JP7358952B2 - A trolley equipped with a lifting mechanism and a method for attaching and detaching a quantitative feeder using the same - Google Patents

Description

The present invention relates to a trolley equipped with a lifting mechanism used when inspecting and replacing a quantitative feeder attached to the bottom of a silo, and a method for attaching and detaching a quantitative feeder using the same.

A Circulating Fluidized Bed boiler, as described in Patent Document 1, uses a cyclone to collect solid fuel such as particulate fluidized sand and coal that has flowed out from the upper part of a furnace (combustor) together with combustion exhaust gas. It has a structure in which the gas is returned to the furnace again, and the gas velocity (superficial velocity) in the furnace can be increased compared to a fluidized bed boiler. This makes it possible to more actively mix the combustion air with combustion air, thereby improving combustion efficiency.

In addition, while the combustion temperature in the furnace of a typical boiler is 1,400 to 1,500 degrees Celsius, a circulating fluidized bed boiler can suppress the combustion temperature to around 800 to 900 degrees Celsius. It is possible to suppress the generation of thermal _NOx caused by high-temperature oxidation of air. Furthermore, in the circulating fluidized bed boiler, by supplying limestone powder (calcium carbonate (CaCO ₃ )) as a desulfurization agent into the furnace, the desulfurization reaction shown in the following reaction formula 1 can be caused in the furnace. A flue gas desulfurization device for reducing the SOx concentration in the combustion exhaust gas discharged from the circulating fluidized bed boiler becomes unnecessary, or a simplified desulfurization device can be used.
[Reaction formula 1]
CaCO ₃ +SO ₂ +1/2O ₂ →CaSO ₄ +CO ₂

As a method of supplying the above-mentioned limestone powder to the furnace of a circulating fluidized bed boiler, for example, as described in Patent Document 2, limestone powder in a flexible container bag is first put into a hopper, and then stored under pressure from the hopper. Transfer the limestone powder to a silo for use. A common method is to supply a fixed amount from the bottom of the silo into the furnace using a fixed amount feeder.

Japanese Patent Application Publication No. 6-281108 JP 2019-105399 Publication

In normal operation of a circulating fluidized bed boiler, the supply amount of limestone powder is controlled to be a constant ratio to the supply amount of solid fuel such as coal, and the supply amount of limestone powder is controlled according to the SOx concentration in the exhaust gas. It is done to control. Therefore, the quantitative feeder for limestone powder is required to supply limestone powder quantitatively over a long period of time. However, volumetric quantitative feeders, such as rotary feeders, table feeders, screw feeders, etc., have a rotating part such as a rotor that quantitatively cuts out powder and granules, a housing part that houses the rotor, and a housing part that drives the rotating part. Since it is composed of a driving part such as a motor, the part that comes into contact with the powder may wear out during continued operation, making it difficult to supply it quantitatively (also called cutting). Therefore, it is generally necessary to replace internal parts such as the above-mentioned rotating parts about once every six months, and to periodically inspect and maintain them.

Since the quantitative feeder is attached to the bottom of the silo as described above, it is extremely difficult to replace only the internal parts during the above-mentioned replacement or maintenance. Therefore, usually after removing the quantitative feeder from the silo in its assembled state, a separately prepared new or refurbished spare quantitative feeder in the assembled state is installed, and repairs such as overlaying are performed on the removed quantitative feeder. things are being done. In this way, when replacing or servicing the metered feeder attached to the bottom of the silo, it is necessary to remove and replace the metered feeder in an assembled state with a spare metered feeder.

Conventionally, when attaching and detaching the quantitative feeder attached to the bottom of the silo in the assembled state as described above, it was necessary to suspend the quantitative feeder using a chain block or the like. At that time, as mentioned above, the quantitative feeder was attached to the bottom of the silo, so it could not be hung directly below, and it was necessary to work while balancing it from two locations diagonally above. Therefore, attaching and detaching the conventional quantitative feeder requires a lot of effort and time.

