Monday, July 18, 2011

BOILER DRUM ROLES

BOILER DRUM ROLES





- Steam drums are used on recirculating boilers that operate at subcritical pressures. The primary purpose of the steam drum is to separate the saturated steam from the steam-water mixture that leaves the heat transfer surfaces and enters the drum. The steam-free water is recirculated within the boiler with the incoming feedwater for further steam generation. The saturated steam is removed from the drum through a series of outlet nozzles, where the steam is used as is or flows to a superheater for further heating. (By definition, saturated steam is pure steam that is at the temperature that corresponds to the boiling temperature at a particular pressure. For example, saturated steam at a pressure of 500 psia has a temperature of 467°F.)

The steam drum is also used for the following:

1- To mix the saturated water that remains after steam separation with the incoming feedwater.
 
2- To mix the chemicals that are put into the drum for the purpose of Corrosion control and water treatment.

3-  To purify the steam by removing contaminants and residual moisture.

4-  To provide the source for a blowdown system where a portion of the water is rejected as a means of controlling the boiler water chemistry and reducing the solids content.

5- To provide a storage of water to accommodate any rapid changes in the boiler load.





- The most important function of the steam drum, however, remains as the separation of steam and water. Separation by natural gravity can be accomplished with a large steam-water surface inside the drum. This is not the economical choice in today’s design because it results in larger steam drums, and therefore the use of mechanical separation devices is the primary choice for separation of steam and water.

- Efficient steam-water separation is of major importance because it produces high-quality steam that is free of moisture.

This leads to the following key factors in efficient boiler operation:

1. It prevents the carry-over of water droplets into the superheater, where thermal damage could result.

2. It minimizes the carry-under of steam with the water that leaves the drum, where this residual steam would reduce the circulation effectiveness of the boiler.

3. It prevents the carry-over of solids. Solids are dissolved in the water droplets that may be entrained in the steam if not separated properly. By proper separation, this prevents the formation of deposits in the superheater and ultimately on the turbine blades.



**(carry-over is the passing of water and impurities to the steam outlet)
**The term critical pressure is the pressure at which there is no difference between the liquid and vapor states of water; i.e., the density is identical. This occurs at 3206 psia.



This information source from ‘’ Steam Plant Operation  8th Ed. - Everett B. Woodruff  ‘’ .

Monday, July 11, 2011

AUTOMATIC VOLTAGE REGULATOR (AVR)

-I will talk today with you about an important equipment used in power system utilities, it is the Automatic Voltage Regulator (AVR). From its name it is a regulator which regulates the output voltage at a nominal constant voltage level.
Role of AVR
AVR (Automatic voltage regulator) has following roles.

1- To regulate generator terminal voltage.
Mainly generator under no-load condition, AVR regulates the generator voltage to voltage setter (90R).






*AVR detects terminal voltage and compare with voltage setter (90R).
*AVR regulates field current via the Exciter.
*Generator terminal voltage is regulated by field current.


Vt < 90R _ Field current will be increase
Vt > 90R _ Field current will be decrease



2-To adjust MVars (Reactive power).
When the generator connected to power grid, AVR adjust reactive power by regulate generator voltage.





MVar (Reactive power: Q) is regulated by generator terminal voltage. Therefore AVR can regulate MVars.

Vt is increased _ MVars will be increase
Vt is decreased _ MVars will be decrease

Hence;

To increase MVars _ 90R raise
To decrease MVars _ 90R lower



3-To improve the power system stability.
There are two stability
-Transient stability …… Improved by AVR
-Dynamic stability ……. Improved by PSS (power system stabilizer)







  * Improve the Transient stability






Transient stability is improved by high initial response characteristic. In the fault condition, Field voltage is increased to keep the generator voltage constantly. If the excitation response is slow, it will not able to keep voltage and the generator cannot keep synchronizing.


 * Improve the Dynamic state stability







Dynamic stability is improved by Power System Stabilizer (PSS). PSS is provided in order to improve the power system dynamic stability. PSS will control the excitation to reduce the power swing rapidly.



4-To suppress the over-voltage on load rejection.
When the load rejection, field current and field voltage should be reduced rapidly to keep terminal voltage constantly and prevent overvoltage.







