A Cast Iron Case - part one

High voltage (HV) flameproof electric motors, operating at voltages of up to 11 kV, are the workhorses of the oil and gas industry. Installed in hazardous atmospheres, and capable of withstanding extreme temperatures, corrosive dust, and humidity, they drive a wide variety of pumps and drills in the upstream sector. It is a global business worth approximately €150 million a year.

The key feature of flameproof motors is that they are designed to withstand the pressure caused by an internal explosion without incurring any damage, and are certified for gas groups IIB (atmospheres containing ethylene, or gases and vapours of equivalent hazard)and IIC (atmospheres containing acetylene or hydrogen, or gases and vapours of equivalent hazard). They are a safe choice for applications in potentially explosive atmospheres and meet all relevant international standards, including IEC, EN, NEMA, and ATEX Directive.

Crucially, the motor is not designed to prevent an explosion, but rather to confine any explosion that might occur within its enclosure. The motor is not sealed, so its internal atmosphere becomes the same as that of the hazardous environment where it is installed. Therefore, a motor fault could result in an explosion.

Figure 1. The modified length and angle of the fins increase the surface area and allow for more efficient cooling. 

Designing a flameproof motor requires careful selection of the enclosure materials. Furthermore, should an explosion occur, the design must allow any hot gases generated to escape the enclosure. This is achieved by including flame paths: long, narrow openings that both quench the flame and cool the escaping gases to a temperature low enough so that they will not cause a further explosion when they reach the external atmosphere. Flame paths might be created to run along the motor shaft, or through flanged and threaded joints in the enclosure.

Figure 2. Lower channels in feet, designed to cool the internal ventilation circuit, are present in motors like the AMDR 450 and 500.

Windings are key to a long life

Electric motor technology may be a hundred years old, yet it is always subject to improvements and refinement. In the case of flameproof motors, there has been a particular emphasis in R&D on the motor windings and their method of installation. This is because the windings are the heart of the motor and high-quality windings are directly related to the motor life, with many customers looking for a life of up to 25 years.

Cast iron frames offer improved cooling

Recently, ABB’s main focus has been on improving the cooling efficiency of the AMDR 450 and 500 motors for applications, such as mud pumps or rotary tables, that require an explosion protected (Ex) flameproof motor suitable for installation in zone 1 – an area in which an explosive gas atmosphere is likely to be present in normal operations – or an Ex flameproof motor equipped with an increased safety terminal box. Following a R&D programme, the company has achieved improvements in cooling that provide higher power density and efficiency. These motors offer more watts per kg than have been achieved previously, and lower running costs due to reduced energy consumption. The advantage of high power density is that, for a given output, a customer can now specify a motor one frame size smaller than would otherwise have been the case. This helps to save space and enables more compact installations at a lower cost.

The adoption of a cast iron frame has helped improve cooling in comparison to more traditional welded steel frames. Together with the ribbed cooling fins, the cast iron material provides improved heat dissipation.

Improved cooling that maintains an even temperature across the motor is also vital in ensuring a long life for the motor, particularly in terms of both the windings and the bearings. Test programmes have shown that a differential of just 10°C can result in a 20% reduction in motor life. The AMDR 450 and 500 motors are designed to operate in applications such as mud pumps and rotary tables over a wide temperature range of -55°C to +60°C, to suit the demanding environments found in the upstream sector.

This is part one of a two-part article. Part two will be available shortly.

Written by Patrick Tampik and Marco Nardi, ABB.

Read the article online at: https://www.oilfieldtechnology.com/special-reports/10072019/a-cast-iron-case--part-one/