How to Trap Superheated Steam Lines

How to Trap Superheated Steam Lines
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11/4/03
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Page CG-23
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 CG-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)
28
CG-23
Armstrong Steam and Condensate Group, 816 Maple St., P.O. Box 408, Three Rivers, MI 49093 – USA Phone: (269) 273-1415 Fax: (269) 278-6555
www.armstrong-intl.com
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838_Cons_Guide.qxd
11/4/03
2:49 PM
Page CG-24
How to Trap Superheated Steam Lines
Sizing Superheat Loads to Traps
Table CG-15. Time Period Table
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:
Time Period
Average
Pressure
psig (bar)
Temperature at
End of Time Period
°F (°C)
14" Line
Condensation Rate
lb/hr (kg/hr)
1st 2 hours
2nd 2 hours
3rd 2 hours
4th 2 hours
5th 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 CG-13 on page CG-19 lists the proper drip leg
size for given pipe sizes.
Using:
0.114 Wp (t2-t1)
=
H
C
Where:
C
= Amount of condensate in pounds
Wp
= Total weight of pipe (from Table CG-12 on
page CG-18)
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.
C =
(0.114 btu/lb °F) (21,400 lb) (200°F)
= 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 CG-31 on page CG-19. 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.
29
Armstrong Steam and Condensate Group, 816 Maple St., P.O. Box 408, Three Rivers, MI 49093 – USA Phone: (269) 273-1415 Fax: (269) 278-6555
www.armstrong-intl.com
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