Most recently, PdM professionals have opened their eyes to the benefits ultrasound offers as a predictive technology. Ultrasound often gives an early alert that an impending problem is developing in a bearing or helping to optimize the lubrication of rotating equipment.
All of these valuable applications contribute to billions of dollars saved in downtime, energy efficiency, and improved product quality. Is there room for even more savings? Well, just consider the fact that most of these inspections are carried out on fixed assets only. Are additional savings possible for mobile assets? At many companies such as mines, cement production, quarries, civil engineering contractors, industrial farms, commercial fleets, and oceanic vessels, the production cycle depends on heavy vehicles, loaders, off road vehicles, and seagoing ships. Surely, there must be benefits that Ultrasound inspection could realize on these mobile assets as well.
These vehicles have a wide range of applications like moving goods from land points to sea ports and beyond. They are used to plant, maintain, and harvest crops, excavate earth and move thousands of tons of raw materials in quarries and open pit mines. Although their size can vary between 30 tons to more than 350 megatons, they all have in common an internal combustion engine to provide the power to move the vehicle and power the hydraulics. Most have a cabin to keep the operator safe, dry, warm, or cool, while others have storage volumes which must be weather tight, at the least, and hermetically tight in the case of chilled container transports. And in many cases, compressed air systems are used for braking and suspension systems.
To protect the investment in these mobile assets, preventative and predictive maintenance is performed on a regular basis. Most fleet managers rely on oil analysis for predictive maintenance, while other PdM technologies (Ultrasound, Vibration, Infrared) are seldom considered. Additional investment in these technologies is not currently considered a priority. There are several important applications that can be served with ultrasound technology, but are not currently understood, and definitely not employed by most mobile asset repair shops. These applications fit the PdM tool box perfectly for any maintenance department responsible for keeping a commercial fleet running flawlessly.
Most PdM technologies are symbiotic, which is to say that when used in concert they provide a more complete picture, but when used alone vital data can be overlooked. The purpose of this article is to educate about some important applications where the combination of ultrasound testing and oil analysis can predict major premature engine failures, as well as speed up the inspection time required to find and fix problems. This article also discusses some secondary applications that address issues related to safety of these vehicles, protection of cargo, and comfort for the operator. Hopefully, you to learn the important role that ultrasound technology serves for fleet maintenance managers and mechanics.
Ultrasound Testing...What Is It and How Does It Work?
Many people in maintenance departments responsible for fixed assets of factories know that the principle source of ultrasonic waves is turbulent flow, friction and discharge related to electrical problems. They also know that ultrasound waves are sound waves vibrating over 20,000 Hz, which is impossible for humans to hear without the help of special ultrasonic instrumentation.
Ultrasonic instruments detect potential problems that can lead to shut down of a process or factory. They also detect sources of energy waste and issues that impact negatively on product quality (Figure 1). Many early stage problems produce ultrasonic signals that are transmitted from the source as pressure waves. Ultrasonic instruments detect these waves and translate them into an audible signal that can heard by the inspector, all the while measuring the ultrasound signal so that it can be compared and trended to determine gradual deterioration. Even if that sounds complicated, its really not.
Today in factories with fixed assets, there are thousands of trained ultrasound inspectors working who are extremely sharp and creative when it comes to detecting sources of ultrasound inside their processes, and fixing the problems they find.
Unfortunately, the demographic of qualified and skilled ultrasound inspectors is poorly represented in the mobile maintenance shop, where the technology is virtually unknown. Sadly, many cost saving applications have not been revealed. If you are working in the maintenance department and are responsible for mobile asset maintenance, you will be pleasantly surprised by the applications revealed in this article.
Applying Ultrasound Inspection to Mobile Fleets
We will discuss using ultrasound techniques in several different applications within mobile fleets, including diesel engines, hydraulic cylinders, air braking systems, air suspensions and cabin tighness.
Diesel Engines – Internal combustion engines burn fuel and, regardless of size, they require air - preferably clean air. The air we breathe is the same air engines breath. No matter where we are on the planet, air contains particles in suspension. Some of these particles are harmless but others represent a serious danger. Silica ranks as one of the hardest elements on earth, only surpassed by topaz, corundum, and diamond. Silica is very damaging if it reaches the inside of an engine. Silica also ranks as one of the most abundant elements on earth and ever present in dirt and dust, which is made airborne in the conditions where mobile machines operate. Therefore, engines are equipped with high efficiency filtration systems to prevent silica and other contaminants from reaching the combustion chamber (Figures 2 and 3).
All diesel engines have primary and secondary filters fitted between the air intake vents and the turbocharger (Figure 4). When the engine is operational a negative pressure is created in the air intake system and any leaky orifice (loose clamps, cracked hoses, thinned metal, pin holes) downstream of the filters means the engine is breathing without filtration. This means air full of silica can reach the pistons, rings, sleeves and other engine components causing damage and premature failure. Depending on how much silica is ingested, the life of the engine is dramatically reduced, sometimes lasting only a few days!
