Maintenance Quarterly: Do You Really Know Where Your Machines Are?
Kathy | December 1, 2007
Becoming a “Reliable Plant” and staying there requires keeping abreast of constantly changing and improving technologies and practices.
In today’s leaner maintenance departments, companies rely heavily on the reliability of their machinery. While the practice of reliability engineering has been around for many years, it has never been focused on as much as it is now. In today’s maintenance world, reliability engineering positions—not to mention entire departments—have been created to put 100% of their time and effort toward the prevention of unscheduled machinery downtime and critical failures.
Even though the goal of a “Reliable Plant” remains much the same as it has for years, methods and practices for getting to that state are constantly changing and improving with the development of new technologies and practices. A case in point is proper shaft alignment of rotating machinery in the running condition, through the derivation and application of proper coupling target values.
With today’s laser alignment tools and proper training, alignment of machinery has become an easier task than in years past. However, in some cases, companies are finding that even while machines are within excellent alignment tolerances, they still have problems associated with misalignment. This often is a result of thermal growth issues with the machine, dynamic loads, downstream (or upstream) piping movement and other variables.
Many manufacturers supply their equipment with thermal expansion data and recommended alignment targets. The idea is to purposely misalign a machine when the alignment is done “cold,” or offline, so that when the machine reaches its normal running condition the machine is aligned. Compensating with target values is one step closer to proper alignment, but often these values are not as accurate as they were originally intended to be, due to flaws in the methods of their calculation.
Two identical steam turbine-hot water pump machine trains are sold and supplied with factory-calculated target values. It is late October. One unit is installed in a Louisiana refinery at 90 F, the other in an identical plant in Washington State at 40 F. Both operate at the same temperature, but which machine will be in alignment when it reaches its normal running condition? Consider that the factory calculated the target values using an arbitrary cold temperature of 70 F. Because of the temperature differences, it is possible that both units may be out of alignment at running condition using the factory supplied alignment targets.
Using the “TLC” thermal growth calculation method we can see how much the growth can differ depending on what the ambient temperature is when the alignment is performed. The TLC method is the product of the change in Temperature, the Length of material from base of machine to the centerline of rotation and the Coefficient of expansion for the material involved. Each support foot of each machine needs to be calculated. The calculations for one of the feet at each location are shown in Fig. 1.
These variations at the feet could mean an even greater misalignment at the coupling center, or point of power transmission. The graph in Fig. 2 is based on the thermal growth values shown in Fig 1. It illustrates how these growth values could result in even greater misalignment at the coupling center.
Dealing with “problem” machines
Many companies seem to have some “problem” machines that they too often accept as being uncorrectable. Extra spare parts become part of the yearly budget and it’s no surprise to anyone when those particular machines break a bearing or lose a seal every few months—while similar machines run without a problem for years.
This type of situation became clear for a South California refinery several years ago. As part of its growing reliability program, the refinery decided to do something about the site’s “problem” machines, as well as those machines without accurate target values. The company utilizes the best laser alignment tools and trains its employees to do correct alignment incorporating target values wherever necessary. Even with these good practices in place, however, some of the machines still have high-failure rates.
Whenever refinery personnel identify a machine that is still having problems with failures associated with misalignment, they install a system called PERMALIGN® to accurately measure any relative movement between the machines from cold to hot or normal running condition. This laser-based system measures and records any movement, whether across a coupling or an absolute movement relative to Earth, and is accurate to 1 micron. (It is the only linearized laser monitoring system with a resolution of 1 micron throughout the entire 0.630″ detector range.) The system measures any offset and angular movement over separations of up to 30′, so it can also record data on the site’s large cooling tower fans. Even in the harsh environment that the refinery offers, temperature variations and vibration do not diminish accuracy.
The data collected by the PERMALIGN system can be trended, analyzed and archived using software called WINPERMA®. This software uses the data to translate the relative machine movement into movement at the coupling center in both axes; Vertical Offset, Vertical Angularity, Horizontal Offset and Horizontal Angularity are calculated. A baseline established at the ambient temperature becomes the zero point, then the machines are turned on and allowed to reach their normal running condition. The graph in Fig. 3 shows all four axes of movement so the new alignment targets are easily read. Flags can be marked on the graph to record system events such as when the system was brought on-line, to mark different running loads, a valve opening or any other system event. Let’s look at a recent example of a “problem” machine where the California refinery utilized the PERMALIGN system to measure the movement across the coupling.
In one of its distillation units, the refinery has a set of residuum pumps that are vital to the continuous operation of the unit. If the pumps were to shut down unexpectedly, the whole process would follow suit—leading to a major shutdown, resulting in significantly higher repair cost than just replacing a bearing on a pump. Since these pumps are redundant, if one fails the other picks up the load. On the other hand, when one “problem” pump is out of commission for repair, there is no backup. Of the two pumps, only one of them has a very high failure rate. They are identical pumps and the reason(s) why one of them has a high failure rate and the other does not remains a mystery. They both are aligned using the factory recommended targets, yet only one pump continues to have bearing failures. Vibration readings also are significantly higher on the one pump compared to the other, and vibration analysis points to misalignment. While there are myriad possible causes for this problem, correcting it is the priority. Thus, the PERMALIGN system was installed on the unit to measure the relative movement of the pump and motor.
Once the system was installed on the unit and started recording data, a baseline was established. Since these pumps operate at a very high temperature, they are slowly brought up to operating temperature, as marked on the graph with an event flag. A second flag was placed to note when the pump was brought on line. As the pump reaches its normal operating condition and the data levels out—in this case about eight hours—it can be shut down and allowed to cool.
The data shown in the box near the center of the graph in Fig. 3 are the new target values used for the alignment. These targets were input into the refinery’s ROTALIGN® ULTRA shaft alignment system and the alignment was performed once the unit cooled to ambient temperature. The unit was then put back on line.
A four-month trend of the overall velocity levels measured on the pump using the VIBXPERT® vibration data collector is shown in Fig. 4. The final reading on the trend was taken several days after the alignment was performed using the new target values.
After further investigation into the root cause of the problem pump, it was found that the concrete base had been cracked during a repair on an adjacent machine several years earlier. After the base was repaired, the “cold” position had apparently moved from its original setting, causing the targets to change. This cause was luckily found by a senior millwright reporting the repair after overhearing a conversation concerning the investigation. There was no documentation of the accidental damage or of the repair, so this information may never have been known if not for the millwright coming forward.
Utilizing the latest technologies, the refinery was able to identify a piece of critical machinery that had uncommon characteristics and quickly apply an accurate solution. A complete maintenance history of the machines is now stored in the site’s alignment and condition monitoring software. Proper use of these tools has put this refinery one step closer to what it truly wants to be—a Reliable Plant!
Deron Jozokos is an engineer with LUDECA, INC. Telephone: (305) 591-8935; e-mail: Deron.Jozokos@ludeca.com