Process Description
Purpose
The purpose of the Condenser with Cooling Tower is to remove low-level heat from a steam turbine surface condenser and exhaust it to the atmosphere by exchanging the heat with ambient air through direct contacting of the air and warm cooling circulating water returned from the process. An overview of the simulated cooling system is provided in Schematic 2: Overview that follows this section.
Major Equipment
The turbine system mainly consists of the Steam Turbine, JT-301, Surface Condenser E-301, Hotwell D-301 and Condensate Return Pumps P-301A/B.
The cooling system mainly consists of the Cooling Tower, CT-101 and two Cooling Water Pumps P-101A/B.
Process Overview
Cooled water is collected in a basin at the bottom of the Cooling Tower CT-101 and is pumped by Cooling Water Pumps P-101A/B to a supply header which feeds the Surface Condenser E-301 of the Steam Turbine JT-301. The cooling water picks up heat from the Surface Condenser by heat transfer across the surface of metal tubes in the shell-and-tube heat exchanger. Warm water from the Surface Condenser is collected in a return header and is routed to the top of the Cooling Tower. The warm return water falls to the basin and is cooled by contact with air drawn through the tower by a motor-driven fan on top of the Cooling Tower. The internal packing in the Cooling Tower maximizes contacting between the air and the water to ensure maximum heat exchange. Also, evaporation of a portion of the return water occurs in the Cooling Tower.
High pressure steam from battery limits is fed to the Steam Turbine to produce shaft power. Steam from the last stage is exhausted to Surface Condenser which operates at low (vacuum) pressure. Steam condensate from the Surface Condenser is collected in Hotwell D-301 and is pumped to battery limits for reuse as boiler feedwater by Condensate Return Pumps P-301A/B.
Importance
Cooling towers are an extremely important system for surface condensers of steam turbines. A loss of cooling capacity will result in poor performance of the steam turbine at best, or a shutdown of the turbine at worst. A cooling water system allows more efficient use of plot space compared to using air-cooled exchangers to directly condense the exhaust steam. The overall capital cost and the operating cost is most often lower for a cooling water system, especially when multiple streams need to be cooled. Also, lower cooling temperatures can be attained by using a cooling water system in hotter weather than by directly using air-cooled exchangers. Lower temperatures are a result of the evaporative effect within the cooling tower as ambient air directly contacts the water. The lower temperature results in a lower condenser pressure which results in more power produced per unit of steam fed to the turbine.
Steam Turbine and Condenser
The Steam Turbine JT-301 represents a typical multi-stage steam turbine found in a power plant or process plant. High pressure steam from battery limits drives the Steam Turbine which turns a rotating equipment such as an electric power generator or a compressor. The exhaust from the Steam Turbine is condensed with cooling water and is returned to the boiler feed water preparation section of the plant.
Superheated high pressure (HP) steam from battery limits enters the Steam Turbine JT-301 under control of the turbine control system which regulates the inlet throttle valve JV-301. JT-301 is a three-stage turbine; the first stage exhausts intermediate pressure (IP) steam which is routed to an IP Desuperheater prior to passing through the IP Reheating Coils. Reheated IP steam is readmitted to JT-301 and passes through the IP stage of the steam turbine and then to the low pressure (LP) stage within the same casing. Steam exiting the LP stage is exhausted directly to the shell side of Surface Condenser E-301 which uses cooling water to condense the exhausted steam from JT-301. Note that the details of the HP, IP and LP stages within the Steam Turbine are not shown on the simulator.
Surface Condenser E-301 normally condenses all the steam from JT-301 using cooling water supplied from Cooling Water Pumps P-101A/B. The cooling water returns back to the Cooling Tower CT-101. The condensate from E-301 drains directly into the Hotwell D-301. The pressure of E-301 is essentially determined by the vapor pressure of the condensate leaving E-301 so it normally operates at vacuum conditions. To ensure vacuum conditions are maintained, a line to the Vacuum System at battery limits is provided. Normally, this line pulls a small flow of low pressure vapor from E-301.
At startup or in case of leaks of air into E-301 or in case of non-condensables in the HP steam, the pressure in E-301 may build due to pocketing of the non-condensable vapor. If this occurs, the vacuum line can be fully opened to exhaust the vapor more quickly (commonly called hogging).
Hotwell
The Hotwell D-301 is a vertical drum directly connected to outlet of Surface Condenser E-301 and collects steam condensate from E-301 and E-302. The condensate is pumped by Condensate Return Pumps P-301A/B to the Deaerator in the BFW treatment section at battery limits for reuse as boiler feed water. Either pump can be set to auto-start in case of high level in D-301.
Cooling Tower
Warm cooling water is returned from the Surface Condenser E-301 via the return header and is routed to the top of the Cooling Tower CT-101 via a hand valve controlled by HIC-103. The warm water is split equally and sent to opposite sides of the Cooling Tower and distributed evenly into the two opposing cells.
