs 101

Before discussing VFD installation solutions, it is necessary to understand what a VFD does and the key considerations to determine how and where to install one. Used in a wide range of applications—from small appliances to major upstream processes –VFDs are designed to control the rotational speed of a motor, changing the fixed frequency of the supplied voltage to a variable frequency. The ability to vary the speed and torque of alternating current (AC) induction motors allows for improved process control, energy savings and additional diagnostic capabilities. The improved performance associated with VFDs is the main reason they are used in so many applications, including the different stages of the exploration, processing and refinement of natural resources. To understand the solutions currently available, the key concerns when working with or around VFDs must be considered. In many cases, those concerns relate to the heat generated by these drives and the environmental conditions in which they operate. These conditions can include a range of factors—such as condensation and moisture, dust, dirt, gases or other flammable substances. When encountering these conditions, products are susceptible to performance failure or in some instances, conditions that may result in the ignition of the product. Knowing that safety is of the utmost importance to any operating facility, manufacturers have developed several installation solutions to combat the problem. Installation becomes a critical issue for harsh and hazardous applications. Prior to new technologies in the marketplace, the only real solution was to place the VFD in a non-hazardous control room or use a purge solution. While both are effective, they also include factors that can slow or limit operations and increase cost.

Using a Non-Hazardous VFD Remotely

The first and most commonly-used solution is designed to eliminate the possibility of contact with the hazardous location by housing the VFD in a non-hazardous control room away from the classified jobsite. This non-hazardous facility, usually hundreds of feet from the process, enables continued, safe operations while reducing or eliminating exposure to dangerous environmental conditions and limiting the potential for heat to become a concern. While this solution is effective in isolating the VFD from harsh and hazardous conditions, it has several drawbacks. The most obvious is an increase in cost, a direct result of the distance covered and its resulting conduit and motor cable costs. If the location presents additional challenges—such as navigating conduit and cable around obstacles, roadways or other hazards—that cost will continue to rise. In addition to increased cost, the total distance from the non-hazardous control room to the jobsite can negatively impact the overall performance of the VFD. Depending on the distance, certain conditions may create reflective wave syndrome. In these instances—usually more than 100 to 150 feet away—the current passed through the motor cable will reflect back and forth, causing vital components, such as motors and pumps, to fail. This will affect those pieces of equipment and may require additional sections of the operation to shut down in response.

Using a Purging System

For applications in which access to a non-hazardous control room offsite is unacceptable, purging systems are available. These specialized solutions, much like the name suggests, use pressurized compressed air or an inert gas through a purging process to reduce the hazardous gas inside the enclosure to a safe level. While purging systems offer an effective solution in these applications, the product’s labor-intensive installation process and bulky design is a major drawback. In addition, these solutions tend to have a higher rate of failure, causing costly downtime in field operations and the need for ongoing maintenance.

Using an Explosion-Proof VFD

The previous solutions allow for continued operations. However, what if end users could place their VFDs exactly where they wanted them, mounting the drives next to motors inside harsh and hazardous environments? Thanks to advancements in the field, this placement is a reality with explosion-proof solutions for onshore and offshore applications. These new VFD enclosures offer simple, cost-effective installations, using National Electrical Manufacturers Association (NEMA) Type-7-rated enclosures. They allow the VFD to be installed in the hazardous area. To ensure proper airflow in a wide range of conditions, many are breathable and include filters designed to eliminate the dirt and dust that can react to the heat that can create unsafe conditions. In some instances, these filters can undergo routine maintenance without powering down the motor or drive, which leads to increased uptime. In addition to remote installations, these explosion-proof products can also be used in offshore applications, taking the place of purging systems. With a reduction in the maintenance costs that purge systems may require, these enclosures represent significant savings.

An Easy Choice

Knowing that working conditions vary from jobsite to jobsite, choosing a solution that addresses safety and performance is important. By understanding the key factors for each product, end users can make an informed decision to maximize their productivity and their budgets. Many end users are moving away from remote VFD installations and purged solutions, and selecting an explosion-proof VFD solution.