Latent super heater vs platen super heater vs pendant super heater

Super heater
Super heater is useful for increasing the cycle efficiency of the power plant. super heater converts the saturated steam that is generated from drum into super heated steam and is sent to turbine. Super heater increases the steam temperature by keeping the pressure constant. super heated steam are usually in the range of 540 deg Celsius.

Types of super heater:
Latent super heater: 
This super heater is placed at the back pass of the boiler. It gains heat through convection from flue gas. It is the primary super heater that heats the steam coming from the steam drum. It helps to remove the water carry over, hence the name latent.It is place in low temperature zone of furnace so it is also called as low temperature super heater. The steam from latent super heater passes to the platen super heater for further heating.

Platen Super heater:
platen is a vertical single loop placed in the furnace with many parallel tubes carrying steam. platen has set of closly packed tubes appearing like plate. they are place in the upper part of the furnance. they pick up heat enterily by radiation as it is placed in the furnace top.

The platen coils are place 600mm- 1000mm apart to avoid formation of ash build up.

with increase in steam flow the steam temperature from the platen super heater  decreases. and with decrease in steam flow  the steam temperature increases This is because at lower steam flow steam absorbs greater heat and becoming hotter at low load.

pendant Super heater:
Pendant super heater is a vertical or horizontal multi loop super heater placed in the convection section of boiler with one or many parallel tubes carrying steam. the pendant tubes are place with space. It is normally vertically hanged from top, but also can be found as horizontal. The heat transfer is by a combination of radiation and convection but predominantly by the latter. With increase in steam load the temperature of the steam increases.

both pendant and platen super heater are required in a boiler to get to attain high steam temperature needed.Their characters can complement each other to get a fairly constant temperature at all loads thus minimum attemperation is required. 

Geared variable speed coupling / Geared Hydrodynamic coupling

The principle:
The geared variable speed coupling combines the hydro dynamic coupling and Mechanica Gear in one compact unit.
The geared variable speed coupling controls the speed of the driven machine precisely, quickly.

Uses:
It is used to control the speed of the Driven Machine,Vary the power transmitted Reduce the power consuption, Increases the productivity of operation. Motor can start at no load

Design and function:
*The hydrodynamic geared coupling consist of double helical gear integrated in a housing with a split   horizontal body.
*The gear box is located before the hydrodynamica coupling.
*In hydrodynamic coupling there are two wheels, pump wheel abd turbine wheel. pump wheel is          conected to the drive machine. and the turbine wheel is conected to the driven machine.
*The pump wheel rotates at constant motor speed  and the turbine wheel rotates atvariable speed.
*Using the hydro dynamic principle the fluid is transmitted from the pump wheel to the turbine wheel at variable output speed.
*The scoop varies the coupling filling. The filling amount determines the power transmitted and the    speed of the driven machine.
*An actuator varies the speed of the scoop tube, in this way speed can be controlled.
*Working oil is supplied to the hydrodynamic coupling for varying the speed
*lube oil is supplied to the gear box.
*working and lube oil circuits are seperate.

The actuator is Electro hydraulic actuator for quick and precision operation of the scoop. A simple 4-20 MA is requires to control the scoop position.

The motor will start with no load conditions and smoothly accelerate even with machines with large moment of inertia. This happens because the hydrodynamic coupling is empty during the motor starting, so it seperates the motor from the driven machine so that the motor runs at no load condition.
starting the motor at no load conditions are important for equipments having large moment of inertia or with processes with initially large load.

Classification Steel Pipes and Tubes

Steel pipe & tubes are classified by the following methods:

1. Manufacture: Method Broadly Classified as Seamless & Welded.
• Seamless method is further classified as Hot Finished & Cold Drawn.  
• Welded are classified as ERW, SAW, CDW, SW, MMAW.
2. Steel Chemistry: Carbon steel, Alloy steel, Stainless Steel.
• Carbon Steel are further classified as low, Medium, high carbon, manganese steel.
• Alloy steel are further classified by composition of chrome & moly.
• Stainless steel are classified by their various composition of Nickel, crome, moly etc.   
3. End Use;
• Tubes are used to transferred heat load through its walls (Thickness) 
• Pipes are used to transfer heat loads through its Bore

Precautions to be taken before entering Electrostatic precipitator for maintenance

Following are the safety precautions must be taken before enering the esp
1.Valid work permit must be taken for working
2.Switch off the High voltage power supply using disconnecting switch and lock the disconneting          switch in off position
3.when the disconnecting switch is in off position the emmitting electrode of respective field gets          earthed directly
4.Then discharge the static charge of Electrostatic precipitator field using earthing rods
5.The earthing rod clamps must be properly connected to the earth
6.The ESP internals must be cooled down sufficiently
7.Rapping motors are switched off and power should be isolated.
8.The Oxygen level must be measured before entering the ESP
9. If dust is accumulated, it must be removed and area must be clean                                           10.Hand lamps and tools for work must be connected to 24v supply
11.It is always safe to carry out work atlest with a team of two persons inside ESP.

Proximate and Ultimate Analysis of coal

Coal can be analysed by two methods
(i) Proximate Analysis
(ii) Ultimate Analysis

(i) Proximate Analysis
The analysis of moisture content, fixed carbon, volatile matter and Ash content of coal is called proximate Analysis of coal

Analysis of Moisture Control in coal:
1-2 gms of powdered coal is taken in a dry crucible and heated for 105deg c for 1 hour is cooled over concentrated h2so4.

% of moisture content =( (W-W1)/W) x 100

where W is the 1-2 gms weight of coal powder before heating
W1 - Weight of coal powder after heating at 105 for 1 hour

Analysis of volatile Matter in coal
1-2 gms of coal is taken in a platinum crucible and heated in a furnace at 950 deg c for 7 min and it is cooled over as explained earlier

% of volatile matter in coal (W1-W2)/W1  x100

W1 - Weight of coal powder after heating at 105 for 1 hour
W2 - Weight of coal powder after heating at 950deg for 7min

Analysis of fixed carbon and Ash
1-2 gms powedered coal is taken in a platinum crucible with lid and carefully heated until to remove all volatile matter and then the lid is removed and stongly heated at 750deg c until all carbanaeous content are reonved then it is cooled

% of ash = (ash weight / coal powder weight) x 100
% of fixed carbon = 100 -(% moisture content+ % of volatile matter + % of  ash content)

ULTIMATE ANALYSIS
It is the analysis of element wise coal composition. It is helpful in finding the % of carbon, hydrogen, nitrogen,sulphur and oxygen content.

STEAM DRUM CONTROL ELEMENTS

The purpose of the drum level controller is to bring drum level upto a level during the startup and to maintain the level at the constant load.The decrease in the level will uncover the boiler tubes and expose them to high temperature and leads them to failure. Increase in the level may interfere the steam and moisture separation and will lead to moisture carry over to turbine and reduce boiler efficiency.

Steam flow is the rate of steam leaving the boiler- the demand. water flow is the rate of feed water flow in to the drum- the supply. Drum level reflects the amount of water in the boiler - the invetory

when there is change in the boiler load , suppose boiler load increases, the steam flow demand will be higher than the supply feed water and so the drum water level reduces. so in such a event the controller opens the feed control valve and increases the drum level.

There are types of drum level control.

single element drum level control
Two Element drum level control
Three element drum level control

The use of one of these method will depend upon boiler size and the load changes.

Single element drum level control:

This is the simplest and least effective form of control

Two Element Drum level Control

The two element drum level control can be best applied to single drum boiler where the feed water is at constant pressure. It gets rwo proportional signal from drum trasmitter and mainsteam transmitter.

The steam flow from the boiler transmitts the output signal which acts as a set point for the feed water controller. so that there will be balance between the steam flow and feed water flow. If the steam flow increases due to the load increases, The controller compares the setpoint of the steam flow with the feed flow and to match the set point it opens the feed control valve to increase the feed flow this is called feed forward flow.

The drum level transmitter sends signals to the drum controller which further trims the setpoint of the feed water flow set by the steam flow. Because the drum controller can compensate for the flow measurement errors and unmeasured load disturbances like blow down or steam circuit leakage. this is called cascade controll.

Three Element Drum level control

The three element drum level control maintains the drum level by monitoring the drum level, feed water flow  and steam flow.It provides tighter control for drum level with fluctuating steam load and ideal for a system which suffers from fluctuating feed water pressure or flow.

The three level controller uses three transmitter or process variables to maintain the drum level.

Three Element Drum level control is used in plant where multiple boiler feed water pumps run in parallel to meet the pressure rating of the steam drum, so that it can fill the water in the drum.. 

When the steam flow from the boiler increases , it sets a setpoint to the feed water controller so that the feed water control valve opens according to it. further the drum level controller further trims the set point as mentioned earlier in the two element drum control. If any of the pump pressure reduces and as the feed water valve opens to maintain the water in the drum , the feed water pressure reduces as the valve opens so in order to avois this the valve must be opened gradually for decreasing the feed water pressure this is achieved by taking the feed water pressure transmitter and comparing with the drum and feed water controller set point

water Level gauges in super critical boiler

In plant where steam is used primarily for generating power, water and steam cycle becomes more complicated. Some of these are reffered to supercritical boiler where steam is produced in the pressure exceeding 220 bar. At this pressure the density of water and steam are same. Because there is no difference in density these boilers does not require water level  guages because there is no water level. the supercritical boiler is also called once through boiler as it does not have drum.  After the water is converted into steam in the boiler it passes through the superheater coil, more tubes are exposed to the flue gases to increase the temperature of steam. As steam passes through these tubes the temperature of steam further increases and forms superheated steam. (If the saturated steam which is heated again is converted into supersatured steam.)