Oil analysis is used as a predictive tool that compares the metal content and silica in parts per million (PPM) found in the oil sample against limit values set according to the engine manufacturer. The acceptable silica content is very low, ranging from 15-50 PPM. When a sample shows values over the limit, the source of the contamination needs to be found quickly, and the mobile asset must be removed from service to avoid further costly damage. This introduces the added cost of downtime and lost productivity.
Finding the leaks calls for an exhaustive visual inspection of the entire air intake system. This can take several hours to inspect, and after the inspection, it’s not uncommon to have found nothing. The next oil sample will still show high silica levels and increasing wear metal values indicating the problem is getting worse. As a companion to visual inspection, ultrasound testing to find the leak will net results much faster, and is also useful to confirm the repairs to the leak were performed correctly. Progressive mobile mechanics use ultrasound inspection after any service work is completed on the air intake system.
There are two methods for finding problems in the air intake system with Ultrasound.
• Inspection with the engine running
• Inspection with the engine turned off
Inspection with the Engine Running – Using this method of inspection is based on the premise that any turbulent flow from a potential leak produces ultrasonic sound pressure waves, which are, in turn, detected with the ultrasonic detector. Turbulent flow is produced between two adjacent volumes when those volumes have a) differential pressure, and b) a leak path. Turbulent flow will exist at the leak path for as long as there is differential pressure between the volumes.
Start the engine and leave it to idle. With noise attenuating headphones in place adjust the sensitivity of the ultrasonic instrument according to the ultrasound sources near the engine. Using the flexible sensor for safety (if you have that accessory), inspect the entire intake system starting from the air breather and ending at the turbocharger. Any air ingress will produce an ultrasonic signal that sounds like the hissing, swooshing sound you know from a compressed air leak. A well trained ear will pick out this sound quickly, despite competing noises that may come from the engine itself. Additional training teaches ultrasound inspectors how to deal with parasite noise and harsh environments, and is highly recommended for mobile mechanics that are adopting ultrasound testing symbiotically with oil analysis. Techniques known as “shielding”, “covering”, “blocking”, and “positioning” are learned keys that assist inspectors in high noise areas.
Inspection with the Engine Turned Off – The air intake system can also be inspected for leaks when the engine is not running. In fact, this may be a more desirable method because parasite noise from the engine is loud and can interfere with the inspection.
When the engine is off there is no differential pressure and consequently no turbulent flow. No turbulent flow means no natural ultrasound signals are present at any leak sites. In lieu of turbulent flow you can generate artificial ultrasound signals directly in the air breather system. This is accomplished by means of an ultrasonic transmitter, like the SDT 200mW Bi-Sonic Transmitter, which is a small accessory that generates a 40 kHz signal powerful enough to fill small volumes. The ultrasound signal can be heard and measured directly through the various membranes that make up the air breather system. Wherever the possibility of air ingress exists, the signal detected by the ultrasonic receiver is significantly louder. This is noted in the headphones and the decibel level measured by the instrument.
A large mining company in northern Canada recently shared their experience of inspecting the air intake on a LeTourneau production loader. In response to very high levels of silica and iron from oil sampling on one of their production loaders (Figures 6 and 7), an attempt was made to determine if there were any leaks in the breather system of the loader which would cause the severe dusting. A visual inspection of the breather system failed to produce any definitive results. Then they conducted a second check of the breather system, this time using airborne ultrasound with a transmitter.
“Finding the leaks was easy,” the mining company reported. A 200mW Ultrasonic Transmitter was placed inside the inner air filter. Both air filters were replaced and the breather system was sealed up. The entire breather system from the filters to the engine was probed with the ultrasonic receiver (see Figure 8). All locations along the breather piping and joints displayed ultrasound readings from 20 to 24 dBμV, except for one. The location inside a clamp on the right hand side of the loader gave readings of 34 to 38 dBμV. That is an increase of 14 decibels. 38 decibels is 5 times louder than 24 decibels. This is a strong indication of thinned metal combined with the possibility of pinhole leaks under or around the clamp. Images shown in Figures 8 and 9 indicate where baseline ultrasound readings of 20-24 dBμV were registered versus the leak site, where 34-38 dBμV were observed.
The mechanics at the mobile repair shop reported that, once the leaks were discovered, the fix was relatively simple. And finding the leaks with ultrasound inspection was far quicker than any other method previously used. There was a weld patch where the pipe had been previously repaired. It was removed and the existing patch inspected and rewelded. The pipe was reinstalled on the loader, filters replaced and it was recommended that the loader return to operations for a 12 to 24 hour period. Fresh oil samples would then be taken again, and the results analyzed for further dusting problems.
