Monday, December 29, 2008

Demineralizers

Purpose of Demineralizers
Dissolved impurities in power plant fluid systems generate corrosion problems and decrease efficiency due to fouled heat transfer surfaces. Demineralization of the water is one of the most practical and common methods available to remove these dissolved impurities.

In the plant, demineralizers (also called ion-exchangers) are used to hold ion exchange resins and transport water through them. Ion exchangers are generally classified into two groups: singlebed ion exchangers and mixed-bed ion exchangers.

Demineralizers
A demineralizer is basically a cylindrical tank with connections at the top for water inlet and resin addition, and connections at the bottom for the water outlet. The resin can usually be changed through a connection at the bottom of the tank. The resin beads are kept in the demineralizer by upper and lower retention elements, which are strainers with a mesh size smaller then the resin beads. The water to be purified enters the top at a set flow rate and flows down through the resin beads, where the flow path causes a physical filter effect as well as a chemical ion exchange.

Single-Bed Demineralizers
A single-bed demineralizer contains either cation or anion resin beads. In most cases, there are two, single-bed ion exchangers in series; the first is a cation bed and the second is an anion bed. Impurities in plant water are replaced with hydrogen ions in the cation bed and hydroxyl ions in the anion bed. The hydrogen ions and the hydroxyl ions then combine to form pure water.
The Chemistry Handbook, Module 4, Principles of Water Treatment, addresses the chemistry of demineralizers in more detail.

Figure 13 illustrates a single-bed demineralizer. When in use, water flows in through the inlet to a distributor at the top of the tank. The water flows down through the resin bed and exits out through the outlet. A support screen at the bottom prevents the resin from being forced out of the demineralizer tank.

Figure 13 Single-Bed Demineralizer

Single-Bed Regeneration
The regeneration of a single-bed ion exchanger is a three-step process. The first step is a backwash, in which water is pumped into the bottom of the ion exchanger and up through the resin. This fluffs the resin and washes out any entrained particles. The backwash water goes out through the normal inlet distributor piping at the top of the tank, but the valves are set to direct the stream to a drain so that the backwashed particles can be pumped to a container for waste disposal.

The second step is the actual regeneration step, which uses an acid solution for cation units and caustic solution for anion units. The concentrated acid or caustic is diluted to approximately 10% with water by opening the dilution water valve, and is then introduced through a distribution system immediately above the resin bed. The regenerating solution flows through the resin and out the bottom of the tank to the waste drain.

The final step is a rinsing process, which removes any excess regenerating solution. Water is pumped into the top of the tank, flows down through the resin bed and out at the bottom drain.

To return the ion exchanger to service, the drain valve is closed, the outlet valve is opened, and the ion exchanger is ready for service.

Single-bed demineralizers are usually regenerated "in place." The resins are not pumped out to another location for regeneration. The regeneration process is the same for cation beds and for anion beds; only the regenerating solution is different. It is important to realize that if the ion exchanger has been exposed to radioactive materials, the backwash, regeneration, and rinse solutions may be highly radioactive and must be treated as a radioactive waste.

Mixed-Bed Demineralizer
A mixed-bed demineralizer is a demineralizer in which the cation and anion resin beads are mixed together. In effect, it is equivalent to a number of two-step demineralizers in series. In a mixed-bed demineralizer, more impurities are replaced by hydrogen and hydroxyl ions, and the water that is produced is extremely pure. The conductivity of this water can often be less than 0.06 micromhos per centimeter.

Mixed-Bed Regeneration
The mixed-bed demineralizer shown in Figure 14 is designed to be regenerated in place, but the process is more complicated than the regeneration of a single-bed ion exchanger. The steps in the regeneration are shown in Figure 14.

Figure 14a shows the mixed-bed ion exchanger in the operating, or on-line mode. Water enters through a distribution header at the top and exits through the line at the bottom of the vessel. Regeneration causes the effluent water to increase in electrical conductivity.

