The running alignment position of machines is the aligned position of two or more machines relative to each other at running conditions, which differs from the cold or shutdown aligned position. Often, machines are misaligned in a shutdown condition in hopes that the alignment during operation (or under “running conditions”) is within the acceptable tolerances for that machine. While this is often referred to as “hot alignment,” there are others who define hot alignment as mounting alignment equipment and capturing data immediately after machines are taken off line. Although this method is probably better than nothing if certain rules are followed, it is not very accurate and is about the same as guessing where the alignment of the machines should be.
There are several methods that provide alignment criteria for machines to be misaligned in the cold condition and achieve alignment during operation. These methods are listed below in order of least to most accurate:
• Guess where the machines should be aligned
• Shut down and perform the alignment
• Use the manufacturer’s recommendations
• Monitor the machines from one condition to the other
The manufacturer of a boiler feedwater pump and the manufacturer of a drive turbine tend to give information based on their respective machines. This information is based on either monitored or calculated data, and this data is typical for “thermal growth” considerations of the respective machines. Most often, you will get numbers from the manufacturers that represent some vertical change in the machines relative to ground or to another machine. Some horizontal change might also be provided.
More often than not, however, the horizontal misalignment targets are far from being what the manufacturer of the machines provides or what can be calculated at the power station.
Monitoring alignment changes allows you to determine targets in the horizontal direction. Horizontal types of misalignment are generally due to piping forces (either static or dynamic) that prevent the pump from being aligned where desired or move the pump after startup.
Large pumps present problems in this area. Many large pumps have keys or a combination of pin and key along the bottom of the pump casing. Some pumps even have keys radially projecting
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When targets reveal that a pump must have a given amount of horizontal angularity or horizontal offset, provisions must be made to modify these keys from their as-built configuration (See Section 4).
Figure 5-1 is a graphical representation supplied by one pump manufacturer of the calculated thermal growth of a motor-driven pump with a gear box.
Figure 5-1
Alignment of Shaft Centerline Heights
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Figure 5-2 Expansion Chart
Typically, the horizontal changes supplied by the manufacturer are not close to what is encountered in the field when the machines in question are acting together with the entire system. Although the vertical changes may occasionally be within the range provided by the manufacturer, the horizontal changes seldom are.
The terms “hot alignment” and “thermal growth” do not disclose the complete story of running position alignment. The most accurate terminology is “transient alignment monitoring” because it best describes what running position alignment is about. You must monitor the alignment changes under all conditions to establish an understanding of the behavior of the machines. Capturing alignment changes within all operating parameters gives you an opportunity to explore these changes.
Machines can be monitored starting with the machine cold (at shutdown) and monitoring the changes as the machine reaches its operating condition. Monitoring can also be performed starting with the machine at operating conditions and monitoring to shutdown. Monitoring can also take place anywhere in between, if certain parameters are to be observed without
determining the full amount of changes of the alignment.
The preferred method is to monitor the machine from operating conditions to shutdown. This enables you to analyze and implement the alignment data during shutdown and observe the
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There are two concepts involved in monitoring machine movement and arriving at the alignment targets. For the purpose of clarity and convenience, these methods can be referred to as absolute and relative monitoring.
Absolute monitoring involves the technique of measuring one machine from a fixed reference point from the ground, such as a column, wall, or floor. The types of monitoring equipment that do this include precision sight levels and jig transits, Acculign bars, and Jackson cold water stands.
The relative alignment methods monitor the changes in alignment between two or more machines. The equipment used for relative monitoring includes Dynalign or Dodd Bars, Permalign lasers, and rotating Vernier gages. When absolute measurements are compared between two or more machines, these measurements can also be referred to as relative. See Figures 5-3 through 5-8 for pictures of various types of monitoring equipment.
Figure 5-3
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Figure 5-4
Laser Monitoring Movement Between Steam Generator Feed Pump and Turbine
Figure 5-5
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Figure 5-6
Precision Sight Level Used for Optical Alignment Checks Source: Brunson Instrument Company
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Figure 5-8
Scales Used With Jig Transits and Precision Levels Source: Brunson Instrument Company
Infrared Thermography
Infrared thermography can play a very important part in analyzing misalignment. Thermography can detect problems through temperature rises in the couplings or bearings. It cannot distinguish the amount of misalignment, only the results of misalignment. In many cases, this is just as important as measuring the amount of misalignment.
