Steam Traps

Steam Traps
1.10 STEAM AND WATER LEAKAGE
1.10.3 Use the most efficient type of steam
trap for each application.
MEASURE
Steam traps are vital components of a steam system.
If they do not perform properly, they can be a source of
major energy and water loss. Normal wear of traps is a
major cause of steam leakage. Steam traps can fail in a
way that effectively creates a hole in the steam system.
Using the wrong type of trap can cause improper
operation of the steam-using equipment, damage piping,
and destroy the trap itself. Loss of steam through a trap
is almost invisible, because the steam disappears into
the condensate system.
Make sure that each steam trap is individually
matched to its application in terms of type and size.
About half a dozen different types of steam traps are in
common use today. The variety of types results from
differences in application requirements and differences
in cost. Trap types differ in reliability and time-betweenoverhaul. Some types of steam traps have higher average
leakage than others. The cost of the trap should be a
lesser factor in selecting one type of trap over another,
because the trap cost is dwarfed by the cost of wasted
steam.
In existing systems, you may find traps that are not
the best type for the application. Proper trap application
requires a certain amount of specialized knowledge, so
traps may have been installed without considering all
the relevant factors. Also, facility operators tend to favor
trap types that are cheap and compact.
This Measure gives you an introduction to steam
trap selection, with an emphasis on selecting traps that
have the lowest average leakage over the life of the plant.
Supplement this with manufacturers’ literature for the
specific types of traps that apply to your facility.
187
RATINGS
New Facilities
Retrofit
O&M
A B
SUMMARY
Inappropriate types of steam traps waste
steam. Select trap sizes using manufacturers’
procedures.
SELECTION SCORECARD
Savings Potential ...................
Rate of Return, New Facilities
Rate of Return, Retrofit .........
Reliability ...............................
Ease of Retrofit ......................
Once you understand trap selection, survey all the
steam traps in your facility to determine whether each
is the appropriate type and capacity for its application.
Finally, replace all inappropriate traps with the
proper types and capacities.
This Measure deals with selecting steam traps and
replacing inappropriate or less efficient types. See
Measures 1.10.4 ff for trap maintenance.
The Two Applications of Steam Traps
Steam traps are devices that block the passage of
steam while allowing liquid condensate to pass. Traps
are used in two general applications, which are illustrated
in Figure 1:
• on the outlets of steam-using equipment, to keep
steam confined inside the equipment until it has
Armstrong International, Inc.
Fig. 1 The two functions of steam traps One is to keep steam inside heating equipment until it has condensed and given up
its latent heat. The other is to drain condensed water from steam lines before it damages the system.
© D. R. Wulfinghoff 1999. All Rights Reserved.
1. BOILER PLANT
188
given up its heat, i.e., to keep the steam from
blowing straight through the equipment
• on steam lines, to remove condensate that forms in
the pipe. Condensate forms because of heat loss
from the pipe. If allowed to accumulate, slugs of
condensate propelled by steam pressure may
accumulate enough kinetic energy to destroy valves,
piping, and equipment. Traps used to drain steam
lines are called “drip traps.”
In many steam systems, the traps also serve as an
important means of removing air and other noncondensible gases from the system. If these gases are
not removed, they may block the flow of steam. For
example, in a steam coil with many circuits, air that
accumulates in the coil causes most of the steam to be
routed through a few of the circuits, rendering the rest
of the coil useless. Also, non-condensible gases are the
source of corrosion in steam pipe and equipment.
Oxygen corrodes steel directly, while carbon dioxide
forms carbonic acid, which causes acid corrosion.
To minimize these problems, most applications
require steam traps to have the ability to remove, or
“vent,” non-condensible gases (i.e., gases other than
steam). Traps need a large venting capacity to clear air
out of the steam system after a period of shutdown. Traps
need a smaller venting capacity to vent the system
continuously while it is in operation, to remove gases
that are carried in the steam.
Steam traps usually vent non-condensible gases into
the condensate system. The location of the traps may
not allow complete venting of the system. Parts of a
steam system may trap air by letting it become stagnant,
so it is not carried along by steam flow. Separate air
vents are installed in these parts of the system. These
vents typically discharge air directly to the atmosphere,
rather than to the condensate system.
How Steam Trap Leakage Wastes Energy
Most of the useful energy of steam is in the form of
latent heat, which is the heat required to turn liquid water
into steam inside the boiler. For a steam system to
operate efficiently, all the steam must condense inside
the steam-using equipment, so that the latent heat is
transferred to the heating application. If steam leaks
through a trap, most of the energy of the leaked steam is
wasted.
Little or none of this energy is recovered on the way
back through the condensate system. Steam that leaks
through a trap into the condensate system is condensed
by conductive heat loss through the pipes of the
condensate system. In an atmospheric or vacuum
condensate system, any steam leakage that is not
condensed in the pipes is blown out of the condensate
system vents, which are typically located at the
condensate receiver.
Causes of Steam Trap Leakage
A large part of this Measure deals with the leakage
characteristics of different types of steam traps. Before
getting into the individual types, it is worth noting that
all steam traps may leak for these reasons:
• inherent leakage of the trap design. In principle,
all types of traps, except orifice traps, are capable
of blocking steam completely. However, all traps
will leak eventually. Field experience suggests that
some trap types tend to operate longer before
developing leakage. Also, some types of traps will
leak if they are not installed in a certain way,
whereas the method of installation does not cause
leakage in other types.
• sticking in an open or partially open position. All
types of traps are subject to complete failure as a
result of fouling, corrosion, or mechanical failure.
Failure in the open position is the equivalent of
having a hole in the system the size of the trap’s
internal discharge passages. Failure in the closed
position causes steam equipment to cease operating
because it cannot discharge condensate. Failure of
drip traps in the closed position is dangerous
because slugs of condensate remain in steam lines.
Some traps tend to fail in an open position, while
others tend to fail in a closed position. This
tendency may also be influenced by the
characteristics of the steam system.
• wear and fouling of sealing surfaces. All types of
steam traps (except orifice traps) block the flow of
steam by metal-to-metal contact of sealing surfaces.
Even if a trap continues to close properly, it will
eventually develop leakage because of steam
abrasion, fouling, and hammering of the sealing
surfaces. Once leakage begins, it typically
progresses rapidly. Some types and models of traps
develop this type of leakage more quickly than
others.
Steam loss also depends on the size of traps. Larger
traps can waste more steam, and they cost more to
replace. Survey all your traps, large and small, to make
sure that each is appropriate for its application.
Steam Trap Types and Leakage Characteristics
The following is an introduction to the types of
steam traps that are presently in common use, with
emphasis on their steam leakage characteristics. The
trap types are covered in approximate order of
condensate draining capacity.
This comparison of efficiency characteristics
requires broad generalizations. Information on trap
leakage provided by manufacturers is sketchy and may
lack credibility. One problem is that all types of traps
can leak seriously under certain conditions, so any
comparison depends on assumptions about the
application. The major trap manufacturers tend to be
ENERGY EFFICIENCY MANUAL
1.10 STEAM AND WATER LEAKAGE
189
cautious about pointing the finger at certain trap types,
probably because they make a variety of types
themselves.
Float-and-Thermostatic (F&T) Traps
F&T traps are used for draining steam equipment
and as drip traps. They are one of the most popular
types, and they are used in a wide range of sizes.
Figure 2 shows how they work.
As the name implies, a float-and-thermostatic trap
is a combination of two separate devices, a float trap
and a thermostatic vent. The float trap is the essence of
simplicity. It consists of a chamber with a discharge
valve at the bottom. The discharge valve is actuated by
a float and lever. When the chamber is dry, the weight
of the float keeps the valve closed. When the chamber
fills with condensate, buoyancy lifts the float and opens
the discharge valve. The float is usually spherical in
shape to resist the pressure of the steam.
Float traps are efficient in separating condensate
from steam because the trap directly senses the presence
of condensate through its great buoyancy. Furthermore,
the float ball can be made as large as needed to provide
a strong operating force for the condensate valve. Steam
cannot leak through the trap because the discharge is
located under water.
A simple float trap cannot vent air. It would
eventually fill with air and keep the valve from opening
(“air bind”). To prevent this, virtually all float traps
include a separate thermostatic valve near the top of the
trap. This gives them the name “float-and-thermostatic.”
The thermostatic valve remains open until the trap heats
up. F&T traps are able to vent large amounts of the air
from the system, which is an important feature.
In F&T traps, the thermostatic element can become
a source of steam leakage. Fortunately, the thermostatic
WESINC
Armstrong International, Inc.
Fig. 2 How float-and-thermostatic (F&T) traps work
Fig. 3 Typical F&T traps The chubby shape provides space
for the float ball to swing up and down. The thermostatic
element is on the upper left.
