How to Determine a Vertical Pump’s Natural Frequency

August 29, 2017

There are two commonly used testing methods to determine a vertical pump’s natural frequency. The first method is called a startup or coast down. In order to perform this method a tach signal is required for the speed to be tracked.  The pump is started and the amplitude and phase are recorded during start-up and coast down, however when a pump is started across the line (connected directly to a power source without a drive or soft-start circuit) it is very difficult to use this method.  The problem is that when a pump is started across the line it goes from zero rpm to full speed so quickly that there is not enough time to obtain valid data.   The coast down method is not normally successful in these cases. When the stop is initiated the pump comes to a complete stop in a very short period of time as the liquid inside the pump column falls back to the wet well acting as a brake. However start-up and coast down testing can be performed successfully if a pump is being operated using a VFD (Variable Frequency Drive) as the rate of speed can be controlled.

The other method of determining structural natural frequencies on a vertical pump is to conduct an impact test. This test is more commonly known as a bump test.  This test requires that the pump be stopped and impacted using a block of wood or a large hammer that has a soft tip (modal hammer).  The bump test provides a response curve that will identify the natural frequency and/or frequencies of the pump.  It is recommended that the testing be performed in two separate directions.  One direction would be in-line with the pumps discharge pipe and the other direction should be 90 degrees from the discharge pipe.  The two different directions will usually result in two different natural frequencies as the pumps discharge pipe tends to stiffen the structure. This vibration data can be shown as a higher natural frequency from that direction.  The other direction which is 90 degrees from the pump discharge will usually have a lower natural frequency. This is due to the fact that the pump manufacturers typically cutout part of the structure. This allows access to the coupling or seal which also dampens the structure in that direction.

Both of the mentioned methods can assist with discovering the natural frequencies of a pump. Once the frequencies have been identified on the pump; the proper corrections can be made to make certain that the pump is not operating on a resonance frequency.

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Precision Maintenance: Belts, Chains, and Sprockets

August 22, 2017

Guest post by Brandon Weil, CMRP at Eruditio LLC

Belts, chains, and sprockets, chances are you have at least one if not all of these in your facility, and chances are you’re relying heavily on experience and judgment instead of quantitative inspection criteria. All too often the importance of proper inspection techniques and defined replacement criteria for these critical parts are overlooked. Don’t believe me? Just pull up some of your PM inspection procedure, discuss the topic at a tool box meeting, or observe someone performing the inspection, you might be surprised at the range of answers and opinions. If there isn’t a specific measurement or min/max criteria, then you’re leaving the inspection up to chance. Another thing to consider is if these parts aren’t being installed properly in the first place you will undoubtedly see premature failures and reduced operational life. Precision maintenance installation tools such as laser alignment for shafts, pulleys, and chains make a world of difference in preventing the introduction of infant mortality related failures like premature bearing failures, belt and pulley wear, etc.

The good news is that you can start improving the quality of your preventative maintenance inspections almost immediately; all you need are a few basic low-cost tools [Click Here] and you will find a document with inspection criteria for these three parts to get you started. Improving your PM inspection procedure, putting the right tools in the right hands, and setting quantitative standards for your inspection is a very low-cost high-return activity that can start paying dividends today.

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A Better Black Liquor Process – Vacuum Leak Inspection

August 15, 2017

Reposted from ReliabilityWeb

One method of detecting vacuum leaks is to use airborne ultrasound detection, a technology already widely used for positive leak detection in compressed air systems. But finding vacuum leaks is not as straightforward as finding pressure leaks, and often times, the method is abandoned in frustration.

One problem here is the quality of the ultrasonic instrument which can vary significantly from one manufacturer to another. Lesser quality detectors cannot function well in high noise situations.  They simply have difficulty differentiating a leak sound from ambient plant noise. Since vacuum pumps already generate a lot of background noise, rarely will an inspector perform vacuum leak inspections in a quiet atmosphere.  Another problem is lack of inspector training which really plays a role when searching for vacuum leaks in high noise environments.

Just like positive pressure leaks, vacuum leaks produce a rushing, whooshing ultrasonic signal with peaks around 35-40 kHz. The ultrasound is caused by turbulent flow of air molecules at the leak site. Positive pressure leaks, such as those found in compressed air systems, push the turbulent flow outward making them easily detectable from several feet with a quality ultrasound tool. Vacuum leaks behave quite the opposite, drawing the turbulent flow inward, decreasing the distance of detection as compared with positive pressure leaks. Most of the telltale leak sound is contained within the body which means inspectors must diligently trace an entire installation leaving no stone unturned in the search for ingress.

Read the full story by Allan Rienstra – SDT International and Karl Hoffower – Failure Prevention Associates including details and photos for a Vacuum Leak Inspection on Multiple Effect Evaporator at major Pacific Northwest Pulp & Paper Mill.

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Reliability Project – Don’t Stop the Beatings!

August 8, 2017

The word “thrashing” can mean many things.  Words like flogging, whipping, beating, head banging and many more are always included in the definition.  Do you constantly feel these effects when trying to manage or participate in a reliability project?  As a result, does the project become overwhelming, or lack support, or have steps and goals that keep changing and a desired outcome that is never reached?

One of the most overlooked ingredients for a successful project is overcoming resistance to change by key individuals.  These individuals may resist because they do not agree with the project steps, the outcome, or simply believe they will not benefit.  You must look at things through their eyes!  You cannot wait until the project is near completion to let these individuals “thrash” it and for you to “see” it from their perspective.  Schedule time and welcome thrashing at the beginning of the project.  As a result, you will be able to better convey project value, identify crucial things that should be included in the project design and then focus on the target completion date (ship date) and returning value back to your company.

The unfortunate fact is that we cannot stop our reliability projects from being flogged, beat up and whipped.  Take advantage of this reality and understand key aspects from everyone’s viewpoint early in the project.  Otherwise, you run the risk of having your project delayed and yourself beaten to death at the end.

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Monitoring Plain Bearings With Ultrasound

August 1, 2017

For rotating machines, it is necessary to reduce friction most of the time to increase efficiency, decrease power losses and support loads. The element of choice is the well known team of bearing and lubricant. Bearings, in their different configurations, are one of the most efficient ways to reduce friction between a stationary and a rotational part of a mechanism.

Two broad classes of bearings exist: plain bearings and rolling contact bearings. Which type of bearing is used depends on several factors related to the design of the machine and its process. Sometimes both types are used in the same machine doing different jobs. For this article, the focus is on plain bearings.

Choosing the best technology to monitor friction and condition in plain bearings is a challenge. Due to the physical characteristics of plain bearings, using vibration analysis (VA) is more effective for rolling contact bearings and less so for plain bearings. Ultrasound (US) is trending more frequently for condition monitoring of rolling contact bearings and it also shows promise for plain bearings. Understanding the physical differences between the two bearing categories is critical for developing condition monitoring strategies for plain bearings using ultrasound.

Read on to find out more about plain bearing types, failure modes and how to monitor.

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