Within each cell, falling warm water is contacted with air drawn into the cell by a motor-driven fan located at the top of the tower. When air is drawn in along the outside of each cell the contacting method is known as “cross-flow”. When air is drawn in only at the bottom, the contacting method is known as “counter-flow”. If a cooling tower is constructed without any mechanical way to move the air, the contacting method is “natural draft”. In this type of tower air naturally moves upward as it is warmed. This method requires a much larger, taller, circular tower to accomplish heat transfer because the air flow rates are lower.
The simulator employs the cross-flow contacting method. In a cross-flow tower, adjustable louvers are usually installed to allow control of the air flow into the tower. A hand controller, HIC-108, allows adjustment of the louvers’ position on the simulator. Only one cell is simulated.
There are many design types of cooling tower cells but they all try to accomplish intimate mixing of the warm water and ambient air. Different materials of construction are selected in an attempt to provide a long life of the cooling tower while delivering good heat transfer performance. Packing material (also known as “fill”) can be of highly engineered shapes or sections that are installed inside the framework of the cell structure. The cell structure itself can be metal, wood, plastic, or fiberglass or any combination thereof. The fill is most often made from plastic because it will not rot or rust. The design of the fill often takes installation, fouling, maintenance and heat transfer performance into consideration.
Note that for high-capacity turbines, the cooling tower will consist of a battery of several or more parallel units, each with their own fan and lover system. To facilitate training on the simulator, only one unit is simulated.
Heat Transfer
As warm water contacts ambient air, heat is transferred from the warm water to the cooling tower at the surfaces of the fill material. The more surface area, the more heat transfer will occur until the temperature of the air and the water are the same. In practice, the equilibration of the air and water temperatures does not normally occur in a cooling tower operating at normal loads.
After the water completes its fall through the cell and its fill, it is collected in a basin at ground level for recirculation to the process heat exchangers.
Evaporation of Water
In addition to heat transfer, some of the cooling water will evaporate into the air. As with humans in a very hot climate, evaporation results in a loss of heat that can produces temperatures below the ambient air temperature. The rate of evaporation of cooling water into the air will depend on the temperature of the air and its moisture content. At the same temperature, air with lower moisture content can evaporate more water than air with higher moisture content.
The moisture content of ambient air is represented by its dewpoint. The dewpoint is the temperature at which the moisture in ambient air will begin to produce dew if it is cooled any further.
The maximum evaporative capacity of ambient air is essentially the difference between the saturated moisture content of the air leaving the cooling tower and the actual moisture content of the air supplying the cooling tower. So, a cooling tower whose air leaves at 80 DEG F will be able to theoretically increase its moisture content by 3.45 – 0.83 = 2.62% with ambient air at a dewpoint of 40 DEG F but only increase it by 3.45 – 2.46 = 0.99% with ambient air at a dewpoint of 70 DEG F.
Makeup water must be added to the cooling system to offset evaporation losses. Water can also be lost due to carryover of water mist in the air drawn by the fan and by leaks.
Cold Weather Operation
In cold weather, the Cooling Tower may overcool the water at low process rates even with the louvers at their minimum opening. To raise the cooling water supply temperature in this condition, a portion of the return water can be routed directly to the basin as needed with the manual valve controlled by HIC-102.
Fouling
Fouling of the fill is a major operating problem of cooling towers because the system is open to the atmosphere and dirt and dust will enter the system with the ambient air. Also, biological activity will result in growth of algae and similar organisms on the surfaces of the system over time and affect performance. Specialized water treatment with biocides is typically used to prevent the growth of organisms within the cooling system.
Another source of fouling in cooling towers is scale. Scale is comprised of mineral deposits that can form when soluble minerals in the makeup water build up in the system over time. Regular purging of water (known as blowdown) helps regulate the concentration of minerals. The purging rate and frequency will highly depend on water chemistry and the economics of other chemicals used to treat the cooling water.
pH
The control of the pH of the cooling water is very important to avert corrosion of the piping and equipment. A pH near neutral (7.0) is ideal. pH is affected by a number of things, including the pH of makeup water and the addition of other chemicals. Supply flows of acid and base are provided for adjusting the system pH. The acid solution will decrease the pH while the base solution will increase the pH.
Note that pH is logarithmic with respect to concentration. Therefore, a 1.0 change of pH represents a ten-fold change of concentration and a 2.0 change of pH represents a hundred-fold change of concentration. This non-linearity should be kept in mind when adding chemicals to adjust the system pH.
Cooling Water Pumps
The Cooling Water Pumps P-101A/B are identical electric motor-driven centrifugal pumps with a design capacity of 43,000 GPM at 55 PSIG. Normally only one pump is in operation. The pumps take suction from the basin of the Cooling Tower CT-101 and deliver cooling water to Surface Condenser E-301 via a supply header. The flow of the cooling water to E-301 is adjustable using hand controller HIC-308.
