Transcript KBR 1-16-03 - Armstrong International

®
Process Heat Transfer
The Cause and Effect of Various
Design Concepts
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©2006 Armstrong International, Inc.
Exchanger Variables
• Fouled surface area
• Non-condensible gases
• Flooded surface area
• Variable process inlet and outlet temperatures
• Variable process flow rates
• All of these change the BTU demand on the heater,
changing the pressure and temperature of the heat
transfer media
®
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David M. Armstrong
2
Fouled Surface Area
• Fouled surface area decreases the heat transfer
efficiency of the tube bundle
• This inherently causes adjustments in the
pressure and/or temperature of the heat transfer
media being supplied to the exchanger
®
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David M. Armstrong
3
Fouled Surface Area
• Resulting in more surface exposed to the
transfer media in a level control system. This
will increase the BTU transfer rate.
• Higher delivery pressure from the inlet control
valve decreases the efficiency of the heat
exchanger. Higher pressure lacks the same
latent heat content of lower pressure. Energy
consumption will increase, while production
levels remain unchanged.
®
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David M. Armstrong
4
Non-Condensible Gases
• Presence of non-condensibles’ occupies
valuable steam space
• A reduction of viable heat transfer area can
result due to the insulating properties
• Promotion of carbonic acid formation is inherent
• Excessive amounts can inhibit drainage
®
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David M. Armstrong
5
Flooded Surface Area
• Promotes corrosion and fouling
• Can develop into water hammer
• Controls process temperature by decreasing
available surface area for heat transfer (Level
Control)
• Typically causes process outlet variations
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6
Variable Process Inlet & Outlet
Temperatures
• Changes the BTU exchange rate required or
(Delta T)
• These variable temperatures can increase or
decrease exiting pressure based on condensing
rate of the heater
• Will promote flooding on low exchange rate
demand
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7
Variable Process Flow Rates
• Variable flows will change BTU demand on the
exchanger
• Higher flow rates will increase the surface area
needed, raising or lowering the outlet pressure
based on available surface area
• Lower flow rates will decrease surface area
needed, raising or lowering the outlet pressure
based on available surface area
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8
Control Options
• Level Control
• Steam Control
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Level Control
• Level control systems flood exchangers to
reduce the amount of useable surface area for
BTU transfer
• Exchangers run flooded due to the control valve
on the condensate outlet, modulating to maintain
the desired process outlet temperature
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10
Steam Control
• Allows the exchanger to run at the lowest
possible steam pressure, which maximizes
energy efficiency due to latent heat content
• Less energy consumed for the same amount of
product produced
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11
Process Design Summary
• Utilize all of the surface area
• Eliminate corrosion and fouling by keeping the
exchanger dry
• Eliminate non-condensibles
• Optimize the design by using the lowest
pressure steam, to gain more latent heat content
per pound
®
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David M. Armstrong
12
®
Operating Characteristics
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©2006 Armstrong International, Inc.
Filling
Steam/Air In - Closed
Steam/Air Out - Open
Open Check
Valve
Closed Check
Valve
Step 1. During filling, the steam or air inlet and check valve on
pumping trap outlet are closed. The vent and check
valve on the inlet are open.
®
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14
Begin Pumping
Steam/Air In - Open
Steam/Air Out - Closed
Check Valve
Closed
Open
Check
Valve
Step 2. Float Rises with level of condensate until it passes trip
point, and then snap action reverses the positions
shown in step one.
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15
End Pumping
Steam/Air - In
Steam/Air - Closed
Closed
Check
Valve
Open Check
Valve
Step 3. Float is lowered as level of condensate falls until snap
action again reverses positions.
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16
Repeat Filling
Steam/Air In - Closed
Steam/Air Out - Open
Open
Check
Valve
Closed
Check
Valve
Step 4. Steam or air inlet and trap outlet are again closed while
vent and condensate inlet are open. Cycle begins anew.
