Strategies for Leak Detection, Repair, and Prevention
EP Editorial Staff | November 2, 1999
Leaks cost industry millions of dollars each year. A few small 1/2-in. leaks in a facility using air at 100 psi with an electric production cost of about 6 cents/kilowatt hour (kWh) wastes more than $22,000 per year. A recent compressed air leak survey at a New Jersey manufacturing plant resulted in a savings of more than $40,000 a year and an annual reduction in electrical energy consumption of 496,893 kWh.
Delaying the replacement of a leaking $100 steam trap could waste $50 a week or $2,600 a year. Since an average facility has hundreds of steam traps, leaking ones may be squandering hundreds of thousands of dollars annually. In addition to wasted dollars, unattended leaks also may result in unscheduled downtime, affect product quality, pollute the environment, and endanger people’s lives.
This article deals with leaks from three different perspectives: detecting and pinpointing leaks before they mushroom into major trouble, using mechanical adhesives to repair leaks, and installing hardware to prevent leakage and improve equipment reliability.
Early leak detection Ultrasonics has been industry’s technology of choice to detect and pinpoint leaks for more than 25 years. Inspectors using a hand-held, battery-operated ultrasound instrument, such as the Ultraprobe 2000, from UE Systems, Inc., Elmsford, NY, can hear leaks in vacuum or pressurized systems as well as faults in operating machinery and electric transmission and distribution systems. State-of-the art accessories such as close-focus modules and stethoscope extensions enhance the capabilities of ultrasonic instruments.
An ultrasonic detector senses subtle changes in the ultrasonic signature of a component and pinpoints potential sources of failure before they can cause damage. Longer wavelengths of lower-pitched sounds are gross waves that can be difficult to locate. But higher frequency sounds are short wave signals localized to the source of emission. For this reason, it is possible to use ultrasonic sensors in relatively noisy environments.
Continuous monitoring. While most applications for ultrasonic inspection are focused on hand-held portable instruments, there has been increasing interest in continuous equipment monitoring.
Continuous monitors include two basic components: a processing unit and a sensor, which often is in the contact mode. A wave guide is affixed to a set test point by either bonding it to a surface or by drilling a threaded hole and screw-mounting the wave guide on the object to be monitored. The processing unit may feature adjustments for sensitivity/dB level, a threshold setpoint for alarm, and outputs such as 0-10 V dc, 4-20 mA. Some units provide a heterodyned signal which allows remote listening or downloading to recording devices such as vibration analyzers, tape recorders, or computers.
An example of this type of monitoring device is a valve leak onset alarm. When a valve is shut, there is no sound. A baseline is set when the instrument is installed. If the valve leaks, the onset or increase of sound intensity over the set threshold will set off an alarm. The generated sound is usually localized to the test area where the sensor is affixed. This reduces false alarms produced by irrelevant sound generation.
Long range and close-up detection. While many ultrasonic translators offer a sensitivity range capable of locating gross leaks at a distance, there is a need to locate more subtle distant leaks and to scan electrical apparatus accurately at a safe distance. Also, close scans of low level sounds, usually associated with low level leaks such as vacuum leaks, are a challenge to standard ultrasound microphones. Long range detection devices can detect and enhance the signal of remotely generated ultrasounds. Some applications include locating leaks in overhead pipes and cables, and detecting arcing tracking or corona emitted from high voltage equipment including transformers, insulators, or switchgear.
Since the ultrasound event produced by these emissions can be detected at a distance, these detection devices provide safe scanning around potentially hazardous high voltage equipment. Low level emitting leaks are a different problem. The signal amplitude is extremely low and needs some form of amplification beyond the normal range of most standard microphones. Receptors to enhance low level leak emissions have been developed and offer a reliable method for locating these leaks.
Liquid leak amplification. When low level leaks do not produce turbulent flow, it is not possible to detect them with conventional scanning probes because ultrasonic leak detection of either pressure or vacuum leaks depends on the generation of a turbulent flow as the gas moves from high pressure to low pressure. When it is not possible to locate this type of low level leak (typically below 1×10-3 ml/sec), using a liquid with a low surface tension will help. Only a small amount of liquid must be applied to the leak test area. As the gas migrates through the leak hole and passes into the film of the fluid, bubbles will form and burst. The bursting produces a detectable ultrasound. Leaks with rates as low as 1X10-6 ml/sec have been detected with this method.
Remotely positioned transducers. In some cases, it has been difficult to maneuver and hold a probe at a test position while recording or listening to the generated ultrasounds. Some manufacturers provide multi-directional sensors with a cable that can be positioned in confined spaces. This technique is used to determine the presence of remote leakage without performing the time- consuming procedures required for entering confined space areas.
Valve leak monitoring/trending. An increase in amplitude over a baseline is often a warning signal of impending failure or worsening condition. Valves should be inspected routinely since the information collected can be extremely useful. Aside from go/no-go leak inspection, the worsening condition from acceptable to unacceptable can be determined. For valve monitoring, there should be a consistent test point and conditions within the test object (such as flow rate) should be constant. A baseline should be set and compared to future readings under the same conditions and recording modes
When used in tandem with other technologies such as vibration analysis and infrared thermography, ultrasound has myriad uses. The technology enables knowledgeable inspectors to go beyond the basic applications of leak detection and valve and steam trap inspections, and opens opportunities for improved equipment uptime, energy savings, and safety.
