BY

G. UMAKANTHAN DGM/PP/PE(FB)

 

In general the heat absorbing parts subject to

internal pressure in a boiler will be called as pressure

parts. The main pressure parts in a boiler will be as

follows:-

 

Drums, Water walls, Super heaters, Re heaters,

Economisers and valves & fittings. The Drum, Down

comers, water wall headers and water walls will form the

circulation system and cover the furnace zone. The drum

provides the necessary space for locating the steam

separating equipments for separation of steam from

mixture of steam and water.

INTRODUCTION:

It also serves as a reservoir for the supply of water to circulation system to avoid possible starvation during operation. Super heaters are used to raise the steam temperature above the saturation temperature by absorbing heat from flue gas to increase the cycle efficiency. For utility boilers which are meant for power generation, the super heater outlet temperature is limited to 540 5C due to metallurgical considerations. Re heaters are used to raise the steam temperature to the same super heater temperature but at a lower pressure since steam flow through re heater takes place after HP stage turbine. Economisers are used to absorb heat from the flue gases and add this as sensible heat to the feed water before water enters the evaporation circuit of the boiler.

The Fig.1 shows the arrangement of pressure parts in a boiler. The circuit arrangement is normally classified as water circuit and steam circuit.

LAYOUT OF PRESSURE PARTS:

WATER CIRCUIT:

Feed water is supplied to the economiser inlet header via feed stop and check valves. The feed water flow is upward through the economiser coils that is counter flow to the hot flue gases. After feed water is collected by economiser intermediate headers it passes through Economiser hanger tubes to reach top Economiser outlet header. The water from Economiser outlet header will go to drum through Economiser connecting links to drum.

Water from Drum comes to bottom water wall ring header through vertical down comers. From Ring headers it will rise through front, side and rear water wall panels and absorb radiant heat and the resulting mixture of water and steam will reach top water wall headers above roof. The mixture will be discharged into Drum by riser pipes. Steam will be separated from the mixture inside the drum by drum internals.

The saturated steam which is separated from water steam mixture by drum internals will reach SH Radiant Roof inlet header SHH1 through SH connecting pipes. After steam passes through radiant roof tubes it will reach outlet header SHH2.

STEAM CIRCUIT:

The steam from SHH2 comes to SHH3 (left and right side wall inlet headers) in the second pass. Fin welded panels on either side of second pass will bring the steam to SH side outlet header SHH4. Steam from SHH4 will reach SH front inlet header SHH5 via elbow connections and it will raise up through Front Panels. These panels will be in the form of loose tubes at the corner near SH Final coils and they are called SH Hanger tubes connected to SH Rear Junction Header SHH 7. Steam also will pass through supply pipes from SHH 5 to extended side wall inlet header SHH6. The panels from SHH6 cover the bottom and side portions of extended from either side of SHH 14 region and connected to separate header at top on either side (extended side wall outlet headers) SHH 3A.

The steam from these headers reach SHH7 through connecting pipes and further flow will be through fin welded panels covering the rear roof and rear wall to LTSH Inlet Header SHH9. A portion of steam which enters SH 2nd Pass Side Wall Inlet Headers SHH3 will reach SH Side wall Outlet Header Rear SHH 4A. The steam from SHH 4A on either side enters SH Back pass Rear wall Inlet Header SHH8 through elbows and goes up to SHH9 covering the lower portion of rear wall. The steam from SHH9 passes through (LTSH/Horizontal) super heater lower and upper banks and will come to SHH 10 (LTSH Outlet Header) by SH terminal tubes.

De super heater links are connected between SHH 10 and SHH 11 (SH Platen Inlet Header) at quarter points where DESH are placed. Then steam from SHH 11 will pass through SH platen coil assemblies and reach SHH 12 (SH Platen Outlet header). Two Links which are connected between SHH 12 and vertical spaced SH (Final SH) inlet header SHH 13 will allow the steam to pass through Final SH assemblies to reach SHH 14 (SH Vertical spaced or main steam outlet header). The main steam line will further carry the steam to HP turbine from either side of SHH14. Steam after HP turbine enters Reheater through cold reheat lines and the Reheated steam reaches the next stage/IP turbine Via Hot reheat lines connected on either side of Reheater outlet header.

