DoctorKnow® Application Paper Title: Adventures in Balancing Source/Author:Trish Whaley, Nelson Baxter - Abm Technical Services Product: Corrective Technology: Corrective Classification:
ADVENTURES IN BALANCING BALANCING TECHNIQUES CASE HISTORIES OTHER CONSIDERATIONS BY NELSON L. BAXTER ABM TECHNICAL SERVICES
ABSTRACT: The purposes of this paper are: 1: To introduce the reader to various methods of balancing rotating equipment. 2: To present case histories, which illustrate some of the inherent complicating factors which are experienced in the field? 3: Discuss several situations where the problem might appear as unbalance, but is not. BALANCING METHODS: Prior to getting into case histories, the following balancing methods will be discussed in this paper: 1: SINGLE PLANE VECTOR METHOD 2: COMPUTER SOLUTIONS TO BALANCING 3:STATIC COUPLE APPROACH TO BALANCING SYMMETRIC ROTORS. 4: FOUR RUN NO PHASE BALANCING
FLEXIBLE
SINGLE PLANE VECTOR METHOD The vector method of balancing is the classical approach to this subject. It is very reliable and gives the user an appreciation of how the balancing process works.
The figure to the left shows a vector solution to a balance problem on an ID Fan. The original vector is drawn, a trial weight is added, then the effect of the trial weight is displayed. It can visually be seen that to drive the solution to zero, that the trial weight needs to be rotated counter clockwise approximately 30 degrees and that more weight needs to be added. The vector solution is a very good learning tool in that the real essence of balancing is quickly perceived by the user. This method is also useful to know in that with only a pencil and piece of paper, it can be performed. COMPUTER BASED SOLUTIONS: Computers are readily available in the work place. They are therefore accessible for the solution of balancing problems. The balancing procedure is the same, in that trial weights need to be added to each plane and the effects of the trial weights on each of the response points must be measured and recorded. The computer then solves the balance problem by using the measured effects, which are also called influence coefficients in a matrix solution routine. Advantages of utilizing a computer program to solve balance problems: 1: Computers perform the matrix math accurately. 2: A computer program can store or print out the influence coefficients for use in future balance work.
3: Beyond one or two planes, the graphical method is too complicated, so a computer solution is generally required. Disadvantages of computer based balancing are: 1: It separates the user from the actual balance process. 2: If there are minimal cross effects, some programs blow up and give ridiculous solutions. The following are computer solutions to single and a dual plane balancing problems.
Note that the process of taking initial readings, adding trial weights and then retaking the data to determine the influence coefficients is the same in each case. STATIC COUPLE BALANCING: Static Couple Balancing is a method which can either be performed graphically or on the computer. This procedure is very important when flexible symmetric rotors are being balanced, in that it can reduce the number of shots that need to be installed. It is particularly applicable to balancing large steam turbines. The figure below shows the unbalance vectors from two rotors, which operate above their first critical.
The polar plot on the left shows that the phase angles for the two bearings are nearly in phase with one another. This indicates that equal amounts of weight should be added on each end at exactly the same angular location. By adding a trial weight in each end in this manner, the solution weights can be added in the second run, rather than on the third run, had a standard dual plane approach would have been used. The polar plot to the right shows the unbalance vectors from a rotor operating near its second bending mode. This is typical for a low pressure rotor on a steam turbine. As can be seen, the thing to do is to add equal amounts of weight 180 degrees apart on opposite ends of the rotor. As stated above, once this is performed with a trial weight, the solution weights can be added on the second run. The above method works well with flexible rotors, which operated near their bending modes. It also helps if the rotor and bearing stifnesses are symmetrical. Saving a balance shot on a large turbine can represent tens or hundreds of thousands of dollars, so the above approach is therefore often utilized by the utility industry. FOUR RUN NO PHASE BALANCING: This method is a very old technique, but is being mentioned because sometimes it is the only way out of a bad situation. It is useful when: 1:It is not possible to get a once per revolution tach pulse or view the rotor with a strobe.
