How To Trap: Superheated Steam Lines
How to Trap:
Superheated
Steam Lines
Bringing Energy Down to Earth
Say energy. Think environment. And vice versa.
Any company that is energy conscious is also environmentally
conscious. Less energy consumed means less waste, fewer
emissions and a healthier environment.
In short, bringing energy and environment together lowers
the cost industry must pay for both. By helping companies
manage energy, Armstrong products and services are also
helping to protect the environment.
Armstrong has been sharing know-how since we invented
the energy-efficient inverted bucket steam trap in 1911. In the
years since, customers’ savings have proven again and again
that knowledge not shared is energy wasted.
Armstrong’s developments and improvements in steam trap
design and function have led to countless savings in energy,
time and money. This section has grown out of our decades
of sharing and expanding what we’ve learned. It deals with the
operating principles of steam traps and outlines their specific
applications to a wide variety of products and industries.
You’ll find it a useful complement to other Armstrong literature
and the Armstrong Steam-A-ware™ software program for
sizing and selecting steam traps, pressure reducing
valves and water heaters, which can be requested through
Armstrong’s Web site, armstronginternational.com.
This section also includes Recommendation Charts that
summarize our findings on which type of trap will give
optimum performance in a given situation and why.
IMPORTANT: This section is intended to summarize
general principles of installation and operation of steam
traps, as outlined above. Actual installation and operation
of steam trapping equipment should be performed
only by experienced personnel. Selection or installation
should always be accompanied by competent technical
assistance or advice. This data should never be used
as a substitute for such technical advice or assistance.
We encourage you to contact Armstrong or its local
representative for further details.
2
Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information.
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Instructions for Using the Recommendation Charts
A quick reference Recommendation Chart appears throughout
the “HOW TO TRAP” brochures (857-EN - 868-EN).
A feature code system (ranging from A to Q) supplies you
with “at-a-glance” information.
The chart covers the type of steam traps and the major
advantages that Armstrong feels are superior for each
particular application.
3. Now refer to Chart 3-2 below, titled “How Various Types
of Steam Traps Meet Specific Operating Requirements”
and read down the extreme left-hand column to each
of the letters B, C, E, K, N. The letter “B,” for example,
refers to the trap’s ability to provide energy-conserving
operation.
4. Follow the line for “B” to the right until you reach the
column that corresponds to our first choice, in this case
the inverted bucket. Based on tests and actual operating
conditions, the energy-conserving performance of the
inverted bucket steam trap has been rated “Excellent.”
Follow this same procedure for the remaining letters.
For example, assume you are looking for information
concerning the proper trap to use on a gravity drained
jacketed kettle. You would:
1. Turn to the “How to Trap Jacketed Kettles” brochure,
864-EN, and look in the lower right-hand corner of
page 10. The Recommendation Chart located there is
reprinted below for your convenience. (Each section has
a Recommendation Chart.)
Abbreviations
IB
IBLV
BM
F&T
CD
DC
Inverted Bucket Trap
Inverted Bucket Large Vent
Bimetallic Trap
Float and Thermostatic Trap
Controlled Disc Trap
Automatic Differential
Condensate Controller
CV Check Valve
T
Thermic Bucket
PRV Pressure Reducing Valve
2. Find “Jacketed Kettles, Gravity Drain” in the first
column under “Equipment Being Trapped” and read
to the right for Armstrong’s “1st Choice and Feature
Code.” In this case, the first choice is an IBLV and
the feature code letters B, C, E, K, N are listed.
Chart 3-1. Recommendation Chart
(See chart below for “Feature Code” References.)
