Any component used in oil and petrochemical facilities must pass stringent safety standards to avoid potential ignition of hazardous gasses. So how can high-output electrical generators be designed to operate safely in these conditions?
In order to protect oil and gas facilities, employees and the environment from the risk of explosion, it is essential to ensure all sources of ignition are removed from spaces that could potentially contain flammable gasses.
All electrical equipment, however small, must be tested against rigorous safety standards before it can be used in sensitive areas of a facility, so it is no surprise that high-output electrical generators are no exception to this rule. Without the proper precautions, the mechanical and electrical components of generators could easily ignite an explosive mixture with serious consequences.
Almost all large oil and gas facilities feature on-site power generation capability, so it is essential to specify a generator package with the correct rating for the specific requirements of the space in which it will operate.
But how is the level of risk presented by a potentially explosive atmosphere measured and rated?
Hazardous area requirements
For almost every region of the globe except the USA, the level of explosion threat presented by any given environment is rated according to guidelines laid out by the International Electrotechnical Commisssion (IEC). These assign the level of hazard a rating in three categories.
Environments are designated with a Class, based on the overall type of material present – gas, vapour, dust or fibre – a Group, which gives the severity of the threat posed by the specific material in question, and a Division, which indicates whether the hazard is always present or only in abnormal circumstances.
The atmosphere is also given a temperature rating, which denotes the maximum temperature of any external surface – this must always be safely below the lowest potential ignition point for any given gas mixture.
The area where the electrical machinery is to be operated is classified for Class, Group and Division, and this designation determines the level of precautions that need to be taken in that space. Here is a summary of how atmospheres are classified:
The nature of the hazardous material that is present and broken down into three categories:
Class I – Potentially explosive gas or vapour
Class II – Potentially explosive dust
Class III – Potentially explosive fibre
The specific type of hazardous material which is broken down into seven types of material, named as groups A to G. Each group contains a list of different materials with similar properties, including the temperature at which they will ignite, their level of flammability and how easily they leak through seals.
The Frequency of presence is broken down into two categories:
Division 1 – Potentially explosive material present under normal operating conditions
Division 2 – Potentially explosive material present under abnormal operating conditions
So how can generators be made safe to operate in these hazardous environments?
Designing generators for explosive atmospheres
For a generator in the 8 to 50MW rating class to operate in any area carrying a risk of explosion, it should be a totally enclosed design – that is, the atmosphere surrounding the unit must not be able to contact the internal parts, when the potential ignition sources are present.
Generators produce a great deal of heat during operation, so a cooling system is essential. Often, in safe non-hazardous areas, an open air-cooled design will be used, drawing the cooling air directly from the surrounding atmosphere. Clearly, for spaces with an explosion risk, this is not appropriate.
Therefore, the cooling methods that are employed are Totally Enclosed Air-Air Cooled (TEAAC) and Totally Enclosed Water-Air Cooled (TEWAC).
These totally-enclosed generators are selected for installations where hazardous gas mixtures are normally present, or where they could occur if a process or equipment malfunctions.
Where water-cooling plant is part of the facility design, TEWAC generators will be supplied. When the greater simplicity of air cooling is preferred, this can be achieved with either air-to-air coolers or heat pipe elements. The latter system uses refrigeration to transfer heat outside the generator enclosure.
For hazardous areas, classified by the owner or design authority as Class 1 Group D Division 2, purged and pressurised enclosures must be used to remove the risk of explosive materials remaining within the generator’s enclosure.
Under this arrangement, a clean air supply from a safe area is provided by the facility to ventilate the internals of the generator and volume of air and purge time is monitored. The generator casing and each internal compartment has a manifold used to purge the internal spaces of gases out to atmosphere through a flameproof trap.
A controller then shuts off the generator enclosure and the clean air supply slightly overpressures the compartments, maintaining a small flow to make up any leakage. Failure of the clean air supply or loss of overpressure will cause a generator alarm or trip.
The right tool for the job
We at BRUSH custom-build each generator to include the features, configuration and protective devices to meet purchaser’s specifications and all ancillary devices must also be compliant with the hazardous-area specifications.
Any ancillary equipment supporting the generator, such as anti-condensation heaters, temperature detectors and vibration monitors – including all wiring and connections – must also be carefully selected to support the required hazardous-area classification.
We have developed and supplied generators up to 40MW for service in hazardous areas both on and offshore, including a number of units rated over 30MW for both gas and steam-turbine drivers. Our generator ranges include both salient 4-pole and cylindrical-rotor 2-pole types in direct air-cooled ratings up to 300MW.
During the design and build process, the generator’s features are reviewed and explained to the independent certifying inspection authority - the American Bureau Shipping, Lloyds or Det Norske Veritas for example. The completed product is then fully tested in accordance with the relevant standards to demonstrate compliance with purchase specification and contracted performance points.
Ultimately, potentially explosive atmospheres are an ever-present risk right along the oil and gas supply chain. With this in mind, we are committed to delivering generator systems that will help our customers meet their on-site power requirements without ever having to compromise on safety.
Figure 1 - A BRUSH DG-type 4-pole generator being prepared for packing after testing. The customer required BRUSH to supply and mount a speed-reducing gearbox and integrated sub-base to align and couple to the turbine driver as a single assembly. BRUSH consolidated the complete instrumentation and protection features into a single customer connection point.
Figure 2 - A totally enclosed BRUSH DAX-type 2-pole generator, complete with cooling system, during final assembly, being prepared for testing. In operation, the generator casing experiences a purging cycle before start-up and remains pressurised to ensure air exchange is from inside to outside, with the small positive pressure maintained by a clean external air supply.
Figure 3 - Clean air feeds into the generator enclosure via the flange point shown.