2: Your photocell battery goes down or the unit doesn't work for some reason. 3: Your analyzer or balancing instrument fails and all you have is a hand held unit. Some disadvantages are: 1: It requires 4 runs 2: At some point on one of the runs, the trial weight will end up being either right on the heavy spot or close to it. This can result in high levels of vibration and possibly driving the system nonlinear. Obviously, if the user was not in a bind, this approach would not be used, but it is at least a way out, when things get real tough. THE FOLLOWING ARE THE REQUIRED STEPS FOR THE FOUR RUN METHOD
THE FIGURE BELOW SHOWS THE FINAL SOLUTION FOR A 4 RUN NO PHASE
BALANCE EXAMPLE
The above graphical solution shows that there is a net effect of 1 Mil, as measured from the center of the original circle to the intersection point of the three trial weight circles. Since the original amplitude without the trial weight was three mils, this means that the solution weight required would be three times the trial weight. When the effect vector is projected to the edge of the original circle, it intersects the original circle at approximately 10 degrees clockwise from the 240 degree location. That is the location where the solution weight needs to be installed. The final solution is therefore to install 3 times the trial weight at the 250 degree location. BALANCING CASE HISTORIES
THERMAL VECTOR IN MOTOR The figure to the left shows the presence of a thermal vector on a large 4000 BP motor. This problem occurred following an overhaul to the motor. Initially upon startup, the motor would run smoothly. However, when load was applied, the vibration would increase and the bearings would be destroyed. The motor was returned to the shop for balancing and finally was sent back to the manufacturer for high speed balancing. In each case, when the motor was returned and put under load, the vibration would return and the bearings would be destroyed. Proximity probes were installed on the motor and the vibration level was measured throughout the load range, with the results depicted on the polar diagram above. The solution was to install a compromise balance shot to offset the thermal vector. The after balance shot thermal vector is illustrated on the polar plot. The root cause of the problem was found to be shorted laminations, which were the result of the motor repair shop dropping the rotor.
COMPRESSOR WITH UNBALANCE IN COUPLING. Following an overhaul, the vibration on the proximity probes on a 7000 Hp compressor read high on one end. The compressor had been balanced and there was no evidence of abnormal vibration as the unit passed through its critical speed. The vibration simply increased as the speed increased. The phase angle did not shift. As is shown in the lot above, this can be a sign of
unbalance in the coupling. The addition of 23.7 grams in the coupling reduced the level from 3.8 mils to .6 mils. Note !- Balancing in the coupling should not be used if the rotor is flexible and there is significant response as it passes through its critical.
EFFECT OF EXCESS KEY ON MOTOR VIBRATTON LEVEL, A motor on a condensate booster pump at a nuclear power plant had been installed after an overhaul. The level was .14 in/sec of running speed vibration. It was noted that excess key was present. A calculation was made and it was determined that the extra key was twice the ISO limit for residual unbalance. A step key was installed and the level dropped to .07 IPS.
CROSS EFFECT FROM FAN CAUSING HIGH MOTOR VIBRATION A service technician from a motor repair shop had been working for several days trying to balance a motor which was being installed by his company. The figure to the left shows that there were 2.3 mils of vibration present at the running speed of 1800 RPM. The plant demanded that the levels be
less than 1 mil. The multiple balance attempts in the motor were all unsuccessful. An analysis of the situation indicate t at t e fan was the source of the vibration. The fact that the motor balance attempts were unsuccessful, combined with the fan phase angles leading the motor angles led to this conclusion. The addition of balance weight to the fan reduced both the fan and the motor levels. Note !- on large fans where the motors are mounted on a common pedestal, it is not uncommon for the motor vibration to exceed that of the fan even thou h the fan is the source.