1st Choice and Feature
Code
Alternate Choice
Jacketed Kettles
Gravity Drain
IBLV
B, C, E, K, N
F&T or Thermostatic
Jacketed Kettles
Syphon Drain
DC
B, C, E, G, H, K, N, P
IBLV
Equipment Being Trapped
Chart 3-2. How Various Types of Steam Traps Meet Specific Operating Requirements
Feature
Characteristic
IB
BM
Code
(1)
(2)
(3)
(4)
(5)
(1) Intermittent (2) Intermittent
F&T
Disc
Thermostatic
Wafer
DC
Orifice
A
Method of Operation
Continuous
Intermittent
(2) Intermittent
Continuous
Continuous
B
Energy Conservation (Time in Service)
Excellent
Excellent
Good
Poor
Fair
(3) Excellent
Poor
C
Resistance to Wear
Excellent
Excellent
Good
Poor
Fair
Excellent
Poor
D
Corrosion Resistance
Excellent
Excellent
Good
Excellent
Good
Excellent
Good
E
Resistance to Hydraulic Shock
Excellent
Excellent
Poor
Excellent
(4) Poor
Excellent
Good
F
Vents Air and CO2 at Steam Temperature
Yes
No
No
No
No
Yes
Poor
G
Ability to Vent Air at Very Low Pressure (1/4 psig)
Poor
(5) NR
Excellent
(5) NR
Good
Excellent
Poor
H
Ability to Handle Start-Up Air Loads
Fair
Excellent
Excellent
Poor
Excellent
Excellent
Poor
I
Operation Against Back Pressure
Excellent
Excellent
Excellent
Poor
Excellent
Excellent
Poor
J
Resistance to Damage From Freezing
(6) Good
Good
Poor
Good
Good
Good
Excellent
K
Ability to Purge System
Excellent
Good
Fair
Excellent
Good
Excellent
Poor
L
Performance on Very Light Loads
Excellent
Excellent
Excellent
Poor
Excellent
Excellent
Poor
M
Responsiveness to Slugs of Condensate
Immediate
Delayed
Immediate
Delayed
Delayed
Immediate
Poor
N
Ability to Handle Dirt
Excellent
Fair
Poor
Poor
Fair
Excellent
Poor
O
Comparative Physical Size (7)
Large
Small
Large
Small
Small
Large
Small
P
Ability to Handle“Flash Steam”
Q
Mechanical Failure (Open or Closed)
Fair
Poor
Poor
Poor
Poor
Excellent
Poor
Open
Open
Closed
(8) Open
(9)
Open
NA
Drainage of condensate is continuous. Discharge is intermittent.
Can be continuous on low load.
Excellent when “secondary steam” is utilized.
Bimetallic and wafer traps – good.
Not recommended for low pressure operations.
(6)
(7)
(8)
(9)
Cast iron traps not recommended.
In welded stainless steel construction – medium.
Can fail closed due to dirt.
Can fail either open or closed, depending upon the design of the bellows.
Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information.
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3
Steam Tables
What They Are…How to Use Them
How the Table Is Used
The heat quantities and temperature/
pressure relationships referred to in
this section are taken from the Properties
of Saturated Steam table.
In addition to determining pressure/
temperature relationships, you can
compute the amount of steam that will
be condensed by any heating unit of
known Btu output. Conversely, the
Saturated Steam is pure steam at the
temperature that corresponds to the
boiling temperature of water at the
existing pressure.
Absolute and Gauge Pressures
Absolute pressure is pressure in
pounds per square inch (psia) above
a perfect vacuum. Gauge pressure is
pressure in pounds per square inch
above atmospheric pressure, which is
14.7 pounds per square inch absolute.
Gauge pressure (psig) plus 14.7 equals
absolute pressure. Or, absolute pressure
minus 14.7 equals gauge pressure.
Table 4-1. Properties of Saturated Steam
(Abstracted from Keenan and Keyes, THERMODYNAMIC PROPERTIES OF STEAM,
by permission of John Wiley & Sons, Inc.)
Col. 1
Gauge
Pressure
Inches of Vacuum
Definitions of Terms Used
Pressure/Temperature Relationship
(Columns 1, 2 and 3). For every
pressure of pure steam there is a
corresponding temperature. Example:
The temperature of 250 psig pure
steam is always 406°F.
Total Heat of Steam (Column 6). The
sum of the Heat of the Liquid (Column
4) and Latent Heat (Column 5) in Btu.
It is the total heat in steam above 32°F.
Specific Volume of Liquid (Column 7).
The volume per unit of mass in cubic
feet per pound.
Specific Volume of Steam (Column 8).
The volume per unit of mass in cubic
feet per pound.
4
PSIG
Heat of Saturated Liquid (Column 4).
This is the amount of heat required
to raise the temperature of a pound
of water from 32°F to the boiling
point at the pressure and temperature
shown. It is expressed in British
thermal units (Btu).
Latent Heat or Heat of Vaporization
(Column 5). The amount of heat
(expressed in Btu) required to change
a pound of boiling water to a pound of
steam. This same amount of heat is
released when a pound of steam is
condensed back into a pound of water.
This heat quantity is different for every
pressure/temperature combination, as
shown in the steam table.
table can be used to determine Btu
output if steam condensing rate is
known. In the application portion of
this section, there are several references
to the use of the steam table.
29.743
29.515
27.886
19.742
9.562
7.536
5.490
3.454
1.418
0.0
1.3
2.3
5.3
10.3
15.3
20.3
25.3
30.3
40.3
50.3
60.3
70.3
80.3
90.3
100.0
110.3
120.3
125.3
130.3
140.3
150.3
160.3
180.3
200.3
225.3
250.3
Col. 2
Absolute
Pressure
(psia)
Col. 3
Steam
Temp.
(°F)
Col. 4
Heat
of Sat.
