Introduction

A. Objective and Function of the System

The Feed Water System provides treated high-pressure water to the boiler. It has the following major functions: To supply high-pressure water (free from dissolved gases like oxygen) to the boiler during start-up, normal and emergency operations. To supply attemperation water to:a. Desuperheater sprays to control superheater temperature control system.b. Desuperheater spray for Bypass Control.c. Spray for auxiliary steam pressure reducing and Desuperheating system. Ensure minimum flow of water through recirculation control of BFP. Regulate flow of feed to boiler to maintain boiler drum level. It also accepts chemical dosing to scavenge dissolved gas further as well as to increase pH value of the feedwater.Feedwater can be defined as return condensate plus make-up water. It is the high-pressure deaerated (removal of gases like oxygen) water supplied to the boiler from which steam is generated. The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water) and fresh water, which has been purified in varying degrees (make up water). Feed-water composition depends on the quality of the make-up water and the amount of condensate returned to the boiler. Feedwater is supplied by the Deaerator, which also provides the necessary Net Positive Suction Head (NPSH) to the Boiler Feed Pumps. The Boiler Feed Pumps (BFP) must supply a constant flow necessary to replace water in the boilers that has been changed to steam. The Boiler Feed Pumps (BFP) must also develop the required pressure to overcome head and drum pressure.

System BoundaryFeed Water System starts from outlet nozzle of Deaerator Feed Water Tank and ends at the downstream of HP Heater-2 i.e., upstream of Feed Water Flow Control Station. Minimum Flow Recirculation lines terminate at Feed Water Tank. Balancing Leak Off Line of each Boiler Feed Pump also terminates at Feed Water Tank.This system also includes a branch line from BFP common discharge to Spray for Turbine Bypass.

B. System Overview

3. SYSTEM FLOW PATH DESCRIPTION

4. FEEDWATER SYSTEM COMPONENTS DESCRIPTION

Deaerator and Feedwater Tank One of the main parts of feedwater system is deaerator (LAA10AC001) and feedwater tank (LAA10BB001). The mass balance between steam produced by boiler and the feedwater should be maintained in a fluctuating load conditions based on firing rate and steam demand. Furthermore feed water should be controlled to supply enough water into steam boiler.The required water must face water treatment system before supplied into a steam boiler, so the water has required parameter contents about silica, scaling, conductivity, pH, dissolved oxygen and so on. The main consideration is how to remove oxygen to prevent corrosion and remove scaling which can isolate heat transfer process in the pressure parts of steam boiler (water wall, header, economizer, etc).The main functions of deaerator feedwater tank are:1. Eliminate corrosive gases such as oxygen and non-condensable gases in feedwater before supplied into steam boiler.2. As storage tank to maintain and supply enough feedwater into steam boiler.3. Increase temperature feedwater until saturated temperature.

Figure 1. Shows Deaerator and Feedwater Tank DiagramTHE DEAERATOR PRINCIPLE The removal of dissolved gases from boiler feedwater is an essential process in a steam system. The presence of dissolved oxygen in feedwater causes rapid localized corrosion in boiler tubes. Carbon dioxide will dissolve in water, resulting in low pH levels and the production of corrosive carbonic acid. Low pH levels in feedwater causes severe acid attack throughout the boiler system. While dissolved gases and low pH levels in the feedwater can be controlled or removed by the addition of chemicals, it is more economical and thermally efficient to remove these gases mechanically. This mechanical process is known as deaeration and will increase the life of a steam system dramatically.

Deaeration is based on two scientific principles. The first principle can be described by Henry's Law. Henry's Law asserts that gas solubility in a solution decreases as the gas partial pressure above the solution decreases. The second scientific principle that governs deaeration is the relationship between gas solubility and temperature. Easily explained, gas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature. A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feedwater. The feedwater is sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation. Spraying feedwater in thin films increases the surface area of the liquid in contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes it from the feedwater. The liberated gases are then vented from the deaerator.

Reason to DeaerateThe most common source of corrosion in boiler systems is dissolved gas: oxygen, carbon dioxide and ammonia. Of these, oxygen is the most aggressive. The importance of eliminating oxygen as a source of pitting and iron deposition cannot be over-emphasized. Even small concentrations of this gas can cause serious corrosion problems.