In addition, when replacing the quantitative feeder for limestone, it is necessary to stop the limestone powder supply equipment, and the longer this stop period is, the higher the SOx concentration in the flue gas discharged from the circulating fluidized bed boiler. There is a risk that the boiler may need to be shut down in some cases. On the other hand, when replacing a metered feeder for solid fuel, it is necessary to stop the solid fuel supply equipment and also stop the operation of the circulating fluidized bed boiler itself. It is desirable to reduce downtime.

The present invention was made in view of the conventional situation, and is a method for quantifying powder and granular materials attached to the bottom of a silo, such as limestone powder and solid fuel supply equipment included in a circulating fluidized bed boiler. It is possible to attach and detach the feeder easily and in a short time, thereby avoiding the suspension of operation of the main equipment to which the powder and granular materials are supplied, such as the circulating fluidized bed boiler, or stopping the operation of the main equipment. The purpose of the present invention is to provide a cart having a lifting mechanism (also referred to as a lifter) used when replacing a quantitative feeder, and a method for attaching and detaching a quantitative feeder using the same, which can shorten downtime when replacing a quantitative feeder. There is.

In order to achieve the above-mentioned object, a trolley having a lifting mechanism according to the present invention is a trolley for replacing a quantitative feeder attached to the bottom of a silo, and maintains an attitude in which the quantitative feeder can be attached to the silo. The support part includes a lifting table that is rectangular in plan view, a pantograph mechanism as the lifting mechanism that lifts and lowers the lifting table, and a pantograph mechanism that lifts and lowers the lifting table. A pair of rectangular gate-shaped structures are provided on the top side of the table, and the pair of structures are on the opposite side of the flange surface of the flange for connection with the silo in the quantitative feeder. It is characterized by being spaced apart from each other so as to support only both ends .

Further, the method for attaching and detaching a quantitative feeder according to the present invention is a method for attaching and detaching a quantitative feeder located at the bottom of a silo using a cart having a support section equipped with an elevating mechanism, the support section having a rectangular shape in plan view. It is composed of an elevating table, a pantograph mechanism as the elevating mechanism for elevating the elevating table, and a pair of rectangular gate-shaped structures provided on the upper surface side of the elevating table. , are spaced apart from each other so as to support only both ends of the surface opposite to the flange surface of the flange for connection with the silo in the quantitative feeder, and when the lifting mechanism is lowered, the A trolley is placed, the elevating mechanism is raised below the silo, and the quantitative feeder is supported by the pair of structures while the removal or installation work is performed, and after the work is completed, the elevating mechanism is lowered. The cart is moved from below the silo.

According to the present invention, the quantitative feeder can be replaced easily and in a short time without using large-scale equipment, and thereby the granular material cut out by the quantitative feeder can be supplied to a circulating fluidized bed boiler, etc. It becomes possible to avoid stopping the main equipment, or to significantly shorten the downtime when the main equipment is stopped.

FIG. 2 is a schematic flow diagram of a circulating fluidized bed boiler equipped with solid fuel and desulfurizing agent supply equipment that allows suitable use of a trolley equipped with an elevating mechanism according to the present invention during attachment and detachment work. FIG. 2 is a schematic flow diagram of the solid fuel and desulfurization agent supply equipment shown in FIG. 1. FIG. 3 is a schematic side view (a) and a schematic front view (b) of a table feeder included in each supply facility in FIG. 2. FIG. 1 is a side view (a) and a front view (b) of a specific example of a trolley equipped with an elevating mechanism according to the present invention. FIG. 5 is a perspective view of a truck equipped with the lifting mechanism shown in FIG. 4; FIG. 5 is a side view (a) and a front view (b) showing a state before the cart equipped with the lifting mechanism shown in FIG. 4 is placed below the quantitative feeder. 5 is a side view (a) and a front view (b) illustrating a state in which the cart equipped with the lifting mechanism of FIG. 4 is placed below the quantitative feeder and before the lifting mechanism is raised. FIG. 5 is a side view (a) and a front view (b) showing a state in which the cart equipped with the elevating mechanism shown in FIG. 4 is raising the elevating mechanism and supporting a quantitative feeder at its upper end; FIG. FIG. 5 is a side view (a) and a front view (b) of the truck equipped with the lifting mechanism shown in FIG. 4 in a state where the lifting mechanism is lowered while supporting the quantitative feeder.

