How To Trap: Flash Tanks

How To Trap: Flash Tanks
How to Trap:
Flash Tanks
1
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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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
TEMPERATURE F
Pressure
(psig)
100
0
0
300
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.
8
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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 Flash Tanks
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. The heat content
of flash is identical to that of live steam at the same pressure, although this valuable heat is wasted when allowed to
escape through the vent in the receiver. With proper sizing
and installation of a flash recovery system, the latent heat
content of flash steam may be used for space heating;
heating or preheating water, oil and other liquids; and low
pressure process heating.
If exhaust steam is available it may be combined with the
flash. In other cases, the flash will have to be supplemented
by live make-up steam at reduced pressure. The actual
amount of flash steam formed varies according to pressure
conditions. The greater the difference between initial pressure and pressure on the discharge side, the greater the
amount of flash that will be generated.
To determine the exact amount, as a percentage, of flash
steam formed under certain conditions, refer to page 5 for
complete information.
Trap Selection
The condensate load can be calculated using the following
formula:
Q
LxP
L100
=
Where:
Q = Condensate load in lbs/hr
(to be handled by steam trap)
L = Condensate flow into flash tank in lbs/hr
P = Percentage of flash
EXAMPLE: Determine the condensate load of a flash tank
with 5,000 lbs/hr of 100 psig condensate entering the flash
tank held at 10 psig. From page 5, the flash percentage is P =
10.5%. Using the formula:
Q
=
5,000 - (5,000 x 10.5)
100
=
In some cases, the float and thermostatic type trap is an
acceptable alternative. One particular advantage of the F&T
is its ability to handle heavy start-up air loads.
Chart 10-21. Recommendation Chart
(See Page 3 for “Feature Code” References.)
Flash Tanks
A third type of device that may be the preferred selection
in many cases is the automatic differential condensate
controller. It combines the best features of both the IB and
F&T and is recommended for large condensate loads that
exceed the separating capability of the flash tank.
Safety Factor
The increased amount of condensate at start-up and
the varying loads during operation accompanied by low
pressure differential dictates a safety factor of 3:1 for
trapping flash tanks.
Figure 10-57. Typical Flash Tank Piping Sketch
Reducing Valve
Make-up
Valve Strainer
Alternate
Vent
Location
CV
Relief Valve
Gauge
To Low
Pressure
Steam Use
Air Vent
To Drain
Flash Tank
High Pressure
Condensate
Return Line
4,475 lbs/hr
Due to the importance of energy conservation and operation
against back pressure, the trap best suited for flash steam
service is the inverted bucket type with large bucket vent.
In addition, the IB operates intermittently while venting air
and CO2 at steam temperature.
Equipment Being
Trapped
Refer to Chart 5-3 (page 5) for percentage of flash steam
formed when discharging condensate to reduced pressure.
1st Choice and
Feature Code
Alternate Choice
IBLV
B, E, M, L, I, A, F
F&T or *DC
IBLV Steam Trap
To Low Pressure
Condensate Return
Flash steam tank with live steam make-up, showing
recommended fittings and connections. The check valves
in the incoming lines prevent waste of flash when a line is
not in use. The by-pass is used when flash steam cannot
be used. Relief valves prevent pressure from building up
and interfering with the operation of the high pressure steam
traps. The reducing valve reduces the high pressure steam
to the same pressure as the flash, so they can be combined
for process work or heating.
*Recommended where condensate loads exceed the separating capability of the
flash tank.
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 Flash Tanks
Installation
Flash Tank Dimensions
Condensate return lines contain both flash steam and condensate. To recover the flash steam, the return header runs
to a flash tank, where the condensate is drained, and steam
is then piped from the flash tank to points of use, Fig. 10-57.
Since a flash tank causes back pressure on the steam traps
discharging into the tank, these traps should be selected to
ensure their capability to work against back pressure and
have sufficient capacity at the available differential pressures.
The flash tank can usually be conveniently constructed from
a piece of large diameter piping with the bottom ends welded
or bolted in position. The tank should be mounted vertically.
A steam outlet is required at the top and a condensate
outlet at the bottom. The condensate inlet connection should
be 6’’-8’’ above the condensate outlet.
Condensate lines should be pitched toward the flash tank,
and where more than one line feeds into a flash tank, each
line should be fitted with a swing check valve. Then, any line
not in use will be isolated from the others and will not be
fed in reverse with resultant wasted flash steam. If the trap
is operating at low pressure, gravity drainage to the condensate receiver should be provided.
Generally, the location chosen for the flash tank should
meet the requirement for maximum quantity of flash steam
and minimum length of pipe.
Condensate lines, the flash tank, and the low pressure
steam lines should be insulated to prevent waste of flash
through radiation. The fitting of a spray nozzle on the inlet
pipe inside the tank is not recommended. It may become
choked, stop the flow of condensate, and produce a back
pressure to the traps.
Low pressure equipment using flash steam should be
individually trapped and discharged to a low pressure
return. Large volumes of air need to be vented from the
flash tank; therefore, a thermostatic air vent should be
used to remove the air and keep it from passing through
the low pressure system.
The important dimension is the inside diameter. This should
be such that the upward velocity of flash to the outlet is low
enough to ensure that the amount of water carried over
with the flash is small. If the upward velocity is kept low, the
height of the tank is not important, but good practice is to
use a height of 2’-3’.
It has been found that a steam velocity of about 10’ per
second inside the flash tank will give good separation of
steam and water. On this basis, proper inside diameters
for various quantities of flash steam have been calculated;
the results are plotted in Chart 11-22. This curve gives
the smallest recommended internal diameters. If it is more
convenient, a larger tank may be used.
Chart 11-22 does not take into consideration pressure—only
weight. Although volume of steam and upward velocity are
less at a higher pressure, because steam is denser, there is
an increased tendency for priming. Thus it is recommended
that, regardless of pressure, Chart 11-22 be used to find the
internal diameter.
Chart 11-22.
Determination of Internal Diameter of Flash Tank
to Handle a Given Quantity of Flash Steam
Find amount of available flash steam (in pounds per hour)
on bottom scale, read up to curve and across to vertical
scale, to get diameter in inches.
Figure 11-58. Flash Steam Recovery from an Air Heater Battery
Flash is taken from the flash tank and combined with live
steam, the pressure of which is reduced to that of the flash
by a reducing valve.
High
Pressure
Steam
Low
Pressure
Section
Flash
Tank
Heater
Battery
Air
Flow
INTERNAL DIAMETER OF FLASH TANK IN INCHES
30
25
20
15
10
5
0
Condensate
1,000
2,000
3,000
4,000
5,000
POUNDS FLASH STEAM PER HOUR
6,000
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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and reduce environmental emissions while providing an “enjoyable experience.”
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867-EN
Printed in U.S.A. - 5/15
© 2015 Armstrong International, Inc.
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