Liquid
(Btu/lb)
Col. 5
Latent
Heat (Btu/
lb)
Col. 6
Total
Heat of
Steam
(Btu/lb)
Col. 7
Specific
Volume of
Sat. Liquid
(cu ft/lb)
0.08854
0.2
1.0
5.0
10.0
11.0
12.0
13.0
14.0
14.696
16.0
17.0
20.0
25.0
30.0
35.0
40.0
45.0
55.0
65.0
75.0
85.0
95.0
105.0
114.7
125.0
135.0
140.0
145.0
155.0
165.0
175.0
195.0
215.0
240.0
265.0
300.0
400.0
450.0
500.0
600.0
900.0
1200.0
1500.0
1700.0
2000.0
2500.0
2700.0
3206.2
32.00
53.14
101.74
162.24
193.21
197.75
201.96
205.88
209.56
212.00
216.32
219.44
227.96
240.07
250.33
259.28
267.25
274.44
287.07
297.97
307.60
316.25
324.12
331.36
337.90
344.33
350.21
353.02
355.76
360.50
365.99
370.75
379.67
387.89
397.37
406.11
417.33
444.59
456.28
467.01
486.21
531.98
567.22
596.23
613.15
635.82
668.13
679.55
705.40
0.00
21.21
69.70
130.13
161.17
165.73
169.96
173.91
177.61
180.07
184.42
187.56
196.16
208.42
218.82
227.91
236.03
243.36
256.30
267.50
277.43
286.39
294.56
302.10
308.80
315.68
321.85
324.82
327.70
333.24
338.53
343.57
353.10
361.91
372.12
381.60
393.84
424.00
437.20
449.40
471.60
526.60
571.70
611.60
636.30
671.70
730.60
756.20
902.70
1075.8
1063.8
1036.3
1001.0
982.1
979.3
976.6
974.2
971.9
970.3
967.6
965.5
960.1
952.1
945.3
939.2
933.7
928.6
919.6
911.6
904.5
897.8
891.7
886.0
880.0
875.4
870.6
868.2
865.8
861.3
857.1
852.8
844.9
837.4
828.5
820.1
809.0
780.5
767.4
755.0
731.6
668.8
611.7
556.3
519.6
463.4
360.5
312.1
0.0
1075.8
1085.0
1106.0
1131.0
1143.3
1145.0
1146.6
1148.1
1149.5
1150.4
1152.0
1153.1
1156.3
1160.6
1164.1
1167.1
1169.7
1172.0
1175.9
1179.1
1181.9
1184.2
1186.2
1188.1
1188.8
1191.1
1192.4
1193.0
1193.5
1194.6
1195.6
1196.5
1198.0
1199.3
1200.6
1201.7
1202.8
1204.5
1204.6
1204.4
1203.2
1195.4
1183.4
1167.9
1155.9
1135.1
1091.1
1068.3
902.7
0.016022
0.016027
0.016136
0.016407
0.016590
0.016620
0.016647
0.016674
0.016699
0.016715
0.016746
0.016768
0.016830
0.016922
0.017004
0.017078
0.017146
0.017209
0.017325
0.017429
0.017524
0.017613
0.017696
0.017775
0.017850
0.017922
0.017991
0.018024
0.018057
0.018121
0.018183
0.018244
0.018360
0.018470
0.018602
0.018728
0.018896
0.019340
0.019547
0.019748
0.02013
0.02123
0.02232
0.02346
0.02428
0.02565
0.02860
0.03027
0.05053
Col. 8
Specific
Volume
of Sat.
Steam (cu
ft/lb)
3306.00
1526.00
333.60
73.52
38.42
35.14
32.40
30.06
28.04
26.80
24.75
23.39
20.09
16.30
13.75
11.90
10.50
9.40
7.79
6.66
5.82
5.17
4.65
4.23
3.88
3.59
3.33
3.22
3.11
2.92
2.75
2.60
2.34
2.13
1.92
1.74
1.54
1.16
1.03
0.93
0.77
0.50
0.36
0.28
0.24
0.19
0.13
0.11
0.05
Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information.
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Steam Tables
Flash Steam (Secondary)
What is flash steam? When hot condensate or boiler
water, under pressure, is released to a lower pressure, part
of it is re-evaporated, becoming what is known as flash steam.
Why is it important? This flash steam is important because
it contains heat units that can be used for economical plant
operation—and which are otherwise wasted.
How is it formed? When water is heated at atmospheric
pressure, its temperature rises until it reaches 212°F,
the highest temperature at which water can exist at this
pressure. Additional heat does not raise the temperature,
but converts the water to steam.
The heat absorbed by the water in raising its temperature
to boiling point is called “sensible heat” or heat of saturated
liquid. The heat required to convert water at boiling point
to steam at the same temperature is called “latent heat.”
