FLUID LEAKAGE -- What to do?

FLUID LEAKAGE—What to do?
John C. Cox, Business Development Manager
Swagelok Company, Solon, Ohio
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.
Causes of Leakage
System vibration, pulsation, and thermal cycling are all common causes for
chemical 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.
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
1
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 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 (Figure 2). 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
2
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 3). 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.
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 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.
The Reynolds Number (Re), as depicted (Figure 4), 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
3
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 5) 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.
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 larger variety of materials, and has a
larger seal area (Figure 6), 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.
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 (Figure 7).
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 twoferrule in design. This fitting may also utilize a live-loaded seal characteristic. Fitting pullup 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
4
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. 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 9 % 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 % 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 % 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 3 %.
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.
Case in Point: Gaugeable Tube Fittings
One specific energy survey conducted for a pulp and paper company revealed
23 % 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
5
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 at [email protected]. Additional information and
resources can be found on the Alliance to Save Energy web site at www.ase.org.
6

Download Report