Makeup water introduces appreciable amounts of oxygen into the system. Oxygen can also enter the feed water system from the condensate return system. Possible return line sources are direct air-leakage on the suction side of pumps, systems under vacuum, the breathing action of closed condensate receiving tanks, open condensate receiving tanks and leakage of non-deaerated water used for condensate pump seal and/or quench water. With all of these sources, good housekeeping is an essential part of the preventive program.One of the most serious aspects of oxygen corrosion is that it occurs as pitting. This type of corrosion can produce failures even though only a relatively small amount of metal has been lost and the overall corrosion rate is relatively low. The degree of oxygen attack depends on the concentration of dissolved oxygen, thepHand the temperature of the water.

The influence of temperature on the corrosivity of dissolved oxygen is particularly important in closed heaters and economizers where the water temperature increases rapidly. Elevated temperature in itself does not cause corrosion. Small concentrations of oxygen at elevated temperatures do cause severe problems. This temperature rise provides the driving force that accelerates the reaction so that even small quantities of dissolved oxygen can cause serious corrosion.

Chemical DeaerationBoiler feedwater can be chemically deaerated using a specific type of chemical known as an Oxygen Scavenger. These chemicals will combine with the dissolved oxygen in the boiler feedwater. Oxygen scavengers are normally fed to the preboiler section like the feedwater storage tank, feedwater line or storage section of the deaerator. Outlined below are the commonly used boiler feedwater oxygen scavengers.

SULFITESSodium sulfite (Na2SO3) and sodium metabisulfite (Na2S2O5) are inorganic reducing agents, which will react with dissolved oxygen in alkaline water. The dosage for sodium sulfite is 7.88 ppm per ppm of dissolved oxygen. The reaction is as follows:

2 Na2 SO3 + O2 -> 2 Na2 SO4

Sodium sulfate is the byproduct, which can be found in the boiler water. The reaction is pH dependent requiring the feedwater to be above 7.0. The reaction rate is normally very slow. It takes 2030 minutes for sodium sulfite to combine with dissolved oxygen. However, the reaction can be catalyzed with metal ions like cobalt (Co), nickel (Ni), copper (Cu) and iron (Fe). Sodium metabisulfite is the most economical form of sulfite. However, the product is acidic and may require a supplemental feed of caustic soda to achieve oxygen scavenging.

HYDRAZINEHydrazine is a powerful reducing agent that reacts with dissolved oxygen. It is an inorganic compound containing nitrogen (N) and hydrogen (H). Hydrazine will scavenge oxygen in boiler feedwater according to the following reaction:

N2H4 + O2 -> N2 + 2 H2O

Gaseous nitrogen and water are the byproducts of the hydrazine reaction with oxygen. Because hydrazine or its reaction byproducts do not add dissolved solids to the boiler water, it is widely used in highpressure steam boilers where solids are limited. The reaction rate of hydrazine with oxygen is slow at room temperature. At temperatures typically found in pressurized deaerators, the reaction rate is satisfactory for oxygen scavenging. The reaction rate of hydrazine can be increased using various organic or inorganic catalysts. Cobalt salts and hydroquinone are used to catalyze hydrazine. In addition, reacting with dissolved oxygen, hydrazine can remove oxygen from metal oxides. Red iron oxide (rust) can be reduced to black iron oxide (magnetite) according to the following reaction:

12 Fe3 O (OH) + N2 H4 -> 4 Fe3 O4 + 8 H2O + N2

The reaction ratio of hydrazine to dissolved oxygen is 1:1. Commercial hydrazine is available as a 35% solution in water. Hydrazine is classified as a carcinogen and requires special handling.The Plant will used hydrazine as oxygen scavenger and will be dosed or injected on the exit line of feed water storage tank.

Figure 2. This shows the LP Dosing of Hydrazine in actual P&ID

Boiler Feedwater Pump (LAC10AP001/LAC20AP001)

A boiler feed pump (LAC10AP001/LAC20AP001) supplies feed water to the boiler. The boiler feed pump raises the feedwater to a pressure high enough for it to enter the boiler. Boiler feed pump extract water from deaerator and feed it to the boiler drum via H.P heaters and economizer. An essential part of the Boiler feed water system, a boiler feed pump is selected according to the quantity and the amount of pressure required by the boiler. The type of the boiler also plays an important role in selecting a feed water pump. Multistage centrifugal-type boiler feedwater pumps are mainly used in steam power plants. This type of pump is also used in the Plant.Figure 3. This figure shows a part of Feedwater System P&ID and it highlights two identical boiler feed pump (standby and operating pump).