Sealants block leak paths Though leaks of gas or air at a facility are often overlooked, they can become a significant operating cost especially when the situation is chronic. Once a comprehensive survey to detect and pinpoint leaks in a system is completed, the next step is to stop the leaks. State-of-the-art machinery adhesives can reduce costs by eliminating leak paths.
In the average threaded fitting, metal-to-metal contact is approximately 20 percent. Eighty percent is air space surrounding spiral threads, a potential fluid or gas leak path. Loctite, Rocky Hill, CT, supplies engineering adhesives, sealants, and dispensing equipment.
Many situations can cause loosening and/or cracking of fittings, valves, and other connections which result in leaks. Vibration, shock, thermal, and environmental changes all take their toll. Practically all conventional methods of sealing–cork gasketing, pipe dope, or Teflon tape–have their shortcomings.
Conventional gasketing products like cork, paper, and rubber have a tendency to set even when they are properly torqued. When the bolts relax there may be a minute separation leak path. Having an inventory of all size gaskets is virtually impossible, and gaskets can shrink, tear, or deteriorate before use. Also, cutting gaskets is time consuming.
Pipe dope also is no guarantee against leakage. Pipe dope relies on solvents to carry them and form solid seals. When the solvent evaporates, the product dries to form a tough seal. The rigid, brittle nature of pipe dope causes cracking which creates leak paths. And with pipe dope, disassembly can be difficult.
Teflon tape, originally designed as a thread lubricant and not a sealant, can cold-flow out of the pipe and leak. It also can permit overtightening, a situation that may result in threads that lathe up on each other thus increasing the leak path. Another disadvantage of Teflon tape is that it has a tendency to contaminate systems. If a Teflon shred enters a system, it can foul a check valve or other critical component.
The best sealants are based on anaerobic technology. They are a liquid or paste plastic monomer that changes from a liquid to a solid when it comes in contact with metal and when air is excluded.
Because these anaerobic sealants do not dry out but cure without shrinkage, they are excellent when applied to threaded fittings. These sealants provide correct sealing without cold-flow and offer ongoing lubricity that acts as a mild threadlocker. They are also noncontaminating.
The company’s anaerobic gasket product for use on rigid flanges allows the flanges to be taken down virtually metal to metal. The plastic gasket material uses similar chemistry to fill in all the voids. Seen under a microscope, these voids appear as mountain peaks and valleys. The sealant fills in the voids between the mountain peaks with a liquid or paste that changes to a solid. And the piece of equipment still can easily be disassembled and removed.
One application of these super sealants is sealing and casting of porosities. A liquid anaerobic sealer can be painted on a clean surface. It will penetrate into every porosity and seal it. For extremely large vats of up to 100 gallons, porous metal parts can be submerged to both seal them and to increase their machinability.
The application of anaerobic sealers is a relatively simple process. And for anyone who needs guidance, some manufacturers conduct in-plant training sessions on the proper selection and application of its sealants.
The proactive use of these machinery adhesives provides reliability at the base component level. Manufacturers are discovering that using high-end sealants and adhesives in their equipment can improve equipment reliability, reduce costs, and stem wasted energy.
Gaugeable tube fittings improve reliability The installation of high-end tube fittings and valves often has dramatic results. An energy survey conducted for a pulp and paper company revealed 23 percent leakage in its fluid system. Once gaugeable fittings were put in, the leaks dropped to zero.
Swagelok, Solon, OH, provides connectors for fluid systems ranging from 1/16 in. to 2 in. o.d. Its tube connectors and valves are used in air systems, condensate return systems, hydraulics, pneumatics, analytical instrumentation, acid systems, caustic systems, and small bore process applications.
The company works with maintenance engineers to conduct energy surveys of their facilities. All fittings in a given area of a plant where gas (not liquid) service is common are tested for leaks. Once leaks are identified, the use of gaugeable, two-ferrule tube fittings to reduce problems, improve equipment reliability, and conserve energy is demonstrated.
In a typical scenario, a company representative together with plant personnel checks for air leaks in a compressed air system. Working from as many as 1000 check points, about 24-30 percent leakage is usually identified. This statistic is applied to the company’s cost per kilowatt hour and losses are determined. A performance contract to correct the problems is generated. Studies show that properly installed fittings correct leakage to less than 3 percent.
To ensure reliable performance, a tube fitting composed of four components–nut, back ferrule, front ferrule and body–is recommended. The consistency and quality of matched components permit their use in many difficult services. These fittings become a 5-piece connection when affixed to the tubing
The 2-ferrule design and sequential action of the fitting overcome variations in tube material, wall thickness, and material hardness to ensure safe, reliable, and leak-free connections.
Unlike a bite-type fitting which can cut into the tubing and result in a weak point that occasionally may vibrate and break off, a four-piece tube fitting is gaugeable, i.e., every quarter turn is about a 0.0125-in. movement. Consequently there is a go/no-go gauge that enables the person assembling the fittings to put them together and then gauge each one individually during the first makeup to ensure that every fitting will measure up to its properly installed performance factor (1¼ turns).
Leaks may also result from faulty valves, or more commonly from valves whose sealing and packing mechanisms are subject to wear or unsuitable applications. The challenge is to determine the specific application of each valve and choose the right valve for the job.
There are many different types of valves including shut-off valves, regulating valves, uni-directional valves, and pop-off relief valves available for a variety of applications.
Some valves can be pneumatically operated; some may be electrically operated, and some work manually. How frequently should valves be monitored? It is a good idea to check the valve packing and make adjustments periodically according to the cycle requirements of the valve. MT