Global Processing

What to do about fluid leakage

May 12, 2008
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.

The following discussion is aimed at minimizing leakage from process-piping, analytical –instrumentation, and utility lines, such as those for steam and compressed air. Such connections are typically within the size range of 1/16 to 2 in. (2-50 mm).

Causes of Leakage

System vibration, pulsation, and thermal cycling are all common causes for chemical-processing-system leakage.
One can 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” 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 widely researched. One study conducted has produced the Markl Fatigue relationship. This “stress curve” (Figure 1) provides a vertical axis S, which equates to the amplitude of alternating stress caused by vibration imposed on the test specimen (fitting connection) being examined. On the horizontal axis, N indicates the number of cycles to failure. This S/N curve illustrates the number of cycles generated and how soon the 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 which can relate to the deepness 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, are often overlooked as important factors when developing effective, efficient fluid handling systems. Two of the most critical areas contributing to leakage are:

• 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 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. Adoption of such an energy management program is an important factor in maintaining effective fluid handling systems, and will be discussed later.

Welded Pipe Fitting Considerations

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 specialized 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 of 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 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.

NPT fittings.

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 (Figure 2). 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 is significantly easier for the installer.
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 national pipe tapered 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 in industry for fluid systems, there is an inherent disadvantage to using pipe in both 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, an application with which chemical engineers are well familiar.

The Reynolds Number (Re) is expressed as Re = DV р/µ, where D is the inside diameter of the tube or pipe, V is the average fluid velocity, p is the fluid density, and µ is the kinetic viscosity. 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:

Compression fittings.

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 utilizes a friction grip (Figure 3) 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, it is available in just a few materials of construction (mostly brass), and does not often function well in systems having vibration, thermal cycling, and other dynamic forces. Additionally, disassembly and remake of this type of connection is very difficult because of variables in the fitting that may prevent the reseal in the same line-type area. Factors such as manufacturing inconsistencies, scratches or blemishes on the sealing surface can contribute to the compression design’s inability to reseal.

Flare fittings.

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 greater 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. Sealing with this design is dependent on the effectiveness of the flared tubing. A quality flare by the installer should provide a successful seal.

Bite-type fittings.

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. Original 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 make the grip, and provide a sealing element for the coupling body, an action which can too easily compromise one or both functions. A two-ferrule bite-type fitting performs separation of functions (the first, or front, ferrule to seal, the second, or back, ferrule to grip the tube). This separation permits each of the elements to be designed specifically for the task it is required to address. Once again, a line seal is being created, similar to that of the compression design, which may create resealing problems.

Mechanical grip-type fittings.

Mechanical grip-type fittings are typically two-ferrule in design (Figure 4). This fitting may also utilize a live-loaded seal characteristic, which means the fitting pull-up spring loads the front ferrule as it seals by coning 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.

A hinging-colleting feature of a mechanical grip-type fitting is the most desirable in achieving both tube grip and vibration resistance. Colleting of the gripping or back ferrule simply means that the more material of the back ferrule comes in contact with the tubing to reduce the damaging effects of system dynamics. As an example, if you place a notch in the middle of a wood pencil and apply downward pressure on either side with your hands, the obvious breaking or cracking point would occur at the notch. This same scenario may occur on bite-type fittings, where the gripping or biting ferrule places an indent or a stress riser on a single point on the tubing, which puts the fitting’s grip into a potential failure mode. In this same pencil example, colleting would be described as placing the notch in the middle of the pencil and wrapping your entire hand over the notched area. With this additional support, bending the pencil would not create breakage or cracking at the notch, but rather some place outboard of the notch, reducing the intensity of the indent or stress riser on the tube.

Another strong advantage that this design offers over the bite-type fitting is that disassembly and remake of the fitting after installation can be more successfully accomplished without damage to either the fitting components or the tubing. This is a result of a burnishing or polishing contact between the sealing ferrule and the body bevel, which does create an effective line, or point of contact seal, but also generates a back-up sealing area because the polishing effect provides a longer or secondary sealing. In addition, some manufacturers offer an inspector 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. While there are many types of energy management programs to consider, the following discussion and recommendations are outlined from the viewpoint of:

• process and instrumentation lines
• plant utilities ( compressed air, hot water, steam, and chilled water)
• hydraulic systems

The chemical industry is the second largest energy user within the U.S. manufacturing sector. Energy costs represent approximately nine percent of the value of shipments. To identify opportunities for energy conservation and cost saving measures, consider an energy audit, which can be performed by an experienced entity within your own organization.
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 utilization, 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 of the points can be identified as leaking. 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.

Gaugeable-tube Fittings

Using a gaugeable-tube fitting can often solve many leakage problems at a plant. After the connection has been made, a so-called no-go gauge is inserted between the hex of the fitting nut and body. If the gauge does not fit, in the case of several tube fitting designs, then the tube fitting has been safely and sufficiently installed to the recommended manufacturer’s installation instructions. Although there are multiple gauge types available on the market, the key objective is to assure safe reliable connections. The consistency, the quality and the ease of use in a gauge is imperative in its appropriate use.
As an example of the benefits of an energy survey and the use of gaugeable fittings, consider the following. One specific energy survey conducted for a pulp and paper company revealed 23 percent leakage in its pneumatic systems. When gaugeable tube fittings were installed, the leak rate dropped to zero. Typically, 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 tube fittings can lead to improved equipment reliability and energy conservation.

This case is just one example of how focusing on proper component selection, total system design, and energy management programs can help develop an effective, efficient fluid handling system.

If you would like to conduct an energy audit and require additional information, please contact John Cox, Business Development Manager at Swagelok Company, at john.cox@swagelok.com. Additional information and resources can be found on the Alliance to Save Energy web site at www.ase.org.