As indicated in the P&ID, the suction pressure of the Boiler Feed Pump is 9 kg/cm2 with temperature of 180oC and a flow of 256 tons/hr. Its discharge pressure is raise to 187 kg/cm2 with no change in temperature and flow. Aside from the suction and discharge line, the BFP also have Balancing Leak-off Line, Minimum Recirculation Line and Warm-up Line which is used for different specific purpose.

Balancing Leak Off Line (LAB40BR001/LAB40BR002)

Balance leak off line is fitted to a high capacity BFP to reuse the thrust balancing water from thrust balancing system of a BFP. Generally BFPs are fitted with balance valves or dummy piston arrangement or balancing disc arrangement. The high pressure water comes to one of this balancing system through an annular path in between valve or disc or piston sleeve to balance the thrust towards suction end and after that it has to be discharged through balance leak off line to Deaerator for further use as feed water. A relief valve is also fitted on balance leak off line so, if leak off isolating valve closed due to any breakdown when the pump is running it ensures flow of leak off water hence saves BFP from seizing.

Minimum Recirculation Line (LAB30BR001/LAB30BR002)It is worth noting that a part of each BFP discharge (before the discharge valve) is recirculated back to the deaerator. This is called Boiler Feed Pump Recirculation. Recirculation control is necessary to ensure minimum feedwater flow through the boiler feed pump under all conditions. This recirculation is necessary because of the churning effect inside the heavy-duty pump generates a lot of heat, so some minimum water flow is necessary to take away this heat. Flow for BFP suction is measured, and a set point for example 30% of flow is set to operate each BFP recirculation valve. When flow through the BFP is more that the set point, the recirculation valve closes. It can be ON/OFF type or modulating control type.

Warm Up LineA warm up line is a means for maintaining an idle pump at approximate uniform temperature when started. It is recommended that a warm up procedure for liquid temperatures above 150C (302F). When a pump is to be put on the line, it is desirable to first bring it nearly to the prevailing feed water temperature by circulating water through it before it is started. Likewise, when a pump has been in operation and is taken off the line and shutdown, it is essential that water be circulated through it to maintain a uniform temperature. Otherwise it results in distortion of the close-running parts. If such a pump is started, rubbing would occur inside the pump.

High Pressure Heater

The regenerative feedwater heating or regeneration is one of the most commonly used methods to increase the thermal efficiency of steam power plants. A feedwater heater is a heat exchanger designed to preheat boiler feedwater by means of condensing steam extracted (or bled) from a steam turbine. The heaters discussed here are classified as closed, since the tubeside fluid remains enclosed by the tubes and channel, and does not mix with the condensate, as is the case with open feedwater heaters. They are unfired since the heat transfer within the vessel does not occur by means of combustion, but by convection and condensation. The steam extraction process in a closed feedwater is referred to as uncontrolled extraction. The flow rate of steam into a feedwater heater is not limited by the amount of available steam (as opposed to a surface condenser, for instance). The shellside operating pressure in a feedwater heater is determined by the pressure of the steam supplied to it, not by the amount of heat transfer surface.

CONFIGURATIONMost feedwater heaters are of a standard shell-and-tube configuration. A few employ straight tubes, although the majority uses U-tubes, which are relatively tolerant to the thermal expansion during operation.

RELATION TO POWERPLANT CYCLEThe heating process by means of extraction steam is referred to as being regenerative. The feedwater heaters are an integral portion of the power plant thermodynamic cycle. Normally, there are multiple stages of feedwater heating. Each stage corresponds to a turbine extraction point. These extraction points occur at various stages of the expansion of steam through the turbines. The presence of the heaters in the cycle enhances the thermal efficiency of the power plant; the greater the number of extraction stages, the lower the amount of thermal energy required to generate a given amount of electrical energy. A beneficial by-product of the energy extracted by the heaters is the reduced rate of rejection of energy to the environment.

PRESSURE CLASSIFICATION

Low Pressure Heater: A heater located (with regard to feedwater flow) between the condensate pump and either the boiler feed pump or, if present, an intermediate pressure (booster) pump. It normally extracts steam from the low pressure turbine.High Pressure Heater: A heater located downstream of the boiler feed pump. Typically, the steam source is the high pressure turbine. This heater is used in the Plant in Feedwater System.