Reuben Hale, P.E.
Ph: (510) 507-1300
As vibration consultants, it is always enjoyable to work on the structural dynamics of large structures. The logistics are sometimes challenging, especially when we work during a short planned, or unplanned, outage. Our decades of experience planning and executing testing to measure key dynamics and extract meaningful data is one of our assets.
We also have worked on steam turbine issues in large manufacturing plants debugging structural issues. We work on the support of steam boilers of various designs and have noted that where old-school design meets modern building techniques, structural dynamics issues occur. This may be due to the fact that over the years spans have increased, lowering stiffness, as well as lowering structural damping. This may be due to the quest to be efficient with steel that modern computer modeling has made possible. An honest and thorough analysis of dynamic forces anticipated for the modern structure is what was often lacking.
We work in the solar industry on the structural dynamics of solar arrays of various kinds, performing dynamics testing, monitoring structural response and wind conditions, modeling the dynamic response and structural modifications, and analysing the wind excitation of the structural dynamics. Our analyses have provided important design guidance. We also work on the structural dynamics and mechanical issues of wind turbines and smaller engine/generator sets as well, to help debug unique mechanical issues.
|Oil Pump and Filter Analysis on a Windmill||Dynamic Testing of a Solar Array|
Our work in traditional power generation plants includes many projects associated with support structure dynamics and the excessive vibration of rotating equipment. Elevated rotating equipment is, of course, especially challenging because of the greater flexibility of the base structure with height. Often, however, the way the equipment is installed and tied into the existing larger framing is found to be the key issue and not the stiffness of the main structural members of the building. This is always good news as the structural modifications are often minor and involve subtle details of the attachment.
Less often, however, when our analysis has shown that the main floor beams and columns are not sufficient for the dynamic forces of the equipment installed, we have created custom solutions that involve, structural modification, customized and novel vibration isolation, tuned reaction mass treatments, or custom tuned mass dampers (See our discussion on Damping Treatments at the bottom of the page regarding a large mining building).
Our analysis and testing most often includes Dynamic Testing, often with a partial, or full Modal Analysis, and Operating Deflection Shape Testing (ODS) to visualize, characterize, and quantify the key structural issues so that engineered solution options can be evaluated. Our strain gage testing is often used to quantify the loads passed though key structural members. This often involves crawling around on beams in dark corners of a noisy plant to install strain gages in hard to reach places. Or climbing up 200ft of ladders to test an oil filter mount in a Wind Turbine. Often, the ability to positively identify a force transmission path makes this effort worthwhile.
In some cases, where a significant structural modification is necessary, we create a Finite Element Analysis (FEA) that is then tuned to match the key dynamic characteristics of the structure (such as dynamics stiffness, resonant frequencies, key mode shapes, and existing structural damping). Various structural modifications and/or custom damping treatments can then be evaluated and a quantifiable cost benefit analysis can be done by the engineering team.
|Our FEA model of a 6 story building supporting a large vibration source was guided and tuned using our experimental testing, modal analysis, and dynamic analysis, a custom tuned reaction mass treatment was designed to counter the inertial forces of the shaker screen.|
Problem Solving - When a very complex structure is thought to be the cause of vibration to an adjacent structure, there are often many interconnecting elements/systems that are suspect. To further complicate the problem, there are also often multiple suspected sources of the disturbance. We will often start by forming a number of likely hypothesis based on our initial analysis of the system and discussions with the client. We will then suggest a test plan to prove or disprove these hypothesise by making measurements of the system response to ambient, and known stimulus.
Strain, or stretch, in a structural element is one way to show that force is being transmitted along a particular piece of the structure. We can then ask the questions:
- Does the force transmitted through a structural member have the expected magnitude explain the resulting response on the receiving structure?
- Does the frequency content of the force match the disturbance frequency on the receiving structure?
These are some of the questions we can answer using Strain Gage Testing to prove of disprove a hypothesis. This often involves crawling around on beams in dark corners of a noisy plant to install strain gages in hard to reach places. Or climbing up 200ft of ladders to test an oil filter mount in a Wind Turbine. Often, the ability to positively identify a force transmission path makes this effort worthwhile.
Excessive Vibration Issue of a 120 Mega-Watt Power Plant, Surprisingly Simple Solution
We were called in to help identify the cause of vibration to the control room of a newly constructed 120 Mega-Watt power plant. The vibration in the control room was so bad that the control personnel could not read the monitors. There was also a stairway that was unusable due to high vibration levels, and a few stalls in the mens bathroom that were not usable as well due to the uncomfortable vibration levels. We designed a quick bracing fix for the monitors to make the situation bearable and went to work to characterize the problem.
We started with an Operating Deflection Shape Analysis that led to two surprising findings: First, the turbine pedestal was moving primarily axially and appeared rigid with the control room building, second, the vibration was not at the spinning speed of the turbine but at a lower frequency that appeared to be related to the power load.
We then looked at the structural connections of the many large pipes that connect the adjacent building to the turbine. It was thought that one, or more, of these large pipes may be transmitting the forces to the adjacent building caused by a flow induced instability in the supply steam. Additionally, based on our acceleration measurements on the receiving control room floor, we calculated that it would take about 50,000 lbs to excite the control room structure to the levels of measured vibration. Thus, we had both a disturbance frequency to look for, as well as a total force level to search for in these various connections.
|Vibration Analysis and Testing of 120 MegaWatt Steam Turbine, Pedestal and Building Structure>|
We installed strain gages to estimate the forces passing though each pipe support that was suspected to be part of the load path. We were able to show, to our suprize, that there were no large forces transmitted though the pipes that, combined, came close to the needed 50,000 lbs of alternating force to drive the surounding building.
This left us to re-think the turbine and building structure as a whole. While standing on-site looking at the structure, a final hypothesis came to mind. There were seized up expansion plates around most of the gap between the building and the turbine pedestal. Some hand calculations suggested that these expansion plates may be seized up due a difference in thermal expansion between the slow thermal time constant of the massive concrete turbine pedestal and the relatively low thermal mass of the steel building structure. The building had been constructed during the winter and a calculation of the temperature of the concrete turbine pedestal suggested it was still far thermal equilibrium. Although the expansion plates were thin, the total perimeter of plate was substantial. A calculation of the total maximum frictional force of the expansion plates showed that the expansion plates were a possible force transmission path. Rather than instrumenting these plates it was easy enough to jack up the expansion plates to remove the force path. The vibration problem was solved. Thus, we used the stain gage testing narrow down the list of suspects using quantifable engineering methods and reveal the offending structural load path.
We succeeded where many power generation specialists had failed due to our decades of hard earned experience, a solid grounding in basic physics, and structural dynamics expertise.
- Heavy Industry Consulting Topics Include:
Overview, Manufacturing & Plant Process, Rotating Equipment, Vibration Induced Fatigue, Power Generation