The unit of heat in common use is the Btu, which is the
amount of heat required to raise the temperature of one
pound of water 1°F at atmospheric pressure.
If water is heated under pressure, however, the boiling
point is higher than 212°F, so the sensible heat required
is greater. The higher the pressure, the higher the boiling
temperature and the higher the heat content. If pressure
is reduced, a certain amount of sensible heat is released.
This excess heat will be absorbed in the form of latent heat,
causing part of the water to “flash” into steam.
Chart 5-3.
Percentage of flash steam formed when discharging
condensate to reduced pressure.
Condensate at steam temperature and under 100 psig
pressure has a heat content of 308.8 Btu per pound. (See
Column 4 in Steam Table.) If this condensate is discharged
to atmospheric pressure (0 psig), its heat content instantly
drops to 180 Btu per pound. The surplus of 128.8 Btu
re-evaporates or flashes a portion of the condensate.
The percentage that will flash to steam can be computed
using the formula:
% flash steam = SH - SL x 100
H
SH = Sensible heat in the condensate at the higher
pressure before discharge.
SL = Sensible heat in the condensate at the lower
pressure to which discharge takes place.
H = Latent heat in the steam at the lower pressure
to which the condensate has been discharged.
% flash steam = 308.8 - 180 x 100 =13.3%
970.3
Chart 5-3 shows the amount of secondary steam that
will be formed when discharging condensate to different
pressures. Other useful tables will be found in brochure
873-EN (Useful Engineering Tables).
Chart 5-4.
Volume of flash steam formed when one cubic foot of
condensate is discharged to atmospheric pressure.
400
PERCENTAGE OF FLASH STEAM
25
A
20
B
C
15
D
E
F
G
10
CURVE
BACK PRESS.
LBS/SQ IN
A
B
C
D
E
F
G
– 10
–5
0
10
20
30
40
5
0
– 20
0
50
100
150
200
250
CU FT FLASH STEAM
PER CU FT OF CONDENSATE
30
300
200
100
0
100
200
300
400
PRESSURE AT WHICH CONDENSATE
IS FORMED—LBS/SQ IN
300
PSI FROM WHICH CONDENSATE IS DISCHARGED
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5
Steam…Basic Concepts
Steam is an invisible gas generated by adding heat
energy to water in a boiler. Enough energy must be added
to raise the temperature of the water to the boiling point.
Then additional energy—without any further increase in temperature—changes the water to steam.
When steam reaches the heat exchangers in the system,
the story is different. Here the transfer of heat from the
steam is desirable. Heat flows to the air in an air heater,
to the water in a water heater or to food in a cooking kettle.
Nothing should interfere with this heat transfer.
Steam is a very efficient and easily controlled heat transfer
medium. It is most often used for transporting energy from a
central location (the boiler) to any number of locations in the
plant where it is used to heat air, water or process applications.
Condensate Drainage…
Why It’s Necessary
As noted, additional Btu are required to make boiling water
change to steam. These Btu are not lost but stored in the
steam ready to be released to heat air, cook tomatoes,
press pants or dry a roll of paper.
The heat required to change boiling water into steam is
called the heat of vaporization or latent heat. The quantity
is different for every pressure/temperature combination,
as shown in the steam tables.
Steam at Work…
How the Heat of Steam Is Utilized
Heat flows from a higher temperature level to a lower
temperature level in a process known as heat transfer.
Starting in the combustion chamber of the boiler, heat
flows through the boiler tubes to the water. When the
higher pressure in the boiler pushes steam out, it heats
the pipes of the distribution system. Heat flows from the
steam through the walls of the pipes into the cooler
surrounding air. This heat transfer changes some of the
steam back into water. That’s why distribution lines are
usually insulated to minimize this wasteful and undesirable
heat transfer.
Condensate is the by-product of heat transfer in a steam
system. It forms in the distribution system due to unavoidable
radiation. It also forms in heating and process equipment
as a result of desirable heat transfer from the steam to the
substance heated. Once the steam has condensed and
given up its valuable latent heat, the hot condensate must
be removed immediately. Although the available heat in a
pound of condensate is negligible as compared to a pound
of steam, condensate is still valuable hot water and should
be returned to the boiler.
Definitions
n The Btu. A Btu—British thermal unit—is the amount of
heat energy required to raise the temperature of one
pound of cold water by 1°F. Or, a Btu is the amount of
heat energy given off by one pound of water in cooling,
say, from 70°F to 69°F.
n Temperature. The degree of hotness with no implication
of the amount of heat energy available.
n Heat. A measure of energy available with no implication
of temperature. To illustrate, the one Btu that raises one
pound of water from 39°F to 40°F could come from the
surrounding air at a temperature of 70°F or from a flame
at a temperature of 1,000°F.