Hereinafter, a specific example of a truck equipped with the lifting mechanism of the present invention will be described in detail. First, with reference to FIG. 1, a circulating fluidized bed boiler equipped with a supply facility to which a truck equipped with an elevating mechanism according to an embodiment of the present invention is suitably applied during attachment/detachment work will be described. The circulating fluidized bed boiler (hereinafter also simply referred to as a boiler) shown in FIG. It mainly consists of a cyclone 2 that recovers solid content such as sand, and a heat recovery section 3 that recovers heat from the combustion exhaust gas.

In the combustor 1, solid fuel such as coal is fed into the lower part of the combustor 1 from a solid fuel supply facility 20, and a desulfurizing agent such as limestone powder for removing sulfur oxides from the combustion exhaust gas is fed from a desulfurizing agent feeding facility 30. be done. Fluidized sand such as silica sand is charged in advance into the combustor 1, and the solid fuel and desulfurization agent together with this fluidized sand are fluidized by air introduced from below the combustor 1, thereby combusting the solid fuel. progresses. Due to the radiant heat transfer of the combustion exhaust gas generated by this combustion, the boiler water flowing in the water tube group (not shown) is heated and partially evaporated, thereby generating saturated steam from the boiler drum 1a communicating with the water tube group.

The combustion exhaust gas whose heat has been recovered by the above-mentioned radiation heat transfer exits the combustor 1 as combustion exhaust gas and is introduced into the cyclone 2, where solids such as fluidized sand are removed, and then heat is recovered by convection heat transfer. The heat is introduced into the heat recovery section 3. The heat recovery section 3 is provided with a super heater 3a that heats the saturated steam discharged from the boiler drum 1a by heat exchange with combustion exhaust gas to generate superheated steam for private power generation. Further, a first economizer 3b preheats the boiler water supplied from the boiler water tank 4 via the deaerator 5 through the heat exchange, and a second economizer 3c preheats the air supplied from the blower 7 through the heat exchange. is provided. The superheated steam generated by the superheater 3a is introduced into the turbine generator 6 and used to rotate the turbine, and then is condensed and recovered into the boiler water tank 4.

The combustion exhaust gas whose heat has been recovered by the heat recovery section 3 is introduced into an electrostatic precipitator 8 to remove dust, and then is released into the atmosphere from a chimney 9. This chimney 9 is provided with an SOx concentration meter 9a, and the SOx concentration of the combustion exhaust gas flowing inside the chimney is measured. The SOx concentration value measured by this SOx concentration meter 9a is input to a control means 10 such as a DCS, where calculations are performed according to a predetermined algorithm, and based on the calculation results, the SOx concentration value is The rotation speed of the motor M that drives the rotating part of the quantitative feeder is controlled.

Next, the above solid fuel supply equipment 20 and desulfurization agent supply equipment 30 will be described with reference to FIG. 2, taking as an example a case where the solid fuel is coal and the desulfurization agent is limestone powder. The solid fuel supply equipment 20 that supplies granular coal to the combustor 1 of the circulating fluidized bed boiler described above includes a fuel hopper 21 that temporarily receives coal transported from a stockyard or the like by a belt conveyor, and A fuel silo 22 that receives and stores coal discharged from the bottom, and a quantitative fuel feeder 23 such as a rotary valve or table feeder that is provided at the bottom of the fuel silo 22 and cuts out the coal quantitatively. , and a fuel pneumatic transport piping system 24 that transports the coal cut from the fuel quantitative feeder 23 using compressed air as a transport medium.

Similarly, the desulfurizing agent supply equipment 30 that supplies limestone powder includes a desulfurizing agent hopper 31 that temporarily receives limestone powder in the form of granules delivered in a packaging such as a flexible container bag; A desulfurizing agent silo 32 that receives and stores limestone powder discharged from the bottom of the desulfurizing agent silo 32, and a desulfurizing agent such as a rotary valve and a table feeder installed at the bottom of the desulfurizing agent silo 32 to quantitatively cut out the limestone powder. The desulfurization agent quantitative feeder 33 is mainly composed of a desulfurization agent quantitative feeder 33 and a desulfurization agent pneumatic transportation piping system 34 that conveys the limestone powder cut out from the desulfurization agent quantitative feeder 33 using compressed air as a transportation medium.