The production loader’s oil was re-sampled on May 22, 2009, after approximately 48 hours operating in the field, and test results were received back from the lab six days later. All indications of dusting had disappeared from the oil sample results. As can be seen in Figure 10 comparing the before and after samples, substantial drops in aluminum, nickel, chromium, iron, copper, lead, & silica (see Figure 11) were observed, indicating the air leak had been patched successfully, and was indeed the cause of the dusting. It was suspected that the contaminants in the initial samples were a combination of dusting ingress and wear particles from engine components.
Bottom Line Savings
This is an easy procedure to implement at any mobile repair shop because of the relative cost of good quality ultrasound inspection equipment. I say “relative cost” because the dollars spent on ultrasound equipment and training can save a company millions. As an example, this northern Canadian mining operation reported savings well in excess of one million dollars only a few months after implementation of ultrasonic testing.
Their numbers are pro-rated estimates based on expected remaining engine life and do not include any additional labor costs. In other words, the remaining value of only the engines after depreciation. The equipment was the one Letorneau 1850 Production Loader we just discussed and four Komatsu 830E Haulage Trucks. All of the engines are Cummins QSK60’s and the dusting issues were discovered and repaired between May and November 2009 using a combination of Oil Analysis and Airborne Ultrasonic Inspection. The cooperation of these two predictive technologies contributed to a savings of $1,147,029 over this time period. Actually, business analysts at the company put the figure much higher as they included all possible costs such as labor, parts, and downtime etc..., but this additional cost savings could not be disclosed.
Let’s now discuss the Komatsu 830 E Haulage Trucks. To give you an idea oh how difficult it is to find these tiny air breather leaks with visual inspection, take a look at Figures 12 and 13.
The two pipes that are touching in Figure 12 are supposed to be separated by at least an inch. However, in the field, they were actually touching, which caused rubbing. Now look at Figure 13. These breather pipes are 10” in diameter. Once the leak site was detected with ultrasound they were pried apart with a 4’ crowbar. This could have caused hundreds of thousands of dollars in engine damage and downtime. Ultrasound inspection of air intake systems is now standard practice at this Canadian mine site, as are some other interesting applications.
Hydraulic Cylinders
Hydraulic cylinders (Figure 14) are used in fixed and mobile hydraulic systems. They provide force through a linear stroke. Their operation is based on Pascal’s Law, which states if you apply pressure to confined fluids then the fluids will transmit the same pressure in all directions at the same rate. Hydraulic cylinders are an efficient way to multiply force and move heavy loads.
Seals are one of the most important components in hydraulic cylinders. They create a barrier between the high pressure chamber and the low pressure chamber. When the integrity of seals is compromised, the cylinder no longer transmits its full force potential.
Symptoms of Problems in Cylinders
A sure symptom that the cylinder has problems is a loss of power and or slow operation. In severe cases the cylinder can stall even under light loads. An increase in pump noise and temperature is also a sign of leaking cylinders. The leading cause for hydraulic system failures is contaminated hydraulic fluid. Hard contaminants in the fluid, such as silica, wear out the barrel and the seals making it hard for the hydraulic pump to maintain the necessary pressure.
How to Troubleshoot the System
A conventional method to check for leaks in hydraulic cylinders requires an operator to run the piston to one end of its stroke and leave it stalled in this position under pressure (Figure 15). Then he would crack open the fitting at the same end of the cylinder and check for fluid leaks, which would indicate hydraulic oil has passed the wiper seal. After checking, the fitting is re-tightened and the procedure is repeated on the other end, and the middle of the stroke. This procedure is quite time consuming and requires the asset to be out of service for a longer time than necessary.
Ultrasound speeds the time required for the inspection, and in many instances, the inspection is performed in the field, avoiding the cost and delay to float the equipment back to the repair bay. This has added benefit if the inspection reveals a leak, and the leak can be repaired in the field.
Using ultrasound, the inspector places the contact sensor or magnetic sensor over the barrel near to the piston (see Figure 16). The system is put under pressure and the sensor scans around the barrel 360° while listening for the characteristic sound produced by a leak when the fluid passes from the high pressure to low pressure chamber. This sound could be that made by small bubbles of oil bursting on the non-pressure side of the wiper. In the case of larger leaks, the sound is more like a squishing sound as oil is forced across a small orifice in the seal. The point where the signal is most intense indicates the integrity breach of the seal.
Air Operated Brake Systems
Air break systems are primarily used in all types of trucks, buses and rail cars (see Figure 17). For an efficient and safe operation, the system must be absolutely tight. Brake systems manufacturers establish guidelines for pressure, and this working pressure must be maintained under all circumstances. Air break systems have several parts including the compressor, an air dryer, valves, air reservoir tank, pipes, fittings and the brake system itself. All of these components are susceptible to leaks. The compressor is designed to run loaded up to 25% of the engine’s running time, but air leaks can cause the running time to increase, adding operational costs in the form of fuel and maintenance. Of course, this is not as important as the fact that leaks cause the braking system to be an unreliable safety risk.