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Figure 6-1. These drawings show how much heat is
required to generate one pound of steam at atmospheric pressure. Note that it takes 1 Btu for every
1°F increase in temperature up to the boiling point,
but that it takes more Btu to change water at 212°F
to steam at 212°F.
6
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Figure 6-2. These drawings show how much heat is required to
generate one pound of steam at 100 pounds per square inch
pressure. Note the extra heat and higher temperature required
to make water boil at 100 pounds pressure than at atmospheric
pressure. Note, too, the lesser amount of heat required to change
water to steam at the higher temperature.
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Steam…Basic Concepts
The need to drain the distribution system. Condensate
lying in the bottom of steam lines can be the cause of one
kind of water hammer. Steam traveling at up to 100 miles
per hour makes “waves” as it passes over this condensate
(Fig. 7-4). If enough condensate forms, high-speed steam
pushes it along, creating a dangerous slug that grows larger
and larger as it picks up liquid in front of it. Anything that
changes the direction—pipe fittings, regulating valves, tees,
elbows, blind flanges—can be destroyed. In addition to
damage from this “battering ram,” high-velocity water may
erode fittings by chipping away at metal surfaces.
The need to drain the heat transfer unit. When steam
comes in contact with condensate cooled below the temperature of steam, it can produce another kind of water hammer
known as thermal shock. Steam occupies a much greater
volume than condensate, and when it collapses suddenly,
it can send shock waves throughout the system. This form
of water hammer can damage equipment, and it signals
that condensate is not being drained from the system.
Obviously, condensate in the heat transfer unit takes up
space and reduces the physical size and capacity of the
equipment. Removing it quickly keeps the unit full of steam
(Fig. 7-5). As steam condenses, it forms a film of water on
the inside of the heat exchanger. Non-condensable gases
do not change into liquid and flow away by gravity. Instead,
they accumulate as a thin film on the surface of the heat
exchanger—along with dirt and scale. All are potential
barriers to heat transfer (Fig. 7-3).
$
The need to remove air and CO2. Air is always present
during equipment start-up and in the boiler feedwater.
Feedwater may also contain dissolved carbonates, which
release carbon dioxide gas. The steam velocity pushes
the gases to the walls of the heat exchangers, where they
may block heat transfer. This compounds the condensate
drainage problem, because these gases must be removed
along with the condensate.
Figure 7-3. Potential barriers to heat transfer: steam heat and
temperature must penetrate these potential barriers to
do their work.
%
Figure 7-4. Condensate allowed to collect in pipes or tubes
is blown into waves by steam passing over it until it blocks
steam flow at point A. Condensate in area B causes a pressure differential that allows steam pressure to push the slug
of condensate along like a battering ram.
Figure 7-5. Coil half full of condensate can’t work at
full capacity.
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Figure 7-6. Note that heat radiation from the distribution system causes condensate to form and, therefore, requires steam
traps at natural low points or ahead of control valves. In the heat exchangers, traps perform the vital function of removing the
condensate before it becomes a barrier to heat transfer. Hot condensate is returned through the traps to the boiler for reuse.
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7
Steam…Basic Concepts
Effect of Air on Steam Temperature
When non-condensable gases (primarily air) continue to
accumulate and are not removed, they may gradually fill
the heat exchanger with gases and stop the flow of steam
altogether. The unit is then “air bound.”
When air and other gases enter the steam system, they
consume part of the volume that steam would otherwise
occupy. The temperature of the air/steam mixture falls below
that of pure steam. Figure 8-7 explains the effect of air
in steam lines. Table 8-2 and Chart 8-5 show the
various temperature reductions caused by air at various
percentages and pressures.
Corrosion
Two primary causes of scale and corrosion are carbon
dioxide (CO2) and oxygen. CO2 enters the system as
carbonates dissolved in feedwater and, when mixed with
cooled condensate, creates carbonic acid. Extremely
corrosive, carbonic acid can eat through piping and heat
exchangers (Fig. 9-9). Oxygen enters the system as gas
dissolved in the cold feedwater. It aggravates the action of
carbonic acid, speeding corrosion and pitting iron and steel
surfaces (Fig. 9-10).
Effect of Air on Heat Transfer
The normal flow of steam toward the heat exchanger
surface carries air and other gases with it. Since they do
not condense and drain by gravity, these non-condensable
gases set up a barrier between the steam and the heat
exchanger surface. The excellent insulating properties of
air reduce heat transfer. In fact, under certain conditions
as little as 1/2 of 1% by volume of air in steam can reduce
heat transfer efficiency by 50% (Fig. 9-8).
Eliminating the Undesirables
To summarize, traps must drain condensate because
it can reduce heat transfer and cause water hammer.
