Successful field engineering requires many of the same tools and techniques as traditional development engineering. For the purposes of this article, “field engineering” can be thought of as the application of engineering to solve a problem with a machine or system after it has been designed and installed.
A logical, detailed analysis approach must be used in conjunction with the appropriate engineering analysis hardware and software. Often, field engineering efforts degenerate into a trial-and-error approach, which does not achieve an optimal, cost-effective solution. Pressure from management to get a system online and running or to ensure adherence to a project schedule often force temporary fixes or poorly developed solutions, which ultimately detracts from the long-term reliability and increases life-cycle costs of the equipment.
If a field engineering effort is approached with the same methodology as a new project or design problem, the outcome will be a superior solution. This is true regardless of whether the problem lies within hydraulic system or is a mechanical equipment issue.
To illustrate this concept, this article will start with a basic “engineering analysis” procedure:
Figure 1. Original base with first “field fix”
- Clear problem statement
- Information gathering
- Defining assumptions and limitations
- Selecting appropriate governing equations
- Calculations (or simulation) and solution generation
- Alternative selection and implementation
The list shows a clear progression from the problem to the solution and the steps required to accomplish this.
Consider the typical trial-and-error approach:
- Vague problem statement
- Trial solution
- Feedback (Did it work?)
- Another trial solution
Figure 2. Drawing of new motor saddle
This cycle generally continues until either a “good enough” solution is implemented or the project over-runs its cost thresholds and higher level corrective actions are initiated. The time, effort and cost required to solve a field problem is usually much less if the engineered approach is used as opposed to the trial-and-error approach.
This occurs because most trial solutions are not based on a thorough engineering analysis, but instead are implemented based on past experiences with similar equipment or because fixes are designed without complete information. Statements such as, “We reduced the vibration on another pump like this one a few years ago by changing the coupling to a different style, so we decided to do the same to this one,” are common.
While well-intended, the chances of successfully mitigating the problem were extremely low because a rigorous engineering approach was not used to solve the problem. The first step, which was a clear definition of the problem to be solved, was missed.
Figure 3. New motor saddle installed
A Case Study
The following case history is offered to illustrate the concept. A new manufacturing plant was being constructed near Kansas City in early 2012 and the plant utilities building was close to completion. However, a problem was noted with the newly-installed chilled water circulator pumps. The pumps were between bearings, double suction centrifugal, 1,800 RPM, size 8x10-17 operating on VFDs. The reported problem was elevated turning speed vibration at higher operating speeds. Amplitudes of greater than 0.40 IPS-RMS were reported. This was unacceptable and the contractor was facing penalties on the project unless the issue was corrected in a short time frame.
The first field engineering effort proved unsuccessful in reducing the vibration and took approximately two weeks to work through at a cost of several thousand dollars.
The problem was investigated, but not enough to facilitate the engineering solution. In this case, the problem was identified as a resonant frequency using standard spectral vibration analysis, but no further investigation was performed to clearly define exactly which component(s) were involved, the mode shapes involved or the operating deflection shape of the equipment. The result was a vague definition of the problem.
The next steps were contacting the original equipment manufacturer (OEM) for support. This was a positive step, but without complete information and a clear definition of the problem, the suggested field fix from the OEM was ineffective. It involved welding gussets to several areas of the base (see Figure 1).
Figure 4. Experimentally measured first mode at 25 hertz
Later, when questioned about the field fix, the OEM stated that it had been their experience that this specific baseplate sometimes requires increased stiffening measures to minimize vibration levels when “larger” NEMA frame motors are used.
The additional bracing did not correct the vibration levels. Data that was provided after the first trial fix indicated that a resonance condition was still present within the operating speed range of the pump. Further corrective measures were examined, and it was decided to fabricate new motor supports from a thicker plate to further increase the rigidity of the motor base. Again, this was based on past experience in which thicker supports did not suffer this same problem.