This information source is MELCO CO. for CEPC generator

Sunday, July 3, 2011

Gas Recirculation Fan ( Structure & Control )


Gas Recirculation Fan

- Gas Recirculation Fan (GRF) draw gas from a point between the economizer outlet and the air heater inlet and discharge it into the bottom of the furnace outlet.

- Recirculation gas introduced in the vicinity of the initial burning zone of the furnace is used for steam temperature control, while re circulated  gas introduced near the furnace outlet is used for control of gas temperature.












-RH outlet steam temperature is normally controlled by regulating the gas recirculation flow. Increased flue gas flow over Reheater of the convection heating surfaces increases the heat absorption and RH temperature is increased.

-The following picture will show the full specification of the GRF ( motor and impeller ).







-Due to large GRF duty , Bearings are equipped with water cooling flexible line , the bearing at the motor side is fixed to the shaft, while at the counter to the motor side is set free for the shaft expansion caused by the heat of high temperature gas in the casing during the continuous operation.






-Because the fan treats the high temperature gas, it is adopted with a turning device, If the fan stopping remain the high temperature gas in the casing, the impeller has the unbalance by heat effect. So the purpose of the no unbalance of the heat effect drive the turning device regular time at about 50 rpm until the economizer heat decreased to a safety limit about 100 centigrade.



Gas Recirculation control

-RH outlet steam temperature is normally controlled by regulating the gas recirculation flow. Increased flue gas flow over Reheater of the convection heating surfaces increases the heat absorption and RH temperature is increased. The amount of gas recirculation flow is controlled by positioning the inlet dampers on the two Gas Recirculation Fans (GRF).

(1)During start -up (approximately < 25% boiler load), GRF inlet damper position is control led by the function generator from Boiler Master. Because RH temperature feedback control is difficult during the start-up due to the slow response.

(2) Dynamic feed forward signal (BIR) is added to GRF inlet damper to improve RH steam temperature control during load change.

(3)High and low limit from Boiler Master for GRF inlet damper are provided. High limit is to avoid the unstable combustion and the over current of GRF. And low limit is to protect the water wall and avoid the high NOx.

Saturday, July 2, 2011

Keeping Control of Drum Levels


Keeping Control of Drum Levels

 “THIS ARTICLE GIVES US DIFFERENT TECHNIQUES ABOUT BOILER CONTROL ESPECIALLY DRUM LEVEL CONTROL,”

-Power plants are designed to operate for decades, provided they undergo regular repair, upgrade and improvement. Much of the time, those maintenance actions are minor. But plant managers expect a few big-ticket expenditures.
-Public Service Electric & Gas Co. (PSEG), for instance, spent $1.3 billion and more than 7 million hours of labor during the 2008 to 2010 period upgrading emissions controls at its Hudson and Mercer, N.J. coal-fired plants. By installing scrubbers, selective catalytic reduction, baghouses and activated carbon injection, PSEG reduced NOX emissions by more than 95 percent, SO2 by more than 94 percent, particulates by more than 99 percent and mercury by more than 90 percent. On a smaller scale, PSEG’s 753 MW Mercer Generating Station experienced problems controlling the water levels in its four Foster Wheeler drum boilers. In that case, it was simply a matter of replacing the original actuators on the feedwater valves with new electraulic actuators.
-“We would have excursions in drum levels and had to wait until it settled down before the operator was comfortable with moving on line,” said PSEG Controls Engineer, Mark Maute. “It’s not a problem anymore. There is no slop in the controls and it runs right where it is supposed to.”