Trouble Shooting the Brake System
Finding leaks in the compressed air system in any mobile equipment is easy and fast. In fact, many manufacturers, including Volvo Trucks and Mack Trucks already use SDT170 detectors on the assembly line to ensure leak free brake systems. Start the engine and let the compressor run until the required pressure is reached in the system. Turn off the engine, and using the ultrasonic instrument with the flexible sensor, scan from the compressor side to the brakes in the wheels. The hissing sound of any leak will be easily heard, and, because it is ultrasonic, it is directional, and, therefore, easy to localize.
Air Suspension Systems
Air suspension systems (see Figure 18) provide a much smoother ride, which can add protection to cargo that is sensitive to transportation shocks. The air spring is basically a bellow filled with compressed air and runs off of the same compressor that the braking system uses. Leaks in the air suspension system affect the smooth ride, but can also draw on the brake system, making it unreliable, and, therefore, unsafe. Of course, this adds risk for a vehicle transporting several tons of cargo. When the air spring loses its pressure there is the chance of balance loss and tipping. Troubleshooting air suspension systems is essentially the same procedure as that used for braking systems.
Cabin Tightness
The final application to discuss here is an equally important one where ultrasound inspection is usefully employed to ensure the tightness of cabins and cockpits. In smaller vehicles tightness is important to prevent noisy interiors from wind noise and water leaks. On larger mobile assets like loaders and tractors, keeping micron sized dust particles out of the cabin is a comfort and health issue for the operators. On ships and other oceanic vessels, weather tightness is vital to ensuring safety of crew and preventing damage to cargo.
The inspection is similar to, and as simple as, the procedure used for inspecting air breather systems with artificial ultrasound. Place the 200mW tone generator inside the cabin and close all windows, doors, and vents. Using the ultrasound instrument and flexible sensor, scan the outside seals on all windows and doors. The artificial ultrasound source is powerful enough to fill the entire cabin, but it is also powerful enough to transmit directly through glass and steel. Use the following procedure to understand the difference between a leak and non-leak.
1. Set the 200mW transmitter inside the cabin.
2. Take a dBμV reading through an open door or window. That
is your OHV (open hatch value).
3. Now close the doors and windows and take a dBμV reading
at an area where there could not possibly be any leak (the
middle of the door glass will suffice). That is the CHV
(closed hatch value).
4. Now scan around doors and seals with the flexible sensor.
The baseline reading will hover around the CHV. Listen in
the headphones and when you hear a signal louder than
CHV observe it. Does it approach the OHV? If so you have
found a potential leak path.
Conclusion
This article has covered four reasons to implement ultrasound inspection for major fleet maintenance centers. The inspections discussed have positive impact for cost reduction through faster inspections as compared to traditional methods. They also aid in the prevention of premature failure in diesel engines which, as we learned from our Canadian mining operation, saved more than one million dollars over a seven month period.
Ultrasound inspection already shares a symbiotic relationship with vibration and infrared inspection of fixed assets. The examples here cite an excellent argument for marrying ultrasound inspection and oil analysis data for better control of internal combustion engines.
Finding hydraulic issues is time consuming. If ultrasound inspection can isolate leakage in the wiper seals, it makes sense to implement the technology to win more inspection time and find additional problems faster.
Finally, inspecting cabins for tightness enhances comfort and safety for the operators. Tightness of refrigeration units means less drain on the compressor motor and better fuel efficiency. Tightness of hatches on cargo carrying ships means dry cargo arriving in port on safer ships.
While great progress has been made applying ultrasound to inspections on fixed assets, we have learned from this article that there are equal wins to be gained from applications on mobile assets. As ultrasound technology proliferates around the globe, we can’t help but wonder what other simple applications exist that will help save the next million dollars.
Gustavo Velasquez has more than 20 years professional experience working for several Companies in Latin-American and Canada such as Mobil Corporation, Saybolt Consultores, Lubrication Engineers of Canada, Battenfeld Grease Canada, LubriSupport, and lately as Account Manager for Latin-American Region with SDT North America. Gus is a member of STLE and SMRP. He lives in Cobourg, Ontario with his wife and two daughters.
Allan Rienstra is the CEO of SDT Ultrasound Systems. He has been involved with airborne ultrasound methods for nearly two decades and has helped thousands of ultrasound inspectors achieve inspection greatness through his unique coaching techniques. He is co-author of two certification training manuals, founder of the SDT certification training and implementation guide. His writing appears in maintenance journals around the world. He lives in Cobourg, Ontario Canada with his wife and two sons. Allan can be reached at 905-377-1313 x 221 or allan@sdtnorthamerica.com