Traps should evacuate air and other non-condensable
gases because they can reduce heat transfer by reducing
steam temperature and insulating the system. They can
also foster destructive corrosion. It’s essential to remove
condensate, air and CO2 as quickly and completely as
possible. A steam trap, which is simply an automatic valve
that opens for condensate, air and CO2 and closes for
steam, does this job. For economic reasons, the steam trap
should do its work for long periods with minimum attention.
Table 8-2. Temperature Reduction Caused by Air
Temp. of
Steam, No Air
Present (°F)
10.3
25.3
50.3
75.3
100.3
240.1
267.3
298.0
320.3
338.1
Temp. of Steam Mixed With Various
Percentages of Air (by Volume) (°F)
10%
234.3
261.0
291.0
312.9
330.3
20%
228.0
254.1
283.5
304.8
321.8
30%
220.9
246.4
275.1
295.9
312.4
E–%
LUM
VO
R BY
T AI
CEN
PER
Figure 8-7. Chamber containing air and
steam delivers only the heat of the partial pressure of the steam, not the total
pressure.
100
90
80
70
60
50
40
30
20
PRE
10
0 300 2
50
SSU
200
RE–
150
PSIG
100
75 5
0 25
0
450
425
400
450
375
425
400
350
375
325
350
300
275
250
325
300
225
275
200
250
225
150
200
100
150
Steam chamber 100% steam
Total pressure 100 psia
Steam pressure 100 psia
Steam temperature 327.8°F
100
0
0
300
Steam chamber 90% steam and 10% air
Total pressure 100 psia
Steam pressure 90 psia
Steam temperature 320.3°F
8
TEMPERATURE F
Pressure
(psig)
250
200
150
100
75 5
0
0 25
0 100 9
80
70
60
50
40
30
20
10
0
Chart 8-5. Air Steam Mixture
Temperature reduction caused by various percentages of air at differing
pressures. This chart determines the percentage of air with known pressure
and temperature by determining the point of intersection between pressure,
temperature and percentage of air by volume. As an example, assume system
pressure of 250 psig with a temperature at the heat exchanger of 375°F. From the
chart, it is determined that there is 30% air by volume in the steam.
Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information.
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Steam…Basic Concepts
What the Steam Trap Must Do
1. Minimal steam loss. Table 9-3 shows how costly
unattended steam leaks can be.
7. Freedom from dirt problems. Dirt is an ever-present
concern since traps are located at low points in the
steam system. Condensate picks up dirt and scale in
the piping, and solids may carry over from the boiler.
Even particles passing through strainer screens are
erosive and, therefore, the steam trap must be able to
operate in the presence of dirt.
2. Long life and dependable service. Rapid wear of
parts quickly brings a trap to the point of undependability.
An efficient trap saves money by minimizing trap testing,
repair, cleaning, downtime and associated losses.
A trap delivering anything less than all these desirable
operating/design features will reduce the efficiency of the
system and increase costs. When a trap delivers all these
features the system can achieve:
3. Corrosion resistance. Working trap parts should be
corrosion-resistant in order to combat the damaging
effects of acidic or oxygen-laden condensate.
1. Fast heat-up of heat transfer equipment
2. Maximum equipment temperature for enhanced steam
heat transfer
3. Maximum equipment capacity
4. Maximum fuel economy
5. Reduced labor per unit of output
6. Minimum maintenance and a long trouble-free service life
The job of the steam trap is to get condensate, air and CO2
out of the system as quickly as they accumulate. In addition,
for overall efficiency and economy, the trap must also provide:
4. Air venting. Air can be present in steam at any time
and especially on start-up. Air must be vented for
efficient heat transfer and to prevent system binding.
5. CO2 venting. Venting CO2 at steam temperature will
prevent the formation of carbonic acid. Therefore, the
steam trap must function at or near steam temperature
since CO2 dissolves in condensate that has cooled
below steam temperature.
Sometimes an application may demand a trap without
these design features, but in the vast majority of
applications the trap which meets all the requirements will
deliver the best results.
6. Operation against back pressure. Pressurized return
lines can occur both by design and unintentionally. A
steam trap should be able to operate against the actual
back pressure in its return system.
Figure 9-8. Steam condensing in a
heat transfer unit moves air to the heat
transfer surface, where it collects or
“plates out” to form effective insulation.
Figure 9-9. CO2 gas combines with
condensate allowed to cool below
steam temperature to form carbonic
acid, which corrodes pipes and heat
transfer units. Note groove eaten
away in the pipe illustrated.
Figure 9-10. Oxygen in the system
speeds corrosion (oxidation) of pipes,
causing pitting such as shown here.
Figs. 9-9 and 9-10 courtesy of Dearborn
Chemical Company.