The first regeneration step is backwash, as shown in Figure 14b. As in a single-bed unit,backwash water enters the vessel at the bottom and exits through the top to a drain. In addition to washing out entrained particles, the backwash water in a mixed-bed unit must also separate the resins into cation and anion beds. The anion resin has a lower specific gravity than the cation resin; therefore, as the water flows through the bed, the lighter anion resin beads float upward to the top. Thus, the mixed-bed becomes a split bed. The separation line between the anion bed at the top and the cation bed at the bottom is called the resin interface. Some resins can be separated only when they are in the depleted state; other resins separate in either the depleted form or the regenerated form.

The actual regeneration step is shown in Figure 14c. Dilution water is mixed with caustic solution and introduced at the top of the vessel, just above the anion bed. At the same time, dilution water is mixed with acid and introduced at the bottom of the vessel, below the cation bed. The flow rate of the caustic solution down to the resin interface is the same as the flow rate of the acid solution up to the resin interface. Both solutions are removed at the interface and dumped to a drain.


Figure 14 Regeneration of a Mixed-Bed Demineralizer

During the regeneration step, it is important to maintain the cation and anion resins at their proper volume. If this is not done, the resin interface will not occur at the proper place in the vessel, and some resin will be exposed to the wrong regenerating solution. It is also important to realize that if the ion exchanger has been involved with radioactive materials, both the backwash and the regenerating solutions may be highly radioactive and must be treated as liquid radioactive waste.

The next step is the slow rinse step, shown in Figure 14d, in which the flow of dilution water is continued, but the caustic and acid supplies are cut off. During this two-direction rinse, the last of the regenerating solutions are flushed out of the two beds and into the interface drain.

Rinsing from two directions at equal flow rates keeps the caustic solution from flowing down into the cation resin and depleting it.

In the vent and partial drain step, illustrated in Figure 14e, the drain valve is opened, and some of the water is drained out of the vessel so that there will be space for the air that is needed to re-mix the resins. In the air mix step, (Figure 14f) air is usually supplied by a blower, which forces air in through the line entering the bottom of the ion exchanger. The air mixes the resin beads and then leaves through the vent in the top of the vessel. When the resin is mixed, it is dropped into position by slowly draining the water out of the interface drain while the air mix continues.

In the final rinse step, shown in Figure 14g, the air is turned off and the vessel is refilled with water that is pumped in through the top. The resin is rinsed by running water through the vessel from top to bottom and out the drain, until a low conductivity reading indicates that the ion exchanger is ready to return to service.

External Regeneration
Some mixed-bed demineralizers are designed to be regenerated externally, with the resins being removed from the vessel, regenerated, and then replaced. With this type of demineralizer, the first step is to sluice the mixed bed with water (sometimes assisted by air pressure) to a cation tank in a regeneration facility. The resins are backwashed in this tank to remove suspended solids and to separate the resins. The anion resins are then sluiced to an anion tank. The two batches of separated resins are regenerated by the same techniques used for single-bed ion exchangers. They are then sluiced into a holding tank where air is used to remix them. The mixed, regenerated, resins are then sluiced back to the demineralizer.

External regeneration is typically used for groups of condensate demineralizers. Having one central regeneration facility reduces the complexity and cost of installing several demineralizers.

External regeneration also allows keeping a spare bed of resins in a holding tank. Then, when a demineralizer needs to be regenerated, it is out of service only for the time required to sluice out the depleted bed and sluice a fresh bed in from the holding tank. A central regeneration facility may also include an ultrasonic cleaner that can remove the tightly adherent coating of dirt or iron oxide that often forms on resin beads. This ultrasonic cleaning reduces the need for chemical regeneration.

Demineralizers Summary
Demineralization of water is one of the most practical and common methods used to remove dissolved contaminates. Dissolved impurities in power plant fluid systems can generate corrosion problems and decrease efficiency due to fouled heat transfer surfaces. Demineralizers (also called ion-exchangers) are used to hold ion exchange resins and transport water through them. Ion exchangers are generally classified
into two groups: single-bed ion exchangers and mixed-bed ion exchangers.

