Mar-2008

Thwarting fluid-system leaks, saving energy, reducing leaks

Fluid leakage costs industry millions of dollars every year. For example, a few small leaks in a facility using air at 100 pounds per square inch gauge (psig), with an electric consumption cost of approximately 6 cents/kilowatt-hour (kWh), can waste more than $22 000 annually. Delaying the replacement of a leaking $100 steam trap could waste $50 per week.

John C Cox
Swagelok Company

Article Summary

Because an average facility typically has hundreds of steam traps throughout its operations, leaking traps could be squandering hundred of thousands of dollars each year. In addition to wasting money, unattended leaks can increase downtime, affect product quality, pollute the environment and cause injury.

Leakage culprits
System vibration, pulsation and thermal cycling are common causes of chemical processing system leakage.

Any type of fitting connection might leak, regardless of whether pipe or tube is used — especially when mechanical vibration is present. This “vibration fatigue” is an unavoidable factor that can be aggravated by poor metallurgical consistency within the fitting material construction, undue stress imposed on the connection from side-load or other system design characteristics, or simply improper installation practices.

Stress intensification and fatigue have been researched widely. One study produced the Markl Fatigue relationship. This relationship is a “stress curve” that illustrates the number of cycles generated and how soon the test specimen will fail after repeated stressing. The findings suggest that the greater the amplitude of alternating stress on a specimen, the sooner it will fail. A stress intensification factor, as it pertains to fittings, shows an exacerbated onset of failure that can relate to the depth of the groove or notch made in the pipe or tubing line by the fitting as it is tightened.

Preventing leakage
Proper selection of components and total system design, as well as product technology, often are overlooked when developing effective, efficient fluid-handling systems. Two of the most critical areas contributing to leakage are:
• The types of connecting devices used in joining process pipe throughout the system
• The level of knowledge and practical experience of those installing and maintaining the application.

Although the ideal connection — offering total leak-free operation in every system parameter requirement — realistically does not exist, it is worthwhile to evaluate the various fitting connection types available. In addition, regardless of the connection type selected, proper and effective system energy management must be a high priority. Adoption of such an energy management program is an important factor in maintaining effective fluid-handling systems.

Welded pipe fittings
The fitting connection most resistant to both vibration and fatigue is a pipe butt weld fitting. Its ability to resist vibration and fatigue is determined by the strength and integrity of the connection made.

However, pipe butt weld fitting connections do have some disadvantages. The welding equipment and specialised training required to make the connection can be costly. Additionally, the amount of time required to install pipe butt weld fittings into a system is greater than that required for other fitting installation options. The degree of knowledge required by the installer should be factored into the equation as well. Thorough training is essential to ensure quality weld connections are achieved. Finally, accessibility for maintenance in fluid system piping is minimal, unless maintenance people are prepared to carry a torch or hacksaw to cut their way into a system line.

Threaded pipe fittings
One of the most common types of connections found in process fluid handling systems is the threaded or screwed pipe fitting connection.

NPT fittings Used as a workhorse in industry since the inception of joining pipe, National Pipe Thread (NPT) fittings have a tapered thread on both the male and female ends. The seal is actually a “crush seal” between the joining metal surfaces, and occurs on the flank, crest and root of the tapered thread.

Because of the affinity metal has for itself, especially when mating carbon steel or stainless steel, galling and tearing of the metal will take place during the installation procedure. When joining NPT threaded connections, it is imperative for plants to apply lubricant or a sealant with a lubricating agent on the male threads to prevent damage to them. A popular thread sealant is polytetrafluoroethylene (PTFE) tape.

When using tape to lubricate or fill voids in the thread crest, root and flanks, plant personnel must be sure to:
• Limit tape application to two to three wraps of the male threads, which is sufficient with most tapes
• Never wrap tape over the end of the first thread, because the tape eventually will splinter and enter into the fluid-handling system and could damage the internals of system components
• Wrap tape in a clockwise direction when viewing the thread from the end of the fitting. If not wrapped in the correct direction, the tape will not properly lubricate, potentially resulting in leaks
• Cut off excess tape and draw the free end of the tape around the threads tautly to conform to the thread. Then, press on the tape firmly with thumb and index finger at the overlay point. If the crests of the threads protrude through the tape, galling might occur, so additional tape will be required
• Be sure to remove all excess tape and apply new tape prior to reassembling the threaded connections if the threads are disassembled for maintenance. Tape that has not been removed from initial installation could act as a leak point on subsequent assemblies.

SAE straight thread fittings Another thread type gaining popularity is the Society of Automotive Engineers (SAE) straight thread. The SAE straight threads are mechanical types, designed to hold only the fitting in place; SAE threads do not provide a seal. The sealing function is provided by an elastomer, typically located at the base of the male thread (see Figure 1). The elastomer compresses against a boss or flat surface near the entrance to the female port. This type of threaded seal offers the advantages of an NPT connection in that maintenance, accessibility and remake of the fitting are significantly easier for the installer.