December 21, 2011
Checking Your Thermal Targets
By Patrick Lawrence
Any equipment that goes through large temperature differences between startup and normal operation should have accurate thermal targets calculated to allow for proper shaft alignment. Shaft alignment data can be taken when the machine is at operating temperature and compared to readings taken when the machine is at ambient temperature. The resulting calculations provide alignment targets to be used for subsequent alignments on that piece of machinery. No special tooling is needed.
Typically, the thermal offsets used to set the movable piece of equipment is an educated guess based on OEM recommendations or you have calculated where you think the equipment will settle once it has reached operating temperature (see the “What’s So Hot About It” post). One of the assumptions is that the equipment will grow straight up as it reaches operating temperature. This may or may not the case. The dynamic forces of rigid flanged connections, different metals expanding at different rates, uneven temperature distributions and varying process conditions could cause the equipment to move both horizontally and vertically.
Shaft alignment data taken from the machine while it is at (or very close to) operating temperature compared to alignment data taken when the machine is at ambient temperature will provide a more accurate representation of how the machine actually moves. The Fixturlaser XA Pro has a nifty function called Hot Check that does the math work for us.
Here’s how it works: the shaft alignment is performed as soon as the pump is shut down and safe to work on. The goal is to quickly measure the alignment condition as close to operating temperature as possible. That shaft alignment data is saved. Let’s call it ‘Pump-Hot’. The machine is allowed to cool completely to ambient conditions and another alignment is performed and saved just as before. Let’s call it ‘Pump-Cold’. We now have two sets of data to compare: one at ambient conditions, where most alignments will be made; and, one at normal conditions, where the pump and motor finally settle in during operation.
With these two sets of data, we can use the Hot Check function to compare and build the thermal targets needed to allow for the proper alignment—both horizontal and vertical. How do these targets compare to the OEM’s? That’s up to you to determine.
One limitation to this method is how quickly the machine cools down from the time it is shut down to the time the ‘hot’ measurement can be completed. It is recommended that both the movable and stationary pieces of equipment (motor/pump, motor/blower, etc.) be protected from heat loss (welding blankets work) just before shut down to limit the heat loss during lock-out and setting up the alignment equipment. Being prepared to move quickly will yield great results. Also, since both readings are taken while the machine is not running, any dynamic effects of load on the piping, ducting, foundation, etc. are not taken into account.
Need an even better solution for measuring thermal targets? Take a look at the OL2R method …