DRUM AND DRUM INTERNALS:

CIRCULATION SYSTEM:

The circulation system of water through the

furnace water wall is due to either natural circulation

or controlled circulation. Natural circulation boilers

employ the effect of density differences between water

and steam to produce circulation.

This density differences together with the height

of the boiler constitutes a head which overcomes the

losses occurring in the circuit and makes circulation

possible. In controlled circulation boilers this density

difference is augmented by a pump output energy to

ensure positive circulation through the water wall

tubes.

DRUM:DRUM:

The circulation system for a natural circulation boiler

comprises of the Drum, Downcomers, Furnace walls

and connecting pipes at the top of the boiler furnace.

The boiler drum serves three main functions:  To provide the necessary space for separation of

steam from mixture of steam and water.  To provide the required space for the separated

steam till it leaves the drum as purified steam. 

  To house the equipment needed for purification of

steam after separation from the mixture of steam and water.

  To provide a water storage for preventing the

starvation of tubes during operation.   The Drum size is determined to meet all the above

requirements. Drum diameter and length should be sufficient to provide accessibility for installation and inspection. The drum is checked for stresses induced due to self weight and external loads.

DRUM INTERNALS:DRUM INTERNALS:

The following are the functions of Drum Internals.   Separation of water from steam water mixture.

Removal of water bubbles from water volume of boiler from entering downcomer.

  Mixing and distributing the feed water.  Adding chemicals.  Facilitating blow down. 

FURNACE WALLS: FURNACE WALLS:

(Fig.4) Normally for boiler furnace enclosures

membrane wall construction is adopted. They are

called fusion welded panels in which a number of

tubes are joined together by a process of fusion

welding. The fusion welded panels have been built

with tube sizes from 31.8 mm OD to 63.5 mm OD and

the panels are made in width on length is because of

transportation.

The fusion welded wall construction offers many

advantages, over tangent tube construction.

Pressurised furnace is possible with the related advantages of (a) increase in efficiency (b) better load response with similar combustion control (c) quicker starting and stopping (d) Forced draft fan handle less air.

  Heat transfer is better.  Erection is made easy and quick.  Gas tight enclosure.  Less insulation weight.

Presently, the membrane walls are also made from fin welded panels using PEMAMEK fin welding machine. All the above advantages are applicable to this PEMAMEK made fin welded panels also.

These heating surfaces are in the form of coils which are made by bending the tubes in cold or hot condition. The superheater is composed of four basic sections. 

The platen section is located directly above the furnace in front of the furnace arch (Fig.5a). It absorbs heat mainly by radiation. The pendant spaced section is located in back of the screen wall tubes (Fig.5b). The mode of heat transfer is convection.

The horizontal section of the superheater is located in the rear gas pass above economiser. The steam cooled wall sections form the side, front and rear walls and roof of the vertical gas pass. The combination of convective and radiant superheater is generally used in all boilers to give flat superheat curves over wide ranges in load.

SUPERHEATER AND REHEATER:SUPERHEATER AND REHEATER:

The fundamental considerations governing superheater design apply also to reheater design. The reheater is composed of single stage but two parts for ease of manufacture and erection (front & rear pendant vertical spaced section) (Fig.5c). The rear pendant vertical spaced section is located above the furnace arch between water wall screen and rear wall hanger tubes. The front pendant vertical spaced section located between the rear water wall hanger tubes and the superheater platen section. The outside diameter of reheater tubes will be larger than that of superheater tubes as more volume is to flow through reheaters, operating at low pressures.

Spacer are used to maintain pitches along and across

coil assemblies. The type of spacers generally used

are transverse spacers and alignment ties. Fluid

cooled spacers or mechanical spacer bar are used as

transverse spacers. Alignment ties are used to

maintain pitch between tubes in the same assembly.

Flexible connector and alignment band are used as

alignment ties. Flexible connectors in combination

with fluid cooled spacers are used and mechanical

spacer bars in combination with alignment band are

used. The spacers are all made of stainless steel

(Fig.6). 

SPACERS FOR SH & RH: