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 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 because power is generated on site, and the need to run additional power lines is non-existent.
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.
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.
Figure 3.1.1 – Overall P & I Diagram
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 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 -fuel is shut off upon loss of air flow
· Low fuel supply -fuel is shut upon loss of fuel supply
· Loss of flame -all fuel is shut off upon loss of flame in furnace and or to an individual burner
· 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 rackRack # / 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
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 / Name / Type / DescriptionI:001/0 / LALL / DI / Level Alarm High - High
I:001/1 / LALL / DI / Level Alarm High
I:001/2 / LAH / DI / Level Alarm Low
I:001/3 / LAHH / DI / Level Alarm Low - Low
N10:5 / CV1 / AO / Boiler Feed Water Valve
N10:6 / CV2 / AO / Boiler Blowdown Valve
N10:7 / CV3 / AO / Boiler Blowdown Valve
N10:22 / FT1 / AI / Steam Output Flow Transmitter
N10:23 / FT2 / AI / Boiler Blowdown Flow Transmitter
N10:24 / FT3 / AI / Boiler Blowdown Flow Transmitter
N10:25 / FT4 / AI / Boiler Feed Water Flow Transmitter
N10:26 / LT / AI / 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.
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.
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.
Figure 6.2.1 – Transmitter Wiring Diagram
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 LossesComponent / K / Ft / Equiv. lenth / Quantity
Entry / 0.78 / 20 / 1
Exit / 1 / 25 / 1
Elbow (90*) / 30 / 14 / 1
5" sch 40 / 65 / 1
Total Equivalent Pipe Length: / 124’
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
Table 7.1.2 – Pump Discharge Fittings Losses
Pump Discharge – Fittings LossesFitting / K / Ft / Equivalent Length / Quantity
Entry / 0.78 / 20 / 1
Exit / 1 / 25 / 1
Elbow (90*) / 30 / 14 / 2
5" sch 40 / 150 / 1
Total Equivalent Pipe Length: / 223’
DP Le = 3.06 psi = 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
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)
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.
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
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
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
Table 9.1.2 – Losses through Fittings
Pump Discharge to Boiler Pipe – Fittings LossesFitting / K / Ft / Equivalent Length / Quantity
Entry / 0.78 / 20 / 1
Exit / 1 / 25 / 1
Elbow (90*) / 30 / 14 / 2
5" sch 40 / 150 / 1
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.
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.
9.2 Pump Inlet Pipe