Steam Boiler

Design Soot Blowing System

The design soot blowing system should furnish well experienced and approved steam jet type sootblowers for the superheater, generating bank, economizer and air heater. The design soot blowing system should arrange the sufficient number of sootblowers for the above mentioned equipment.

If additional soot blowing equipment is necessary to maintain the proper surface conditions of superheater, generating bank, economizer, or air heater, additional equipment with all piping and wiring should be furnished without any addition to the contract price during the guaranteed period.

A complete soot blowing system consisting of all controls, actuators, piping, valves, control panels, including all necessary starters, switches, relays, protection devices, etc., should be provided. Steam from high pressure auxiliary steam header should be used as the cleaning medium. An air operated isolating valve controlled from the remote control panel should be provided to allow isolation of the soot blowing steam.

Complete remote control system for each sootblower, including all switches, relays, signal lamps, sequences, etc., should be provided and installed in the central control room to supervise operation of blowers.

The system should be able to allow local manual operation of each sootblower in case of test or failure of remote control system. The remote control panel should be of the mimic type suitable for flush mounting. The panel should be complete and fully wired and tubed with clearly identified terminal blocks and strips.

The panel should be mounted on the auxiliary control panel in the central control room. All motor starters for the boiler sootblowers should be furnished. Motor starters should be of magnetic type mounted on each sootblower with thermal trip relays and start-stop push buttons for local operation.

Retractable sootblowers should be provided with a hand crank or drive nut for withdrawing the lance tube in case of power failure. Sootblowers should be installed in such a manner as to permit boiler expansion without binding or unbalanced loading. A complete system of sealing air piping should be furnished to seal the wall sleeves if necessary.

A complete system of aspirating air piping to permit removal of blower of should be furnished if necessary. Sealing air and aspirating air piping should have flexible hoses to permit boiler expansion without binding.

Any drain attack due to soot blowing on superheater, generating bank, economizer and air heater element should be absolutely prevented. Special consideration should be given to the method of efficient draining and heat insulation. Any other cleaning system on heating surfaces of flue gas side should be given a special consideration and should be approved by the engineer.

Construction of LP and HP Heater

Construction of LP and HP Heater should be performed as procedure and design. The selection of LP heater materials and method of construction remains with the design although the use of shells with a minimum number of joints, welded tube plate to channel joint, and welded feedwater pipe attachments are preferred. The corrosion resistance of the proposed tube material selection should make due allowance for the oxygen content that may prevail in the LP feedwater, particularly at low load and during start up.

The LP and HP heater should be of the conventional U tube and shell type for which the selection of tube and tube plate material, and their method of attachment should be described. The proposed selection of materials for the tubes, tube plate or header should be in accordance with Code and Standards, provided together with full details of the method of tube attachment.

This selection should be based on service experience with waterside velocities not exceeding previous good service experience. All tubes should be produced from single lengths of tubes and be hydraulically tested to the requirements of ASME standard or equivalent standard.

The bled steam inlets of both the LP and HP heaters should incorporate provisions to prevent erosion, impingement and vibration damage of the tubes by either steam or water. All bled steam and drain inlets should incorporate arrangements to prevent direct impingement of fluid against the tube nest.

The method of attaining access into the channel or headers should be described, to permit inspection and plugging of all tube ends. With divided channel designs, details of the construction and attachment of the dividing wall or chamber to the channel should be provided.

For both the LP and HP surface type heaters, only designs with proven service reliability whilst of similar size and rating should be proposed and reference listings should be made available during the contract.

Design Surface Type LP and HP Heater

Design surface Type LP and HP heater should be described with reference to sectional arrangement drawings. These drawings should be representative in terms of heater disposition, channel division plate details, baffling arrangements, the location of any de-superheating and drain cooling sections arid their shrouding, etc.

The design surface Type LP and HP heater must be performed by manufacturer which has relevant and successful experience with the type of heaters being proposed. Heaters are required to last the life of the station with minimal maintenance. The design surface Type LP and HP heater should therefore describe the design features incorporated into the design of the feed water heaters to minimize or eliminate problems of:

Tube vibration and corrosion
Tube fretting at baffles and shroud plates
Tube erosion and fretting adjacent to bled steam and flash drain inlets
Distortion and cracking at the tube plate
Stagnant pockets of non-condensable gases.
Division plate cracking and damage

Design Deaerator or Feed Water Storage Vessel

The design daerator should consist of the dearating unit and a feed water storage vessel. A feed water storage vessel should be provided within the feed water heating system. The dissolved oxygen content in the feed water effluent from the heater should not be more than 0.007 mg/liter at any load condition, measured in accordance with the "Method and Procedure for the Determination of Dissolved Oxygen" of the Standards of the Heat Exchange Institute. The feed water storage vessel should be integrated with a deaerating unit to fully de-aerate the feed water, if an alternative water chemistry regime is proposed the design should substantiate his provisions for control of dissolved oxygen in the feed water system.

