Economizer

The Economizer located on the last stages of the exhaust gas path of the WHRB.

Feed water from the feed regulating station, enters the Bottom header of the ECO ßows up wards.

Economizer is provided with air vent and drains. ECO is hung from the top by two guide supports

and anchor support with provision for downward thermal expansion.

The drain valves are used for draining the ECO tubes when the WHRB is not in service, if required for maintenance.

Feed water, after picking up heat from the Economizer, enters the Drum through t Economizer outlet pipe. Temperature Indication / Recording Instruments 11-TE-138 / 141 are meant for indicating feed water temperatures before and after Economizer respectively. There are pressure gauges 11-PG-139 / 140 at inlet and outlet header of economiser to monitor the feed water pressure.

3.2 Steam Drum

The Steam Drum is a long (4000mm approx.) all welded cylindrical vessel made of SA-516 Grade 70 material. The steam drum is supported by the main down comers and the down comers are placed on the WHRB structure over beams. The sliding arrangement permits a limited shift due to thermal expansion. The drum is insulated by lightly resin bonded mineral wool mats. Two manholes, at either end of the drum, provide access to the drum. The drum is closed tight at either end cover plates bolted against the manhole rim by two holding bars. A gasket is Þtted between the cover plate and the mating machined surfaces in the dished ends. The cover plates swing inside, for convenience during opening. Steam Drum is Þtted with several components to perform important functions, which are listed below:

(a) Steam Drum receives feed water from the Economizer outlet through feed pipes and distributes the feed water along the length of the drum by a perforated pipe 80NB to maintain a near constant level (Normal water level) for continuous supply to the evaporator. (to be described further later) through down comer pipes. While ßowing through the evaporator panels, by absorbing heat from the coke oven exhaust gas, the hot water gets converted to water / steam mixture and ßows back to the Drum through riser tubes.

(b) Steam drum receives the water – steam mixture from the evaporator panels through the riser tubes, the water – steam mixture ßows tangentially through the Diemeister pad installed in the steam drum. In this tangential ßow, water, which is heavier, is separated from steam and trickle down to mix with the water in the steam drum. Saturated dry steam collects at the top of the drum and distributed to the Superheater I.

(c)Conditioning of Boiler Water

Due to continuous evaporation of boiler water in the drum, minor impurities present in the feed water, concentrate to high impermissible levels in the boiler water. Rise in hardness of water (conductivity), content of chlorides, silica etc., have to be kept to a minimum to prevent scale formation or deposits, in the evaporator tubes and drum.

Sample of Boiler water is collected from the continuous blow down line through a sample cooler. If the analysis indicate high conductivity, (chlorides, silica) etc., small pre-determined amount of water is continuously drained from the steam drum through the continuous Blow down valve CBD-104 with a needle valve for controlling the ßow to reduce their concentration to permissible levels in the steam drum.

Tri-Sodium phosphate is dosed into steam in the boiler drum to maintain a phosphate concentration and a pH. The Phosphate has the capacity to convert hardness producing insoluble calcium/ magnesium salts to soluble sodium salts, which are drained through the blow down. A typical reaction can be as follows.

3 CaSO4 + 2 Na3 PO4 →Ca3 (PO4)2↓ + 3Na2 SO4.

The dozed phosphate also provides desired alkalinity to the boiler water. An alkaline pH minimizes the possibilities of corrosion.

The following facilities have been provided in the steam Drum for the above operations:

(d) Emergency Blow Down (EBD)

During WHRB startup situations arise resulting in high drum water levels. As high drum water levels are not permissible provision has been made for quickly draining some water from the boiler drum under this condition. The EBD line, drawn from the entire length of the drum consists of a manually operated inlet isolating valve EBD-VG-101, a manual operated parallel slide emergency blow down valve EBD-102, The EBD line drains to the blow down tank. The isolating valves are normally kept closed and are opened only when emergency blow down has to be done. Ensure that the EBD should be close to Blow Down tank so that the operator can easily operate the valve during emergency.

(e) Level Gauges, Level Indicators, Level Transmitters

As maintaining normal water level in the steam drum is one of the important parameters to be

monitored and controlled, elaborate provisions for level instrumentation has been made on the Steam Drum. Brief mention of these instrumentation will be made in this section. (f)Continuous Blow Down Line

To enable the water drained from the drum to reßect the true composition of Boiler water, a perforated pipe 25NB is laid along the water space of the drum and connected to the CBD line to the Blow down tank. There is one isolating valve CBD-VG-101,a needle valve CBD-104. The valve for Boiler water continuous Blow down (CBD) is positioned to drain continuously a pre-calculated quantity.

(g)Sampling Line

The CBD line provided to the sample cooler through two isolating valves CBD-VG-102 & CBD-VL-103.

(h)HP (Phosphate) Dozing Line

Dosing of phosphate to the Boiler water is to be done in a manner that it quickly mixes with the whole of Boiler water. To enable this, a perforated pipe, 25 NB has been laid along the length of the drum and connected to the HP dosing line through a non-return valve HPC-VC-117 and an isolating valve HPC-VG-118.

(i)Level Gauges (11-LG 143 / 144)

The Level Gauges is of multi-port type. The top of the gauge glass is connected to the steam side of the drum through two isolating valves. The bottom portion of the gauge glass is connected to the water side of the drum through two isolating valves. Care is taken to ensure that the center line of the center port coincides with the center line of the drum which is the required normal water level. Twin drain valves are Þtted to each gauge. The drains normally remain shut when the gauge is in service with steam side and water side isolating valves open.

