TIP 0416-19 ISSUED – 2009 ©2009 TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.
Recovery boil er s ootblo wers: the basics Scope The purpose of this TIP is to provide a brief description of sootblower components and the mechanisms by which a sootblower jet removes deposits in recovery boilers.
Safety Safety p recautions A recovery boiler sootblower is connected with high pressure steam piping system and a power supply to drive its motor(s). Before any work is performed on a sootblower, it is important to the sootblower be put in the zero energy state. Zero energy state means that (1) the isolation valve is closed and a lock tag is placed in the valve to prevent the high pressure steam to enter the sootblower, and (2) the power supply to the sootblower is disconnected with a lock tag placed in the power supply to prevent inadvertent or unexpected start-up or energization of the motor.
Definitions Poppet valve pressure: The poppet valve pressure is the steam pressure measured at the location immediately downstream of the valve seat. This is the steam pressure supplied to the feed tube. Nozzle pressure: The nozzle pressure is often used interchangeably with lance pressure or blowing pressure. It is defined as the steam pressure supplied to the sootblower nozzles, and is the pressure measured just before the steam enters the nozzle (Points A shown in Figure 1). Note that the nozzle pressure is always lower than the poppet valve pressure due to minor minor losses and the the pressure drop in in the feed and lance tubes.
Fig. 1. Nozzle 1. Nozzle pressure Reverse and rest positions . The reverse position is the point where the limit switch is triggered to send a signal to change the carriage forward direction and retract the sootblower lance. The rest position is the point where the limit switch is triggered to send a signal to stop the carriage retracting movement and park the sootblower. Helix. The helix is the distance traveled by a nozzle to complete one full rotation (360°). See Figure 2.
Fig. 2. Helix 2. Helix
TIP Category: Data and Calculations TAPPI
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Sootblower: basic principle Function and operation
The entrainment of fly ash particles from the recovery boiler lower furnace to the convection sections of the boiler is an inevitable process. The accumulation of these particles in the fireside heat exchanger surfaces may reduce the boiler thermal efficiency, create a potentially corrosive environment at the boiler tube surfaces and, if the accumulation is not properly control, it may also lead to costly unscheduled boiler shutdowns due to plugging of the gas passages. Sootblowers are by far the most widely used equipment to control the fireside deposit accumulation in recovery boilers. A sootblower consists of a lance tube with two opposing nozzles mounted near the tip of the lance. During the deposit removal process, the sootblower lance rotates and extends, through a small opening in the boiler wall, while blowing high pressure steam directed into the tube banks. After the lance is fully extended, it rotates in the opposite direction as it is inserted and retracts to its original inactive state. Figure 3 illustrates the cleaning process of a tube bank by a sootblower.
Lance movement during retraction
Lance movement during insertion
Sootblower Lance
Figure 3. Cleaning process of a boiler bank by a sootblower Deposit removal mechanism
When a sootblower jet hits a deposit that adheres to a boiler tube, the jet velocity is converted into impact pressure that is distributed throughout the jet/deposit contact area. To better understand the deposit removal mechanism, we define a deposit/tube control volume as shown in Figure 4.
Deposit/tube control volume
Fig. 4. Deposit/tube control volume
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Two main deposit removal mechanisms have been identified; debonding and brittle fracture ( 1). The deposit will be removed from the boiler tube by debonding if the weakest link in the deposit/tube control volume is the strength of deposit adhesion to the tube (Fig. 5a). Brittle fraction occurs when the deposit tensile strength is the weakest link. In this case, the deposit will be broken into smaller pieces as the jet impacts the deposit (Fig. 5b). The deposit will stay attached to the tube if the stress imposed by the jet on the deposit is less than both the deposit adhesion strength and deposit tensile strength (Fig. 5c).
a) Debonding (S adhesion is the weakest link) Stensile > S jet > Sadhesion
b) Brittle Fracture (Stensile is the weakest link) Sadhesion > S jet > Stensile
c) Jet fails to remove the deposit Both Sadhesion and Stensile > S jet Fig. 5. Three scenarios of deposit removal attempt by a sootblower The deposit removal criterion can be written as follows: S jet > S d
(1)
where S d
= either S adhesion or S tensile, whichever is least.