Hitting the Set Point

-Boiler efficiency and overall plant performance depend on being able to accurately control the water levels in the drum. Several problems affect that ability. To begin with, there is the issue of shrink and swell. As steam demand increases, the drum pressure drops initially, causing bubbles to form below the surface of the water and producing a rise in the drum level (swell). When demand decreases, pressure in the drum increases and water levels drop (shrink). This is usually addressed by using a cascade/feed-forward control strategy that takes readings from steam flow, feedwater flow, drum level and drum pressure transmitters and adjusts the feedwater accordingly.
-The feedwater control strategy must also integrate with the combustion control strategy. The firing rate set point demand is calculated as a function of the steam flow and the main steam header pressure. Accurate control of the feedwater loop is necessary to maintain stable combustion flow. If the plant requires steam pressure and flow to remain fairly constant but the feedwater loop is unstable due to poor controllability, the combustion controls will have to continually adjust in attempting to follow unstable changes in the feedwater flow and keep the steam demand at the set point.
-To accurately control the volume of water entering the drum, the feedwater regulator valve needs to accommodate a wide range of operating conditions. During start-up and low-fire conditions, the valve sees high inlet pressures but low flow, so the valve requires anti-cavitation trim to address the full pressure drop across the valve. As the boiler load increases, the valve passes more flow, the inlet pressure drops and the outlet pressure to the drum rises. As a result, the valve must have a large capacity with minimal pressure drop. Some plants use two valves in parallel, one designed for start-up conditions and the other for mid- to full load. The other approach is to use a single valve with a characterized disk stack designed to accommodate varying flow conditions.
-Whichever approach is taken, the feedwater regulator valve must have an actuator that can smoothly and accurately adjust feedwater flow. The valve doesn’t need to be extremely fast, because the digital control system will tune the loop for optimum operation. But the actuator should respond to the command without added delay and execute the command without overshooting or undershooting.
-Pneumatic actuators cannot achieve the highest level of control performance because air is compressible. When the command is given to increase the air pressure and move the valve, there is a lag while the air pressure increases to the point where it is high enough to overcome the static friction of the actuator and start it moving. Pneumatic actuators also tend to overshoot the set point. This dynamic is known as hysteresis, and is a common occurrence in pneumatic control valves. “Smart” pneumatic positioners slow the actuator as it nears the set point to reduce the amount of overshoot, but the adverse affect is that they also increase the “deadtime” in the process loop and ultimately inhibit the controls engineer in optimum loop-tuning. It is common for pneumatic actuators to add seconds of deadtime into the process loop, making them more difficult to tune for high levels of process performance.
-One alternative for improved control is for a plant to use hydraulic actuators. Hydraulic fluids are incompressible by nature, have immediate response and move to position in a stable and repeatable manner without overshooting and without hysteresis. The detriments of electro-hydraulic control systems (EHC’s) are in that they are more expensive than pneumatic systems and require a network of components: an external gravity fed reservoir, constantly running motors/pumps, expensive filtering systems and typically handling of fire retardant oil. Because of these components, EHCs have given maintenance department’s headaches for years. Although the performance of hydraulic systems is undeniably more robust and more accurate, the trade-offs associated with maintaining these systems offset their inherent benefit.

Gaining Control

-To address its feedwater problems, PSEG decided to use an “electraulic” (electro-hydraulic) actuator from Rexa. The Rexa actuators are self-contained units that combine hydraulic, electronic and mechanical technologies. Rexa actuators provide performance commensurate with typical hydraulic systems (0.05 percent resolution and 70mSec deadtime), but eliminate the maintenance-intensive requirements associated with EHCs.
-Electraulic actuators have two main components, a power module and a control enclosure and operate by moving hydraulic fluid from one side of a double-acting cylinder to the other.
-Inside the power module are a motor, gear pump, flow match valve, 60cc thermal expansion/make-up oil reservoir, heater and thermostat. Upon receipt of a control signal, the pump delivers oil at a nominal 2,000 psi to one side or the other of a hydraulic cylinder, causing motion in the desired direction. The hydraulic cylinders come in either linear or a rack-and-pinion rotary design. A position sensor is mounted within or adjacent to the cylinders and provides position feedback to the control electronics.
-The control sub-assembly contains the central processing unit (CPU), power supply and motor drivers. The CPU typically contains a microprocessor, an analog-to-digital converter, a position transmitter, limit switches and warning and alarm systems. The power supply takes the incoming AC power and coverts it to the voltages required by the control components. The motor driver receives commands from the CPU and sends control signals to the motor. Outside of the enclosure (or inside when conditions require) is a two-line display giving actuator status and a five-button keypad to set up and calibrate the actuator.
-When used to control a feedwater regulator valve, the CPU receives a control signal from the DCS and converts it to a target position for the actuator. It then compares current position of the actuator as reported by the feedback assembly with the desired new position. If the difference is outside the preprogrammed range the CPU will send a signal to start the motor, which then drives the reversible hydraulic pump to pressurize one side of the cylinder or the other, moving the piston in the desired direction. Once it reaches the new position, the pump shuts down and check valves close. This locks the hydraulic fluid in the cylinder and maintains the actuator position, without having to keep the motor running, until a new signal is received from the DCS.

By Joe Zwers , freelance writer
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