Note that in high capacity cooling systems, there is normally more than one cooling water pump in operation to improve service reliability and for purchase cost and capacity capability reasons. On the simulator, only one pump is in service. This improves training since the loss of or trouble with the operating pump will have immediate and stronger consequences on the operation of the Surface Condenser and Steam Turbine compared to the loss of one pump out of two or three operating pumps.
The cooling water supply header pressure is controlled to 50 PSIG by returning a portion of the cooling water back to the Cooling Tower CT-101. Keeping the supply pressure constant minimizes cooling water flow disturbances to the process heat exchangers. Also, a line is provided off the supply header for sending blowdown water to disposal facilities as needed.
Cooling Water Filter Circuit
The cooling water filter circuit consists of the Cooling Water Filter Pumps P-102 and the Cooling Water Filter F-101. The circuit circulates water from the basin of the Cooling Tower through the filter to remove any solids that are suspended in the cooling water. The filtered water is returned to the basin. This ensures that fouling of the Cooling Tower and the process heat exchangers is minimized. P-102 normally circulates 250 GPM of water. The pressure drop across F-101 is around 5 PSIG when clean.
Instrumentation
Steam Turbine Controls
The HP steam flow to JT-301 is indicated on FI-301. The supply pressure of HP steam is indicated on PI-301 and its temperature is indicated on TI-301. HP steam passes through trip valve XV-301 which is controlled by HIC-301. HP steam flow can also pass through a smaller warm-up line at startup. HIC-302 controls the warm-up valve HV-302 on this line. Power controller JIC-301 regulates the opening of power control valve JV-301 inside the turbine casing. JV-301 controls the flow of HP steam to the first stage of JT-301.
The power produced by JT-301 is measured by JI-301. This instrument is also used by the power control system for JT-301.
The pressure of cooling water to Surface Condenser E-301 is indicated on PI-305 and its temperature is indicated on TI-305. The flow of cooling water to E-301 is controlled by HIC-305 which adjusts the valve opening of HV-305. The flow of cooling water is indicated on FI-305. The temperature of steam condensate leaving E-301 is indicated on TI-304 and its pressure is indicated on PI-304. The temperature of cooling water leaving E-301 is indicated on TI-306.
The rate of flow pulled from E-301 to the vacuum system is controlled by HIC-307 which adjusts the opening of HV-307. Note that the net capacity of the vacuum line decreases as the pressure of E-301 decreases since the density of vapor decreases with pressure. Therefore, depressuring E-301 occurs faster the higher the pressure, and vice-versa. HV-307 is normally open 12%. In case the pressure of the Surface Condenser becomes high due an air leak or steam pocketing, fully open HIC-307 until the pressure returns to normal.
Hotwell Controls
The level of condensate in Hotwell D-301 is indicated and controlled by LIC-301 which adjusts the position of LV-301 on the discharge of the Condensate Return Pumps P-301A/B. The flow of condensate taken to the Deaerator is indicated on FI- 309.
The motors of Condensate Return Pumps P-301A/B are operated by switches HS- 301A and HS-301B, respectively. LAH-301 also indicates the level of condensate in D-301 and its high alarm signal (80% setpoint) is used to auto-start P-301A or P- 301B by selecting the AUTO state of switch HS-302A for P-301A or by selecting the AUTO state of HS-302B for P-301B. Normally one pump is in service with its auto- start switch in the MAN state and the other pump is on standby with its auto-start switch in the AUTO position.
Cooling Tower Controls
The flow rate of makeup water to the Cooling Tower’s basin is controlled by FIC-104. The temperature of the makeup water is assumed to be the same as ambient conditions. The level of the Cooling Tower’s basin is indicated on LI-101.
The filtration system flow is indicated on FI-102 and the filter pressure drop is indicated on PDI-103.
Acid solution flow is controlled by FIC-105 and base solution flow is controlled by FIC-106. These solutions are added to the filtration system’s return line to the basin. When adding these chemicals, the filter system should be in operation to avoid locally high/low pH situations in the return line.
The supply header pressure is controlled by PIC-101 which sends excess cooling water pumped by P-101A/B back to the Cooling Tower. The setpoint is normally 50 PSIG. The total flow of cooling water sent to the supply header is indicated on FI- 101 and the temperature of the supply water is indicated on TI-101.
Blowdown is taken off the discharge of the Cooling Water Pumps P-101A/B and controlled by FIC-107.
The return header pressure is indicated on PI-102 and the return water temperature is indicated on TI-102. Return water is directed to the top of the Cooling Tower with HIC-103 or to the basin with HIC-102.
The opening position of the louvers of the Cooling Tower is adjusted with HIC-108. Maximum air flow is at 100% output. At 0% output, air flow will be at a minimum but will be non-zero to avoid starving the Cooling Tower’s fan. HIC-108 is used to adjust the cooling water supply temperature. However, in very cold weather regulation of the supply temperature to a desired value may not be possible. In this case, some of the return cooling water can be bypassed around the Cooling Tower by opening HIC-102.