The PHWB series boiler is hot water fire tube type, of three-pass and wet back of welded construction design. It has one furnace tube(s) where combustion is started and completed. The gases then pass to the diverter section at the rear of furnace tube. Here, it is reversed and pass along fire tubes – as second pass - to a chamber in front of the boiler. The third pass is along tubes arranged in two banks to the outlet chamber at the rear, and hence to a chimney or exhaust stack.
All equipment essentials for operation of the boiler are included as parts of a totally self-contained unit and all those required such as external supplies of electricity, suitable fuel and water are available.
Control of the boiler functions is normally maintained by the automatic control System and whilst the boiler is designed to give a long and trouble-free life it can only do so if checked and maintained carefully and correctly.
The main parts of the boilers are: Shell, furnace, reversal chamber, access tube, smoke tubes, tube plates, stay bars and saddles described below.
One fire tube / wet back type hot water include the followings:
• Complete set of pressure parts including:
- Tube passes
- Combustion chamber
- Stay bars & plates
- Holes & connections
- Instrument branches
•Non-pressure parts including:
- Insulation & cladding
- Platforms & ladders
- Gas reversing chamber & stack inlet flange
- Lifting lugs
• Necessary observation ports, manhole & hand hole for maintenance, access doors and explosion vents.
• Steel structures, platforms, ladders, stairs, walkways and handrails.
• Boiler firing equipment including
- Dual fuel burners complete
- FD fans with electro motor drive, coupling and guard, inlet silencer (if required), expansion joints, flow metering devices, dampers and actuators and all complete accessories
- Ignition system and the associated piping & valves
- Flame safeguards system
- Gas train unit complete
The boiler shell is a rolled plate with longitudinal and circumferential weld seems.
Tube plates are welded to the shell at both ends. The shell contains most of boiler pressure parts. 25% of circumferential weld seems and 100% longitudinal weld seems will be examined by radiography method (According to BS 2790-12953).
There is one set of furnace tubes in this type boiler. Furnace tube is the place where combustion occurs. About half of the boiler heat transfer takes place in furnace because of its high temperature and radiation. The furnace is cylindrical in shape.
The hot flue gas, leaving furnace, enters the reversal chamber. The flue gas enters the second pass of boiler, the smoke tubes which are welded to front wrapper plate. The welding of smoke tubes to front wrapper plate will be of seal weld type. The rear wrapper plate is attached to the boiler rear tube plate using short stay bars. 25% of longitudinal weld seems will be radiography tested. The access tube at the rear wrapper makes the maintenance of furnace and reversal chamber very easy.
The boiler being a three-pass type utilizes the furnace as the first pass. Two passes of straight tubes are fitted. Second pass tubes are fitted between the boiler front plate and reversal chamber front plate. Third pass tubes are fitted between the boiler front and rear plates. All plain tubes are expanded or welded into the tube plate holes.
Stay tubes are expanded and welded into position. To cater for gas firing all second pass tubes are seal welded into the combustion chamber after being expanded.
Tube plates are welded to both ends of shell. The weld of tube plate to shell will be examined by ultrasonic method.
Two kinds of stay bars are used in the boiler. Short stay bars connect the reversal chamber to rear tube plate and long stay bars, which connect the front and rear tube plates together. The ends of stay bars will be examined by magnetic particle method before welding to tube plates. The weld of stay bar to tube plate will be examined by penetrate method.
To provide a safe release of steam in the event of high pressure, a single or double spring safety valve is fitted to the top of the boiler shell, near to the front.
The boiler pressure gauge is mounted on the front of the boiler. It is connected to the boiler by a cock, which allows the gauge to be isolated. The gauge is mounted on plate.
Two or four lugs on top of the boiler are provided for lifting purposes.
Two inspection ports with bolted covers are provided to allow access to the waterside of the boiler at furnace level. One port is positioned in each side of the boiler.
Two saddles are located under the boiler. Some holes are considered beside them for transport chains.
An elliptical or circular manhole with bolted cover is positioned in the crown of the boiler shell between the safety valve and main steam stop valve.
An electrical terminal box is mounted on the proper side of the boiler. For high capacity boilers it would be located in the electric room.