Table 9-3. Cost of Various Sized Steam Leaks at 100 psi
(Assuming steam costs $10.00/1,000 lbs)
Size of Orifice
1/2"
7/16"
3/8"
5/16"
1/4"
3/16"
1/8"
Condensate
Steam
12, 7 mm
11, 2 mm
9, 5 mm
7, 9 mm
6, 4 mm
4, 8 mm
3, 2 mm
Lbs Steam Wasted
Per Month
Total Cost Per Month
(USD)
Total Cost Per Year
(USD
553,000
423,500
311,000
216,000
138,000
78,000
34,500
$5,530.00
4,235.00
3,110.00
2,160.00
1,380.00
780.00
345.00
$66,360.00
50,820.00
37,320.00
25,920.00
16,560.00
9,360.00
4,140.00
The steam loss values assume typical condensate load for drip trap applications.
Armstrong methodology for steam trap management and condensate return is sanctioned by the Clean Development
Mechanism of the United Nations Framework Convention on Climate Change.
Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information.
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9
How to Trap Superheated Steam Lines
At first glance, this may seem confusing due to the idea
that superheated steam produces no condensate; therefore,
the steam lines carrying superheated steam should not have
any condensate in them. This is true once the system is
up to temperature and pressure, but condensate removal
is necessary up to this point. This section will explain what
superheated steam is and the applications for its use.
This water can be removed with separators and traps
in the steam outlets, but they are not 100% efficient. In
applications where dry steam is a necessity, additional
superheating coils are placed in the boiler furnace as
convection passes. More heat is added to the steam to
vaporize the water carryover, which adds a small amount
of superheat to guarantee absolutely dry steam.
The specific heat of any substance (using Btu standards)
is the quantity of heat required to raise the temperature of
1 pound by 1 degree F. With this definition, the specific heat
of water is 1, and the specific heat of superheated steam
varies according to temperature and pressure. Specific heat
decreases as the temperature rises but increases as the
pressure goes up.
Because superheated steam can give up so little heat
before it converts back to saturated steam, it is not a good
heat-transfer medium. Some processes, such as power
plants, require a dry heat in order to do work. Whatever
the type of power unit, superheat helps reduce the amount
of condensation when starting from cold. Superheat also
increases the power output by delaying condensation during
the expansion stages in the equipment. Having drier steam
at the exhaust end will increase the life of turbine blades.
Superheated steam is customarily made by the addition of
an extra set of coils inside the boiler or in the exhaust area
of the boiler so as to use the “waste” heat from the boiler.
Or, by the addition of a superheat chamber somewhere after
the boiler, attached to the steam main. A schematic diagram
of a steam generator with a superheated section of coil is
shown below.
Superheated steam can lose heat without condensing
whereas saturated steam cannot. Therefore, superheated
steam can be transported through very long steam lines
without losing sufficient heat to condense. This permits the
delivery of dry steam throughout the entire steam system.
Properties of Superheated Steam
Why Trap Superheated Systems?
Superheated steam has several properties that make it
unsuitable as a heat energy exchange medium yet ideal
for work and mass transfer. Unlike saturated steam, the
pressure and temperature of superheated steam are independent. As superheat is formed at the same pressure as
the saturated steam, the temperature and volume increase.
The primary reason for traps on superheat systems is the
start-up load. It can be heavy because of the large size
of the mains. On start-up, manual valves will most likely be
used since time is available to open and to close the valves.
This is known as supervised start-up. A second reason for
steam traps is to handle emergencies such as superheater
loss or by-pass, which might require operation on saturated
steam. In these unscheduled events, there is no time
available for manually opening valves; therefore, steam
traps are a necessity.
In high heat release boilers with relatively small drums,
separation of steam from water is extremely difficult.
The combination of the small volume of water in the drums
and rapid load swings produces severe shrink and swell
conditions in the drum, which promotes water carryover.
Figure 10-39. Steam Generator
Stack
Gases Outlet
Preheated
Air
These are the situations for which proper trap sizing is a
must. Condensate must be removed as it forms in any steam
system to keep efficiency high and to minimize damaging
water hammer and erosion.
Air
Inlet
Superheated Steam
(High Pressure)
Warm
Air
Turbine
Hot Water
Generator
Steam
(Low Pressure)
Vapor
Cool Water
Fuel
Condenser
Low-Temperature
Water
(High Pressure)
Pump
Superheater
Pump
Cool Water
(From Tower or
Lake/River)
10
Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information.
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How to Trap Superheated Steam Lines
Sizing Superheat Loads to Traps
The condensate load to a trap used on superheat will vary
widely from severe start-up loads to virtually no load during
operation. Consequently, this is a demanding application for
any steam trap.
