Maintenance Fundamentals 2011 Part 12 ppsx

Installation Most baghouse systems are provided as complete assemblies by the vendor. While the unit may require some field assembly, the vendor generally provides the structural supports, which in most cases are adequate. The only controllable installation 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 must absorb the vibrations generated by the cleaning system. This is especially true when using the shaker-cleaning method, which can generate vibrations that can adversely affect the structural supports, foundation, and adjacent plant systems. Connections Efficiency and effectiveness depend on leak-free connections throughout the system. 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. Because installation varies with the type of bag and baghouse design, consult the vendor’s O&M manual for specific instructions. Operating Methods The guidelines provided in the vendor’s O&M manual should be the primary reference for proper baghouse operation. Vendor-provided information should be used because there are not many common operating guidelines among the various configurations. The only general guidelines that are applicable to most designs are cleaning frequency and inspection and replacement of filter media. Cleaning As previously indicated, most bag-type filters require a pre-coat of particulates before they can effectively remove airborne contaminants. However, particles can completely block air flow if the filter material becomes overloaded. Therefore the primary operating criterion is to maintain the efficiency of the filter media by controlling the cleaning frequency. Most systems use a time sequence to control the cleaning frequency. If the particulate load entering the baghouse is constant, this approach would be valid. However, the incoming load generally changes constantly. As a result, the straight time sequence methodology does not provide the most efficient mode of operation. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 324 324 Maintenance Fundamentals Operators should monitor the differential-pressure gauges that measure the total pressure drop across the filter media. When the differential pressure reaches the maximum recommended level (data provided by the vendor), the operator should 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 abrasive wear. Therefore, regular inspection and replacement is needed to ensure continu- ous, long-term performance. Any damaged, torn, or improperly sealed bags should be removed and replaced. One of the more common problems associated with baghouses is improper installation of filter media. Therefore it is important to follow the instructions provided by the vendor. If the filter bags are not properly installed and sealed, overall efficiency and effectiveness are significantly reduced. CYCLONE SEPARATORS A widely used type of dust-collection equipment is the cyclone separator. A cyclone is essentially a settling chamber in which gravitational acceleration is replaced by centrifugal acceleration. Dust-laden air or gas enters a cylindrical or conical chamber tangentially at one or more points and leaves through a central opening. The dust particles, by virtue of their inertia, tend to move toward the outside separator wall from where they are led into a receiver. Under common operating conditions, the centrifugal separating force or acceleration may range from five times gravity in very large diameter, low-resistance cyclones to 2,500 times gravity in very small, high-resistance units. Within the range of their performance capabilities, cyclones are one of the least expensive dust-collection systems. Their major limitation is that, unless very small 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 satisfactory and less subject to abrasion. Configuration The internal configuration of a cyclone separator is relatively simple. Figure 16.2 illustrates a typical cross-section of a cyclone separator, which consists of the following segments:  Inlet area that causes the gas to flow tangentially  Cylindrical transition area Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 325 Dust Collectors 325  Decreasing taper that increases the air velocity as the diameter de- creases  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 valve affixed to the bottom of the drop-leg. This is a motor-driven valve that collects the particulate material and discharges it into a disposal container. Performance Performance of a cyclone separator is determined by flow pattern, pressure drop, and collection efficiency. Flow Pattern The path the gas takes in a cyclone is through a double vortex that spirals the gas downward at the outside and upward at the inside. When the gas enters the cyclone, the tangential component of its velocity, V ct , increases with the decreas- ing radius as expressed by: V ct % r Àn Clean gas outlet Dust shave-off Shave-off- dust channel Inlet for dust-laden gases 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. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 326 326 Maintenance Fundamentals In this equation, r is the cyclone radius and n is dependent on the coefficient of friction. Theoretically, in the absence of wall friction, n should equal 1.0. Actual measurements, however, indicate that n ranges from 0.5 to 0.7 over a large portion of the cyclone radius. The spiral velocity in a cyclone may reach a value several times the average inlet gas velocity. Pressure Drop The pressure drop and the friction loss through a cyclone are most conveniently expressed in terms of the velocity head based on the immediate inlet area. The inlet velocity head, h vt , which is expressed in inches of water, is related to the average inlet gas velocity and density by: h vt ¼ 0:0030rV 2 c where h vt ¼ Inlet-velocity head (inches of water) r ¼ Gas density (lb=ft 3 ) V c ¼ Average inlet gas velocity (ft=sec) The cyclone friction loss, F cv , is a direct measure of the static pressure and power that a fan must develop. It is related to the pressure drop by: F cv ¼ Dp cv þ 1 À 4A c pD 2 e  2 where: F cv ¼ Friction loss (inlet-velocity heads) Dp cv ¼ Pressure drop through the cyclone (inlet-velocity heads) A c ¼ Area of the cyclone (ft: 2 ) D e ¼ 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 geometric proportions, F cv and Dp cv , are essentially constant and independent of the actual cyclone size. Collection Efficiency Since cyclones rely on centrifugal force to separate particulates from the air or gas stream, particle mass is the dominant factor that controls efficiency. For Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 327 Dust Collectors 327 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, fan limitations dictate a maximum allowable pressure drop corresponding to a cyclone 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 small amount. The primary design factor that controls collection efficiency is cyclone diameter. A small-diameter unit operating at a fixed pressure drop has a higher efficiency than a large-diameter unit. Reducing the gas-outlet duct diameter also increases the collection efficiency. Installation As in any other pneumatic-conveyor system, special attention must be given to the piping or ductwork used to convey the dust-laden air or gas. The inside surfaces must be smooth and free of protrusions that affect the flow pattern. All bends should be gradual and provide a laminar-flow path for the gas. TROUBLESHOOTING This section identifies common problems and their causes for baghouse and cyclonic separator dust-collection systems. BAGHOUSES Table 16.1 lists the common failure modes for baghouses. This guide may be used for all such units that use fabric filter bags as the primary dust-collection media. 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.), and ambient conditions (i.e., temperature, humidity, etc.). Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 328 328 Maintenance Fundamentals Table 16.1 Common Failure Modes of Baghouses Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 329 Dust Collectors 329 Table 16.2 Common Failure Modes of Cyclonic Separators Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 330 330 Maintenance Fundamentals 17 PUMPS CENTRIFUGAL PUMPS Centrifugal pumps basically consist of a stationary pump casing and an impeller mounted on a rotating shaft. The pump casing provides a pressure boundary for the pump and contains channels to properly direct the suction and discharge flow. The pump casing has suction and discharge penetrations for the main flow path of the pump and normally has a small drain and vent fittings to remove gases trapped in the pump casing or to drain the pump casing for maintenance. Figure 17.1 is a simplified diagram of a typical centrifugal pump that shows the relative locations of the pump suction, impeller, volute, and discharge. The pump casing guides the liquid from the suction connection to the center, or eye, of the impeller. The vanes of the rotating impeller impart a radial and rotary motion to the liquid, forcing it to the outer periphery of the pump casing, where it is collected in the outer part of the pump casing, called the volute. The volute is a region that expands in cross-sectional area as it wraps around the pump casing. The purpose of the volute is to collect the liquid discharged from the periphery of the impeller at high velocity and gradually cause a reduction in fluid velocity by increasing the flow area. This converts the velocity head to static pressure. The fluid is then discharged from the pump through the discharge connection. Figure 17.2 illustrates the two types of volutes. Centrifugal pumps can also be constructed in a manner that results in two distinct volutes, each receiving the liquid that is discharged from a 180-degree region of the impeller at any given time. Pumps of this type are called double Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 331 331 volute pumps. In some applications the double volute minimizes radial forces imparted to the shaft and bearings because of imbalances in the pressure around the impeller. CHARACTERISTICS CURVE For a given centrifugal pump operating at a constant speed, the flow rate through 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. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 332 332 Maintenance Fundamentals manual for a specific pump usually contains a curve of pump flow rate versus pump head called a pump characteristic curve. After a pump is installed in a system, it is usually tested to ensure that the flow rate and head of the pump are within the required specifications. A typical centrifugal pump characteristic curve 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 pump operating at a set speed. Pump run-out is a the maximum flow a centrifugal pump can develop without damaging the pump. Centrifugal pumps must be designed to be protected from the conditions of pump run-out or operating at shutoff 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 other flow 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 in the pump casing to the point that it will flash to vapor. If the pump is run in this condition 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 to periodic shutoff head conditions, it is necessary to provide some means of pump protection. One method for protecting the pump from running deadheaded is to Figure 17.3 Centrifugal pump characteristic curve. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 333 Pumps 333 [...]... positive-displacement pump consisting of a single reciprocating piston in a cylinder with a single suction port and a single discharge port as shown in Figure 17 .12 Figure 17 .12 Reciprocating positive-displacement pump operation 344 Maintenance Fundamentals During the suction stroke, the piston moves to the left, causing the check valve in the suction line between the reservoir and the pump cylinder to... components with distinct purposes The impeller contains rotating vanes that impart a radial and rotary motion to the liquid The volute collects the liquid discharged from the impeller at high velocity and gradually causes a reduction in fluid velocity by increasing the flow area, converting the velocity head to a static head 342 Maintenance Fundamentals               A diffuser increases the efficiency... Typical mixed flow pump Pumps 337 Figure 17.7 Multi-stage centrifugal pump COMPONENTS Centrifugal pumps vary in design and construction from simple pumps with relatively few parts to extremely complicated pumps with hundreds of individual parts Some of the most common components found in centrifugal pumps are wearing rings, stuffing boxes, packing, and lantern rings These components are shown in Figure 17.8... Casing Volute Impeller Stuffing Box Pump Shaft Stuffing Box Gland Packing Lantern Ring Impeller Wearing Ring Pump Casing Wearing Ring Inlet Volute Figure 17.8 Components of a centrifugal pump 338 Maintenance Fundamentals Impellers Impellers of pumps are classified based on the number of points that the liquid can enter the impeller and also on the amount of webbing between the impeller blades Impellers... energy The increase in flow energy can be observed as an increase in the pressure of an incompressible fluid Figure 17.11 shows a centrifugal pump diffuser Figure 17.11 Centrifugal pump diffuser 340 Maintenance Fundamentals Wearing Rings Centrifugal pumps contain rotating impellers within stationary pump casings To allow the impeller to rotate freely within the pump casing, a small clearance is maintained... contact This wear is due to the erosion caused by liquid leaking through this tight clearance and other causes Eventually the leakage could become unacceptably large and maintenance would be required on the pump To minimize the cost of pump maintenance, many centrifugal pumps are designed with wearing rings Wearing rings are replaceable rings that are attached to the impeller and/or the pump casing to allow...334 Maintenance Fundamentals provide a recirculation line from the pump discharge line upstream of the discharge valve back to the pump’s supply source The recirculation line should be sized to allow enough flow... The impeller of a typical axial 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 336 Maintenance Fundamentals Mixed Flow Mixed flow pumps borrow characteristics from both radial flow and axial flow pumps As liquid flows through the impeller of a mixed flow pump, the impeller blades push the liquid... pressure generated by reciprocating pumps is independent of fluid density It is dependent entirely on the amount of force exerted on the piston Figure 17.13 Single-acting and double-acting pumps 346 Maintenance Fundamentals ROTARY Rotary pumps operate on the principle that a rotating vane, screw, or gear traps the liquid in the suction side of the pump casing and forces it to the discharge side of the casing... suction lines and producing a high suction lift In pumps designed for systems requiring high suction lift and self-priming features, it is essential that all clearances between rotating parts, and between rotating and stationary parts, be kept to a minimum to reduce slippage Slippage is leakage of fluid from the discharge of the pump back to its suction Because of the close clearances in rotary pumps, it is . etc.). Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 328 328 Maintenance Fundamentals Table 16.1 Common Failure Modes of Baghouses Keith Mobley /Maintenance Fundamentals Final. finer particles) following eddy current Figure 16.2 Flow pattern through a typical cyclone separator. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:07pm page 326 326 Maintenance Fundamentals In. pump. Figure 17.2 Single and double volute. Keith Mobley /Maintenance Fundamentals Final Proof 15.6.2004 6:12pm page 332 332 Maintenance Fundamentals manual for a specific pump usually contains