Pumps & Systems, July 2008
Routine checkups, a healthy diet, and regular exercise are the prescriptions for a healthy life. The same prescriptions hold true for healthy pumps. Your pumps' "diet and exercise" is a function of how you operate them. If the fluid specific gravity, temperature, pressure or power quality are not within specifications, then their "diet" is not optimum. If they are running off the pump curve, then they are "over-exercised." Just like you, they may experience an injury or failure if they are running misaligned or are overworked.
Think how you would feel if one heel of your running shoes was an inch shorter than the other as you tried to run a 10K. By the end of the race, your body would be aching, perhaps with some serious back issues.
Machinery Health Monitoring
Your pumps benefit from regular checkups to ensure no underlying problems exist. Diagnostics indicate the health of the pump, providing answers to questions like "Is this asset suitable for continued operation?" or "Will this pump be available for service when needed?"
Some plants (and pump manufacturers) still promote rigid schedules for equipment teardowns. However, seasoned professionals contend that regularly scheduled teardowns or machinery rebuilds are not necessarily a prescription for reliability. Some anecdotally cite that approximately 25 to 30 percent of the time after a rebuild there is a new problem. It is not hard to understand that opening, inspecting and rebuilding some machinery can actually introduce a problem.
Scheduled teardowns were mandatory in the past because there were no non-invasive tools to determine the condition of internal components while the pump was in operation. Through the years, all sensor technology has advanced. Today there are infrared imagers, on-line lube analysis, pressure transducers and flowmeters in the growing arsenal of diagnostic instruments. One of the most reliable indicators of mechanical machinery health is vibration. Thanks to advancements in diagnostic techniques, vibration analysis has become a widely accepted tool for condition monitoring.
Here are a few condition monitoring methods that use vibration analysis.
24/7 Continuous Spectral Monitoring of Pump Vitals
This offers the best defense against unscheduled downtime. It is probably a safe bet that a few select pieces of equipment in your plant, including some pumps, are considered critical assets. It is also highly likely that these lucky few pieces of equipment have a protection system that includes spectral-based continuous monitoring with shutdown capability if something extraordinary is detected. These pumps and other similarly equipped pieces of machinery benefit from this advanced spectral monitoring. As a potential fault develops, these systems have the ability to notify plant personnel well in advance of failure. However, this costly health monitoring program is usually reserved for critical equipment deemed worthy of constant monitoring.
Walk-Around ("Route-based") Vibration Programs
These programs have gained widespread acceptance as a way to trend pump health. Over time, the data collected from a route-based program can be beneficial in early detection of adverse operating conditions like imbalance or misalignment, as well as specific machinery defects, like bearing damage or gear problems.
The downside to this approach is the possibility of a machine failing between machine health checkups. Add the cost of completing a full physical checkup on a month-to-month basis and you can see the rapidly expanding health monitoring cost. In this case, instead of focusing on what is good for the "patient," a decision to include or exclude a pump is made based on the cost of adding another machine to the monitoring program.
With both spectral-based and route-based monitoring, vibration analysis requires a skilled operator. Vibration experts are educated about the complex analytical monitoring equipment and undergo significant education and ongoing training so that their analysis is trustworthy.
An Economical Continuous Monitoring Option
Vibration analysis, like an advanced medical test, comes at a price. Just like using your insurance, you have to weigh the benefits against the costs. Not every muscle ache dictates a full-blown MRI because you would quickly go broke from the co-pays alone! Likewise, not all of your machinery needs a full blown-vibration analysis every month. The company could not justify the expense.
A practical means of watching your pumps' health continuously is to utilize 4-20 mA loop powered vibration sensors. When connected to a monitoring system, such as a plant PLC, DCS or SCADA system, they continuously observe the health of your pumps and other plant equipment. When the overall vibration level (a number between 4 and 20) is observed with the rest of the pump performance indicators, the output of loop powered vibration sensors will indicate when a change in the operating condition has occurred.
A change in the pump's vibration level (like a "heart rate" for the pump) indicates something in the machinery has changed. While the exact cause of the problem cannot be determined immediately, you have been alerted to a problem. Given this knowledge, it is now advisable to closely investigate the pump to find the problem. A maintenance professional can sometimes locate the cause of the change by inspecting the pump. One might find a minor problem from day-to-day operations, or the pump may require immediate repair.
Loop powered sensors for many types of physical parameters have been around for a long time. Loop powered sensors for vibration are relatively new but are gaining widespread acceptance because they provide simple data at a fraction of the cost.
While the early vibration sensors outputted a 4-20 mA signal proportional to peak velocity, a steady stream of improvements have led to vibration sensors that output a signal proportional to root-mean-square (rms) velocity, true peak velocity, rms acceleration, peak acceleration and true peak acceleration. The latest addition is a loop powered sensor that outputs a 4-20 mA signal proportional to the peak-to-peak displacement of the monitored machine, which is equivalent to monitoring your blood pressure in millimeters of mercury or pounds per square inch. The units may be different, but the interpretation is the same. A higher or lower trend indicates further investigation is needed.
When you implement vibration monitoring with loop powered sensors, the plant receives vital, previously overlooked information. This additional knowledge provides 24/7 access to instantaneous operating information that is easy to understand, trend and alarm. Expanding your knowledge base to include pump health determined by vibration levels brings you closer to a more comprehensive health assessment and a healthy "lifestyle" for all plant equipment.
You already check and chart amperage, discharge pressure, suction pressure or even shaft rpm of your pumps. At a minimum, operating and maintenance personnel know the normal ranges and instinctively look for variations outside these ranges. Vibration is also a key indicator which can now be easily monitored in the same manner-even if you are not a vibration expert. By using cost-effective loop powered vibration sensors, a plant's existing process control system can continuously checkup on machinery and alert maintenance and reliability professionals about a pump operating outside of normal vibration levels.