Basic Steam Trap
Operation
©2008 Armstrong International, Inc.
What MUST a Steam Trap do?
A Steam Trap must remove
condensate, air, and CO2 out of
the system as quickly as it
collects.
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A Steam Trap Must Also:
Minimize steam lose
Long life and dependable
service
Corrosion resistance
Air Venting
CO2 Venting
Operation against
backpressure
Freedom from dirt problems
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What makes up a Steam Trap
• A Steam Trap has an orifice
• An orifice alone is not a
steam trap
Inlet
Connection
– If flow changes, orifice is not
correct
Body
– If pressure drop changes,
orifice is not correct
Outlet
Connection
– The orifice must change size
as conditions change
Orifice
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What makes up a Steam Trap
• A Steam Trap should have a
valve
– A valve may be fully opened
and fully closed or modulated
to vary the size of the orifice
as conditions change
Inlet
Connection
Body
Valve
Outlet
Connection
Orifice
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What makes up a Steam Trap
• A Steam Trap should have
an Operator
– An operator senses when to
move the valve and supplies
the power to move the valve
Inlet
Connection
Operator
Body
– Traps are different in the
types of valve and the types
of operator they use
Valve
Outlet
Connection
Orifice
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Trap/Operator Types
Thermodynamic
Mechanical
Thermostatic
Steam (flash) –
flow operates
valve
Use difference
in density
between steam
and condensate
to operate
valve. A float
operates the
valve.
Sense
temperature
change of
condensate to
operate valve
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Thermostatic Trap Types
• Bellows balanced pressure
– High capacity
• Wafer/Diaphragm balanced
pressure
– Low capacity
• Bi-metallic
– High and low capacity
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Bellows Balanced Pressure
Steam
Condensate
Bellows
Liquid fill
Valve
Seat
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Bellows Balance Pressure
Bellows Balance
Pressure
Modulation
Poor
Backpressure
Good
Dirt
Fair
Wear
Fair
Water Hammer
Poor
Freezing
Good
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Simple Bi-Metal Element
High expansion metal
Low expansion metal
Cold
Hot
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Simple Bi-Metal Operation
Valve inside trap
• Pressure holds valve closed;
valve toggles
• Back pressure can blow trap
open
• Opening and closing
temperatures wide apart
• Must sub-cool deeply to
prevent steam loss
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Bi-Metal Disks
Valve in outlet
• Pressure opposes closing.
• Does not toggle.
• Some thermodynamic
action; roughly follows
steam saturation curve.
• Acts as check valve when
pressure is lost.
• Good for use in superheated applications.
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Disk Trap Orifices in Series
A. Trap body
B. Disk bonnet
C. Disk
D H
P2
E
C
B
A
F G
P1
D. Inner seat ring
P3
E. Outer seat ring
F. Inlet port
G. Outlet port and groove
E
D
H
F
E
D
G
Disk Trap Start-up
• Condensate pressure
pushes disk off seat
• Full capacity flow
• Two pressure drops
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Disk Trap Flashing Steam
• Condensate near steam
temperature flashes to
steam in inlet port
• Pressure drops with flow
through restriction under
disk
• Pressure difference (P2 (<<P2)) slams disk onto
seats
P2
<<P2
P1
P3
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Disk Trap Closed
• P2 – P3 x disk area > P1 x inlet
port area + P3 x outlet port
area
• Closing force greater than
opening forces so disk stays on
seat
• Condensing flash steam above
disk relieves pressure until
opening forces are greater;
disk snaps open
• A bleed groove is used to help
with control of the pressure in
the control chamber.