As expressed in Equation 1 and conceptually shown in Fig. 5, the success of deposit removal process depends on three key parameters. They are the stress imposed by the jet on the deposit ( S jet), the strength of deposit adhesion to the tube (S adhesion), and the deposit tensile strength (Stensile). The magnitude of these parameters determines whether or not the deposit accumulation in the boiler tubes can be controlled by the sootblowers. Out of these three parameters, only S jet depends on the sootblower performance, while the magnitudes of both Sadhesion and Stensile are controlled by factors outside the sootblower equipment. Sadhesion and Stensile are influenced by the design of the boiler, operating conditions, and the liquor chemistry. Detailed and more elaborated discussion can be found in references (2, 3)
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In order to achieve high recovery boiler thermal efficiency and prevent unscheduled interruption due to fouling or plugging, the recovery boiler system should be designed to ensure that the rate of deposit removal by sootblowers is greater or at least equal to the rate of deposit accumulation.
Major compo nents Sootblower major components can be divided into three parts: steam carrying, mechanical, and auxiliary components. Figures 6 and 7 show the schematic diagram of sootblower major components. Rest position
Reverse position
Rear hanger
Sootblower Beam
Boiler wall
Wall box
Gear rack
Poppet valve
Lance tube
Feed tube Supplied steam
Nozzles Front Roller
Carriage
Front View
Lance tube
Front Roller
Fig. 6. Sootblower major mechanical components (side view)
Poppet valve
Carriage
Lance tube
Front Roller
Wallbox
Fig. 7. Sootblower major mechanical components (top view)
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Steam carrying components
As the steam enters a sootblower, it is directed through four components in the following order: • • • •
Poppet valve Feed tube Lance tube Nozzles
Poppet valve
The poppet valve serves two purposes: (a) to open and shut the supply steam to the sootblower, and (b) to adjust the blowing pressure of the sootblower. Note that in the effort to reduce the sootblower operating cost, many pulp mills have stopped using the poppet valve as a mean to adjust the blowing pressure. Centralized control valve is used instead to adjust the flow rate (and thereby the blowing pressure), of the sootblowers. For the discussion on how the sootblower operating cost may be reduced by means of adjusting the steam flow rate through centralized control valve, please see TAPPI TIP on Recovery Boiler Sootblowers: Practical Guideline. A schematic diagram of a poppet valve is shown in Fig. 8. The valve stem, valve plug, and pressure control disk are the moving parts of the poppet valve. The valve seat and the body are the stationary parts. To open the valve, the valve stem and plug have to be pushed downward against the incoming steam flow. The blowing pressure is adjusted by means of a pressure adjustment nut, which moves the pressure control disk up or down. The pressure control disk serves as a flow restrictor. The farther it is moved downward, toward the valve seat, the higher the pressure drop, resulting in a lower sootblowing pressure.
Pressure adjustment nut Valve bod
Valve stem
Steam outlet to the feed tube
Pressure control disk
Valve seat Valve plug
Supplied steam
Fig. 8. Schematic diagram of a poppet valve (externally adjusted)
When a signal to initiate an operation from the control room is received by a sootblower, the motor is energized and the carriage rotates and moves the lance tube forward into the boiler. At a certain distance during the insertion process, the carriage engages the poppet valve latch and rotates the lever arm assembly. As the lever arm assembly rotates, it pushes the poppet valve stem and plug down, allowing the steam to enter the sootblower. Fig. 9 illustrates the opening mechanism of the poppet valve. Note that there are many different designs of lever arm assembly; however, the mechanism to open the v alve is practically the same. During the retraction process, just before the sootblower reaches the rest or inactive state, the carriage releases the poppet valve latch, which in turn pulls the lever arm back to the rest position.