®
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17
®
Pump Trap Applications
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©2006 Armstrong International, Inc.
Process Heat Exchanger
with 100% Turndown Capability
Steam
Control
Valve
Thermostatic
Air Vent
Condensate
Return
Heat
Exchanger
Motive
Steam
Thermostatic
Air Vent
Process
Inlet
Vent
Reservoir
F&T
Trap
Pump Trap
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19
Vacuum Reboiler Construction Comparison
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20
Hydrocarbon Knockout Drum/Separator
Gas
Outlet
Gas
Inlet
Seperation
Chamber
Nitrogen or
Inert Gas
Motive Supply
Equalizing
Vent Line
Expanded
Reservoir
Piping
To Reclamation
Destination
Pump Trap
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21
Flare Header Drain
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22
Flash Vessels
Steam
Outlet
Condensate
Inlet
Flash
Tank
Air
Vent
Motive
Steam
Vent
Reservoir
Pump Trap
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23
Steam Turbine Casing
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Pump Trap Applications
• Process Heat Exchangers
• Liquid Separators
• Sumps
• Vacuum Systems
• Condensate Drum – Flash Tanks
• Vented Systems
• Closed Loop Applications
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25
®
Understanding and Benefiting from
Equipment Stall
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©2006 Armstrong International, Inc.
Q=UADT
Q = Design Load (BTU/Hr)
U = Manufacturer’s Heat Transfer Value (BTU/ft2/°F/Hr)
A = Heat Transfer Surface Area (ft2)
DT = (Ts – T2) Approaching Temperature (°F)
Ts = Operating Steam Temperature (°F)
T2 = Product Outlet Temperature (°F)
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27
What is wrong with this application?
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Effects of “Stall”
• Inadequate condensate drainage
• Water hammer
• Frozen coils
• Corrosion due to Carbonic Acid formation
• Poor temperature control
• Control valve hunting (system cycling)
• Reduction of heat transfer capacity
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29
Factors Contributing to “Stall”
• Oversized equipment
• Conservative fouling factors
• Excessive safety factors
• Large operating ranges
• Back pressure at steam trap discharge
• Changes in system parameters
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30
Finding “Stall”
Where does Stall occur??
• Air heating coils
• Shell & tube heat exchangers
• Plate & frame heat exchangers
• Absorption chillers
• Kettles
• Any type of heat transfer equipment that has
Modulating Control
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31
®
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What is the “Stall” Solution?
• Use a bigger steam trap?
• Use a vacuum breaker?
• Implement a safety drain?
• Install a Posi-Pressure system?
• Use an electric pump?
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33
Keys to Operation
• How quick it can fill: This is dictated by head
pressure & inlet pipe and check valve size
• Vent/Equalization: Vent connection must
always be in vapor space
• Pump Out: Motive vs. back pressure and gas
used
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34
Vocabulary
Filling Head: Distance between the top of the
pump and the bottom of the receiver or reservoir
pipe
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Vocabulary (Continued)
Receiver/Reservoir Pipe: This is a temporary
holding place to store condensate while the pump
is in the pump down cycle. The receiver/reservoir
pipe is designed and sized to prevent condensate
from backing up into the system.
®
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36
®
Open System Configuration
Closed System Configuration
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©2006 Armstrong International, Inc.
