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Accommodating Thermal Movement:
Reliability developments for district energy systems
Edward W. Patnode, PE,
President
Advanced Thermal Systems Inc.
Third Quarter 2007, District
Energy Magazine
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In any steam or water energy
distribution system, whether heating or cooling, provisions
must be made for the thermal movement of the pipeline due to
the differential change in temperature between the minimum
design, installation and operating temperatures. Depending on
the piping configuration and system operating conditions,
there are several methods for accommodating the piping's
thermal expansion or contraction.
The most reliable method is to
allow the system to expand or contract freely, provided there
is sufficient natural flexibility inherent in the piping
arrangement. This requires a stress analysis to ensure the
pipe stresses are within the piping code limits. If the piping
fails to meet the code limits, the use of expansion
compensation devices such as bellows joints or packed-type
expansion joints should be considered. Of these, packed slip
joints and ball joints have proven to be among the most
reliable methods of accommodating the thermal changes.
Developments in the design of these expansion devices have
resulted in the uninterrupted operation of district energy
systems and a service life comparable to the piping in which
they are installed.
Today's trend is to distribute
steam and hot water at higher temperatures and pressures than
were common 20-plus years ago. This is especially true with
the resurgence of combined heat and power systems. In addition
to the elevated design conditions, the demand for reliability
and uninterrupted service has also increased due to steadily
increasing energy and maintenance costs. To meet these
demands, the 'packed' expansion joint and flexible ball joint
have undergone major improvements resulting in their increased
reliability for district energy systems today.
- Today's trend is to
distribute steam and hot water at higher temperatures and
pressures than were common 20-plus years ago.
The Evolution of Packed Expansion Joints
The establishment of the district heating
industry prompted development of a device to absorb the axial
expansion and contraction of the steam distribution system
pipes. The first such device was referred to as a gland packed
expansion joint. It was constructed of a cast-iron body with a
stuffing box having several rings of graphite/asbestos rope
packing and an adjustable gland. Refined over the years, this
was the most widely used expansion device for many decades.
Early steam distribution systems operated at
low pressures and temperatures, and the gland packed expansion
joints performed well. Their major drawback was the need to
depressurize the system to add or replace packing, which
resulted in loss of revenue due to downtime and maintenance
costs.
Over the years many changes were made to
help reduce the maintenance issues associated with gland
packed expansion joints. One of the first attempts was the use
of a double-wall slip design. This arrangement included a dead
air space between two cylinders to reduce the temperature to
which the packing would be subjected, thereby extending the
life of the packing. Although this worked in some
applications, the system still had to be shut down to add or
replace packing.
A further attempt was the introduction of
the piston ring-type expansion joint. This design incorporated
a series of piston rings at the inner end of the slip,
permitting the joint to be unpacked and repacked under full
line pressure. This design was popular for a short time only;
after a few years of service the piston rings would foul,
making it necessary once again to depressurize the system to
add packing.
Injectable packing was first introduced
during the late 1940s. With this design, a flowable
semiplastic packing material was injected into the stuffing
box. The concept was widely accepted by the district heating
industry as a more efficient method to accommodate pipe
expansion and contraction and contain leakage without system
depressurization. A problem with this design, however, was the
need to lubricate the packing to keep it in a flowable
semiplastic state. The lubrication requirement proved to be a
'damned-if-you-did-and-damned-if-you-did-not' scenario. If the
joint was not lubricated two to three times a year, the semiplastic packing would become hard and no longer flow
towards a leak when additional packing was injected. If on the
other hand, the joints were lubricated as prescribed, in time
the lubricant would carbonize and coat the slip with a hard,
rough deposit that would often cut a leak path when the slip
moved. In any event, this type of expansion joint proved to be
more maintenance-free than any previous design of the gland
packed type of expansion joint.
In 1968 another major improvement to the
'packing-injection-under-pressure' expansion joint (fig. 1) was introduced through the development of a self-lubricating injectable packing consisting of asbestos fibers encapsulated
in Teflon®. By the mid-1970s the Teflon/asbestos injectable
packing had become widely accepted and was offered by all
manufacturers of packed expansion joints. This packing
material, however, did have limitations. Teflon-based
materials are only recommended for service temperatures up to
500 degrees F; once the steam distribution system operating
conditions rose above this level it became necessary to look
for an alternative.
Figure 1. Typical Packed
Slip-Type Expansion Joint Assembly.
Source: Advanced Thermal
Systems Inc.