A demineralizer is basically a cylindrical tank with connections at the top for water inlet and resin addition, and connections at the bottom for the water outlet. The resin can usually be changed out through a connection at the bottom of the tank. The resin beads are kept in the demineralizer by upper and lower retention elements, which are strainers with a mesh size smaller then the resin beads.

The water to be purified enters the top at a set flow rate, flows down through the resin beads where the flow path causes a physical filter effect as well as a chemical ion exchange. The chemistry of the resin exchange is explained in detail in the Chemistry Fundamentals Handbook.

There are two types of demineralizers, single-bed and mixed-bed.
Single-bed demineralizers have resin of either cation or anion exchange sites. Mixed-bed demineralizers contain both anion and cation resin.

All demineralizers will eventually be exhausted from use. To regenerate the resin and increase the demineralizer's efficiency, the demineralizers are regenerated. The regeneration process is slightly different for a mixed-bed demineralizer compared to the single-bed demineralizer. Both methods were explained in this chapter.
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Boilers

Introduction
The primary function of a boiler is to produce steam at a given pressure and temperature. To accomplish this, the boiler serves as a furnace where air is mixed with fuel in a controlled combustion process to release large quantities of heat. The pressure-tight construction of a boiler provides a means to absorb the heat from the combustion and transfer this heat to raise water to a temperature such that the steam produced is of sufficient temperature and quality (moisture content) for steam loads.

Boilers
Two distinct heat sources used for boilers are electric probes and burned fuel (oil, coal, etc.)This chapter will use fuel boilers to illustrate the typical design of boilers. Refer to Figure 9 during the following discussion.

The boiler has an enclosed space where the fuel combustion takes place, usually referred to as the furnace or combustion chamber. Air is supplied to combine with the fuel, resulting in combustion. The heat of combustion is absorbed by the water in the risers or circulating tubes. The density difference between hot and cold water is the driving force to circulate the water back to the steam drum. Eventually the water will absorb sufficient heat to produce steam.

Steam leaves the steam drum via a baffle, which causes any water droplets being carried by the steam to drop out and drain back to the steam drum. If superheated steam is required, the steam may then travel through a superheater. The hot combustion gasses from the furnace will heat the steam through the superheater's thin tube walls. The steam then goes to the steam supply system and the various steam loads.

Some boilers have economizers to improve cycle efficiency by preheating inlet feedwater to the boiler. The economizer uses heat from the boiler exhaust gasses to raise the temperature of the inlet feedwater.

Figure 9 Typical Fuel Boiler

Fuel Boiler Components
Figure 9 illustrates a typical fuel boiler. Some of the components are explained below.

Steam drum - The steam drum separates the steam from the heated water. The
water droplets fall to the bottom of the tank to be cycled again, and the steam leaves the drum and enters the steam system. Feedwater enters at the bottom of the drum to start the heating cycle.

Downcomers - Downcomers are the pipes in which the water from the steam drum travels in order to reach the bottom of the boiler where the water can enter the distribution headers.

Distribution headers - The distribution headers are large pipe headers that carry the water from the downcomers to the risers.

Risers - The piping or tubes that form the combustion chamber enclosure are called risers. Water and steam run through these to be heated. The term risers refers to the fact that the water flow direction is from the bottom to the top of the boiler. From the
risers, the water and steam enter the steam drum and the cycle starts again.

Combustion chamber - Located at the bottom of a boiler, the combustion chamber is where the air and fuel mix and burn. It is lined with the risers.

Boilers Summary
*Boilers are vessels that allow water in contained piping to be heated to steam by a heat source internal to the vessel. The water is heated to the boiling point. The resulting steam separates, and the water is heated again. Some boilers use the heat from combustion off-gasses to further heat the steam (superheat) and/or to preheat the feedwater.

*The following components were discussed:
The steam drum is where the steam is separated from the heated water.

Downcomers are the pipes in which the water from the steam drum travels to reach the bottom of the boiler.

Distribution headers are large pipe headers that carry the water from th downcomers to the risers.