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There are three different poppet valve designs:
Poppet valve latch
Lever arm assembly
Fig. 9. Poppet valve opening mechanism 1. Internally adjusted. This design requires the blowing pressure to be adjusted in an iterative fashion. First, the sootblower has to be put out of service before any pressure adjustment is made. Once the sootblower is out of service, the operator would need to remove a small lock pin plug, located at the side of the valve body, and rotate the pressure control disk manually to the desired position. The sootblower is then put back in service, and the pressure setting is verified. Should the blowing pressure is too high or too low, the operator will have to follow the same procedure, discussed above, until the desired pressure is achieved. 2. Externally adjusted . Unlike internally adjusted valves, an externally adjusted valve is equipped with a pressure adjustment nut located outside the valve, as shown in Fig. 8. The blowing pressure can be adjusted without the need to put the sootblower out of service. To adjust the blowing pressure, (i) install a pressure gauge on the poppet valve, (ii) run the sootblower, (iii) observe the pressure at the gauge, and (iv) rotate the pressure adjustment nut to the desired poppet valve pressure. 3. Low pressure poppet valve. This is the latest poppet valve design to date. It is mainly used in boilers that implement the low pressure sootblowing system ( 4). The valve is designed with a large valve seat and generally is not equipped with a pressure control disk. The blowing pressure is controlled by means of control valve or an orifice installed in the flange between the poppet valve and the supply steam piping. The externally adjusted valve is the preferred design versus an internally adjusted valve, since it significantly reduces the time required to adjust the blowing pressure and it does not require the sootblower to be put out of service during the pressure adjustment process. Feed tube
The feed tube is a stationary tube connected to the poppet valve with the main function is to deliver the steam to the lance tube, refer to Fig. 6. The standard material for feed tube is stainless steel with an outside tube diameter (O.D.) of either 23/8 in. (60.3 mm) or 2¾ in. (70 mm). As the steam exits the feed tube and enters the lance tube, it pressurizes the gap between the feed and lance tubes. Packing housing, which is an integral part of the carriage (discussed in “Carriage and gear rack” section), is the main component that prevents steam from leaking to the back side of the lance tube. Feed tubes are available in three different types of finishing: 1. Hardened surface finishing. This is surface treatment is applied to the outer part of the feed tube to make the surface hard and scratch-resistant. A scratched feed tube promotes rapid deterioration of the packing performance, and shortens its service life.
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2 . Insulated . The main purpose of insulating the feed tube is to improve the service life of the packing by protecting it from the high steam temperature. The insulation material is usually installed on the inner part of the feed tube. This insulation is needed particularly for sootblowers with conventional packing made out of materials that are susceptible to high temperature environments. The insulation, however, makes the inner diameter of the feed tube smaller, resulting in a higher pressure loss. 3. Non-insulated and non hardened. In this case, the feed tube is installed with a bare stainless steel tube material (typically SS304). Maintenance tip: It is important that the carriage and the gear rack are aligned properly. The misalignment between the carriage and gear rack causes the feed tube to bend and scratch, which in turn, shorten the packing life. Lance tube
The lance tube is the main component that supplies the sootblower nozzles with high pressure steam and directs the jets toward the boiler tubes. During the cleaning process, the lance extends into the boiler and forms a structure similar to a cantilevered beam. Hence, the lance has to be designed to have sufficient strength to support its own weight in a high temperature environment. To avoid the overheating of the lance tube during the operation, the steam, which also acts as a cooling medium, needs to be supplied continuously to the lance. The minimum amount of the steam required to prevent the overheating of the lance is called minimum cooling flow. The minimum cooling flow of a lance tube depends on the material, the length of the lance tube, the steam and flue gas temperatures. The majority of sootblowers installed in recovery boilers are equipped with lance tubes having an outside diameter (O.D) of 3½ in. (88.9 mm). However, there is an increasing trend to replace them with a larger size tube, 4 in. (101.6 mm) OD. The larger lance tube will reduce the pressure drop and increase the nozzle efficiency, thereby, improve the jet cleaning power. Maintenance tip. Lance tubes are typically made of Cr-Mo alloy steels, such as T11, which has good material strength to support its weight at elevated temperatures while the lance is inside the boiler. However, it may be susceptible to corrosion if the flue gas from the boiler is allowed to enter the lance tube through the nozzles. To prevent lance tube corrosion, it is important that recovery boiler sootblowers are equipped with a scavenging air system. The scavenging system continuously supplies pressurized air into the feed and lance tubes, while the sootblower is inactive state, to create a barrier that prevents the flue gas from entering the lance tube. It is important that the scavenging air system be properly maintained and the air pressure is set to at least 10 in.WC (or a flow rate of 27 SCFM per sootblower). Nozzle
The main function of a sootblower nozzle is to convert the high pressure steam inside the lance tube into a highvelocity jet. An ideal nozzle is defined as a nozzle that fully expands the blowing medium from the pressure inside the lance tube to the outside ambient pressure; thereby, converting the lance pressure completely into velocity (100% efficiency). In order to fully expand the pressure inside the lance and accelerate the steam to a supersonic velocity, a convergentdivergent type of nozzle is used (Fig. 10). In the convergent section, the steam is accelerated to a speed of sound. The divergent section then accelerates it further to a supersonic velocity. A 100% nozzle efficiency (ideal nozzle), however, can only be achieved in a laboratory or a wind tunnel settings where both the inlet pressure to the nozzle and the ambient conditions are carefully set and maintained according to the design of the nozzle. In practice, it is impossible to achieve 100% nozzle efficiency; however, research in the last several years has advanced our understanding of sootblower jet dynamics, and as a result, it is made possible to design a sootblower nozzle with efficiency in the range of 90%, which is a significant improvement to its conventional nozzle counterparts.
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Steam inlet Superson ic Jet
conver ent
diver ent
Fig. 10. Convergent-divergent nozzle.
Mechanical and auxiliary components Carriage and gear rack
The carriage consists of one or two electric motor(s), a gearbox and a packing housing. The electric motor is the main drive that moves the lance tube forward and backward during the cleaning cycle. The motor converts electrical energy into rotation motion, which is then used by the gearbox to rotate and move the lance tube along the gear rack. Since the lance moves together with the carriage, while the feed tube stays stationary, the carriage has to be equipped with a packing set to prevent the steam from leaking through the gap between the feed and lance tubes. The packing set is located inside the packing housing, which is an integral part of the carriage. Maintenance TIP: It is important that the gear rack is sufficiently lubricated and properly aligned with the carriage travel path. Misalignment of the rack may damage the carriage. Carriage and gear rack should be lubricated only with non-greasy type of lubricant. This is to prevent dust, abrasive or corrosive materials to accumulate in the rack. Limit switch
When the lance tube has been fully inserted inside the boiler, a signal to change the motor direction and retract the lance is sent to the carriage by means of limit switch. The limit switch is also used to inform the carriage when it should stop retracting and park the sootblower to the rest position. A limit switch is an electromechanical device that sends a signal transmission to the carriage when its mechanical leg is physically pushed by a lever arm (Figure 4.6) There are two possible limit switch arrangements: 1. Two limit switches with one lever arm. This arrangement requires two limit switches: one is mounted in the rest position and the other is mounted in the reverse position. The lever arm, which is used to trigger the switches, is attached to the carriage and travels together with it (Fig. 11). 2. One limit switch with two lever arms. In this arrangement, the limit switch is attached to the carriage while the lever arms are stationary mounted in the rest and reverse positions (Fig. 12) Front roller
The main function of the front roller is to support and direct the lance tube to the centerline of the wallbox as it is inserted into or retracted from the boiler (Fig. 13) Maintenance TIP. To minimize friction in the front rollers, a sufficient amount of lubricant must be constantly maintained. Refer to the sootblower manufacturer’s manual on how to properly lubricate the front rollers and use
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only the lubricant that can withstand the hot and hostile environments in recovery boilers. δRoller should also be set according to the supplier’s recommendation to prevent excessive friction between the rollers and the lance tube. Rest position
Reverse position
Carriage is in retraction process. Signal sent to stop the carriage
Carriage is in insertion process. Signal sent to retract the lance
Lever arm Switch mechanical leg Limit Switch
Fig. 11. Two limit switches and one lever arm Reverse position
Rest position
Carriage is in retraction process. Signal sent to stop the carriage
Carriage is in insertion process. Signal sent to retract the lance
Limit Switch Switch mechanical leg Lever arm
Fig. 12. One limit switch with two lever arm
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δ
Fig. 13. Front roller set at an angle
δRoller
Wallbox
The main function of a wallbox is to prevent the hot flue gas, fume and carryover particles from escaping the boiler through the openings designed for sootblower operation. This can be achieved by continuously supplying the wallbox with pressurized air (seal air); hence creating an air wall that prevents the flue gas from leaking through the lance/wall sleeve gap. The seal air is important especially for positive pressure boiler (a boiler which has pressure greater than the atmospheric pressure). The wallbox is also used to direct a small amount of jet steam into the lance and wall sleeve. This jet is used to clean both the lance tube and the wall sleeve during sootblower operation. It is important that the steam supplied to this wallbox is free from excessive condensate. Plugging of the wall sleeve may occur if the condensate is mixed with the fume in the wall sleeve or on the surface of the lance tube to form a hard-to-remove material. Sootblower beam
The sootblower beam provides rigid support and protection for all sootblower components. Since some of the components require regular maintenance or may need repair work, the body of the beam should be designed to allow easy access to all of the components. The sootblower beam is generally installed with a negative blower slope. The purpose of the slope is to force the condensate, from the residual steam, to flow toward the nozzles and exits the lance via small holes located near the tip of the lance. The condensate in the feed and lance tubes not only can create a corrosive environment, but may also adversely impact the boiler tube during the cleaning process. Fig. 14 illustrates a negative slope with an exaggeration to better clarify the concept. Note that the wallbox may move downward due to boiler thermal expansion. Hence, the setup of the housing in a cold condition should account for boiler thermal expansion so that proper blower slope can be achieved when the sootblower is operated in a hot condition. Typically, the blower slope is set to around -0.4o for proper drainage of the condensate. Boiler on cold condition Blower slope
Thermal expansion causes the wallbox moves downward
Boiler on hot condition
Fig. 14. Blower slope
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Sootblower control system
There has been significant development in recovery boiler control hardware in the past seventy years. The three phases of development are pneumatic control, electronic control, and microprocessor-based control. Distributed Control System (DCS) is now the most widely used control system for recovery boilers and other pulp mill processes. In a DCS, the controller elements are not central in location, but they are distributed throughout the system. A DCS may comprise a hierarchy of controllers from single-loop to multi-loop and interactive microprocessor-based controllers. Sootblower operation is usually controlled through the DCS, as part of the package originally supplied to pulp mills along with other recovery boiler control units. A basic control system will allow the operator to set the sootblowing sequence, which determines how often a particular sootblower will be run. Sootblowers can be run either one at a time, two, or even six at a time with a delay time between sootblower operations. The delay time is incorporated into the control system to prevent collision between two sootblower operating at the same level, but opposite to each other (one in the left and the other is in the right side of the boiler). In the past several years, the advancement in sootblowing strategies has enabled many mills to achieve better cleaning performance and reduce the steam consumption. Most strategies, however, can only be implemented by means of advanced control system. If the strategy to be implemented is not part of the original package supplied with the recovery boiler DCS system, the mill will h ave an option to incorporate the algorithm to either the existing DCS-based sootblowing control system or Programmable Logic Controller (PLC) program. Since the upfront cost to re-program the existing DCS system is generally high, many mills prefer to implement the strategies through a PLC. The PLC can then be integrated into the existing DCS system. In this case, even though the algorithm is written in and control by the PLC, the operator can still interact and monitor the sootblowing activity through their existing DCS.
Literature cited 1. Kaliazine, A., Cormack, D.E., Ebrahimi-Sabet, A., Tran, H.N., “Mechanics of Deposit Removal in Kraft Recovery Boilers”. 1998 International Chemical Recovery Conference Proceedings, Tampa, FL, USA (ICR98641). 2. Mao, X.S., Lee, S., Tran, H.N. “Effects of Carryover Liquid Content and Particle Size on Deposit Removability in Kraft Recovery Boilers,” 2007 International Chemical Recovery Conference Proceedings, Quebec City, QC, Canada 3. Tran, H.N., “Fouling of Tube Surfaces in Kraft Recovery Boilers,” 40th Anniversary International Recovery Boiler Conference, Porvoo, 2004 4. Tran, H.N., Tandra, D.S., Jones, A.K., “Development of Low-Pressure Sootblowing Technology,” 2007 International Chemical Recovery Conference Proceedings, Quebec City, QC, Canada
Keywords Soot blowers, Recovery boilers
Ad di ti on al inf or mat io n Effective date of issue: April 27, 2009 Working Group: Alarick Tavares, Chairman, Georgia Pacific Danny S. Tandra, Clyde Bergemann, Inc. Honghi Tran, University of Toronto Andrew K. Jones, International Paper g