A BALANCER'S NIGHTMARE The situation in the figure to the left is truly a balancing person's vision of a nightmare. The machine train is a motor generator set on a dragline. There are six rotors solidly coupled sharing seven bearings, all of which are sitting on a metal deck. In this case to complicate factors even more, a structural resonance was found to be present near the operating speed of the MG set. Balance weight was added to the plane next to the bearing with the highest vibration. Levels went down in the horizontal direction, but up in the vertical direction. The first two generator units were uncoupled from the train in hopes of narrowing down the problem. Even though they were uncoupled, their levels were still very high. The problem was found to be unbalance in the motor combined with a structural resonance.
BALANCING MOTORS ON VERTICAL PUMPS.
Due to their structural characteristics, vibration levels on vertical pumps are often higher than what is considered acceptable on horizontal units. The installation of a balance plate on the top of the motor will often provide a means to lower the vibration levels. Running speed vibration can often be significantly reduced by trim balancing in the balance plate.
MID SPAN BALANCE SHOT REQUIRED TO REDUCE TURBINE VIBRATION The high pressure rotors in large steam turbines often operate above their
first critical speed. In addition, the steam temperature at the center of the rotor is 1000 degrees F. After a number of years, these rotors will become bowed a few thousanths of an inch. The phase angles from end to end are in phase and the problem looks like a standard static unbalance where equal amounts of weight need to be added in each end. However, when the static shot is installed, their is almost no effect. What is required is to install a shot at the middle of the rotor.
STATIC SHOT REQUIRED IN A LARGE GENERATOR ROTOR. Large generator rotors often run near their third critical speed. The location of the unbalance is usually near the retaining rings on the end of the rotors. This can result in the need to add a static pair at the end planes. The example to the left is an actual case of a balance shot installed in a 650 Megawatt generator rotor and the final results.
LOW PRESSURE TURBINE ROTOR REQUIRES COUPLE BALANCE SHOT. If there was ever a case where a rotor would require a couple shot, then low pressure rotors on large steam turbines would be it. Low pressure turbine rotors are symmetrical, they usually operate near their second critical and the last stage blades where erosion occurs are located near the end planes. They are therefore the ideal candidate for the installation of couple shots. The figure to the left shows actual data from balancing an LP rotor on a large turbine generator.
COUPLING SHOT REQUIRED IN TURBINE Sometimes a situation which appears to require a set of static couple shots in two or more rotors can be solved by a single shot in a coupling. At first glance, the left rotor in the figure on the left might appear to need a couple shot because its phase angles are out of phase. In addition, the right rotor might appear to need a static shot. Further analysis would however show that the installation of a single shot in the coupling might solve the problem. This is often the case in large steam turbines. On some units it is is possible to to affect the entire train of up to five rotors by adding a weight in one of the couplings.
COOOLING TOWER BALANCING Balancing of low speed cooling tower fans is another example of where Synchronous Time Averaging can be used. Sometimes several tower fans will be in operation at a time, so synch time averaging can eliminate the cross effects from the other cells.
FOUR RUN NO PHASE BALANCING This type of balancing can be utilized when it is not possible to obtain a phase angle. In the figure to the left, the motor was enclosed so that there
was no part of the shaft visible for a phase reference. The fan was in the roof, so it was also very hard to obtain phase measurements from it. The four run no phase method was therefore used to solve the problem.
The figure on the left shows the circle from the original run, along with the three circles from the three trial runs. The distance from the center of the original circle to the intersection of the three trial circles represents the effect of the trial weight. The ratio of the original amplitude to the effect times the trial weight gives the amount of solution weight. The projection of that effect to the edge of the original circle shows where the final weight should be located.
PROBLEMS WHICH APPEAR AS UNBALANCE BUT ARE NOT There are many things which appear as unbalance; however, attempts to balance out the problems will prove impractical. The following are a series of problems, which fall into this category.
Getting good data for use in the balancing process is often challenging. The most difficult part of the process is often to obtain a reliable once per revolution pulse. The following are some of the potential problems which can occur in regards to getting a good tach pulse.
Note !- On the market are now available phase locked loop strobe lights which fire off of a filtered peak from the accelerometer which can overcome some of the problems associated with normal tach pulse devices.