© D. R. Wulfinghoff 1999. All Rights Reserved.
1. BOILER PLANT
190
element in an F&T trap wears out more slowly than the
element in a thermostatic trap. This is because it remains
closed most of the time, opening only when a volume
of air accumulates inside the trap.
Float traps are vulnerable to dirt. The discharge
valve is located near the bottom of the trap, where large
dirt particles may accumulate. Also, float traps can
operate steadily in a partially open position, which allows
debris to become lodged between the plug and seat of
the discharge valve. The protection against dirt is to
buy a trap that has an integral strainer, or to install a
strainer ahead of the trap.
You can recognize F&T traps from their large,
rounded shape, which accommodates the spherical
float ball inside. Figure 3 shows typical units.
Armstrong Inernational, Inc.
Fig. 4 How inverted bucket traps work
ENERGY EFFICIENCY MANUAL
1.10 STEAM AND WATER LEAKAGE
191
and be discharged. The vent hole also allows a small
quantity of steam to escape from the bucket into the
main body of the trap, where it condenses. The presence
of the vent hole also allows the trap to open more quickly
when condensate enters it. In normal operation, loss of
steam through the vent is limited by the rate at which
steam condenses in the trap, which is small.
Inverted bucket traps are resistant to dirt because
the discharge valve is located at the top of the trap, away
from dirt that settles in the bottom of the trap body. The
discharge valve opens abruptly and fully, so dirt carried
in the condensate does not become lodged in the valve
seat.
Inverted bucket traps are most likely to fail in the
open position. They fail in the open position if they run
dry, because then the bucket cannot float. (This problem
is most likely to occur if the steam is superheated.) They
may also fail from misalignment of the internal
mechanical linkage. When they fail in the open position,
the size of the discharge orifice is the only factor that
limits steam loss.
You can recognize inverted bucket traps from their
cylindrical shape, which conforms to the shape of the
bucket inside. Figures 5 and 6 show two different
models.
Armstrong International, Inc.
Fig. 5 A small inverted bucket trap draining a steam-fired
space heater
Inverted Bucket Traps
Inverted bucket traps are used for draining steam
equipment and as drip traps. They are used in a wide
range of sizes. Figure 4 shows how they work.
An inverted bucket trap is built around a floating
bucket that has its open side facing downward. Steam
entering the trap is fed into the bucket, causing it to
float to the top of the surrounding pool of condensate
and close the discharge valve. When only condensate
enters the trap, the steam in the bucket condenses and
the bucket sinks, opening the discharge valve. Thus,
condensate drains in cycles.
Inverted bucket traps allow little steam leakage when
they are operating properly because the buoyancy of
the bucket provides strong closure of the discharge valve.
As with float traps, inverted bucket traps are relatively
reliable because the bucket can be made as large as
needed to provide a strong operating force for the
condensate valve.
The top of the bucket has a small vent hole. This
allows non-condensible gases to escape from the bucket
Armstrong International, Inc.
Fig. 6 A stainless steel inverted bucket trap installed as a
drip trap Note the test valve installed on the discharge (left)
side of the trap.
© D. R. Wulfinghoff 1999. All Rights Reserved.
1. BOILER PLANT
192
Thermostatic Traps
Thermostatic traps are used for draining steam
equipment and as drip traps. They are most common in
smaller capacities. Figure 7 shows typical units, which
are characterized by small physical size.
Thermostatic traps operate by sensing the difference
between the temperature of live steam and the
temperature of condensate or non-condensible gases that
cool inside the trap. Their operating principle is simple:
steam cannot cool below its condensation temperature,
but condensate and non-condensible gases can cool.
When a thermostatic element inside the trap senses a
temperature lower than steam temperature, it assumes
that it is surrounded by water and opens the discharge
valve.
Presently, there are two main categories of
thermostatic traps:
• bimetallic traps, which consist of a valve that is
moved by a simple bimetallic element that is located
on the steam side of the valve. This type has a
fixed operating temperature, so it cannot respond
to changes in the pressure and temperature of the
steam.
Bimetallic traps are compact because they require
space only for the small thermostatic element. The
housing may have any shape.
Armstrong International, Inc.
Fig. 7 A pair of thermostatic wafer steam traps The main
attraction of these is their small size and low cost.
• bellows traps close a discharge valve using the
pressure of a boiling fluid contained inside a
bellows. The fluid in the bellows is selected to boil
at a temperature lower than steam temperature, so
the bellows wants to expand when steam is present.
The pressure inside the bellows is partially balanced
by the steam pressure, so the opening temperature
of the bellows trap can adapt to changes in steam
pressure. Figure 8 shows how a bellows trap works.
This type of trap should not be exposed to
superheated steam, because the pressure inside the
bellows may exceed the steam pressure enough to
burst the bellows.
Armstrong International, Inc.
Fig. 8 How a thermostatic bellows trap works
ENERGY EFFICIENCY MANUAL
1.10 STEAM AND WATER LEAKAGE
193
Bellows traps are small. Some have a cylindrical
housing that conforms to a cylindrical bellows.
Others have a compact housing that surrounds
capsules of different shape, one of which resembles
a large button.
Thermostatic traps are prone to leakage because they
open and close relatively slowly. This provides time
for erosion by high velocity steam and water while the
sealing surfaces are barely separated. The partially open
discharge valve may trap dirt, preventing tight closure.
The piping layout of thermostatic traps may be
critical. The trap does not open until the condensate
cools somewhat. If the condensate does not cool quickly
enough, condensate may back up into the steam
equipment or steam line. Therefore, install thermostatic
traps so that they are surrounded by air that is
substantially cooler than steam temperature. With lowpressure steam, the trap may have to be installed at a
certain distance from the equipment or steam lines. Do
not insulate the trap or the pipe that leads to it.
A standard thermostatic steam trap may be used as
an air vent. You can recognize this application from the
fact that the trap is installed at a high point on the steam
equipment or pipe, rather than at a low point.
Disc Traps
Disc traps are used primarily as drip traps and for
low steam loads, such as steam tracing lines. They are
used in smaller capacities. (Disc traps are sometimes
called “thermodynamic” traps. This may be a derivation
of the name Thermo-Dynamic, which is a trade mark of
Sarco Spirax, the original producer of disc traps.)
A disc trap consists of a flat disc resting on a circular
seat that is smaller than the disc. The disc is enclosed
in a chamber above the seat, where it moves freely.
Figure 9 shows how simple these traps are in
construction. Figure 10 shows how they work.
Condensate or steam enters the trap through the
center of the seat, flows over the seat, and discharges
through ports located under the perimeter of the disc.
The disc is confined in a small chamber, into which it
fits loosely so that steam can leak into the space above
the disc. When condensate is present, it lifts the disc
and exits. When steam enters the trap, Bernoulli effect
reduces the pressure between the disc and seat. (Perhaps,
this type of trap should be called “aerodynamic” rather
than “thermodynamic.”) Steam at full steam pressure
leaks into the space above the disc, aiding the Bernoulli
effect, and the trap snaps shut. Once the trap is shut,
the disc is held down by the steam pressure above the
disc.
The main advantage of disc traps is their small size
in relation to their condensate capacity. You can usually
identify them from the disc chamber, which is a small
cylindrical housing that forms the top of the trap. Some
disc traps are installed in-line, and these may be barely
Spirax Sarco, Inc.
Fig. 9 Disc trap The thin disc sits freely on the seat. Steam
and condensate rise through the center hole, turn around under
the disc, and bleed off through the surrounding space. The
cap over the disc forms a chamber in which the disc is
enclosed. At the top is an insulating cover, which improves
trap performance.
larger than the steam pipe itself. Figure 11 shows an
installed unit.
The reliability of disc traps is a controversial issue.
One leading trap manufacturer (who manufactures all
the major types) asserts that disc traps are as reliable as
other trap types. Other parties assert that leakage
increases after a relatively short time because the sealing
surfaces become deformed from hammering and steam
abrasion. Also, the large contact area of the sealing
surface makes disc traps vulnerable to leakage caused
by fouling. Increasing leakage at the sealing surface
allows the steam above the disc to vent quickly to the
outlet, causing the trap to cycle more and more rapidly.
Some manufacturers admit to this weakness, and they
design their models for quick replacement of the disc
and seat.
Even the way that disc traps operate is controversial.
All agree that disc traps operate in a cyclic manner.
Many parties say that disc traps cycle open periodically
even when there is no condensate in the trap. This occurs
because the steam above the disc condenses, eliminating
the pressure that holds the disc closed. A puff of steam
is lost with each operating cycle. However, one major
© D. R. Wulfinghoff 1999. All Rights Reserved.
194
1. BOILER PLANT
manufacturer denies that their traps open in the absence
of condensate. This manufacturer states that live steam
never reaches the trap, but that the trap is closed by the
flashing of condensate that is near steam temperature.
This manufacturer insists that a water seal should be
created ahead of the trap.
All seem to agree that trap cycling increases if heat
loss through the trap body causes the steam above the
disc to condense. For this reason, some manufacturers
offer insulating caps for the disc cover. Other
manufacturers offer more expensive models that
surround the disc chamber with an outer chamber that
is filled with inlet steam.
The design of the seat is also controversial. Some
manufacturers assert that the operation of disc traps
depends on a controlled rate of leakage between the disc
and seat to ensure that the trap will open. They create
this small leak with a tiny groove or a carefully
roughened surface. However, one leading manufacturer
uses sealing surfaces with no deliberate leakage. All
agree that hardness of the sealing surfaces is important
to resist deformation.
WESINC
Fig. 11 Disc trap installed This type is very small, as you
can see in comparison with the valves and strainer.
The most common failure mode of disc traps is
increasingly rapid cycling that is caused by fouling or
deformation of the sealing surfaces. When this occurs,
steam is lost in puffs. The rate of steam leakage is limited
by the interrupted flow and by the small size of the
passages. If the steam plant or the application is shut
down periodically, the trap might also stick shut or fully
open.
Try to install disc traps so that the disc lies on its
seat horizontally, equalizing the forces on the disc. Disc
traps are sometimes installed with the disc vertical, but
this hastens deterioration.
Orifice Traps
Armstrong International, Inc.
Fig. 10 How a disc trap works
Orifice traps are used almost exclusively as drip
traps. They are made only in smaller sizes.
An orifice trap, as its name implies, consists of
nothing more than a small hole. A screen typically is
installed upstream of the hole to prevent clogging. This
type was introduced formally in the 1970’s. The fixed
orifice is essentially an adaptation of the practice of
draining condensate manually by slightly opening a
drain valve.
Orifice traps can be considered a controlled leak,
whose principal merit is that the volume of the leak is
known in advance. They are limited to use as drip traps,
and they must be sized closely to match the expected
condensation rate. They are not practical for steamusing equipment, which has large and variable steam
consumption.
The housing of an orifice trap is very compact,
typically only slightly larger than the pipe diameter. A
sloppy insulation job may hide the device entirely.
The ASHRAE Handbook explains the operation of
orifice “traps” as follows: “... an orifice of any size has
much greater capacity for condensate than it does for
steam because of the significant differences in their
densities and because flashing condensate tends to choke
the orifice ...” This explanation is not persuasive,
because steam flows much more easily than water, in
terms of volume. It is true that condensate chokes an
orifice. However, a continuously open orifice is passing
dry steam most of the time in a drip trap application,
ENERGY EFFICIENCY MANUAL
1.10 STEAM AND WATER LEAKAGE
especially if the steam has any superheat. The ASHRAE
Handbook goes on to say that “the steam loss is usually
comparable to that of most cycling-type traps,” but does
not state which types.
Orifice traps are vulnerable to dirt because the
orifices are quite small. The orifice must be protected
by a screen that has a smaller mesh than the orifice size,
and such a screen easily becomes clogged.
Other Types of Steam Traps
Various other types of traps have emerged over the
years. Some, such as piston traps and open bucket traps,
have become obsolete because of poor reliability,
complexity, or large size. Other types of traps are limited
to specialized applications, such as a variant of the
inverted bucket trap designed to lift condensate.
Consider replacing obsolete or inappropriate traps, rather
than repairing them.
Other Selection Characteristics
From the standpoint of efficiency, ability to block
steam flow is the main consideration in selecting traps.
In addition, you need to consider other characteristics,
especially these:
• reliability. Most steam is wasted by trap failure,
rather than by inherent leakiness of certain types
of traps. Different trap types vary in susceptibility
to the three failure modes discussed previously.
All traps can be expected to fail, but the average
time between failures may vary widely among
different types of traps. Many people feel that F&T
traps and inverted bucket traps have the longest
intervals between failure. Thermostatic traps
probably have shorter intervals between
maintenance because of their gradual closing
characteristics. The reliability of disc traps is
controversial, as discussed previously. Orifice traps
are very vulnerable to clogging.
All traps, except orifice traps, will eventually fail
by leaking steam, but they may also stick fully open
or fully closed. The latter modes are more likely
in systems that shut down periodically, because this
allows the mechanism to corrode into position.
Inverted bucket traps tend to fail in the open
position, because this is their shut-down position.
Float traps tend to fail in the closed position, which
is their shut-down position. A float trap can also
fail by corrosion of the float ball, which closes the
valve. Thermostatic traps can fail by failure of the
thermostatic element. Orifice traps, of course, can
fail only to a closed state by clogging.
If an F&T trap fails in the closed position, its
thermostatic element will cause it to behave like a
thermostatic trap. This may reduce its capacity
considerably, but will make the failure difficult to
diagnose.
195
No trap is reliable unless it is properly matched to
its application. For example, float traps are
vulnerable to water hammer, freezing, and dirt,
whereas inverted bucket traps are resistant to these
problems.
• capacity range. Float, bucket, and thermostatic
traps block steam efficiently from zero condensate
flow up to their maximum rated capacity. Disc traps
adapt to different drainage rates, but they are limited
to small capacities because of the way they operate.
Orifice “traps” continuously leak steam when
condensate is not present, so they must be sized
accurately for the maximum expected condensate
flow rate.
• system pressure. Inverted bucket traps are available
for any pressure. Float traps are limited in pressure
by the possibility of crushing the float. Bellowstype or encapsulated thermostatic traps are limited
in pressure by the possibility of crushing the
thermostatic element. Orifice and disc traps are
limited in pressure by the erosion that occurs when
high-pressure steam passes through narrow
passages.
Some disc traps require a minimum pressure drop
between the steam side and the condensate side to
operate properly, typically 10 PSI or more. In
addition, disc traps are vulnerable to back pressure
because proper operation requires steam to be able
to exit from the trap at high velocity.
• venting a cold system at start-up. Thermostatic
traps provide rapid venting of cold systems.
Thermostatic elements are included in F&T traps
for cold system venting.
Inverted bucket traps do not vent air rapidly because
the smallness of the vent hole in the bucket limits
the flow of gases through the trap. To compensate
for this, a thermostatic elements can be fitted to the
bucket that increases the size of the vent hole when
the bucket is cold. This added complication reduces
the reliability of the trap, all other things being
equal.
Disc traps vent a cold system very slowly because
the trap is closed by air in the same way as by live
steam.
Orifice traps are poor for venting a cold system
because of the typically small size of the orifice.
You can vent a cold system by using separate air
vents, which gives you greater latitude in selecting
trap types.
• venting a warmed-up system. F&T traps, inverted
bucket traps, and orifice traps all do a good job of
venting non-condensible gases from a system that
is operating at normal temperature.
If a thermostatic trap is kept flooded by condensate,
air never has a chance to reach the thermostatic
element, so the trap cannot vent a warmed-up
© D. R. Wulfinghoff 1999. All Rights Reserved.
196
1. BOILER PLANT
system. This limitation is characteristic of most
thermostatic traps because the thermostatic element
must be set to close at some temperature lower than
the steam temperature.
Disc traps vent an operating system slowly for the
same reason that they vent a cold system poorly,
namely, that non-condensible gases cause the trap
to close rather than to open. Venting becomes
impossible if the trap is installed with a water seal
ahead of it to reduce cycling, as one manufacturer
suggests.
• vulnerability to freezing. Inverted bucket traps and
float traps remain partially filled with water when
they are idle, which invites freezing damage. Of
the two, float traps are more vulnerable because
they contain a float ball and a thermostatic element
(in F&T traps) that are easily crushed by ice
expansion.
Bimetallic thermostatic traps, disc traps, and orifice
traps are less likely to be harmed by freezing.
Installation practice is a major factor in avoiding
freeze damage, both for traps and for other steam
equipment. The basic principle is to completely
drain the portions of the system that may be exposed
to freezing temperatures. Competent steamfitters
have a variety of techniques for accomplishing this.
Major trap manufacturers publish guidance in
avoiding freezing in steam systems.
• operation with superheated steam. Superheated
steam can be a problem for several trap types when
they are used in drip legs. If superheated steam
reaches an inverted bucket trap, the steam will rush
through and keep the trap dry, causing the trap to
remain in the open position. Superheated steam
may cause a disc trap to chatter and pass steam.
Superheated steam may burst the expanding
element of a bellows trap. Superheated steam may
increase the loss through orifice traps.