Open System
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Open System
Advantages:
• Drain multiple pieces of equipment
• Can use Air or Steam for pump trap operation
• Easiest to understand
Disadvantages:
• Lose valuable flash steam
• Must run a potentially expensive atmospheric vent line
• Size the pump trap based total design load
• Must compete with electric pumps
®
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39
Closed System
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40
Closed System
Advantages:
• No flash steam loss
• No need to run long expensive vent lines
• Use a smaller pump than in a open system*
• Return condensate hotter
Disadvantages:
• Dedicated pump for a single piece of equipment
• More complex
• Cannot use air as motive force
®
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41
Pump Sizing / Receiver Sizing
Pump Sizing
• Determine head available from equipment
(distance from equipment outlet to grade)
• Select either closed loop or vented design (Note:
If multiple sources of condensate, vented system must
be used to prevent short circuiting)
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42
Pump Sizing / Receiver Sizing
Pump Sizing
• Determine maximum pumping load
• Calculate maximum back pressure (including lift)
• Determine motive pressure and gas to be used
(use capacity correction factor if using a medium
other than steam)
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43
Pump Sizing / Receiver Sizing
Pump Sizing
• Check and specify head pressure (distance from
bottom of receiver/reservoir to top of selected
pump)
• Make sure to use capacity correction if more or
less head is available than standard catalog
dimension
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David M. Armstrong
44
Pump Sizing / Receiver Sizing
Pump Sizing
• Calculate maximum flash rate & needed vent
size – if vented system
• Determine and size reservoir – if closed loop
system
• Size downstream F&T trap if needed for closed
loop system
®
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45
Vented Receiver Sizing
Vented Receiver Sizing for Open Systems
Flash Steam
Receiver
Diameter
Receiver
Length
Vent Line
Diameter
lb/hr
up to
75
kg/hr
in
mm
in
mm
in
mm
34
4
102
36
914
1-1/2
40
150
68
6
152
36
914
2
50
300
136
9
229
36
914
2-1/2
65
600
272
10
254
36
914
3
75
900
408
12
300
36
914
4
100
1200
544
16
405
36
914
6
150
2000
907
20
508
36
914
8
200
Note: When draining from a single or multiple pieces of equipment in an “open” system, a vented
receiver should be installed horizontally above and ahead of the pump trap. In addition to sufficient
holding volume of the condensate above the fill head of the pump trap to hold the condensate
during the pump trap cycle, the receiver must also be sized to allow enough area for flash steam
and condensate separation.
®
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David M. Armstrong
46
Closed Loop Receiver Sizing
Inlet Reservoir Pipe Sizing for Closed Systems
Condensate
Load
lb/hr
up to
in
2
mm
50
in
3
mm
75
ft
m
ft
m
Reservoir Pipe Diameter
in
mm
in
mm
4
100
6
150
Length of Pipe
ft
m
ft
m
in
8
mm
200
in
10
mm
250
ft
m
ft
m
500
227
4
1.2
2-1/2
0.7
1-1/2
0.4
1,000
453
4-1/2
1.4
2
0.6
1-1/2
0.4
1,500
680
7
2.1
3
0.9
2
0.6
2,000
907
9
2.7
4
1.2
2-1/2
0.7
2,500
1,134
11
3.4
5
1.5
3
0.9
1-3/4
0.5
3,000
1,360 13-1/2
4.1
6
1.8
3-1/3
1.1
2
0.6
4,000
1,814
5.5
8-1/2
2.6
5
1.5
2-1/2
0.7
5,000
2,268
10
3.0
6
1.8
3
0.9
1-1/2
0.4
6,000
2,722
12
3.7
7
2.1
3-1/2
1.1
2
0.6
7,000
3,175
14-1/2
4.4
8-1/2
2.6
4
1.2
2
0.6
8,000
3,629
16-1/2
5.0
9-1/2
2.9
4-1/2
1.4
2-1/2
0.7
1-1/2
0.4
9,000
4,082
11
3.4
5
1.5
3
0.9
2
0.6
10,000 4,536
12
3.7
5-1/2
1.7
3
0.9
2
0.6
11,000 4,990
13
4.0
6
1.8
3-1/2
1.1
2
0.6
12,000 5,443
14
4.3
6-1/2
2.0
4
1.2
2-1/2
0.7
18
Note: When draining from a single piece of equipment in a closed loop system, to achieve maximum
energy efficiency a reservoir should be installed horizontally above and ahead of the pump trap.