The level gauges are simple direct reading instruments and serve for quick and accurate reading of the drum level. During the start up of WHRB, level gauges may be the only instruments which can be relied upon as other instruments may not be accurate. The level gauges are also used to verify the readings of other instruments. The level gauges being located at the drum level are not convenient for regular operation of the Boiler. The level gauges however must be maintained in service as IBR requires that atleast one of the level gauges must be in service to operate the WHRB.

3.2.1 Drum Level Control

Control of water Level in the steam relies on the following Instruments.

Level Transmitters and indicators 11-LI- 142A / 142B.

WHRB ID fan trip has been envisaged on Drum level very Low conditions. To avoid a false trip from malfunction of any one instrument, two out of the above three instruments must vote for a trip action.

The level transmitters provide drum level signal to the single element and three element controllers. The above level instruments are connected to the steam drum, steam and water space through twin isolating valves. The reading of the steam drum water level by the above instruments are sensitive to the drum pressure.

Transmitter 11-PT-145 (through twin isolating valves) mounted on the steam drum, provide a pressure compensation signal to the level transmitters, so that their signals represent true level neutralizing variations due to pressure changes. They also provide steam drum pressure signal to DCS.

11-PG-146 is a local instrument indicating Drum pressure at the drum elevation.

3.2.2 Drum Safety Valve

To protect the boiler and personnel against consequences of abnormal pressure increases caused by sudden load decrease, malfunction of Þring system, closure of steam valves etc., two spring loaded safety valves have been Þtted on the drum. On increase of steam pressure beyond a pre- determined set value, the safety valve opens automatically to relieve steam from the drum to the atmosphere. The safety valve closes when the steam pressure falls by around 4% of the set value. IBR prescribes norms for installation, care and testing of the safety valves, which are mandatory. Safety valve, 11-PSV-001 and 11-PSV-002 along with the 11-PSV-003 have the capacity, as per IBR, to relieve steam from the WHRB in such a manner that pressure rise above 103% of the working pressure is prevented on any condition.

As the spring loaded safety valves result in high noise levels when they open, the exhaust of the safety valves are connected through a silencer to substantially reduce the noise level.

Installation, adjustment and maintenance instructions for safety valves are enclosed which

may be referred for a full understanding of the safety valves.

3.3 Silencer

Mention was made that the exhaust of various safety valves, steam dump & startup valves are exhausted through Silencers. The Silencers are acoustically & mechanically designed to attenuate the large noise made during operation of these valves

The silencers are made out of suitable casing in which the sound absorbing materials are packed in a certain pattern & wrapped by scrim cloth and wire mesh to avoid ‘ßy off’ of sound absorbing materials during operation of silencer at high ßow rates

The process ßuid enters the annular space between the sound absorbing materials packing where the sound energy is absorbed throughout the length of the silencer.

The Silencers are mounted on separate structures on top of the WHRB and the exhaust pipes from the valves are connected to the silencers. As the silencer contain no moving parts, no operational care is needed except opening the drain plug provided in the drain line, once in three months to drain the line.

3.4 Air Vent

An air vent on the drum to vent out air during initial boiler Þlling, before start up and during start up. During start up, the air vents are closed at a drum pressure of 2 Kg/cm² (g) and when copious steam is passing. The air vents are opened after shut down of the boiler when the boiler pressure falls to 2 kg/cm2.

3.5 Evaporator

EVAPORATOR Figure 1

The Evaporators convert hot boiler water received from the Drum through down comer pipes into a steam water mixture, by absorption of heat from the Coke oven exhaust gas. The steam water mixture is led back to the drum from the evaporators through riser pipes.

There are two sections of Evaporators, Evaporator 1 & Evaporator II.

The Evaporators are hung from the top headers in the ßue path, on two guide supports and one anchor support with provision for thermal expansion downward & in the sides.

Hot water ßow to the evaporators from the drum and steam / water mixture to the drum from the Evaporators through risers. A down comer header of the Evaporator spans all the Evaporator panels. The top headers of the panel are connected to the drum by riser tubes .The circulation through Evaporator panels takes place as follows

• Heated Boiler water from the drum ßows through the two down comer pipes to down comer header.

• From the down comer header, the hot water ßows to the lower headers, and then through Evaporation panel tubes, to the Evaporation panel top headers. During its passage through

the Evaporation panel tubes, the hot water absorbs heat from the exhaust gas of the coke oven and gets converted to a water/steam mixture. This circulation is assisted by the higher density of water in the down comer compared to the lower density of water / steam mixture in evaporator and riser tubes

• The water / steam mixture from the top headers of the Evaporation panel, ßows in the steam drum.

• In the steam drum, the steam water mixture ßows through the separators where water & steam are separated and saturated steam ßows to the Superheaters. Seperated water mixes with boiler water to ßow through the Evaporator panels again.

3.6 Super Heater

Superheating of saturated steam from drum is done in two stages in Superheater I & in Superheater II. Between Superheater I & II, an attemperator is located to control the temperature of Þnal steam outlet at 485°±5°C.

Superheaters are made of modules, each consisting of a top header and a bottom header, with tubes between the headers. Superheater modules are hung from their top headers with provision for thermal expansion down wards & in the sides.