Reliability maintenance, energy conservation and asset-health management programs have taken engineering and maintenance departments by storm. On a seemingly daily basis, corporate management is demanding greater results related to production, safety and cost savings — but typically providing fewer resources and less personnel. Lower production costs, higher production rates and less downtime are all good benchmarks to have. Greater corporate success, however, often leads to more headaches for the individual professionals who are charged with keeping things running efficiently and on time.
Airborne and structure-borne (A&SB) ultrasound technology has been used effectively for decades in industrial facilities such as petrochemical and power generation plants and commercial manufacturing. Due to its versatility, ease of use and diagnostic capacity, airborne and structure-borne ultrasound has been used extensively to implement solutions affecting key performance indicators (KPI) related to energy consumption, man-hours, spare parts, reliability, safety and production.
One of the most common — and also most underestimated —capabilities of A&SB ultrasound in process cooling applications is in the field of leak detection. ISO 29821-1:2011 states: “A&SB ultrasound can be used to detect abnormal performance or machine anomalies. [These] are high frequency acoustic events caused by turbulent flow, ionization events and friction, which are caused, in turn, by incorrect machinery operation or alignment, leaks, improper lubrication, worn components or electrical discharges.”
Before implementing an airborne and structure-borne ultrasound condition-based monitoring (CBM) program, however, knowledge is required. The technician must have a basic understanding of ultrasound and how it propagates through both atmosphere and solid structures. This article will provide that overview.
Airborne and Structure-Borne Ultrasound Technology
So, how does a leak generate ultrasound? In short, the answer is fluid velocity and turbulence.
When a velocity gradient occurs between two moving particles — that is, one is moving faster than the other — these tangential forces generate friction. As these forces attempt to introduce rotation between the moving particles, the viscosity tries to prevent it. Depending on the relative values of these forces, different flow states may occur.
When the velocity gradient is low, the inertial force is greater than the friction. The particles move, but they do not rotate, or they rotate with very little energy. The end result is that the particles follow definite trajectories, and all particles passing through a single point in the field of flow follow the same path. This is referred to as laminar flow, meaning that all of the particles move in the form of layers or sheets.
As the velocity gradient increases, the friction between the particles also intensifies. As they acquire an appreciable rotational energy, the viscosity loses its effect, and the particles change direction. As they pass from one trajectory to another, the particles collide with each other and change courses. This process is known as turbulent flow.
Turbulent flow generates ultrasound that is detectable in the high frequency range above 20 kHz (the high end of the human auditory range). As the velocity of a turbulent flow leak drops, the fluid will return to a laminar flow, thus eliminating the ultrasonic propagation.
Given the effects of flow on ultrasound detection, the following is true:
- Airborne and structure-borne ultrasonic inspection instruments can be used to locate turbulent flow leaks in pressurized gas systems — regardless of which type of gas is used. As long as the leak creates turbulence, ultrasound detection technology is capable of detecting it. This is especially beneficial in areas where there is a saturation of gases, or a where many pressurized vessels and vacuum processes exist.
- A&SB ultrasound detection is directional. This means that the high frequency sound created by a leak will travel in only one direction. It will not be dispersed throughout the immediate atmospheric area of the leak. This characteristic makes it relatively simple for the inspector to pinpoint the exact source of a leak when it is heard.
- Systems can be tested while they are operational, saving time and offering convenience. High quality ultrasound technology can locate leaks from up to 300’ away with corresponding parabolic accessories.
Airborne and Structure-Borne Ultrasound Inspection Techniques
In order to perform basic airborne and structure-borne ultrasound leak testing, the system under test either must be pressurized or placed under vacuum. Using a high quality ultrasound receiver, the technician can scan the system to listen for ultrasound indicating a leak. Some information to keep in mind:
- The ultrasound receiver should have an analog or digital meter and the ability to convert an ultrasonic signature into the audible range through a sound-dampening headset.
- The technician should be able to hear a rushing sound that correlates with a spike in the meter. Once a leak is heard, the technician can pinpoint the exact location of the leak by using a focusing accessory such as a concentrator or hollow acoustic probe. While using the focusing accessory, the technician reduces the receiver’s sensitivity (or gain) as he follows the ultrasound to its loudest point. The meter will indicate a higher reading, and the volume in the headset will increase as the technician draws closer to the leak.
- To confirm that the leak has been accurately located, the detector should be placed close to the suspected leak site. Then, the technician should move the detector slightly back and forth as well as up and down around the location. The headset volume and meter indications should slightly increase and decrease in intensity as the receiver passes over the source.
- This source-discrimination technique will eliminate the identification of false leaks that are a result of reflected ultrasonic signals from other locations near the suspected leak site. It also allows the technician to locate multiple leak sites that may be in close proximity to one another. Listening from a distance with a higher gain setting on the receiver can sometimes present multiple sources as a single leak.
It may not always be possible to modify a system’s pressure for testing. In these conditions, an ultrasonic transmitter can be used instead. In this case, the technician places an ultrasonic transmitter inside the component under test. The signal from the transmitter instantly floods the space of the component and can penetrate any existing orifice, indicating the location of any leaks. It is important to note, however, that thin spots in certain metals also can be vibrated by the signal from the transmitter.
Application Example: Ammonia Cooling Systems
Ammonia is one of the most efficient refrigerants available, with a range of applications for both high and low temperatures. Ammonia is not a universal refrigerant, however. It is mostly suitable for industrial and heavy commercial applications. For those, it possesses a number of benefits that have been proven by many decades of use:
- Ammonia is an environmentally friendly, natural refrigerant.
- It possesses both a global warming potential (GWP) and an ozone depletion potential (ODP) equal to zero.
With an ever-increasing focus on energy consumption and environmental consciousness, ammonia systems are proven to be a safe and sustainable choice.
Nevertheless, ammonia’s toxicity, flammability and material compatibility must be taken into account. Some of the widely used methods for detecting ammonia leaks, while effective, can be dangerous for personnel. For instance, in some facilities, detection protocols are limited to personnel detecting the ammonia odor by smell alone. In other facilities, an inspector will follow pipelines until he is able to see a visible chemical response from sulfur candles that produce smoke upon contact with ammonia. Unfortunately, these candles also produce sulfur dioxide, a hazardous gas associated with difficulty breathing, an increased risk of respiratory disease and, in some cases, death.
Ammonia vapor has a sharp, irritating and pungent odor that acts as a warning of potentially dangerous exposure. The average odor threshold is 5 ppm — well below any danger or damage. However, exposure to higher concentrations of gaseous ammonia can result in lung damage and even death. The U.S. Occupational Safety and Health Administration (OSHA) has set a 15-minute exposure limit for gaseous ammonia of 35 ppm by volume in the environmental air and an eight-hour exposure limit of 25 ppm by volume.
Airborne and structure-borne ultrasound provides a safe and efficient solution for detecting ammonia leaks. A&SB ultrasonic detection methods allow the user to detect ammonia leaks in less time and at a safer distance than the alternatives noted.
In conclusion, ammonia leak detection is just one example of A&SB ultrasound helping industrial operations to become safer, greener and more reliable. With myriad other applications for any type of gas leak detection, airborne and structure-borne ultrasound is an option for efficient and cost-effective leak detection.