The main functions of the feed water vessel should be to:

Condition feed water for start-up
Provide a reserve to compensate for fluctuating feed flows
Ensure boiler feed pump suction requirements are met at all times
Removal of oxygen via a steam heating/de-aerating process

Whichever arrangement of feed water / deaeration vessel in the design deaerator or feed water storage vessel to provide a full description of the normal function, including:

Level indication and level control
Conditioning of feedwater prior to start-up (cold start)
Flows of condensate, bled steam, auxiliary heating steam
System responses to transient conditions
Any specific arrangements proposed for part load operation
Disposal of scrubbed or vented non condensable gases

The design deaerator or feed water storage vessel should state the provision for deaeration and heating the stored water on plant starts when the LP heater is out of service. The description should also describe the provision included to monitor and control the condensate level within the storage tank and any recirculation system if necessary to ensure homogenous conditions in the stored water.

The feed water storage vessel should be located at or as close to the turbine operating floor level as possible, consistent with satisfying the feed pump net positive suction head (NPSH) requirements. The tank should store a minimum quantity of feed water corresponding to 7 minutes of rated (MCR) feed water flow or that quantity of feed water which should permit a controlled and safe shut down of the boiler, whichever is greater and assuming that the condensate is initially at the normal working level.

The feed water storage vessel should be designed to operate with freedom from condensate surging and vessel vibration. The freeboard above the top of the working level range should be sufficient to accommodate the total condenser hot well content with margin.

Boiler feed pump leak-off returns should be introduced into the feed water storage tank in a controlled manner to prevent damage from high velocity evolved steam or water impingement. At all other points when steam or water enters the deaerator / feed water storage vessel, suitable precautions such as baffles or diffusers should be provided to prevent direct impingement on the tank plates, internals or water surface, internal baffles should be arranged within the feed water storage tank to prevent surging of the condensate.

Provisions to protect the steam turbine from the risk water induction arising from any bled steam pipe work connecting the deaerator / feed water storage vessel should be as stated in Section "Bled Steam supply Lines".

The deaerator should be of the spray/tray type and should include storage tank, supports, vent condenser and fittings. The design should be to the Heat Exchange Institute standard and suitable for full vacuum.

The deaerator should be designed and arranged for the efficient removal of non-condensable gases from the feed water under all conditions of operation, including the admission of auxiliary steam during starting and low temperature condensate under fault or restart conditions.

If a part load deaerator is offered then the design should include a full description of the start-up and operation with increasing load up to full load on the steam turbine-generator.

The design deaerator or feed water storage vessel should describe features of the deaerator head which facilitate the removal of non-condensable gases from the circulating feed water and the provision if any, for recovering heat from the vented gases and vapor. Deaerator level indicators and alarms should be provided in the CCR, these alarms should be fully functional at all times when the plant is available for operation, including periods when the plant is on standby duty.

Safety valves should be provided to protect the deaerator and feed water storage vessel from over pressure from any source. All parts of the deaerator exposed to oxygen or corrosive gases should have an adequate corrosion allowance or be of corrosion resistant materials.

Requirement Design of Tubes

Requirement design of tubes is made to make sure design tube can be used in certain temperature and pressure in steam boiler. Tubes are one pressure parts. These provisions should apply to any tubular pressure part that is either exposed over much of its length to hot gases for purposes of heat transfer or is directly butt welded to such a tubular pressure part.

Requirement design of tubes should comply with the requirements of the ASME Boiler and Pressure Vessel Code, Section I, Power Boilers. The calculation of tube thickness should be based on ASME BPV Section I.

The requirement design of tubes should describe the design basis for controlling mechanical wastage (eq. grit and soot blower erosion) and chemical wastage (e.g. fireside corrosion, dew point corrosion) of tubing. During the design phase, the requirement design of tubes should supply details of the wastage provision for each tube design.

Membrane panel construction should be either by a fusion welded fin or integral fin method. Resistance welded fin construction will not be allowed. No tube bend should contain a circumferential weld. Parallel down-flow circuits subject to significant variations in heat absorption and/or resistance to flow between these circuits should be avoided. A staggered arrangement of tubes in the gas pass should not be used.

Durable caps suitable for transportation should be provided on each end of the tubes to prevent damage and rust on inside surface of tube and to prevent entry of debris. Corrosion or erosion margin of tube thickness should be provided for the boiler tubes.