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FEEDWATER BY - PASSING

A feedwater heater may be severely damaged by erosion and/or vibration, if it is

operated for any significant period or time with the next lower heater’s feedwater flow

by passed. When a heater is by-passed, its normal feedwater is passed on to the next

higher heater. This next higher heater will come close to making up the duty of both

heaters. This single heater will tend to draw a total amount of extraction of steam

approximately equal to the flow to both heaters.

In the case of heaters with desuperheating zones, the increased steam load due to by-

passing the previous heater can cause an excessive pressure drop in the

desuperheating zone, which in turn can cause condensation. The condensate flowing

at high velocity can lead to severe tube erosion.

Excessive steam flow to a heater, resulting from by-passing the feedwater side of the

previous heater, can result in :

• Localised high velocity leading to vibration of the tubing.

• Flows which cannot be adequately handled by the drain control valve.

• Condensation in the desuperheat zone and high velocity impingement.

DEAERATORS

PRINCIPLE OF DEAERATION

The deaerating heater utilises steam by spraying the incoming water into an

atmosphere of steam in the preheater section (first stage). It then mixes this water

with fresh incoming steam in the Deaerator section. (Second stage).

In the first stage the water is heated to within 2O of steam saturation temperature and

virtually all of the oxygen and free carbon dioxide are removed. This is accomplished

by spraying the water through self adjusting spray valves which are designed to

produce a uniform spray film under all conditions of load and consequently a constant

temperature and uniform gas removal is obtained at this point.

From the first stage the preheated water containing minute traces of dissolved gases

flows into the second stage. This section consists of either a distributor or several

assemblies of trays. Here the water is in intimate contact with an excess of fresh

gas/free steam. The steam passes into this stage and it is mixed with the preheated

water. Deaeration is accomplished at all rates of flow if conditions are maintained in

accordance with design criteria. Very little steam is condensed here as the incoming

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DEAERATOR AND FST CONNECTION

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water has a high temperature caused by the preheating. The steam then rises to the

first stage and carries small traces of residual gases. In the first stage most of the

steam is condensed and the remaining gases pass to the vent where the non-

condensable gases flow to the atmosphere. A very small amount of steam is also

discharged to the atmosphere which assures that the deaerating water is adequately

vented at all times.

The water which leaves the second stage falls to the storage tank where it is stored for

use. At this time the water is completely deaerated and is heated to the saturated

steam temperature corresponding to the pressure within the vessel.

The condensate pressure just before the entry to Deaerator shall be atleast 3 psi more

than the Deaerator steam pressure.

OPERATION OF THE DEAERATOR

1. Flush out all lines and tanks with water until there is no apparent indication of foreign matter or rust. Spray nozzles should be free from all foreign material.

2. Manually check all controls to see that each is working freely.

3. Check to see that all instruments are operating and indicating correctly.

4. Open the Deaerator vent valves or open orifice bypasses to atmosphere.

5. Admit condensate water and slowly increase from 15 % to 30 % of design inlet

flow fate.

6. Put one boiler feed pump in service with recirculation to Deaerator.

7. After making certain that adequate steam pressure is available, open steam

valve slowly admitting steam into the Deaerator.

8. When a strong flow of steam issues from the vents, start throttling the vent and

check feed storage tank temperature.

9. The gauge should read 2 to 3oF below saturation temperature at the existing

pressure.

10. Throttle back vent valves to operating positions. The final position is

determined in conjunction with oxygen tests during operation.

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RECOMMENDED NORMS FOR TEMPERATURE CHANGES

Changing cold or hot water admission to the water box must be accomplished in a

controlled manner. The control must assure that the rate of temperature change of

the metal in the shell or the water box does not exceed 400oF per hour with

instantaneous changes not greater than 50OF per minute for a total excursion of

150oF.

Cold start-up can severely stress a Deaerator. To avoid a severe thermal shock it is

recommended that cold start ups be preceded by a warm up steam with the vents

open and no flow in to the water box. The steam flow should be regulated to permit

the steel shells to heat at a rate of 50OF per minute upto about 200oF. Water in the

storage tank should also be heated to the same value. When the entire vessel and its

contains are heated, the steam supply should be shut off and any remaining steam

vapour should be vented.

Accelerated cooling is often desirable for repair work. Accelerated cooling can be

accomplished using a cooling fluid which is 100OF to 150OF lower than the metal

temperature until the metal has cooled to about 250OF. The rate of change of metal

temperature should stay in the 100O F/Hr range. Once the metal is at or below

250OF, cooling of 60OF to 70OF may be used.

FEED SYSTEM OPERATION

SYSTEM DESCRIPTION

The purpose of the Feedwater System is to provide an adequate flow of properly heated

and conditioned water to the boiler and maintain boiler drum level compatible with the

boiler load. This system also conveys water to the boiler reheater at temperators,

superheater at temperators, auxiliary steam desuperheaters, the high pressure bypass

desuperheater and HP fill & purge SGCW pump cooler. Feedwater is heated to achieve

an efficient thermodynamic cycle.

Under normal operating conditions, feedwater flows from the outlet nozzles of the

Deaerator storage tank, through the boiler feed booster pumps (BFBP), to the boiler

feed pumps (BFP). From the discharge of the boiler feed pumps, the flow continues

through both high pressure feedwater heater strings to the boiler economiser inlets. A

bypass line around the heaters is provided for removal of either or both heater strings

from service.