In the quantitative feeder 23 for fuel and the quantitative feeder 33 for desulfurizing agent, the cutting amount is controlled by controlling the rotation speed using an inverter motor, for example. In addition, the above-mentioned pneumatic transport piping systems 24 and 34 are placed before and after the portions to which the quantitative feeders 23 and 33 are connected, respectively, in order to be cut off from the pneumatic transport piping systems 24 and 34 when replacing the quantitative feeders 23 and 33, which will be described later. Shutoff valves 24a, 24b and 25a, 25b are provided, respectively. Furthermore, the solid fuel supply equipment 20 and the desulfurization agent supply equipment 30 are equipped with a bypass piping system so that the circulating fluidized bed boiler can continue to operate without stopping when the quantitative feeders 23 and 33 are replaced, respectively. Preliminary supply facilities 25 and 35 each consisting of a simple hopper are provided.

Next, the schematic structure of the quantitative feeder 23 for fuel and the quantitative feeder 33 for desulfurizing agent will be described, taking as an example the case where the quantitative feeder is a table feeder. In addition, since these quantitative feeder 23 for fuel and quantitative feeder 33 for desulfurization agent have almost the same structure, the quantitative feeder 33 for desulfurization agent will be explained below.

As shown in FIG. 3, the desulfurizing agent quantitative feeder 33 includes a granular material supply section 41 consisting of a cylindrical housing and a rotating section that rotates inside the housing, and a powder supply section 41 that rotates around an axis that extends in the vertical direction. It mainly consists of a drive section 42 consisting of a motor and gears, and has a high center of gravity due to its structure, making it easy to fall over when removed and placed on a stand. The above-mentioned powder supply section 41 has an input port 41a that is located on the upper side and serves as a reception port for limestone powder falling from the desulfurizing agent silo 32, and limestone that is received into the cylindrical housing from the input port 41a. The powder passes through the discharge nozzle 41b when it is quantitatively cut out toward the desulfurizing agent pneumatic transportation piping system 34 by the rotation of the rotating part, and the internal pressure of the cylindrical housing is transferred to the inside of the desulfurizing agent silo 32. A pressure equalizing nozzle 41c to which a pressure equalizing line for equalizing the pressure is connected is provided.

By providing the pressure equalization line as described above, there is no pressure difference between the inside of the cylindrical housing of the table feeder and the inside of the desulfurizing agent hopper 31. It is possible to suppress the problem that limestone powder is difficult to be supplied to the cylindrical housing, or that limestone powder flows backward from the cylindrical housing toward the desulfurizing agent silo 32 and blows out within the silo.

Next, a specific example of a trolley equipped with the lifting mechanism of the present invention will be described with reference to FIGS. 4 and 5. A trolley 50 equipped with an elevating mechanism according to one embodiment of the present invention includes a substantially rectangular flat base portion 51 having four wheels 51a on the lower surface side and a handle portion 51b on the upper surface side; The support section 52 is located on the upper surface side of the table section 51 and supports the quantitative feeder 33 and is movable up and down. The structure of the support part 52 is not particularly limited as long as it can support and move the quantitative feeder 33 up and down while maintaining the attitude in which it is attached to the desulfurizing agent silo 32. A lifting table 53 having a substantially rectangular shape smaller than the base portion 51, a pantograph mechanism 54 for raising and lowering the lifting table 53 with respect to the base portion 51, and a pantograph mechanism 54 provided on the upper surface side of the lifting table 53. It is preferable that the structure is composed of a pair of rectangular gate-shaped structures 55.

The above-mentioned pair of structures 55 are arranged so that the quantitative feeder 33 can be supported on the side opposite to the flange surface of the connection flange with the desulfurizing agent silo 32 located at the upper end thereof. It is preferable that the lift table 53 be provided so as to face each other in the width direction of the lift table 53. Each structure 55 preferably includes a contact portion 55a that extends in the horizontal direction, and two leg portions 55b that respectively support both ends of the contact portion 55a from below.

The pair of structures 55 are spaced apart from each other so that their contact portions 55a can support only both ends of the flange F of the quantitative feeder without interfering with the drive unit 42. The distance W is preferably about 150 to 450 mm, more preferably about 300 mm. In addition, the pair of structures 55 have a height H of 200 to 200 mm so that the driving section 42 does not interfere with the lifting table 53 when the quantitative feeder 33 is supported by the abutting portions 55a of the structures 55. It is preferably about 600 mm, more preferably about 400 mm.

The pair of structures 55 described above may be reinforced by connecting the legs 55b facing each other in the width direction of the lifting table 53 with a reinforcing member such as a rod-shaped member extending horizontally or diagonally. . At this time, it is preferable that of the two legs 55b of each structure 55, only the above-mentioned handle portion 51b side is reinforced, and the side opposite to the handle portion 51b side is not reinforced. Thereby, even if the lifting table 53 is hardly lowered, the cart 50 can be moved directly below the quantitative feeder 33 without interfering with the drive part 42 located at the lower part of the quantitative feeder 33. Therefore, it becomes possible to complete the replacement work in a shorter time. In addition, since the reinforcing material is not present, the bolts and nuts connecting the quantitative feeder 33 and the desulfurizing agent silo 32, the discharge nozzle 41b, the pressure equalization nozzle 41c, etc. can be easily removed or attached. It becomes possible to do so.

Next, a method for attaching and detaching a quantitative feeder using the cart 50 equipped with the above-mentioned lifting mechanism will be described with reference to FIGS. 6 to 9. First, a case will be described in which the quantitative feeder 33 is removed from the desulfurizing agent silo 32. When the quantitative feeder 33 is removed from the desulfurization agent silo 32, the following steps 1 to 4 are performed in order. That is, in the first step, as shown in FIG. 6, with the pantograph mechanism 54 folded and contracted, that is, with the lifting table 53 lowered, the trolley 50 is moved in the direction of the white arrow to move the quantitative feeder 33. A pair of structures 55 are arranged directly below.

In the second step, as shown in FIG. 7, the pantograph mechanism 54 is extended in the direction of the white arrow to raise the elevating table 53. As a result, without interfering with the drive unit 42, both contact portions 55a of the pair of structures 55 are brought into contact with the surface of the flange F at the upper end of the quantitative feeder 33 on the opposite side to the flange surface, and the flange It becomes possible to support F. At that time, depending on the structure of the metering feeder 33, the bolts and nuts of the flange F for attaching to the desulfurizing agent silo 32 may interfere with the abutting part 55a, and the flange F may not be stably supported by the abutting part 55a. be. In this case, it is preferable to remove several interfering bolts and nuts before lifting the lifting table 53. Further, in order to improve workability, the bolts and nuts of the discharge nozzle 41b and the pressure equalization nozzle 41c of the quantitative feeder 33 may be removed before the lifting table 53 is raised.

In the third step, all the bolts and nuts of the flange F are removed while the quantitative feeder 33 is supported by a pair of structures 55, as shown in FIG. In the fourth step, the pantograph mechanism 54 is contracted in the direction shown by the white arrow in FIG. 8, and the elevating table 53 is lowered. Then, as shown by the white arrow in FIG. 9, the cart 50 is moved from below the desulfurizing agent silo 32.

When attaching the quantitative feeder 33 to the desulfurizing agent silo 32, the procedure is basically the reverse of the procedure for removing the quantitative feeder 33, as shown in the fifth to eighth steps below. That is, in the fifth step, the trolley 50 with the replacement quantitative feeder 33 supported by a pair of structures 55 is attached to the desulfurizing agent silo 32 with the lifting table 53 lowered. Place it directly below the position.

In the sixth step, by extending the pantograph mechanism 54 and raising the elevating table 53, the flange F at the upper end of the quantitative feeder 33 is connected to the mating flange provided at the bottom of the desulfurizing agent silo 32. The flange surfaces are brought into contact with each other via a gasket. In the seventh step, these flanges are fastened together with bolts and nuts. In the eighth step, the pantograph mechanism 54 is contracted to lower the elevating table 53.

