Cogen Heat Recovery Boiler Three Element Feed Water Control
1.0 Introduction
This technical report describes how a cogeneration power plant functions. Component
sizing as well as an appropriate control system for the heat recovery boiler is determined
in this report. Several control loops are used to properly control the heat recovery boiler.
Three element feed water control combines a level loop and two flow loops to properly
maintain the water level inside the boiler. Continuous boiler blowdown has also been
used to maintain boiler efficiency. Proper pump sizing, pipe sizing, as well as orifice
plate sizing are calculated throughout the sections of this report. Certain loops that I
have selected are also animated using the PLC-5, which is available in our lab.
This report has been written to meet the requirements of the third year Automation
Technology course. Cogeneration power plants are an environmentally friendly method
of producing power; this is because they produce two types of energy. Heat and power
using only one fuel source are produced. The heat produced can then be turned into
steam, which powers a second turbine and produces additional power. Basically a
cogeneration plant takes wasted energy and turns it into usable power.
My report contains a full P&I diagram, wiring diagrams, PLC programming, component
selection and sizing for valves, pipes as well as orifice plates. I have also documented the
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separate loops which automate my system. Several drawings are also found throughout
my technical report to illustrate certain parts of the process.
2.0 Cogen Power Generation
2.1 Definiton
Cogeneration power plants are power generating facilities that produce both heat and
electricity, using a single fuel such as natural gas. Heat produced from the production of
electricity, such as the firing of a gas turbine, is recycled and used to produce steam. This
additional steam can be used for additional plant processes, for domestic purposes, or to
power a second turbine which produces additional electricity.
2.2 Benefits
The benefits of cogen are numerous. Single purpose thermal electric power plants reject
between 50% and 65% of the fuel heat to rivers, lakes, the ocean or the atmosphere.
Cogeneration systems use this rejected heat into a usable power source. By using wasted
heat, and turning it into a usable source of energy, the cogen power plants can increase
their efficiency This added efficiency is extremely desirable nowadays because of the
ever – increasing price of fuel, as well as the growing concerns for our environment.
Another benefit of cogeneration type power plants is the lack of line losses; this is
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because power is generated on site, and the need to run additional power lines is nonexistent.
2.3 Sequence of Operation
Basically, all cogen power plants have some form of primary fuel that is burned, (this can
be fuel, natural gas, coal ect.). The primary fuel is burned, which creates a lot of heat and
pressure, this heat and pressure is then used to spin a turbine, which in turn spins a
generator. Power is then produced from the generator. All the while the primary
combustion is occurring, the cogen power plant uses as waste heat boiler. The hot
exhaust gases, left over from the primary combustion process, are sent through heat
exchangers, which heat up steam in a boiler. It is only after that most of the heat energy
is removed from the hot exhaust gasses is it sent up the stack and released to atmosphere.
The additional steam produced by the heat recovery boiler is then used to power a steam
turbine, which powers a second generator. Additional power is generated from the
second generator making the cogen power plants more efficient. For a visual on how the
system actually works, refer to Figure 2.3.1.
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Figure 2.3.1 – Cogen Overview (http://www.cogen.org/cogen-challenge/support/images.htm)
3.0 Heat Recovery Boiler Control
The overall P& I drawing for my control system can be found in the following section.
You will also find 3 element feedwater, boiler blowdown, and basic boiler safety in the
following section of this report.
3.1 Process and Instrumentation Diagram
The overall process and instrumentation diagram for the boiler control system is pictured
in figure 3.1.1.
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Figure 3.1.1 – Overall P & I Diagram
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3.2 Three Element Feed Water Control
Three element feed water control has been selected in this case. The water level inside
the boiler is critical. If boiler level is too low, the heating tubes will be exposed, which
will damage them. Too high a level will interfere with steam separation. Both cases can
prove disastrous.
