LEAKAGE costs industry millions of dollars every year. For example, a few small leaks in a facility using air at 100 psig, with an electric consumption cost of about 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; since an average facility typically has hundreds of steam traps throughout its operations, leaking traps may be squandering hundred of thousands of dollars each year. In addition to wasted dollars, unattended leaks can result in downtime, affect product quality, pollute the environment and cause injury.
System vibration, pulsation and thermal cycling are all common causes for processing system leakage.
Assume that any type of fitting connection may leak, regardless of whether pipe or tube is used, especially when mechanical vibration is present.
This “vibration fatigue” would be 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.
PROPER selection of components and total system design, as well as product technology, are often overlooked as important factors when developing effective fluid handling systems.
Two of the most critical areas contributing to leakage are:
Types of connecting devices used in joining process pipe throughout a 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 a quest to help prevent system leakage.
In addition, regardless of the connection type selected, proper and effective system energy management must be a high priority.
Welded pipe fitting considerations
THE fitting connection most resistant to vibration and fatigue is a pipe butt weld fitting. Its ability to resist is determined by the strength and integrity of the connection made.
However, such connections have 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 is greater than other fitting installation options.
The degree of knowledge required by the installer should be considered as well. Thorough training is essential to ensure that 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 fitting considerations
ONE of the most common types of connections found in process fluid handling systems is the threaded or screwed pipe fitting connection.
Used as a workhorse in industry since the inception of joining pipe, NPT (National Pipe Thread) 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. Due to 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 to apply lubricant, or a sealant with a lubricating agent, on the male threads to prevent damage to them. A popular thread sealant is PTFE tape.
The following factors are important to consider when using tape to lubricate or fill voids in the thread crest, root and flanks:
When applying tape to the threads, two to three wraps of the male threads is sufficient with most tapes.
Never wrap tape over the end of the first thread, as tape will eventually splinter and enter into the fluid handling system, which may damage the internals of system components.
Wrap tape in a clockwise direction as you are viewing the thread from the end of the fitting. If not wrapped in the correct direction, the tape will not properly lubricate, potentially causing 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 may occur, so additional tape will be required.
If threads are disassembled for maintenance, be sure to remove all excess tape and apply new tape prior to reassembling the threaded connections. Tape that has not been removed from initial installation may act as a leak point on subsequent assemblies.
SAE STRAIGHT THREAD FITTINGS
Another thread type gaining popularity is the SAE (Society of Automotive Engineers) straight thread. The SAE straight threads are mechanical types, designed to only hold 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. 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.
There’s more to come
OTHER types of threads found in fluid handling systems include ISO parallel and tapered threads, NPTF dryseal threads, and 37o AN flare fittings.
ISO parallel and tapered thread fittings: ISO (International Standards Organization) thread fittings work similarly to NPT tapered thread fittings, relying on threads to perform the sealing characteristics, and SAE straight threads, using either an elastomer, bonded metal washer or gasket as a backup seal.
NPTF dryseal fittings: Dryseal threads have roots that are more truncated than the crests, so an interference fit causes the roots to crush the crests of the mating threads. The theory behind this thread concept is that when the crest, root and flank of the threads are engaged, there is always mating contact, creating a seal without lubrication. Unfortunately, due to inherent properties of some metals such as carbon steel and stainless steel, galling will occur in this type of seal without lubrication, making initial installation difficult and remake impossible.
37o AN flare fittings: These fittings use straight mechanical threads similar to the SAE and ISO straight or parallel thread design. These straight threads are used only for holding, while a 37o male flared end, machined on the end of the fitting, mates with a female flared surface at the base of the female threaded port. This type of connection is found predominantly in hydraulic applications and is commonly referred to as an AN [Army-Navy] fitting.
Disadvantages of threaded connections
ALTHOUGH threaded connections of any type have been a popular fitting choice for fluid systems, there is an inherent disadvantage to using pipe in process and instrumentation lines.
Pressure drop or head loss due to friction from the internal surface of a piping system can prevent applications from achieving necessary flow characteristics. This pressure drop effect may be illustrated through application of the Reynolds Number, combined with internal geometry.
The Reynolds Number (Re) is equal to the inside diameter of tube or pipe, multiplied by average fluid velocity [V], multiplied by fluid density [p], divided by kinetic viscosity [µ]. An internal friction factor is calculated by first determining the Reynolds Number for the fluid flow in the pipe. Then by combining the relative roughness of the pipe surface with the Reynolds Number, the friction factor is determined.
Tests conducted with this formula indicate that due to the internal surface roughness of pipe versus tube, flow in pipe typically will be more turbulent and will require greater pressure drop.