Risers are the piping or tubes that form the combustion chamber enclosure. Water and steam run through the risers to be heated.

The combustion chamber is located at the bottom of the boiler and is where the air and fuel mix and burn.
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CLASSIFICATION OF ROLLING-ELEMENT BEARINGS

CLASSIFICATION OF ROLLING-ELEMENT BEARINGS
Ball bearings can operate at higher speed in comparison to roller bearings because they have lower friction. In particular, the balls have less viscous resistance when rolling through oil or grease. However, ball bearings have lower load capacity compared with roller bearings because of the high contact pressure of point contact. There are about 50 types of ball bearings listed in manufacturer catalogues. Each one has been designed for specific applications and has its unique characteristics. The following is a description of the most common types.

12.2.1 Ball Bearings


12.2.1.1 Deep-Groove Ball Bearing
The deep-groove ball bearing (Fig. 12-2) is the most common type, since it can be used for relatively high radial loads. Deep-groove radial ball bearings are the most widely used bearings in industry, and their market share is about 80% of industrial rolling-element bearings. Owing to the deep groove in the raceways, they can support considerable thrust loads (in the axial direction of the shaft) in

addition to radial loads. A deep-groove bearing can support a thrust load of about
70% of its radial load. The radial and axial load capacity increases with the bearing size and number of balls.
For maximum load capacity, a filling-notch type of bearing can be used that has a larger number of balls than the standard bearing. In this design, there is a notch on one shoulder of the race. The circular notch makes it possible to insert more balls into the deep groove between the two races. The maximum number of balls can be inserted if the outer ring is split. However, in that case, external means must be provided to hold and tighten the two ring halves together.

12.2.1.2 Self-Aligning Ball Bearings
It is very important to compensate for angular machining and assembly errors between the centerlines of the bearing and the shaft. The elastic deflection of the shaft is an additional cause of misalignment. In the case of a regular deep-groove ball bearing, the misalignment causes a bending moment in the bearing and additional severe contact stresses between the balls and races. However, in the self-aligning bearing (Fig. 12-3), the spherical shape of the outer race allows an additional angular degree of freedom (similar to that of a universal joint) that prevents the transfer of any bending moment to the bearing and prevents any additional contact stresses.

Self-aligning ball bearings have two rows of balls, and the outer ring has a common spherical raceway that allows for the self-aligning characteristic. The

inner ring is designed with two restraining ribs (also known as lips), one at each
side of the roller element, for accurately locating the rolling elements’ path on the
inner raceway. But the outside ring has no ribs, in order to allow for selfalignment.
A wide spherical outer race allows for a higher degree of selfalignment.

Self-aligning ball bearings are widely used in applications where misalignment
is expected due to the bending of the shaft, errors in the manufacture of the shaft, or mounting errors. The design engineer must keep in mind that there are always tolerances due to manufacturing errors. Self-aligning bearings can be applied for radial loads combined with moderate thrust loads. The feature that self-aligning bearings do not exert any bending moment on the shaft is particularly important in applications that require high precision (low radial run-out) at high speeds, because shaft bending causes imbalance and vibrations.

The concept of self-alignment is useful in all types of bearings, including sleeve
bearings.

12.2.1.3 Double-Row Deep-Groove Ball Bearing
This bearing type (Fig. 12-4) is used for relatively high radial loads. It is more sensitive to misalignment errors than the single row and should be used only for
applications where minimal misalignment is expected. Otherwise, a self-alignment
bearing should be selected.

The design of double-row ball bearings is similar to that of single-row ball
bearings. Since double-row ball bearings are wider and have two rows, they can

carry higher radial loads. Unlike the deep-groove bearing, designs of split rings (for the maximum number of balls) are not used, and each ring is made from one piece. However, double-row bearings include groups with larger diameters and a larger number of balls to further improve the load capacity.

12.2.1.4 Angular Contact Ball Bearing
This bearing type (Fig. 12-5) is used to support radial and thrust loads. Contact angles of up to 40 (from the radial direction) are available from some bearing manufacturers, but 15 and 25 are the more standard contact angles. The contact
angle determines the ratio of the thrust to radial load.

