In part one we discussed the different pitfalls that can come up when performing a vibration analysis and also what may or may not make for a competent analyst. In this article I am referencing an actual case where different service providers came to distinctly different conclusions based on same data.
What are we dealing with? The unit is an engine driven, 4 throw – 3 stage recip compressor mounted on piles; very typical installation. Enhanced Maintenance Solutions Inc. (EMSi) was called to site with concerns about high vibration levels on the unit. Excessive grating vibration and cylinder movement were noted by the operations staff and generally high vibration across the skid. The unit typically runs at 1200 Rpm but EMSi would take measurements throughout the speed range of the unit.
What was found?
Excessive vibration was detected on the 2nd stage discharge bottle which was exceeding 2.0 inches per second (ips) peak. The 3rd stage suction bottle was at 1.6 ips peak and the 3rd stage cylinder was approaching 1.0 ips peak. Other measurements taken on the skid showed high vertical vibration on select skid beams that were as high as 1.8 ips peak. All of this vibration was measured at 2x unit run-speed (remember this because it is an important factor in the outcomes of the different analysts). Also noted was high vibration at 1x run-speed on the bypass valve and the flare valve assemblies.
Generally speaking the vibration levels would subside as the unit speed was reduced with the exception of some of the 1x order vibration which would peak near 1150 Rpm and then subside varying amounts depending on what side of that speed range you were on.
In part one of this article, there was reference made to “forcing functions” and understanding where the vibration is coming from. Is it a structural issue? Is the vibration due to excessive forces or inadequate stiffness or both? If you have a beam in place isn’t it safe to assume that the stiffness of that support should be a fixed value? What is already in place for structural support to the components? These are all questions that need to be considered and evaluated.
As an example, on the 2nd stage discharge bottle, there is a common support system via use of wedge supports between the bottle and a support beam below, similar to the illustrations below minus the clamping.
When EMSi first encountered the unit, it was noted that the wedge supports had become dislodged from below the bottle. Surrounding grating vibration was excessive to the point of feeling like there were pins in your feet as you were walking on it. So what happens when we reinstalled the wedge supports? The grating vibration reduced a significant amount and you could now walk on the surface without any real notable issues. Problem solved right? In a word – NO.
By doing this there was actually an increased chance of a catastrophic failure.
We have heard the phase “just chasing the vibration to a different spot”, well in some cases its true and it gets back to understanding the forcing function. By installing the wedge supports back to their intended location, the vibration on the discharge bottle which was moderately high before was now excessive and over 2.0 ips. The wedge supports by themselves were not a successful means of controlling vibration. They removed the symptom of the high grating vibration but in actuality, made the situation worse. High grating vibration by itself, other than the discomfort of walking on it does not represent a high risk for failure of major components.
It is important to note that in both cases, the wedge supports had become dislodged from beneath the bottle when the analysts first encountered the unit. In the original analysis with the other service provider, the recommendation was to re-install the wedge supports with tabs to prevent them from becoming dislodged and the grating vibration would subside. This was not an incorrect statement but would it lead to the desired outcome?
After the EMSi analysts ascertained that the discharge bottle vibration would actually become more severe with additional support, there was more evaluation required. How could vibration actually get higher with increased stiffness? By adding the connection between the bottle and the beam, there was now another dynamic to consider. How does the additional stiffness factor into the forcing function of the vibration? Why do the wedge supports repeatedly become dislodged after installation. Is it a design problem with the supports? Are they double nutted? Based on the review of the installation of the supports and the design, it was determined that they were adequate and installed correctly. So, what causes them to become loose again? The answer is again – force.
Vibration was measured on this beam and the vertical vibration here was found to be extremely high for a structural member. Upon further investigation it was found that there were other locations on the skid where structural beams had high vibration that was contributing the high levels encountered across the unit.
It was determined that the structural members that were in place, were insufficient to dampen the vertical input force that was being effected at 1200 Rpm. Additional vertical stiffness was necessary to reduce vibration on the beams, as well as the bottles, piping, and grating that was being supported by them. A support similar to what is noted below was recommended. There are other ways to improve vertical stiffness on a beam, this is just an example.
Remember back at the beginning where it was discussed that there the predominant frequency that the vibration was happening at was at 2x run-speed? When reviewing the potential forces and what the resultant vibration could be for rotating equipment, one of the most common for 2x is a misalignment between the driver and the driven piece of equipment. Its true that there was abundant data at 2x run-speed that was over guideline. There was however, no issues identified on either the engine or compressor frame with vibration amplitudes at 2x or any other order. This gets back to understanding the forcing function or what could be causing the high amplitudes. You cannot apply rotating equipment vibration analyses techniques and root causes to reciprocating equipment by themselves. While some of the theory is transferrable, it does not consider the complete picture of a reciprocating compressor.
You cannot apply rotating equipment vibration analyses techniques and root causes to reciprocating equipment by themselves. While some of the theory is transferable, it does not consider the complete picture of a reciprocating compressor.
This machine was re-evaluated at the problem locations following the modifications and the resultant values were all found to be within industry guideline at the rated speed. There was additional bracing recommended to deal with some piping issues as well as cylinder vibration that will not be discussed in detail.
This article does not outline the entire analysis, however what it does illustrate is that one size does not fit all where analysis is concerned. After evaluating the same machine, there were entirely different solutions proposed to deal with the vibration. Understanding the behavior and the forces in a machine are critical in determining what steps to take for reducing vibration. This is irrespective of the analysis equipment being employed or represented by the analyst.
One final note regarding why understanding the forces in a compressor and how it relates to vibration is important. It certainly allows an analyst to evaluate how a structure will respond but it also allows him/her to better weigh when its best to try and contain the forces or maybe the better route is to look at steps to reduce the forces. Simply adding more steel is not always the best approach when considering the operating range of the machine and the life cycle of the unit.
Written by Steven Pluister, R.E.T. Enhanced Maintenance Solutions Inc.
Steven is President and Co-Owner of EMS Inc. and has been in the industry for over 20 years providing vibration analysis and condition monitoring services.