In 1977, the use of
self-lubricating, flake graphite injectable packing was
introduced. In addition to being asbestos-free, flake graphite
is suitable for continuous operation at temperatures up to
1,000 F in a nonoxidizing environment. Since its introduction,
flake graphite injectable packing has proved to be suitable
for a wide variety of service conditions including steam, hot
water, chilled water, gases, oils and jet fuel.
Today, all injectable packing
materials are self-lubricating, but not all are effective at
containing leakage. Currently flake graphite is the only
asbestos-free injectable packing that has a proven record of
performance for service temperatures over 500 F.
Avoiding Slip Collapse
Prior to the introduction of the
'packed-under-pressure' expansion joints there was no standard
applied to the design of the sliding slip portion of the
joints. One manufacturer machined the sliding slips from extra
heavy wall pipe for all sizes of expansion joints; another
machined slips from extra-heavy pipe to 16-inch size and
Schedule 40 for larger sizes.
The advent of the
packed-under-pressure expansion joint resulted in a new
phenomenon called 'slip collapse'. Slip collapse occurs due to
the localized high-pressure load directly beneath the packing
cylinders – the result of either an improper factory packing
injection process or the addition of packing over a period of
time. In most cases the user did not realize the slip had
collapsed until a major temperature change occurred (such as
slip extension during depressurization of the system for
yearly maintenance or major temperature spike), at which time
the slip would freeze, causing anchor failures.
To reduce the potential for
slip collapse, the current standard, as outlined in ASTM
F1007, is Schedule 80 pipe for all sizes to 16-inch inclusive
and Schedule 60 for sizes 18-24 inches inclusive. It is
suggested that all users incorporate the slip wall thickness
(prior to machining) in their specifications. Heavier wall
thickness may be required for high-pressure and/or
large-diameter applications. For example, 30-inch joints are
usually furnished with slips having a wall thickness of 1
inch; and 36-inch joints with 1-1/4-inch wall slips regardless
of operating pressure.
Packing slip-type expansion
joints through the packing cylinders requires that several
pounds of injectable packing (depending on joint size) be
forced through the small opening at the tip of the cylinder.
This can result in localized pockets of highly compacted
injectable packing as well as poor packing distribution. To
ensure even packing density and distribution, it is
recommended that the design of the slip joints include
factory-fill connections in addition to the packing cylinders.
These connections should be plugged and welded prior to
shipment to preclude the possibility of field removal.
The Intricacies of Chrome
Plating
The term 'hard chrome plate'
has become so well-established for engineering applications of
chrome that we now use it without apologizing for its
inexactness. The term 'thick chrome plate' would be more
appropriate. A minimum of 2 mils (0.002 inches) of hard chrome
is recommended to ensure sufficient corrosion protection.
Some manufacturers of
slip-type expansion joints offer what is essentially
decorative chrome plating as an alternative to hard chrome.
This extremely thin chrome plate serves as a nontarnishing
outer coat for the underlying nickel or copper, which prevents
the steel base metal from rusting. Although this flash coating
of chrome over a softer metal offers excellent corrosion
resistance, it will not provide wear resistance to preclude a
tendency to peel as the slip moves through its integral guide
surfaces. As a result, decorative chrome plating is not
recommended.
Prior to any type of plating,
it is extremely important that the base metal be properly
prepared. Grinding and polishing to a 16 root mean square
finish plus a second polish after plating will produce the
best surface for lubricity, wear and corrosion resistance.
Guiding Slip-Type Joints
Packed slip-type expansion
joints are intended to only accommodate axial movement. Thus
it is essential that the pipe entering the slip end of the
joint be properly guided. Inadequate guiding can result in the
slip binding, which can cause damage to the sliding slip and
possible anchor failure. There are two types of guides
currently available for use in piping systems: radial and
low-friction (fig. 2). Radial guides are essentially
noncontact guides. They are designed to include a gap and are
intended to limit only the lateral and vertical displacements
of the pipe. Radial guides should never be used as a pipe
support. Low-friction guides, on the other hand, are actually
designed to act as both a guide and support. This type of
guide limits the lateral and vertical upwards movement of the
pipe while providing support for the system. The guide design
selected is dependent on the piping system arrangement, and it
is recommended that the expansion joint manufacturer be
contacted if there are any questions regarding guide
selection.
Figure 2. Typical Pipe
Guide Designs.
Source: Advanced Thermal Systems Inc.
There are currently no
published industry standards covering the spacing of guides
when used with slip-type expansion joints. Each manufacturer,
however, offers recommended guide spacing in part based on the
design clearances within their expansion joints.