In a three element system, input flow, output flow, as well as level are
measured. Measuring and controlling three elements will ensure tight boiler level
control. Figure 3.3.1 shows three element feed water control. You can see the outlet
steam flow, feed water flow, as well as boiler drum level are all monitored.
Figure 3.2.1 – Three Element Feed Water
3.3 Continuous Boiler Blowdown
Boiler feedwater, even after having been treated, will contain impurities and minerals. If
these minerals aren’t removed from the boiler scaling and corrosion will occur on it’s
inside surfaces. This buildup can be avoided with proper boiler blowdown. Blowdown
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will occur in 2 areas of the boiler drum. The first blowdown line will lead up to the top
watermark in the boiler, when this valve is opened, all froth will be evacuated from the
drum. The second blowdown line will be installed into the bottom mud drum, where all
the heavier solids will accumulate. Continuous boiler blowdown will be used in this
case; this signifies that a set ratio of blowdown will occur in proportion to the input of
feedwater flow. A set ratio of 100:1 will suffice for continuous blowdown versus inlet
flow.
3.4 Basic Boiler Safety
As was explained in an AETY in class handout, because of the energy in boilers, safety
during start-up, shutdown and normal operation is very important. Safety is a go / no go
situation. If safety limits are exceeded ON/OFF controls disable the operation of the
boiler.
Below you will find the basic safety interlocks:

Purge interlock
-prevents fuel from being admitted to an unfired furnace until the
furnace is thoroughly purged with air

Low air flow

Low fuel supply -fuel is shut upon loss of fuel supply

Loss of flame
-fuel is shut off upon loss of air flow
-all fuel is shut off upon loss of flame in furnace and or to an
individual burner
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
Fan Interlock
-stop forced draft upon loss of induced draft fan

Low water
-shut off fuel on low water level in boiler

Damper interlock -shut dampers if fans are not operating
4.0 PLC 5 Programming
Allen Bradley’s PLC 5 as well as Rockwell Automation’s RSlogix PLC programming
software is used for automating the system.
4.1 PLC Information
The following table lists all cards installed into the PLC rack:
Table 4.1.1 PLC Information
Processor Type : Allen Bradley PLC-5/40C - 16 slot rack
Rack # Slot #
Description
Part #
00
C0
Ethernet Adapter Card
00
C1
AC Input Module
1771-IA2
00
C2
Analog Input Module
1771-IFE/C
00
C3
Analog Output Module
1771-OFE/B
00
C4
Empty
00
C5
Empty
00
C6
Empty
00
C7
Empty
01
C8
Empty
01
C9
Empty
01
C10
Empty
01
C11
Empty
01
C12
Empty
01
C13
Empty
01
C14
Empty
01
C15
Empty
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4.2 Symbol Table
The following table lists all the I/O addresses as well as descriptions.
Table 4.2.1 – PLC Symbol Table
Address
I:001/0
I:001/1
I:001/2
I:001/3
N10:5
N10:6
N10:7
N10:22
N10:23
N10:24
Name
LALL
LALL
LAH
LAHH
CV1
CV2
CV3
FT1
FT2
FT3
Type
DI
DI
DI
DI
AO
AO
AO
AI
AI
AI
N10:25
N10:26
FT4
LT
AI
AI
Description
Level Alarm High - High
Level Alarm High
Level Alarm Low
Level Alarm Low - Low
Boiler Feed Water Valve
Boiler Blowdown Valve
Boiler Blowdown Valve
Steam Output Flow Transmitter
Boiler Blowdown Flow Transmitter
Boiler Blowdown Flow Transmitter
Boiler Feed Water Flow
Transmitter
Boiler Level Transmitter
4.3 Network Descriptions
The complete PLC ladder logic programming as well as individual network descriptions,
is found in Appendix A – “PLC Program” at the end of this report.
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5.0 RSview
Rockwell Automation’s RSview software is used to create a GUI (Graphical User
Interface). Figures 5.0.1 shows a screen capture of the completed GUI.
Figure 5.0.1 – RSView Screen Capture
6.0 Wiring Diagrams
6.1 Boiler Level Switch Wiring:
In figure 6.1.1 you can see both level alarms, which each contain two probes; they are
wired into the AC input module in slot C1 of the PLC 5.
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Figure 6.1.1 – Level Switch Wiring Diagram
6.2 Level and Flow Sensing Elements:
Figure 6.2.1 shows the wiring for all flow and level transmitters. They all happen to be
Rosemount 1151 differential pressure transmitters.
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Figure 6.2.1 – Transmitter Wiring Diagram
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7.0 Pump Calculations
7.1 Pump Sizing
A) Static Suction Lift = 50’ of liquid + 700 psia =
-2059.924’
B) Suction Side Losses =
Pipe Size: 5” sch 40
Table 7.1.1 – Pump Inlet Fittings Losses
Pump Inlet – Fittings Losses
Component
K
Entry
Exit
Elbow (90*)
5" sch 40
0.78
1
Ft
30
Equiv. lenth
Quantity
20
25
14
65
1
1
1
1
124’
Total Equivalent Pipe Length:
DP Le = 1.70 psi =
5.003’
C) Total Dynamic Suction Lift =
-2054.921’
D) Static Discharge Head =
10’
E) Discharge Side Losses =
Pipe Size: 5” sch 40
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Table 7.1.2 – Pump Discharge Fittings Losses
Pump Discharge – Fittings Losses
Fitting
K
Entry
Exit
Elbow (90*)
5" sch 40
0.78
1
Ft
Equivalent Length
Quantity
20
25
14
150
1
1
2
1
30
Total Equivalent Pipe Length:
DP Le = 3.06 psi =
223’
9.005’
F) Total Dynamic Discharge Head =
19.005
G) Total Dynamic Suction Lift =
-2054.921
H) Total Discharge Head = 950 psia =
2795.56’
TDH =
760.565’
7.2 Calculating pump motor horsepower
Water Horsepower:
(Q*TDH*S.G.) / 3960
= 75.365 HP
Brake Horsepower:
=137.027 HP
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7.3 Pump Specifications
Figure 7.3.1 is the pump curve for the selected pump. Note that I have made a mark on
the drawing where 500 GPM and 760 THD meet, so that I may gather the rest of the
information that is required to purchase a correct pump.
Figure 7.3.1 – Gould Pump Curve (AETY in class handout)
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Here is a list of the specifications needed for this application:
Make: Gould
Model: 3700
Size: 3X4-16
Impeller Size: 14”
Speed: 3550 RPM
Horsepower: 137.027
Efficiency: 55%
8.0 Valve sizing
There are 3 control valves in the overall P&I drawing. You will find the specifications,
as well as size calculations for these valves in the following section.
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8.1 Feedwater valve sizing
Cv = Q sqr. Root (Gf / Dp)
Q = 500 gpm
Gf = 0.7848
Dp = 10 psi
Solution = Cv = 140.07
8.2 Blowdown valves sizing
Cv = Q sqr. Root (Gf / Dp)
Q = 2.5
Gf = 0.7848
Dp = 10 psi
Solution = Cv = 0.700
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8.3 Valve Selection
For complete valve selection order code breakdown, see appendix B entitled “Control
Valve Selection” at the back of this report. Table 8.3.1 contains required valve
specifications.
Table 8.3.1 – Valve specifications
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9.0 Pipe sizing
In the following section the size or various pipes is calculated, losses through the fittings
are also taken into account.
9.1 Pump discharge to boiler pipe
Figure 9.1.1 – Pump Discharge to Boiler Piping Diagram
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Table 9.1.2 – Losses through Fittings
Pump Discharge to Boiler Pipe – Fittings Losses
Fitting
K
Entry
Exit
Elbow (90*)
5" sch 40
0.78
1
Ft
Equivalent Length
Quantity
20
25
14
150
1
1
2
1
30
Total Equivalent Pipe Length:
223’
Here are the required formulas to size the pipe, as found in the Crane manual:
DP = 0.000216 (fLpQ^2) / d^5
(Crane 3-2)
f = I will use a friction factor of 0.016 (Crane A-26) to begin my calculation
L = 223 feet
p = 48.948 (Specific gravity of boiler feedwater @ 500*F in cu.ft/lb).
Q = 500 gpm
DP = 10 psi.
Solution: 3.93” i.d.
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This is a ballpark figure, the Reynold’s number must be found to ensure a more accurate
calculation. This formula is:
Re = 50.6 (Qp) / du (Crane 3-2)
Solution = 7 900 000
Crane A-25 shows that the friction factor will change from .016 to .017. After plugging
the correct friction factor into the first equation, then solving the equation once again,
yields a result of 3.98” inner diameter.
The correct pipe size in this case is a 5” sch40, which has an inner diameter at 5.047
inches.
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9.2 Pump Inlet Pipe
Figure 9.2.1 – Piping Diagram
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Table 9.2.2 – Losses through Fittings
Pump Inlet Pipe – Fittings Losses
Component
K
Entry
Exit
Elbow (90*)
5" sch 40
0.78
1
Ft
30
Equiv. lenth
Quantity
20
25
14
65
1
1
1
1
Total Equivalent Pipe Length:
124’
Here are the required formulas to size the pipe, as found in the Crane manual:
DP = 0.000216 (fLpQ^2) / d^5
(Crane 3-2)
DP = 5 psi
f = 0.017
L = 124’
p = 48.948
Q = 500 gpm
Solution: 4.019” i.d.
Once again, the pipe size will be 5” sch40, which has an inner diameter of 5.047”.
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9.3 Superheated steam boiler outlet pipe
Figure 9.3.1 – Superheated Steam Boiler Outlet Pipe
Table 9.3.2 – Losses through Fittings
Boiler Discharge to Steam Turbine Pipe
Component
Entry
Exit
Elbow (45*)
Elbow (90*)
8" sch 40
K
Ft
Equiv. lenth
0.78
1
Quantity
32
44
9
20
56
1
1
2
2
1
Total Equivalent Pipe Length:
150
16
30
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I used the “S” curve on page 3-22 of the Crane manual to get a ballpark pipe size of 8”
sch 40 pipe. This pipe size (8” sch 40) will be used in the rest of the calculations.
Reynold’s # for 194 336 lbs / hr of steam through 8” sch 40 pipe.
Re = 6.31 (W/du)
W = 194 336 lbs/hr
d = 7.981”
u = 0.028 (Crane A-2)
Solution = 5 487 408
Friction factor (Crane A-25) = 0.014
DP = 0.000 003 360 [(fLW^2Vbar) / (d^5)]
F = 0.028
L = 150’
W = 194 336 lbs/hr
Vbar = 1.0306 (Crane A-17)
d = 7.981”
The pressure lost through the pipe and fittings is : 14.9 psi.
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9.4 Boiler blowdown pipes
The flow rate through the boiler blowdown pipes is only 2.5 gallons per minute. Using
2” sch 40 pipe will suffice for this application. This size of pipe is certainly oversized for
the flow rate, because of this; the pressure loss through the pipe and fittings is negligible.
The larger than needed 2” pipe will assure clear flow of boiler blowdown.
10.0 Component Selection
10.1 Boiler Point Level Detection
Four point level detection points are incorporated into the boiler :
1. LALL – Level Alarm Low
2. LAL – Level Alarm Low
3. LAH – Level Alarm High
4. LAHH – Level Alarm High High
Both the LALL and LAL are measured with one level switch, same goes for the LAH
and LAHH. One single switch has 2 probes inside of it, which makes it ideal for this
application. The switch is made by Clark – Reliance part # EA101 with socket
welded connections, pictured in figure 10.1.1, meets the requirements needed to
assure safe operation for the high temperature and pressure conditions. See Appendix
C – “Level Switch Ordering Information” for data sheet.
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Figure 10.1.1 – Clark – Reliance EA101 Levelswitch (http://www.clarkreliance.com/products/reliance/Product_Line/Levelarms.htm)
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Figure 10.1.2 – Level switch dimensions –
(http://www.clarkreliance.com/products/reliance/Catalogs/D3.1C.pdf)
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Figure 10.1.3 – Level Switch Installation
10.2 Flow Element Selection
Orifice plates are used for flow measurement exclusively. Software available through
Foxboro was used to size the orifice plates, and all of the ordering information, as well as
sizing information can be found at the enof this report in appendix D – “Orifice Plate
Selection
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10.3 Flow Transmitter Selection
FT1 – Steam Outlet Flow Transmitter
The Rosemount 1151 differential pressure transmitter is used for steam outlet flow
measurement. It is connected to the process via flange taps, as noted in the orifice sizing
section. The calibration settings are as follows:
Zero: 0” w.c.
Span: 1000” w.c.
The following order number is required to assure the proper transmitter is ordered:
1151HP 6 S 52 B7 M2
The model number table can be found in appendix D – “Transmitter Ordering
Information” at the end of this report.
FT2 & FT3 – Boiler Blowdown Flow Transmitters
Rosemount 1151 differential pressure transmitters are used for blowdown flow
measurement. They are connected to the process via flange taps, as noted in the orifice
sizing section. The calibration settings are as follows:
Zero: 0” w.c.
Span: 100” w.c.
The following order number is required to assure the proper transmitter is ordered:
1151HP 4 S 52 B7 M2
The model number table can be found in appendix D – “Transmitter Ordering
Information” at the end of this report.
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FT4 – Boiler Feedwater Flow Transmitter
The Rosemount 1151 differential pressure transmitter is used for boiler feed water flow
measurement. It is connected to the process via flange taps, as noted in the orifice sizing
section. The calibration settings are as follows:
Zero: 0” w.c.
Span: 150” w.c.
The following order number is required to assure the proper transmitter is ordered:
1151HP 5 S 52 B7 M2
The model number table can be found in appendix D – “Transmitter Ordering
Information” at the end of this report.
10.4 Boiler Level Transmitter
FT1 – Boiler Water Level Transmitter
The Rosemount 1151 differential pressure transmitter is used to measure the water level
in the boiler via a wet leg setup as pictured in figure 10.4.1
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Figure 10.4.1 – Level Transmitter Installation
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The calibration settings for this level transmitter are as follows:
Zero: 7.848” w.c.
Span: 31.392” w.c.
With the specific gravity of the boiler water being .7848, the calibrations above will
monitor the boiler level between 0” and 30” inches of fluid. The following order number
is required to assure the proper transmitter is ordered:
1151HP 4 S 52 B7 M1
11.0 Conclusion
This report has been written to meet the requirements of the third year Automation
Technology course. I learned a lot while researching the various components that I have
selected throughout the report. There is without a doubt a lot of information to take into
account when sizing, and putting into play a control strategy such as three element feed
water. Paying close attention to detail in a report like this would pay off if the project
was ever undertaken. Wiring diagrams and programming need to be perfect in order to
have a successful installation.
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References
1.
2.
3.
4.
5.
6.
7.
8.
9.
http://www.energy.rochester.edu/cogen/chpguide.htm
http://www.cogeneration.org/
http://www.cogen.org/Downloadables/Projects/EDUCOGEN_Cogen_Guide.pdf
http://www.cogeneration.net/
http://www.hatch.ca/Energy/Energy_Conservation/Energy_Efficiency/power_cog
en.htm
http://appsci.queensu.ca/ilc/sustainability/energy/cogen.php
http://www.software.rockwell.com/
http://www.clark-reliance.com/products/reliance/Product_Line/Levelarms.htm
http://www.emersonprocess.com/Rosemount/
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