Furthermore, to create a directional change with pipe, 45o or 90o elbows must be used. Elbows impose abrupt ID changes and rough edges, adding to turbulence and even greater pressure drop. Although directional elbows are available for tubing systems, the ability to bend tubing provides a smoother transition, reducing the amount of pressure drop or turbulence created.
Tube fitting considerations
TUBING also offers a variety of fitting selections for making connections:
The compression fitting, which was the first tube fitting to be developed, is made up of three components: nut, body, and gasket ring or ferrule. This design utilises a friction grip on the tube. One advantage is that no special tools are required in assembly, unlike pipe connections, which require thread chasers and dies to make the threads. Further, the seals can be (but are not always) line-type, which creates a dominant force in one small area and is one of the most effective metal-to-metal seals available. However, the disadvantages of this type of connection are that it can withstand only minimal pressure due to the friction grip only, is available in just a few materials (mostly brass), and does not often function well in systems having vibration, thermal cycling and other dynamic forces.
The flare fitting was the next variation in tube fitting designs. As compared to the original compression fitting, the flare fitting can handle higher pressures and wider system parameters, is available in a larger variety of materials, and has a larger seal area, which provides remake capabilities in maintenance applications.
The fitting is made up of three components: nut, sleeve, and body with a flare or coned end. In some instances, the sleeve is used as a self-flaring option, usually on thinner wall or softer tubing materials. The disadvantage of this fitting is that ease of assembly takes a step backwards. Special flaring tools are required to prepare the tubing for installation. Additionally, flaring of the tubing may cause stress risers at the base of the flare or cause axial cracks on thin or brittle tubing. Uneven tube cuts with poorly designed rotational tube cutters or ineffective hacksaws will create an uneven sealing surface.
The bite-type fitting needs no special tools for assembly and accommodates higher pressure ratings than the original compression design. This design is comprised of a fitting with a nut, body and ferrule(s) having a sharp leading edge, which bites into the skin of the tubing to achieve holding ability.
A second seal is made on the long, deep surface between the ferrule and internal body taper. Bite-type fittings are typically single ferrule in design. This requires the nose of the ferrule to perform two functions: bite into the tube to hold it and provide a sealing element for the coupling body, an action which can too easily compromise one or both functions. A two-ferrule separation of functions (the first to seal, the second to hold the tube) would solve this problem, as the separation would permit each of the elements to be designed specifically for the task it is required to address.
MECHANICAL GRIP-TYPE FITTINGS
Mechanical grip-type fittings are typically two-ferrule in design. This fitting may also utilise a live-loaded seal characteristic. Fitting pull-up spring loads the front ferrule as it seals by coining the surfaces of the tubing and coupling body. A radial colleting or holding action of the back ferrule grips the tube for a distance just out-board from the tube holding point of the ferrule nose to enhance vibration resistance.
Another strong advantage that this design offers over the bite-type fitting is that break and remake of the fitting after installation can be more successfully accomplished without damage to either the fitting components or the tubing. In addition, some manufacturers offer a gauge to ensure proper and sufficient pull-up on initial installation. Under-tightening of tube fittings, especially in harder materials such as stainless steel, is considered a major cause for tube fitting leakage.
Energy management programs
IN addition to selecting the proper fitting for a system, process system energy management can also be an important factor in maintaining effective fluid handling systems.
Periodic maintenance plays an important role in reducing energy consumption and costs. For example, consider compressed air leaks, clogged filters and warm air leaks into the compressor. Steam system auditors have documented that a typical plant, without a preventive, predictive maintenance program in place, will have approximately 28 percent of its steam traps in a failure mode at any given time.
To significantly improve steam utilisation, employ proper testing of steam traps to identify leakage, repair the leaks and, when appropriate, replace steam traps not working properly.
Another example of important periodic maintenance can be found in checking for air leaks in a compressed air system. Working from as many as 1000 check points in a typical system, about 24 to 30 percent leakage can be identified. This statistic is then applied to the company’s cost per kilowatt-hour and losses are determined. A performance contract is established to correct the problems. Studies show that properly installed fittings from certain manufacturers correct leakage to less than three percent.
The audit should encompass energy supply and consumption, including a detailed analysis of the past year’s energy bills. Energy supply considerations will show the current rate schedule and costs from alternative suppliers. Opportunities for energy efficiencies will begin surfacing as this work continues. Energy and cost savings calculations should include estimated costs for implementation.
* Commentary by John Cox, business development manager, Swagelok Company. Australian distributor is Fluid System Technologies (ES) .