- There are currently no
published industry standards covering the spacing of guides
when used with slip-type expansion joints.
To reduce the potential for slip damage when misalignment
occurs, manufacturers developed internal guides built into the
expansion joint body. These guides were initially made from
either a brass or bronze material. Although the use of these
guides significantly reduced the potential for slip damage,
the coefficient of friction between the guides was not
significantly different from steel on steel; any misalignment
and subsequent binding of the slip that occurred still
resulted in elevated anchor loads.
In 1977 a new low-friction guide insert was introduced made
of nonmetallic bronze-filled Teflon, providing protection to
the slip of packed joints operating at all temperatures to 500
F. For temperatures above 500 F, the only material available
to date for low-friction inserts from any manufacturer is
solid brass or bronze. For temperatures above 650 F, aluminum
bronze is recommended. Brass or bronze guides will only resist
scoring of the chrome-plated slip and prevent corrosion of the
guide surfaces. The density of solid brass or bronze will not
compress to relieve a binding condition. All manufacturers can
offer the low-friction bronze-filled Teflon for guide inserts,
but this material must be specified.
Flexible Ball Joints Enter the Mix
Ball joints have been used to
accommodate movements in piping systems for more than 50
years. During this time ball joint design and sealing concepts
have changed. Many of these changes can be directly associated
with the design improvements incorporated on slip-type
expansion joints. The concept of packing under pressure, the
types of packing cylinders offered, as well as the
chrome-plating process used were all taken directly from slip
joints.
The first ball joints were
designed to seal using soft seats, which centered the ball
within the body of the joint and provided the sealing
surfaces. Since the seals needed to be relatively resilient to
effect a seal, as the ball joint flexed during service, the
materials used were limited in terms of the temperatures they
could withstand. In general, these early designs were limited
to a maximum service temperature of 500 F. This design
required periodic adjustment of the bolted retainer flange to
contain leakage and removal of both flanged and weld-end ball
joints from the line to replace either of the soft seals.
In 1978 the first ball joints
designed for packing injection under full line pressure were
introduced. The original design of the packed ball joints was
developed using many of the same parts as were used for the
soft-seated design, including the bolted retainer flange. The
injectable packing used was the same flake graphite packing
that had been employed for years with slip-type expansion
joints. With this design, ductile iron rings replaced the soft
seats and centered the ball within the body of the joints, and
injectable packing was used to provide the required sealing.
The development of the packed ball joint significantly
extended the pressure temperature capabilities of the joints
in addition to providing more dependable leak-free
performance. Packed-under-pressure ball joints are in use
today for systems operating in excess of 3,000 psig at 800 F.
Because it was no longer
necessary to adjust the retainer flange to ensure proper
sealing, manufacturers further improved packed ball joints in
1985 by introducing an integral socket retainer design. The
bulky, bolted-retainer flanges or retainer caps were
eliminated in favor of a more compact welded design (fig. 3).
Figure 3. Typical
Packed Ball Joint Assembly.
Source: Advanced
Thermal Systems Inc.
The integral welded-socket
retainer design offers the following advantages over the older
bolted-retainer flange design:
- Eliminates the in-service
field error of over-tightening the retainer flange bolting,
which greatly increases the flex torque and may cause
‘freezing’ of the ball within the socket.
- Eliminates the need for
stainless steel bolting when the ball joint is installed in
a corrosive environment and/or must handle a corrosive
fluid.
- Results in a more compact,
lighter ball joint that is easier to install and insulate.
Today packed-under-pressure
ball joints have all but replaced the older soft-seated
design. Soft-seated ball joints are still in use, but only for
special applications where cleanliness is required or where
the introduction of the graphite injectable packing into the
system would be unacceptable. Clean steam and oxygen supply
lines in hospitals and filtered jet fuel are just a few of the
systems where soft-seated ball joints are still specified.
Many of the improvements made
to both slip- and ball-type expansion joints over the years
are the direct result of the dedication of member utility
companies, universities, consulting engineers and expansion
joint manufacturers that comprise the International District
Energy Association. Their input and support have resulted in
safer and more reliable expansion joint options.
Edward W. Patnode, PE,
is president of Advanced Thermal
Systems Inc. Prior to joining the company in 1995, he worked
for General Electric Co. in the large steam turbine
department. A licensed professional engineer in New York, Patnode holds both bachelor’s and master’s degrees in
mechanical engineering from Union College in Schenectady, N.Y.
He may be contacted at
edpatnode@advancedthermal.net.
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