An Efficient Compressed Air System = A Quieter Plant
By Al Perez, and Paul Tashian
Do you experience a constant ringing in your ears?
Do you often have difficulty hearing/understanding normal voices in public places?...
Or argue with family and friends about the volume of TV and radio?
These are questions regularly asked by Al Perez of AGC Developments, Inc., a noise control consulting firm in New Brighton, MN. "I have, and continue to be fascinated by the answers and attitudes I encounter when asking these questions." says Mr. Perez. "If you were to experience bright, non-stopping flashes of light in your eyes, when you know there is nothing going on in the outside world, you would run to a doctor for an explanation and a cure. People on the other hand, go years without telling anybody, much less their doctor, of a similar experience with their ears".
Hearing problems are the result of many causes, from birth defects to sharp no. 2 pencils. Ask any arthritic friend who has consumed large quantities of aspirin through the years to control pain, and you will find an arthritic hard-of-hearing individual. Talk to parents who were given the choice between ototoxic (toxic to the hearing system) antibiotics and the death of their loved one, and you will meet the parents of a deaf child.
No matter how one goes about scrutinizing and cataloging the data related to hearing loss, by far the overwhelming source of these problems in our country is NOISE. And even though we continue to torture our hearing mechanisms at home through new and innovative methods (leaf blowers, loud music, etc.), our primary exposure to noise is the workplace.
At the top of the list of "Bad Jobs for Your Ears" reigns undisputed, the military. Frequently, military experience results in "ringing" in the ears. This disease is called tinnitus. In many cases it is triggered by sharp extremely loud events such as explosions. Employees in industrial plants are also exposed to these singular occurrences called "acoustic traumas". Tinnitus can also result from long term exposure to less intense sounds. It is a disturbing, seldom curable, audible reminder that the sufferer is, in most cases, gradually going deaf to middle, and higher frequency sound.
COMPRESSED AIR NOISE
If you don't feel your air system is noisy, consider yourself one of the few. In over 15 years as a noise control consultant, Mr. Perez has found that compressed air is consistently one of the top noise control problems facing manufacturing facilities. Compressed air noise can be categorized as follows:
- Noise from the compressor itself.
- Noise from operating nozzles, blow guns, and exhausts.
- Noise from nozzles not doing work.
Compressor noise can be dealt with in two ways. The first, and most common way is by building an acoustical enclosure, or separate room around the compressor. Secondly, the total noise produced by the compressor can be controlled by reducing its "On-Time". To do this you must stop wasting air! The two most common ways air is wasted, are very correctable. These are the above categories (c.) and (d.) that is, nozzles not doing work, and leaks. Noise from category (c) can simply be avoided by making sure that the nozzles are activated ONLY when needed. Reducing noise from leak sources (d.) can be accomplished with an ultrasonic leak detector, which is described in more detail later in this article.
BLOW GUNS AND NOZZLES...
One might think that the selection of nozzles to be used would play a major role in reducing plant noise.
In September of 1993 AGC Developments asked 20 manufacturers/suppliers of blow guns, nozzles, and silencers to participate in a test program to rate the noise levels of such products for a major client. The goal was to develop a simple procedure the client could use to judge the best product for a given job. Nine responded positively and sent samples, and some also supplied technical information and suggested test methods.
Tests were performed at 60psig line pressure since this is a common goal in many plants. Naturally higher pressures mean higher noise levels. Since nozzles can be installed in the body of a blow gun, a nozzle was defined as being stationary, and mounted directly on a supply air line, while a blow gun was considered that which is portable and used in a hand held mode. Nozzles and blow guns were aimed 4 inches from two targets, a flat 10" x10" aluminum block, and the same block with a hole 3/8" diameter x 2" deep (to produce a more realistic situation). When testing nozzles, sound levels were taken from a 3 foot distance at a 90 angle from the airstream. For blow guns, sound levels were taken at a 90 angle from the airstream and also at 180 (3 feet behind the gun).
Figures 1 and 2 summarize the results of the tests.
