Maintenance Fundamentals 2011 Part 12 ppsx

In some applications the double volute minimizes radial forcesimparted to the shaft and bearings because of imbalances in the pressure aroundthe impeller.CHARACTERISTICSCURVE For a given

Most baghouse systems are provided as complete assemblies by the vendor.While the unit may require some field assembly, the vendor generally providesthe structural supports, which in most cases are adequate The only controllableinstallation factors that may affect performance are the foundation and connec-tions to pneumatic conveyors and other supply systems

Foundation

The foundation must support the weight of the baghouse In addition, it mustabsorb the vibrations generated by the cleaning system This is especially truewhen using the shaker-cleaning method, which can generate vibrations that canadversely affect the structural supports, foundation, and adjacent plant systems.Connections

Efficiency and effectiveness depend on leak-free connections throughout thesystem Leaks reduce the system’s ability to convey dust-laden air to the bag-house One potential source for leaks is improperly installed filter bags Becauseinstallation varies with the type of bag and baghouse design, consult the vendor’sO&M manual for specific instructions

As previously indicated, most bag-type filters require a pre-coat of particulatesbefore they can effectively remove airborne contaminants However, particlescan completely block air flow if the filter material becomes overloaded Thereforethe primary operating criterion is to maintain the efficiency of the filter media bycontrolling the cleaning frequency

Most systems use a time sequence to control the cleaning frequency If theparticulate load entering the baghouse is constant, this approach would bevalid However, the incoming load generally changes constantly As a result,the straight time sequence methodology does not provide the most efficient mode

of operation

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Operators should monitor the differential-pressure gauges that measure the totalpressure drop across the filter media When the differential pressure reaches themaximum recommended level (data provided by the vendor), the operatorshould override any automatic timer controls and initiate the cleaning sequence.Inspecting and Replacing Filter Media

Filter media used in dust-collection systems are prone to damage and abrasivewear Therefore, regular inspection and replacement is needed to ensure continu-ous, long-term performance Any damaged, torn, or improperly sealed bagsshould be removed and replaced

One of the more common problems associated with baghouses is improperinstallation of filter media Therefore it is important to follow the instructionsprovided by the vendor If the filter bags are not properly installed and sealed,overall efficiency and effectiveness are significantly reduced

CYCLONESEPARATORS

A widely used type of dust-collection equipment is the cyclone separator Acyclone is essentially a settling chamber in which gravitational acceleration isreplaced by centrifugal acceleration Dust-laden air or gas enters a cylindrical orconical chamber tangentially at one or more points and leaves through a centralopening The dust particles, by virtue of their inertia, tend to move toward theoutside separator wall from where they are led into a receiver Under commonoperating conditions, the centrifugal separating force or acceleration may rangefrom five times gravity in very large diameter, low-resistance cyclones to 2,500times gravity in very small, high-resistance units

Within the range of their performance capabilities, cyclones are one of the leastexpensive dust-collection systems Their major limitation is that, unless verysmall units are used, efficiency is low for particles smaller than 5 microns.Although cyclones may be used to collect particles larger than 200 microns,gravity-settling chambers or simple inertial separators are usually satisfactoryand less subject to abrasion

Configuration

The internal configuration of a cyclone separator is relatively simple Figure 16.2illustrates a typical cross-section of a cyclone separator, which consists of thefollowing segments:

Inlet area that causes the gas to flow tangentially

Cylindrical transition area

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Decreasing taper that increases the air velocity as the diameter creases

de-
Central return tube to direct the dust-free air out the discharge port.Particulate material is forced to the outside of the tapered segment and collected

in a drop-leg located at the dust outlet Most cyclones have a rotor-lock valveaffixed to the bottom of the drop-leg This is a motor-driven valve that collectsthe particulate material and discharges it into a disposal container

Vct rn

Clean gas outlet Dust shave-off

dust channel Inlet for dust-laden gases Shave-off-

Shave-off-reentry opening

Pattern of coarser dust mainstream Dust outlet

Pattern of dust stream (principally the finer particles) following eddy current

Figure 16.2 Flow pattern through a typical cyclone separator

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In this equation, r is the cyclone radius and n is dependent on the coefficient offriction Theoretically, in the absence of wall friction, n should equal 1.0 Actualmeasurements, however, indicate that n ranges from 0.5 to 0.7 over a largeportion of the cyclone radius The spiral velocity in a cyclone may reach avalue several times the average inlet gas velocity.

Pressure Drop

The pressure drop and the friction loss through a cyclone are most convenientlyexpressed in terms of the velocity head based on the immediate inlet area Theinlet velocity head, hvt, which is expressed in inches of water, is related to theaverage inlet gas velocity and density by:

Vc¼ Average inlet gas velocity (ft=sec)

The cyclone friction loss, Fcv, is a direct measure of the static pressure and powerthat a fan must develop It is related to the pressure drop by:

Fcv¼ Dpcvþ 1  4Ac

pD2 e

where:

Fcv¼ Friction loss (inlet-velocity heads)

Dpcv¼ Pressure drop through the cyclone (inlet-velocity heads)

Ac¼ Area of the cyclone (ft:2)

De¼ Diameter of the gas exit (ft:)

The friction loss through cyclones may range from 1 to 20 inlet-velocity heads,depending on its geometric proportions For a cyclone of specific geometricproportions, Fcvand Dpcv, are essentially constant and independent of the actualcyclone size

Collection Efficiency

Since cyclones rely on centrifugal force to separate particulates from the air orgas stream, particle mass is the dominant factor that controls efficiency For

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particulates with high densities (e.g., ferrous oxides), cyclones can achieve 99%

or better removal efficiencies, regardless of particle size Lighter particles (e.g.,tow or flake) dramatically reduce cyclone efficiency

These devices are generally designed to meet specific pressure-drop limitations.For ordinary installations operating at approximately atmospheric pressure, fanlimitations dictate a maximum allowable pressure drop corresponding to acyclone inlet velocity in the range of 20–70 feet per second Consequently,cyclones are usually designed for an inlet velocity of 50 feet per second.Varying operating conditions change dust-collection efficiency only by a smallamount The primary design factor that controls collection efficiency is cyclonediameter A small-diameter unit operating at a fixed pressure drop has a higherefficiency than a large-diameter unit Reducing the gas-outlet duct diameter alsoincreases the collection efficiency

Installation

As in any other pneumatic-conveyor system, special attention must be given tothe piping or ductwork used to convey the dust-laden air or gas The insidesurfaces must be smooth and free of protrusions that affect the flow pattern Allbends should be gradual and provide a laminar-flow path for the gas

CYCLONIC SEPARATORS

Table 16.2 identifies the failure modes and their causes for cyclonic separators.Since there are no moving parts within a cyclone, most of the problems associ-ated with this type of system can be attributed to variations in process param-eters such as flow rate, dust load, dust composition (i.e., density, size, etc.), andambient conditions (i.e., temperature, humidity, etc.)

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Table 16.1 Common Failure Modes of Baghouses

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Table 16.2 Common Failure Modes of Cyclonic Separators

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The volute is a region that expands in cross-sectional area as it wraps around thepump casing The purpose of the volute is to collect the liquid discharged fromthe periphery of the impeller at high velocity and gradually cause a reduction influid velocity by increasing the flow area This converts the velocity head to staticpressure The fluid is then discharged from the pump through the dischargeconnection Figure 17.2 illustrates the two types of volutes.

Centrifugal pumps can also be constructed in a manner that results in twodistinct volutes, each receiving the liquid that is discharged from a 180-degreeregion of the impeller at any given time Pumps of this type are called double

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volute pumps In some applications the double volute minimizes radial forcesimparted to the shaft and bearings because of imbalances in the pressure aroundthe impeller.

CHARACTERISTICSCURVE

For a given centrifugal pump operating at a constant speed, the flow ratethrough the pump is dependent on the differential pressure or head developed

by the pump The lower the pump head, the higher the flow rate A vendor

Figure 17.1 Centrifugal pump

Figure 17.2 Single and double volute

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manual for a specific pump usually contains a curve of pump flow rate versuspump head called a pump characteristic curve After a pump is installed in asystem, it is usually tested to ensure that the flow rate and head of the pump arewithin the required specifications A typical centrifugal pump characteristiccurve is shown in Figure 17.3.

Several terms associated with the pump characteristic curve must be defined.Shutoff head is the maximum head that can be developed by a centrifugal pumpoperating at a set speed Pump run-out is a the maximum flow a centrifugalpump can develop without damaging the pump Centrifugal pumps must bedesigned to be protected from the conditions of pump run-out or operating atshutoff head

PROTECTION

A centrifugal pump is deadheaded when it is operated with a closed discharge valve

or against a seated check valve If the discharge valve is closed and there is no otherflow path available to the pump, the impeller will churn the same volume of water

as it rotates in the pump casing This will increase the temperature of the liquid inthe pump casing to the point that it will flash to vapor If the pump is run in thiscondition for a significant amount of time, it will become damaged

When a centrifugal pump is installed in a system such that it may be subjected toperiodic shutoff head conditions, it is necessary to provide some means of pumpprotection One method for protecting the pump from running deadheaded is to

Figure 17.3 Centrifugal pump characteristic curve

provide a recirculation line from the pump discharge line upstream of thedischarge valve back to the pump’s supply source The recirculation line should

be sized to allow enough flow through the pump to prevent overheating anddamage to the pump Protection may also be accomplished by use of an auto-matic flow control device

Centrifugal pumps must also be protected from run-out One method for ing that there is always adequate flow resistance at the pump discharge toprevent excessive flow through the pump is to place an orifice or a throttlevalve immediately downstream of the pump discharge

ensur-GASBINDING

Gas binding of a centrifugal pump is a condition in which the pump casing isfilled with gases or vapors to the point where the impeller is no longer able tocontact enough fluid to function correctly The impeller spins in the gas bubblebut is unable to force liquid through the pump

Centrifugal pumps are designed so that their pump casings are completely filledwith liquid during pump operation Most centrifugal pumps can still operatewhen a small amount of gas accumulates in the pump casing, but pumps insystems containing dissolved gases that are not designed to be self-venting should

be periodically vented manually to ensure that gases do not build up in the pumpcasing

CLASSIFICATION BYFLOW

Centrifugal pumps can be classified based on the manner in which fluid flowsthrough the pump The manner in which fluid flows through the pump isdetermined by the design of the pump casing and the impeller The three types

of flow through a centrifugal pump are radial flow, axial flow, and mixed flow

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Radial Flow

In a radial flow pump, the liquid enters at the center of the impeller and is directedout along the impeller blades in a direction at right angles to the pump shaft Theimpeller of a typical radial flow pump and the flow is illustrated in Figure 17.4

Axial Flow

In an axial flow pump, the impeller pushes the liquid in a direction parallel to thepump shaft Axial flow pumps are sometimes called propeller pumps because theyoperate essentially the same as the propeller of a boat The impeller of a typicalaxial flow pump and the flow through a radial flow pump are shown in Figure 17.5

Figure 17.4 Radial flow centrifugal pump

Figure 17.5 Typical axial flow centrifugal pump

Mixed Flow

Mixed flow pumps borrow characteristics from both radial flow and axial flowpumps As liquid flows through the impeller of a mixed flow pump, the impellerblades push the liquid out away from the pump shaft and to the pump suction at

an angle greater than 90 degrees The impeller of a typical mixed flow pump andthe flow through a mixed flow pump are shown in Figure 17.6

MULTI-STAGEPUMPS

A centrifugal pump with a single impeller that can develop a differential pressure

of more than 150 psid between the suction and the discharge is difficult andcostly to design and construct A more economical approach to developing highpressures with a single centrifugal pump is to include multiple impellers on acommon shaft within the same pump casing Internal channels in the pumpcasing route the discharge of one impeller to the suction of another impeller.Figure 17.7 shows a diagram of the arrangement of the impellers of a four-stagepump The water enters the pump from the top left and passes through each ofthe four impellers, going from left to right The water goes from the volutesurrounding the discharge of one impeller to the suction of the next impeller

A pump stage is defined as that portion of a centrifugal pump consisting of oneimpeller and its associated components Most centrifugal pumps are single-stagepumps, containing only one impeller A pump containing seven impellers within

a single casing would be referred to as a seven-stage pump or generally as a stage pump

multi-Figure 17.6 Typical mixed flow pump

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Centrifugal pumps vary in design and construction from simple pumps withrelatively few parts to extremely complicated pumps with hundreds of individualparts Some of the most common components found in centrifugal pumps arewearing rings, stuffing boxes, packing, and lantern rings These components areshown in Figure 17.8 and are described in the following pages

Figure 17.7 Multi-stage centrifugal pump

Inlet

Volute

Impeller Wearing Ring

Impeller Volute

Pump Casing Wearing Ring

Pump Casing

Figure 17.8 Components of a centrifugal pump

Impellers of pumps are classified based on the number of points that the liquidcan enter the impeller and also on the amount of webbing between the impellerblades

Impellers can be either single-suction or double-suction A single-suction ler allows liquid to enter the center of the blades from only one direction Adouble-suction impeller allows liquid to enter the center of the impeller bladesfrom both sides simultaneously Figure 17.9 shows simplified diagrams of single-and double-suction impellers

impel-Impellers can be open, semi-open, or enclosed The open impeller consists only

of blades attached to a hub The semi-open impeller is constructed with a circularplate (the web) attached to one side of the blade The enclosed impeller hascircular plates attached to both sides of the blades Enclosed impellers are alsoreferred to as shrouded impellers Figure 17.10 illustrates examples of open, semi-open, and enclosed impellers

The impeller sometimes contains balancing holes that connect the space aroundthe hub to the suction side of the impeller The balancing holes have a total cross-sectional area that is considerably greater than the cross-sectional area of theannular space between the wearing ring and the hub The result is suctionpressure on both sides of the impeller hub, which maintains a hydraulic balance

Figure 17.9 Single-suction and double-suction impellers

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