Sufficient reservoir volume is required above the filling head level to hold condensate during the pump
trap discharge cycle. The chart above shows the minimum reservoir sizing, based on the condensate
load, to prevent equipment flooding during the pump trap discharge cycle.
®
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David M. Armstrong
47
Critical Design Criteria Summary
1.
2.
3.
4.
5.
Maximum condensate flow from exchangers and
reboilers
Maximum differential pressure across the system
Minimum differential pressure across the system
(specifically when clean)
Minimum tower height needed to achieve maximum
condensate flow rate at minimum differential
Maximum motive pressure (steam, air, nitrogen, etc.)
available to power pumps
®
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David M. Armstrong
48
Critical Design Criteria Summary
6.
Maximum instantaneous discharge rate for
downstream pipe sizing & trap sizing
7. Temperature differential of condensate source vs.
condensate header design
8. Piping layout to prevent hydraulic shock
9. Total installed cost savings, including construction, on
turnkey jobs
10. Integrity of mechanical design due to the critical nature
of the service
11. Minimize potential problems with proper designs
®
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49
Maximum Differential Pressure Across
the System
• Maximum pressure from control valve, including
minimal drop
• Minimum drop across exchanger
• Maximum pressure – should tube leak occur
• Elimination of back pressure (bypass to grade)
• Consider fouled surface area
®
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50
Minimum Differential Pressure Across the
System
• Consider maximum percentage of turndown on process
flow vs. design flow (plus factor)
• Consider over-surfaced heat transfer area
• Evaluate downstream relief valve settings on condensate
side as traps (etc.) fail and pressurize the return system
• Undersized return lines are common in facility
expansions. Verify effects of additional flow on pipe
velocities and back pressures.
®
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David M. Armstrong
51
Minimum Head Pressure
• Skirt height on reboilers can be minimized by
evaluating discharge capacity needed and
setting height accordingly. This should be done
early in the job scope as it effects tower
construction.
• Additional pump capacity can be achieved by
increasing head pressure
®
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David M. Armstrong
52
Maximum Motive Pressure
Steam, Air, Nitrogen
• Ensure stable source with negligible variations
• Install drip station to insure dry gas is always
present at motive steam valve (pipers often do
not realize it is a dead-end steam line)
®
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53
Maximum Design Pressure
• Utilization of 2/3 Rule can eliminate relief valves
on low pressure side needed for tube rupture
cases
• Use of liquid drain traps can eliminate gas
discharge into return header
®
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David M. Armstrong
54
Maximum Instantaneous Discharge Rate
• Pump discharge rate must be used when sizing
condensate return leads (use bi-phase flow)
• Pump discharge rate also critical to downstream
traps in Pump / Trap combinations
®
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55
Temperature Differential of Source vs.
Header
• Minimize thermal shock by maintaining DT of
150°F or less
• When feasible, run separate headers for vacuum
temperature condensate
• Vacuum condensate headers can be sized on
single phase flow if dedicated solely for vacuum
temperature condensate
®
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David M. Armstrong
56
Piping Layout to Prevent Hydraulic
Shock
• Discharge lead from pumps should be piped into
top of return header
• Flow patterns should be continual – no opposing
flows
• Check valves should be installed at major
elevation changes to disperse hydraulic shock
®
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57
Pipe Sizing
1. Discharge piping should be based on 2-3 times
the normal condensing rate due to
instantaneous discharge rate of the pump
2. Minimize elevation changes to prevent
hydraulic shock
3. Utilize check valves at main header to
minimize backflow
®
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David M. Armstrong
58
Pipe Sizing
4. Run separate lines for vacuum temperature
condensate to minimize thermal shock
potential
5. Always calculate the maximum flash rate in
return lines
6. Insure adequate pipe and nozzle diameters to
facilitate bidirectional two-phase flow
®
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David M. Armstrong
59
®
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®
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David M. Armstrong