As you can see from these charts a direct relationship between thrust, SCFM, and noise can not be established. All nozzles tested produced significant levels of noise. Size, quantity, and physical configuration of the orifices will also play a major role in the amount of noise created from a particular nozzle. The target with the hole consistently produces a higher level of noise from the same jet of air.
OSHA requires an average noise exposure level of less than 85 dBA. Almost all items in the test produce significant noise levels above 85dBA. As a result, the only avenue for most nozzles and blow guns to meet the 85 decibel average is to control their total ON-time. It is common in most of the plants Mr. Perez visits for nozzles to blow constantly when they could be timed to activate only when needed. Another factor to consider is thrust. Are you using more than necessary? Plant Engineers have a tendency to maximize thrust unnecessarily. In most cases this only increases plant noise and wastes air, which can be costly.
Leaks have been found to be one of the most significant contributors to excess noise in plants using compressed air. Some leaks will generate such high levels of noise that they easily compare with those of nozzles and blow guns in figures 1 and 2. It is not only the magnitude of compressed air leaks but the multitude that make them a threat to employees hearing. Several leaks in the same area can produce many times the noise level of just one or two.
Example: If 4 leaks of the exact same configuration (size, shape, pressure, etc.) produce 90dBA, 8 leaks will produce 93dBA, 16 leaks will produce 96dBA, and 32 will produce 99dBA. Because decibels increase on a logarithmic scale, 32 leaks producing 99dBA are almost twice as loud as the 4 leaks producing 90dBA.
Again, OSHA recommends an average noise exposure level of less than 85dBA. This makes the repair of leaks essential because the noise they generate is constant. Leaks contribute significantly to the average plant level in two ways. Leaks themselves create noise, but as mentioned above, they also increase average noise levels by causing additional "on time" for compressors. This workload not only adds to noise levels, but can significantly increase power consumption as well. Compressors must work harder and longer to keep up with the demand for air created by leaks. It is not uncommon to find a plant whose compressors run on the weekend, when the plant is unoccupied, just to feed a leaking system!
Fortunately, with the right equipment, air leaks are one of the easiest sources of noise to reduce. Mr. Perez has found an ultrasonic leak detector to be the fastest and most effective way to accomplish this task. The sonic sound of a leak (that which you can detect with your unaided ear) is almost impossible to detect in a loud industrial environment. Multiple leaks, nozzles/blow guns, and the sound of running machinery, tends to echo throughout the plant making it difficult to pinpoint the source. An ultrasonic leak detector picks up only the high frequency ultrasonic component (20kHz to 100kHz) of the leak, and translates it to the audible range where it is heard by the user through a set of headphones. This feature combined with the directionality of these sound waves makes it much easier to trace the sound to its source.
Newly available DND* (Dynamic Noise Discrimination) technology in ultrasonic detection has made it simple for any member of a maintenance crew to perform effective leak tests. By making one quick adjustment of the DND level, background noise is rejected from the headset. The user can now approach the leaking system by making a sweeping motion with the detector, covering large areas rather quickly. When a sound is detected which is unique in frequency and intensity, the smart DND circuitry of the detector identifies this sound and translates it to the headset.
As with nozzles, leaks produce different sound frequencies and intensities depending on the operating pressure, and the size and shape of the hole. An ultrasonic detector offering DND will be capable of locking on the dominant frequency of the leak automatically. When several leaks are located in close proximity it can capture the sound of individual leaks, even if they are of completely different frequencies.
For air leak detection applications Mr. Perez has found an ultrasonic leak detector with the automatic tuning feature to be much more effective in finding air leaks than traditional manual tuning an ultrasonic leak detector will prove to be a worthwhile investment. It is a low cost way to significantly reduce the amount of plant noise created by leaks, and additional compressor ON-time. If the reduction of plant noise is not cost justification enough, consider the reduction of energy. This will effectively add money back to the company's bottom line.
In conclusion, most blow guns and nozzles can only be used sparingly if one is to avoid a hearing conservation program by OSHA. The ON-time of these devices, as well as air leaks must be strictly controlled.
* DND is a trademark of the Superior Signal Company, LLC