Angular contact bearings are widely used for adjustable arrangements, where they are mounted in pairs against each other and preloaded. In this way, clearances in the bearings are eliminated or even preload is introduced in the rolling contacts. This is often done to stiffen the bearings for a rigid support of the shaft. This is important for reducing the amplitude of shaft vibrations under oscillating forces. This type of design has significant advantages whenever precision is required (e.g., in machine tools), and it reduces vibrations due to imbalance. This is particularly important in high-speed applications. An adjustable arrangement is also possible in tapered bearings; however, angular contact ball bearings have lower friction than do tapered bearings. However, the friction of angular contact ball bearings is somewhat higher than that of radial ball bearings. Angular contact ball bearings are the preferred choice in many important applications, such as high-speed turbines, including jet engines.


Single-row angular contact ball bearings can carry considerable radial loads
combined with thrust loads in one direction. Prefabricated mountings of two or more single-row angular contact ball bearings are widely used for two-directional thrust loads. Two bearings in series can be used for heavy unidirectional thrust loads, where two single-row angular contact ball bearings share the thrust load.

Precise axial internal clearance and high-quality surface finish are required to
secure load sharing of the two bearings in series. The bearing arrangement of two or more angular contact bearings facing the same direction is referred to as tandem arrangement. The bearings are mounted adjacent to each other to increase the thrust load carrying capacity.

12.2.2 Roller Bearings
Roller bearings have a theoretical line contact between the unloaded cylindrical rollers and races. This is in comparison to ball bearings, which have only a theoretical point contact with the raceways. Under load, there is elastic deformation, and line contact results in a larger contact area than that of a point contact in ball bearings. Therefore, roller bearings can support higher radial loads. At the same time, the friction force and friction-energy losses are higher for a line contact; therefore, roller bearings are usually not used for high-speed applications.

Roller bearings can be classified into four categories: cylindrical roller
bearings, tapered roller bearings, needle roller bearings and spherical roller
bearings.

12.2.2.1 Cylindrical Roller Bearings
The cylindrical roller bearing (Fig. 12-6) is used in applications where high radial load is present without any thrust load. Various types of cylindrical roller bearings are manufactured and applied in machinery. In certain applications where diameter space is limited, these bearings are mounted directly on the shaft, which serves as the inner race. For direct mounting, the shaft must be hardened to high Rockwell hardness, similar to that of the bearing race. For direct mounting, the radial load must be high in order to prevent slipping between the rollers and the shaft during the start-up. It is important to keep in mind that cylindrical roller bearings cannot support considerable thrust loads. Thus, for applications where both radial and thrust loading are present, it is preferable to use ball bearings.

12.2.2.2 Tapered Roller Bearing
The tapered roller bearing is used in applications where a high thrust load is present that can be combined with a radial load. The bearing is shown in Fig. 12-7. The races of inner and outer rings have a conical shape, and the rolling elements between them have a conical shape as well. In order to have a rolling motion, the contact lines formed by each of the various tapered roller elements


and the two races must intersect at a common point on the bearing axis. This intersection point is referred to as an apex point. The apex point is closer to the bearing when the cone angle is steeper. A steeper cone angle can support a higher
thrust load relative to a radial load.

The inner ring is referred to as cone, while the outer ring is referred to as cup. The cone is designed with two retaining ribs (also known as lips) to confine

the tapered rollers as shown in Fig. 12-7. The ribs also align the rollers between the races. In addition, the larger rib has an important role in supporting the axial load. A cage holds the cone and rollers together as one unit, but the cup (outer ring) can be pulled apart.

A single-row tapered roller bearing can support a thrust load in only one
direction. Two tapered roller bearings are usually mounted in opposition, to allow for thrust support in both directions (in a similar way to opposing angular contact ball bearings). Moreover, double or four-row tapered roller bearings are applied in certain applications to support a high bidirectional thrust load as well as radial load.

The reaction force on the cup acts in the direction normal to the line of contact of the rolling elements with the cup race (normal to the cup surface). This force can be divided into axial and radial load components. The intersection of the resultant reaction force (which is normal to the cup angle) with the bearing centerline is referred to as the effective center. The location of the effective center is useful in bearing load calculations.

For example, when a radial load is applied on the bearing, this produces both radial and thrust reactions. The thrust force component, which acts in the direction of the shaft centerline, can separate the cone from the cup by sliding the shaft in the axial direction through the cone or by the cup’s sliding axially in its seat. To prevent such undesired axial motion, a single-row tapered bearing should be mounted with another tapered bearing in the opposite direction. This arrangement is also very important for adjusting the clearance.

One major advantage of the tapered roller bearing is that it can be applied in
adjustable arrangement where two tapered roller bearings are mounted in opposite directions (in a similar way to the adjustable arrangement of the angular contact ball bearing that was discussed earlier). This arrangement allows one to eliminate undesired clearance and to provide a preload (interference or negative clearance). Bearing preload increases the bearing stiffness, resulting in reduced vibrations as well as a lower level of run-out errors in precision machining.
However, the disadvantage of bearing preloading is additional contact stresses
and higher friction. Preload results in lowering the speed limit because the higher friction causes overheating at high speeds.

The adjustment of bearing clearance can be done during assembly and even during steady operation of the machine. The advantage of adjustment during operation is the precise elimination of the clearance after the thermal expansion of
the shaft.

12.2.2.3 Multirow Tapered Roller Bearings
The multirow tapered roller bearing (Fig. 12-8) is manufactured with a predetermined
adjustment that enables assembly into a machine without any further adjustment. The multirow arrangement includes spacers and is referred to as a

spacer assembly. The spacer is matched with a specific bearing assembly during
manufacturing. It is important to note that components of these assemblies are not
interchangeable. Other types, without spacers, are manufactured with predetermined
internal adjustment, and their components are also not interchangeable.

12.2.2.4 Needle Roller Bearing
These bearings (Fig. 12-9) are similar to cylindrical roller bearings, in the sense that they support high radial load. This type of bearing has a needle like appearance because of its higher length-to-diameter ratio.
The objective of a needle roller bearing is to save space. This is advantageous in applications where bearing space is limited. Furthermore, in certain applications needle roller bearings can also be mounted directly on the

shaft. For a direct mounting, the shaft must be properly hardened to a similar
hardness of a bearing ring.

Two types of needle roller bearings are available. The first type, referred to as full complement, does not include a cage; the second type has a cage to separate the needle rollers in order to prevent them from sliding against each other. The full-complement bearing has more rollers and can support higher radial load. The second type has a lower number of rollers because it has a cage to separate the needle rollers to prevent them from rubbing against each other. The speed of a full-complement bearing is limited because it has higher friction between the rollers. A full-complement needle bearing may comprise a maximum number of needle rollers placed between a hardened shaft and a housing bore. An outer ring may not be required in certain situations, resulting in further saving of space.

12.2.2.5 Self-Aligning Spherical Roller Bearing
This bearing has barrel-shaped rollers (Fig. 12-10). It is designed for applications that involve misalignments due to shaft bending under heavy loads and due to manufacturing tolerances or assembly errors (in a similar way to the self-aligning
ball bearing). The advantage of the spherical roller bearing is its higher load capacity in comparison to that of a self-aligning ball bearing, but it has higher frictional losses.

Spherical roller bearings are available as single-row, double-row, and thrust
types. The single-row thrust spherical roller bearing is designed to support only thrust load, and it is not recommended where radial loads are present. Double-row

spherical roller bearings are commonly used when radial as well as thrust loads
are present.

The double-row spherical roller bearing has the highest load capacity of all rolling bearings. This is due to the relatively large radius of contact of the rolling element. It can resist impact and other dynamic forces. It is used in heavy-duty
applications such as ship shafts, rolling mills, and stone crushers.
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