Used together, data from both infrared thermography and transient alignment monitoring systems can be very informative to the technician, as well as to management personnel who may need to see more evidence of problems in order to allocate resources necessary to resolve the problems. Past studies involving alignment analysis have determined that some previously held beliefs about misalignment are not necessarily true, including the following:
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Both of these have been used in the past as major selling points for alignment hardware companies.
Alignment and Preloads
A preload is a directional load or force applied to the rotating shaft. Two categories of preloads are internal and external. Internal preloads deal with forces generated from within the machine and go far beyond the scope of this guide.
Only the external type of preload exists for shaft misalignment. There are other types of external preloads that interact with or impact the structure or casing of the machine, including piping loads (forces and moments) and soft foot. The immediate effect of a preload due to misalignment is to force the shaft into one sector of a bearing. A strong indication of preloads, both magnitude and direction, can be determined with the use of proximity probes (x and y) close to the bearing. These preload data are in the form of shaft orbits.
The use of bearing metal thermocouples in conjunction with shaft orbits and infrared
thermography can yield excellent results in determining if misalignment exists. This is easily detected and brought to light in a machine, such as turbine generator train, or a high-energy pump, such as a feedwater pump. The vibration might be low on one bearing accompanied by a high temperature, while the adjacent bearing will have a higher vibration and lower bearing temperature.
The amount of preload can be related directly to the amount of misalignment. Spring-type couplings, such as a diaphragm coupling, exhibit the least amount of preloads on a bearing and its supporting structures, while a rigid-type coupling will impose the most preload.
In Figure 5-9, a circle or ellipse, as shown in the first two orbits, is the norm when no
unidirectional loading or preloads are present. As you move across the page, greater preloads are encountered. The last orbit is where the shaft is located in the bottom of the bearing due to a large amount of misalignment, and the results can show up as twice the shaft speed. An elevation in bearing temperatures can also accompany this scenario. Remember, there are other things that can cause increased vibration. Misalignment occurs perpendicular to the shaft orbit and forces the orbit to flatten; thus, the sensors perceive this as twice the running speed.
Figure 5-9
Shaft Orbits Acquired From Eddy Current Probes on a Sleeve Bearing Machine
A phase difference of 90 degrees between x and y probes should be theoretically true; however, two probes can show a phase difference of 180 degrees. A steady-state preload will cause the shaft to move eccentric to a position within the bearing. This type of orbit is seen most often in
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lower temperature in one bearing and creating an opportunity for an unstable shaft, which may result in oil whirl.
These external preloads can also be due to the misalignment itself. Piping strain and soft foot pose another problem with casing deformation. You have a choice of where you want these forces to enter the pump. They can enter through the piping or through the keys and supporting structures of the pump casing. These forces can also be transmitted into the structure supporting the pump, such as the base and grout of the machine (see Figure 4-4).
Smaller machines with anti-friction bearings pose special problems with preloads. Detection might need to be performed with infrared thermography, as well as vibration analysis, to detect preloads that have an effect on alignment and reliability of the machines. Machines that use a gearbox for speed changes will have preloads associated internally with the machine, which act upon alignment while the machine is in operation.
While piping strain impacts alignment, it also impacts the wear of parts. Piping strain as it relates to misalignment is often overlooked. This is particularly true in the horizontal direction. What appears as a minor amount of piping growth to the piping designer can be a major amount to rotating machinery personnel. Piping growth due to heat can have a severe impact on the misalignment of machines. Some of this misalignment can be accounted for with transient alignment monitoring and corrections.
Cold piping strain in the horizontal direction must be accounted for and remedied as stated in the Induced Loads section. Radial and axial pump keys under the casing do not eliminate these loads; they just enter the casing from another location. The forces against the keys constrain the pump, but they add to the loads on the casing just the same. These keys may require modification from the original installed position.