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Disk and Seat Wear
• Disk slammed hard onto seats
and rolls as it seats
– Edge of disk wears rapidly
– Edge of outer seat ring wears
rapidly
Flat valve
and seats
require near
perfect
flatness to
seal
Two seats
to seal
Uneven contact of disk
on seat causes wear
• High velocity flow of condensate
flash steam and dirt between
disk and inner seat ring
– Inner seat ring wears rapidly
– Disk surface erodes rapidly
Disk Wear
Seat Wear
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Thermodynamic Traps
Bellows Balance
Pressure
Thermodynamic
Traps
Modulation
Poor
Fair
Backpressure
Good
Poor
Dirt
Fair
Poor
Wear
Fair
Poor
Water Hammer
Poor
Good
Freezing
Good
Good
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Mechanical Types
Inverted Bucket
Float & Thermostatic
Air Vent Valve
Linkage
Fixed
Pivot
Valve
Ball
Seat
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Simple Float Trap
Air Vent Valve
Fixed Pivot
Linkage
Valve
Ball
Seat
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Float and Thermostatic
Air Venting
Start-Up
Modulating
Condensate enters
Condensate flow decreases
Ball rises opening valve
to full capacity
Ball moves downward;
valve open less
Bellows cold; vent open
to air and/or condensate
flow
Valve modulates to balance
outlet flow = inlet flow
Steam enters; steam air
mixture closes vent;
temperature below
saturation
Air increases;
temperature drops; vent
opens
Steam lost with air
removal
Dirt falls to bottom; may
stop ball from dropping
holding valve open; may
plug valve
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F&T Traps
Bellows Balance
Pressure
Thermodynamic
Traps
F&T Traps
Modulation
Poor
Fair
Good
Backpressure
Good
Poor
Good
Dirt
Fair
Poor
Poor
Wear
Fair
Poor
Good
Water Hammer
Poor
Good
Poor
Freezing
Good
Good
Poor
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Inverted Bucket
A. Bucket
B. Valve on linkage
• Sized for maximum flow at
maximum pressure
difference
C. Air Vent
D. Body
• Material based on
pressure and cost
E. Inlet
C
B
A
D
Bottom inlet – Top outlet
Other configurations available
E
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IB At Start-Up
•
•
•
•
•
•
•
Condensate fills body
Condensate surrounds all internals;
pressure same on all surfaces of all
internals; no water hammer damage
Valve wide open for maximum flow
rate; quick drain
Air pushed out ahead of condensate
through wide open valve; quick vent
Small dirt particles suspended in flow
and flushed through valve
Large dirt particles are too heavy to
be carried to valve by low velocity
flow around bucket lip, so they stay at
bottom
No strainer needed in most
applications
Condensate
Valve wide open
Low
velocity
flow
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IB Closed
•
Start-up condensate
accumulation gone
Valve tightly closed
•
Steam enters depressing water
level in bucket
•
When bucket approximately 2/3
full of steam, bucket becomes
buoyant
•
Bucket floats valve to seat
•
Pressure difference pushes valve
softly into seat
•
Valve seats tightly
•
Air passes through vent to collect
at top of trap
Condensate
level
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IB Filling
•
Valve closed; condensate continues
to enter trap; no condensate back-up
•
Intermittent discharge but continuous
drain
•
Water condenses and displaces
steam; water level rises
•
Air continually rises through vent to
top of trap
•
If air accumulation becomes
excessive, air will depress water level
and open valve
Valve tightly closed
Steam
Condensate
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IB Opening
•
Water level continues to rise
•
Bucket 2/3 full of water loses
buoyancy; bucket falls pulling valve
wide open
•
Air flushed out
•
Full capacity flow carries small dirt
particles to wide open valve to flush
each cycle
•
Pressure changes affect capacity only
•
Steam in trap continuously;
condensate and air discharged at
steam temperature
•
Cycle rate adjusts immediately with
flow changes
Valve wide open
Small dirt
flushed
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IB Thermic Vent
Bi-metal thermic vent
• Open at start-up for
quick vent of air under
bucket
• Closed at steam
temperature; normal
vent enough for
operating air venting
load
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Inverted Bucket
Bellows Balance
Pressure
Thermodynamic
Traps
F&T Traps
Inverted Bucket
Modulation
Poor
Fair
Good
Good
Backpressure
Good
Poor
Good
Good
Dirt
Fair
Poor
Poor
Good
Wear
Fair
Poor
Good
Good
Water Hammer
Poor
Good
Poor
Good
Freezing
Good
Good
Poor
Good
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Armstrong Promise
We provide intelligent system solutions that improve
utility performance, lower energy consumption, and
reduce environmental emissions . . . while providing
an “enjoyable experience”!
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Intelligent System Solutions ® S T E A M • A I R • H O T W A T E R
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David M. Armstrong
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©2008 Armstrong International, Inc.