Growing up the son of a farm equipment repairman, I learned at an early age to appreciate the versatility of an eight-ounce ball pein hammer, a spanner wrench and a screwdriver. There was practically no fix too difficult for my dad, as long as he had that trusty triumvirate in his travel box. But he also knew there were limitations and he always drew the line where either safety or quality was compromised through the use of the wrong tool. I owe a lot to his practical lessons.
When I started selling ultrasound detectors in the early ‘90s, the technology was little-known and even less understood. Beyond finding compressed air leaks, no one cared about the technology’s other relevant and interesting applications. This frustrated me, as the applications that got me really excited were mechanical in nature, owing to my upbringing diagnosing farm equipment failures. Transferring my enthusiasm to other people in industry proved my biggest challenge. Many roadblocks were put in my way by old-school thinking and outdated practices.
“Testing bearings and steam traps could just as easily be done with a No. 3 Robertson screwdriver strategically placed on one’s delicate cheekbone,” I had been told.
Since then, 20 years have passed and for the most part I don’t see this antiquated thinking any more. Meanwhile I’ve helped pave the way for future generations of ultrasound inspectors the world over. Testimony to the progress made can be found in a book by me and co-author Tom Murphy, Hear More: A Guide to Using Ultrasound for Leak Detection and Condition Monitoring (published by Reliabilityweb.com).
Anyone who has benefited from the advancements made in ultrasound testing will agree that this technology is no longer just for leak testing. It is a complementary companion technology to vibration analysis, infrared imaging, motor circuit testing and oil analysis. Some will assert that the foundation of any predictive maintenance program should be based on ultrasound first – due partly to its versatility, low cost and easy implementation.
Nowadays, meeting reliability professionals who still believe in archaic methods for the detection of machine faults is rare. That made it all the more surprising when I read Dan Wise’s technical article, Knocking Compressor? All you really need is a screwdriver to find the source of the problem (Machinery & Equipment MRO, Nov. 2011, pg. 22). His message compelled me to climb back on my milk box and scream, “You need ultrasound!”
We take safety very seriously. Inspector training is as much about safety as it is about procedure. So it’s hard for me to look past the obvious hazards that could come from sticking a greasy screwdriver on your ear and then placing the other end against a knocking compressor valve. It is also difficult to ignore the added risk of working in a noisy compressor room without wearing approved safety gear.
Mr. Wise’s article suggests that ultrasound is unnecessary because the technology “takes time to learn.” Instead, he urges inspectors to develop their skills with a screwdriver “by spending time, each day, paying attention to the sounds of compressors.” His advice is conflicting. Are we better to spend time each day learning his old-school screwdriver technique? Or are we better off to spend our time learning how to use an ultrasound instrument that can do so much more?
We’ve known for more than 20 years that ultrasound provides the earliest warning signals for mechanical failure. If you can hear compressor valves knocking with a screwdriver, you are too late. But, there is a fundamental difference in philosophy between what ultrasound inspection proposes to accomplish and what can hoped to be achieved with a screwdriver.
Ultrasound monitors the condition of that compressor valve from the time it is new until the time it is ready to be replaced, all the while providing feedback about subtle changes in its operational condition. The screwdriver method proposed by Mr. Wise’s article is used “to find the source of the problem,” which suggests the knocking is already perceivable to the human ear and it is only a matter of days, hours or minutes before the compressor will fail. I call this firefighting. It is certainly not condition-based monitoring, as it leaves no room for planning the repair.
The article by Mr. Wise warns that “Valves can break without warning for a variety of reasons.” It goes on to list some common causes for failure. The real reason machine systems fail “without warning” is because no warning system is in place. Using a predictive technology, such as ultrasound condition-based monitoring to indicate subtle changes in machines over time, allows maintenance to avoid firefighting catastrophes.
Advanced compressor valve testing
A huge benefit of ultrasound is its ability to record dynamic wave files on reciprocating compressors. Using simple time waveform analysis software, it is easy to expand 1/10th or even 1/100th of a second of data to see a valve open, exhaust and close. During that blink of an eye, we can see if there is complete sealing of the valve seat, and we can even assess the strength of the valve spring.
All of this data can be printed out in a logical report, which serves as a summary for management to decide if further action is yet necessary. Because the document becomes an historical archive, the next action decision will have a basis of comparison.
Reciprocating compressors contain one or more cylinders that compress gases. Pistons travel up and down within the cylinder. On the down stroke, new gas is drawn into the cylinder through an inlet valve. On the up stroke, gas already trapped in the cylinder is compressed and pushed through an outlet valve to a receiver or tank.
In a simple example, vacuum pulls the outlet valve closed and the inlet valve open and pressure does the opposite. In more complex reciprocating compressors, more robust valves operate with springs.
Opportunities for failure are many. Seal integrity around the piston cylinder wall reduces the efficiency of compression. Dirty and worn valves do not seat properly due to corrosion and debris build-up around the valve head.
Internal leakage around the piston and the valve head can be detected with ultrasound inspection by recording a dynamic ultrasound signal and analyzing it in the time domain. Early mechanical failures, which can eventually lead to screwdriver-detectable valve knocking, can be seen this way too.
In Figure 1, we can clearly see the impact made by the valve when it is seated and the suction of turbulent air flow when the valve is opened. High-amplitude signals between the valve impacts can reveal mechanical looseness (early knocking) or internal leakage from piston rings or the valve seat. A comparison between similar valves is done easily by scaling the Y-axis of the time signal and presenting it in an overlay or side-by-side mode.
A documented report that can cite the historical evolution of your compressor’s health is an extremely valuable and professional way to present your data. It is a great way to convey to maintenance managers that a problem is escalating and that action is required. It will hold much more credibility than walking into your bosses’ office, grease dripping from your ear lobe, and suggesting that a compressor valve sounds like a “bongo drum” and it should be immediately removed from service.
There is no substitution for walking the plant floor and visually taking account of the state of affairs. Much can be learned by talking with operators. Operators have the best pulse on machine conditions, because they work around them every day.
For those who are ready to take the next step (implementation of a condition-based monitoring program), getting to know the expectations of operations, management and front-line staff is the best approach. From that point, defining the assets that most critically need monitoring is as important a step as figuring out which technologies shall be used to collect the data. Documentation is the final piece of the puzzle and without it, all other initiatives are pointless.
Needless to say, modern techniques for collecting data, which can be trended and reported on, are preferred over archaic methods that rely on individual interpretation and old-school tricks.
Allan Rienstra is the president of SDT Ultrasound Solutions and co-author of Hear More: A Guide to Using Ultrasound for Leak Detection and Condition Monitoring. For more information, visit www.sdthearmore.com.
To view the full layout of this article with images, as it originally appeared, see page 21 of the April 2012 issue. The digital edition of this issue can be found here: