We live in a world of stress. So do machines. As engineers we account for the impact elevated temperatures will have on machinery components design within established known limits.
But it is important to not only account for temperature and temperature swings but also how fast temperature may change for a piece of equipment.
This is known as Rapid Delta T and something worth discussing.
For instance it is not uncommon for a process piece of equipment to see temperature extremes such as 800F, or higher, as well as starting at –40F temperatures
Equipment materials need to be selected to handle the temperature extremes. The lubricant and lubricant system must ensure the right viscosity at the cold end so that the bearings will operate properly while protecting the bearings and lubricant from overheating at the high end.
In addition if the fan experiences the two temperature extremes within a very short period of time, then issues related to different rates of component expansion also have to be addressed.
Even though two components are made from the same material a thin component exposed to a source of heat will see a more rapid increase in temperature than a thick component. A backplate for instance is generally made of steel plate whereas the hub is often made of a massive piece of cast iron or steel forgings.
Under rapid delta T conditions the thinner backplate will act as a conducting fin and grow quicker than the hub. When this uneven expansion takes place the stress is transmitted through the bolts or rivets and can actually shear through the backplate.
Similarly a cast iron hub often has a large diameter end. This large surface area can result in a more rapid change in temperature than the shaft, which is essentially a compact block of steel. In this case the bore of the hub can expand quicker than the shaft and the fit between the hub and shaft may be lost.
Eventually the thicker component will “catch up” but damage can occur while the temperatures are stabilizing. Shaft looseness or movement of a bolted hub/backplate connection can result in a change in the wheel’s center of rotation.
The resulting imbalance can cause damage to the bearings through increased vibration levels.
Rapid delta T events can be accommodated with good design. To combat hub/shaft movement, interference fits are used. To eliminate movement at the hub/backplate bolted connection fitted bolts are often used.
There are many other ways to accommodate differing expansion rates in order to prevent either harmful stresses or mechanical looseness from developing.
Suppliers have various preferences so it is worthwhile to discuss the options with a few.
In conclusion not only is it important to spell out what the equipment will experience, but in some cases, when it will experience it can be just as an important.
Neil Fraser, P.Eng.
Neil is a senior application engineer with CB Power and Industrial Equipment
Buffalo Turbines: Specializing in turbine rotor replacements for either single or multi-stage steam turbines.
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