In principle, you could use any of these types with
superheated steam, provided that you install them
so that liquid condensate always forms ahead of
the trap. However, this is a bad gamble, especially
if the trap can be damaged by contact with
superheated steam. Under some circumstances, the
condensate may not form as you hope.
• vulnerability to water hammer. A slug of
condensate propelled by high steam pressure has
enough energy to crush trap components, especially
thermostatic elements and float balls. Float traps
and bellows-type thermostatic traps are sensitive
to water hammer. Other common types resist
damage much better.
• size. Float traps and bucket traps are bulky, whereas
other common types are small.
• cost. Float traps and bucket traps are more
expensive than other common types.
Other characteristics may be significant, and putting
all the selection factors into perspective requires
experience. Several steam trap manufacturers publish
detailed selection guides. Experienced manufacturers’
representatives can offer valuable advice. Recognize
that vendors are biased toward the types that they offer,
especially if they are proprietary, and toward more
expensive models.
Sizing and Surge Capacity
Do not select traps with excess flow capacity. The
capacity of the trap determines the size of the valve
orifice (or internal passages, in the case of a disc trap).
If the trap fails in the open position, the orifice size
determines the rate of steam loss.
Select trap sizes using the instructions in the
manufacturers’ catalogs. Calculate sizing from the
maximum condensate load, the pressure differential
across the trap, and the nature of the application (which
dictates extra capacity as a safety factor). Typically,
F&T traps and inverted bucket traps are offered in a
range of body sizes, which are combined with a range
of orifice sizes to satisfy the full range of capacity
requirements.
Consider the surge capacity inside the body of the
trap. Surges of condensate must be kept from backing
up into steam equipment or into steam lines. Float traps
and bucket traps have a significant amount of surge
capacity. You can also gain surge capacity by increasing
the volume in the pipe that leads to the trap.
Strainers
As we have seen, dirt is a potential problem with
steam traps. It can keep the valve from closing
completely (all types, except orifice traps) and it can
clog the trap passages (especially orifice and disc traps).
The general solution to these problems is to install a
strainer ahead of the trap. Figure 11 shows a strainer
that is properly installed.
The strainer itself is an item that requires periodic
inspection and cleaning. Unless the steam system has a
particular dirt problem, inspection is needed only at long
intervals. This leads people to forget about them. Make
sure that your maintenance schedule includes such multiyear inspections. Figure 12 shows a strainer that has
likely been forgotten.
To simplify the installation, you can get traps with
integral strainers in most types and sizes. However, these
do not eliminate the need to clean out the strainers
periodically.
Piping Details
Proper operation of a steam trap depends on the way
it is installed. As we have seen, installation practice is
important for freeze protection, and it may be a factor
in providing adequate surge capacity. Piping the trap to
ENERGY EFFICIENCY MANUAL
1.10 STEAM AND WATER LEAKAGE
197
discharge pressure, so if two units discharge to a
common line, the discharge from the higher pressure
unit can block the flow of condensate to the other unit.
This problem is especially severe with modulating
equipment. Sharing traps makes it difficult to diagnose
problems in traps or the equipment they serve.
ECONOMICS
SAVINGS POTENTIAL: Varies widely. For traps serving
equipment, the wrong types of traps may waste 1 to 30
percent of the steam flow to the equipment. For drip
traps, an orifice trap or a malfunctioning trap of any type
may drain much more steam than condensate.
WESINC
Fig. 12 A strainer that needs cleaning The strainer installed for this F&T trap is completely hidden by insulation.
Still, it could be used if it had a blowdown valve, which it lacks.
The crude plug in the blowdown pipe has probably never been
removed.
maintain a water column ahead of the trap may be
necessary for satisfactory operation and longevity of
thermostatic and disc traps. Refer to the manufacturer’s
literature for more details.
Avoid Sharing Traps
COST: F&T traps and inverted bucket traps typically
cost from $100 to $500 each, in sizes where less efficient
types of traps might be used.
PAYBACK PERIOD: Several months to several years.
TRAPS & TRICKS
SELECTING THE TRAPS: Start by educating yourself
about steam traps. Know where to use each type of
trap, and how to size traps. Take one of the courses
offered by major trap manufacturers. Buy your traps
from reputable manufacturers.
INSTALLATION: Trap installation requires special
practices for each type of trap and application. Do your
homework.
Do not use a single trap to serve more than one item
of equipment. Condensate drainage is sensitive to
© D. R. Wulfinghoff 1999. All Rights Reserved.
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