During start-up, very large lines are being filled with steam
from cold conditions. At this time, only saturated steam at
low pressure is in the lines until the line temperature can
be increased. This is done slowly over a long period so
the lines are not stressed. Large condensate flow combined
with low pressure is the start-up condition that requires the
use of large capacity traps. These oversized traps are then
required to operate at very high pressures with very low
capacity requirements during normal superheat operation.
Typical start-up loads can be roughly calculated as follows:
Table 11-15. Time Period Table
Time Period
Average
Pressure
psig (bar)
Temperature at
End of Time Period
°F (°C)
14” Line
Condensation Rate
lb/hr (kg/hr)
1 st 2 hours
2 nd 2 hours
3 rd 2 hours
4 th 2 hours
5 th 2 hours
5 (.35)
140 (9.8)
700 (49)
1200 (85)
1200 (85)
270 (132)
470 (243)
670 (354)
870 (465)
1070 (577)
247 (112)
286 (130)
352 (160)
288 (131)
260 (118)
NOTE: For the average pressure of 1,200 psig (85 bar), assume H to be the latent heat
of 1,200 psig (85 bar) steam plus superheat at temperature at the end of the period.
To ensure the condensate is removed efficiently, proper
drip leg sizing and piping recommendations should also
be followed when installing traps on superheat systems.
The Table 12-13 on page 12, brochure 857-EN lists the
proper drip leg size for given pipe sizes.
Using:
C
=
0.114 Wp (t2-t1)
H
Where:
C
= Amount of condensate in pounds
Wp
= Total weight of pipe (from Table 11-12, page 12,
brochure 857-EN)
H
= Total heat of X pressure minus Sensible heat of Y
Pressure (Latent heat of steam. For long warm-up
times, use the total heat of saturated steam at the
superheat steam supply pressure (X) minus the
sensible heat of saturated steam at the average
pressure (Y) during the warm-up time involved.)
0.114 = Specific heat of steel pipe in btu/lb °F
EXAMPLE:
Assuming a 100°F/hr (37°C/hr) heat-up
14’’ (35 cm) diameter Schedule 80 line
Supply superheated steam at 1200 psig 1070°F (85 bar,
577°C)
Ambient temperature is 70°F (21°C)
200 feet (61 m) of run between traps
For the first two hours:
W = (200 ft) (107 lb/ft) = 21,400 lb (9727 kg)
t(2) - t(1) = 270 - 70 = 200°F (93°C)
H = 1184.8 btu/lb - 196.27 btu/lb = 988.5 btu/lb = (474 kJ)
C =
(0.114 btu/lb °F) (21,400 lb) (200°F)
= 493 lb (224 kg)
988.5 btu/lb
For the second two hours:
The only thing that changes is the sensible heat of the saturated steam at average pressure during the time involved.
(0.114 btu/lb °F) (21,400 lb) (200°F)
C =
= 573 lb (260 kg)
851.1 btu/lb
The question arises whether insulation should be used
on the drip leg, piping leading to the trap, and the trap.
The answer is no; unless it is mandatory for safety reasons,
this section of the steam system should not be insulated.
This ensures that some condensate is continuously being
formed ahead of the trap and going to it, thus prolonging
the trap’s life.
Types of Superheat Traps
Bimetallic
A bimetallic trap is set to not open until condensate has
cooled to a temperature below saturation. For the existing
pressure, it will remain closed whenever steam of any temperature is in the trap. As the steam temperature rises, the
pull of the bimetallic element becomes greater, providing a
greater sealing force on the valve. Superheated steam tends
to seal the valve better. The bimetallic trap also has the ability
to handle large start-up loads. For these reasons, this trap is
a good choice for superheat.
During superheat operation, the condensate in the trap
must cool to a temperature below the saturation temperature before the trap can open. Condensate may back up into
the line and cause damage to the lines, valves and equipment
if drip leg size and length before the trap are insufficient.
Inverted Bucket
A water seal prevents steam from getting to the valve,
promoting no live steam loss and long life. The valve at
the top makes it impervious to dirt and permits removal of
air. Large start-up loads can be handled, and the trap can
still accommodate small running loads. There are problems
associated with its application on superheat, mostly associated with the necessity of maintaining its water seal or “prime.”
Proper piping is necessary to maintain a prime in the IB.
For proper inverted bucket piping on superheat, refer to
Figure 12-31, page 12, brochure 857-EN. When sizing a
superheat trap, size for start-up load with no safety factor.
Body materials should be selected on the basis of maximum
pressure and temperature, including superheat.
Designs, materials, weights and performance ratings are approximate and subject to change without notice. Visit armstronginternational.com for up-to-date information.
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11
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859-EN
Printed in U.S.A. - 4/15
© 2015 Armstrong International, Inc.
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