It only takes a tiny spark or a sufficiently hot surface to ignite a deadly explosion. For many industries that depend on electrically driven pumps, explosion prevention is a primary safety concern.
In the case of chemical plants, refineries and hydrocarbon processors, a high potential exists for explosive vapors to be emitted by process and other liquids. In the case of mining and oil and gas exploration, an explosion's potential fuel source is escaping underground gas from subsurface pockets, which are frequently under extreme pressure.
In response to these potentially destructive situations, electrical equipment, including submersible pump motors, can be designed and manufactured to prevent them from becoming an ignition source in these hazardous environments.
Pumps for Hazardous Zones
Safety in these environments begins with the science of defining the exact nature of the hazardous area. This includes understanding and classifying the explosive material and its potential for being present within defined areas (zones) of the considered site. In the U.S. and some other countries, hazardous locations are defined in an official and comprehensive publication called the National Electric Code (NEC), which is produced and maintained by the National Fire Protection Association (NFPA). The NFPA's mission includes producing fire safety standards that are used as official codes by governments and other authorities with interest in safety for construction, commercial, residential and industrial sites.
The NEC typically classifies areas that can contain the previously mentioned gases as Class 1, Division 1 in Group C or Group D. For submersible pump manufacturers developing products that are suitable for these zones, three key principles guide an engineered product design:
- Spark-free motor design. The pump motor must be designed so its operation will not produce electrical sparks, a source for ignition.
- Engineered flame paths. In the event that explosive gas enters and ignites inside of the motor housing, the motor's design must cause the flame to extinguish before it would exit the vessel.
- Pressure containment. The motor vessel's structure must be capable of containing the pressure generated by an internal explosion without fracturing.
Manufacturers can employ third-party approval agencies, such as Factory Mutual Insurance (FM), to certify that their products adhere to standards that make them suitable for duty in hazardous areas. The agencies scrutinize the design details and the quality-control methods used to produce any electrical device that may be approved for use in hazardous locations.
Submersible pumps manufactured for hazardous locations include special nameplates and other visible certification marks that signify they are designed and built for these zones.
Addressing Safety Concerns
One development that addresses safety concerns is an explosion-proof submersible slurry pump, which can provide end users with another option for hazardous zones. Operators in some cases previously have used a standard pump with an explosion-proof motor—typically side-discharge sewage pumps or pumps designed for water—or units with pump ends better suited for the application but with non-explosion-proof motors.
An explosion-proof submersible slurry pump can provide various benefits for handling applications in hazardous areas. Submersible pumps can go into the liquid or slurry. When dealing with a heavy slurry that contains solids and liquids, it is much more efficient and reliable to have the pump and agitator in the liquid for suction instead of having a self-priming pump that sits above the liquid and relies on a suction hose and atmospheric pressure to pull the slurry into the pump.
A slurry pump using an agitator could improve the handling of abrasive slurries. Not all submersible pumps are able to handle slurries. For high concentrations of solids that tend to settle to the bottom of a pit or holding area, an agitator adds velocity around the pump and keeps the solids suspended for possibly easier pumping and effective removal.
The material used to construct the pipe is also important to consider. Abrasive materials typically erode cast iron and other softer metals. Most dewatering pumps are made from cast iron and do not last long in abrasive applications. The pump must be able to handle abrasive slurries in demanding applications.
When considering designs, some vertical/cantilever pumps have fixed installation and sometimes exhibit shaft breakage and lubrication problems in drilling and mining applications. It is possible that, in moving pumps with a side discharge connection, the connection can break off or be damaged as it is pushed and pulled into place. Top discharge pumps are cooled by the pumped liquid and can pump a pit down to within inches of the bottom. A side discharge pump without a cooling jacket must stay submerged to avoid overheating, leaving as much as 3 feet of unpumped slurry.