Note that in the seventh step, bolts and nuts located in positions where it is difficult to fasten because they interfere with both abutting portions 55a of the pair of structures 55 are fastened after lowering the lifting table 53 in the eighth step. Good too. In addition, tightening of bolts and nuts in the discharge nozzle 41b and the pressure equalization nozzle 41c may be performed in the seventh step or may be performed in the eighth step or later in consideration of workability.

As explained above, quantitative feeders are generally difficult to handle because their center of gravity is located at a high position due to their structure, or they have a shape that makes them prone to falling over when placed on a flat table. However, as described above, the present invention By using a trolley equipped with a lifting mechanism, it is possible to perform attachment and detachment work while stably supporting the flange at the upper end of the quantitative feeder with the flange surface kept horizontal. In addition, simply by raising and lowering the lifting mechanism, the quantitative feeder to be attached and removed can be lifted and lowered in the vertical direction without tilting it in its attached state, so that replacement work can be easily performed with a small number of workers.

[Example]
A hand lifter (HLF-S250B) manufactured by Trusco Nakayama Co., Ltd. is prepared, and as shown in FIG. 4, the upper surface side of the lifting table 53 has a pair of rectangular gate-shaped structures made by processing equilateral angle iron. The body 55 was attached by welding to produce a trolley equipped with a lifting mechanism. At this time, the distance W separating the pair of structures 55 in the width direction of the lifting table 53 was set to 300 mm, and the height H of the pair of structures 55 was set to 400 mm.

By using a trolley equipped with a lifting mechanism created in this way, it is now possible to replace the metered feeder in about 2 hours instead of the 5 hours it used to take. Became. Additionally, the chain block and supporting legs for suspending the quantitative feeder are no longer required. In this way, by using a cart equipped with a lifting mechanism that meets the requirements of the present invention, it has become possible to replace the quantitative feeder easily and in a short time.

1 Combustor 1a Boiler drum 2 Cyclone 3 Heat recovery section 3a Super heater 3b First economizer 3c Second economizer 4 Boiler water tank 5 Deaerator 6 Turbine generator 7 Blower 8 Electrostatic precipitator 9 Chimney 9a SOx concentration meter 10 Control means 20 Solid fuel Supply equipment 21 Fuel hopper 22 Fuel silo 23 Fuel quantitative feeder 24 Fuel pneumatic transportation piping system 24a, 24b, 25a, 25b Shutoff valve 25 Fuel preliminary supply equipment 30 Desulfurization agent supply equipment 31 Desulfurization agent hopper 32 Desulfurization agent Silo 33 Quantitative feeder for desulfurization agent 34 Pneumatic transport piping system for desulfurization agent 35 Preliminary supply equipment for desulfurization agent 41 Powder supply section 41a Inlet 41b Discharge nozzle 41c Pressure equalization nozzle 42 Drive section 50 Cart 51 Base section 51a Wheel 51b Handle portion 52 Support portion 53 Elevating table 54 Pantograph mechanism 55 Structure 55a Contact portion 55b Leg portion

Claims (5)

This trolley is for replacing the quantitative feeder attached to the bottom of the silo, and has a support part equipped with a lifting mechanism that supports the quantitative feeder while maintaining the attitude in which it is attached to the silo. ,
The support section is composed of an elevating table that is rectangular in plan view, a pantograph mechanism as the elevating mechanism that raises and lowers the elevating table, and a pair of rectangular gate-shaped structures provided on the upper surface side of the elevating table. is,
A trolley characterized in that the pair of structures are spaced apart from each other so as to support only both ends of a surface opposite to a flange surface of a flange for connection with the silo in the quantitative feeder. The trolley according to claim 1, wherein the silo constitutes a supply facility for supplying solid fuel or desulfurization agent to a combustor of a circulating fluidized bed boiler. 2. A method for attaching and detaching a quantitative feeder located at the bottom of the silo using the cart according to claim 1, wherein the cart is arranged below the silo with the lifting mechanism lowered, and Then, the elevating mechanism is raised and the quantitative feeder is supported by the pair of structures while the removal or installation work is carried out. After the work is completed, the elevating mechanism is lowered and the trolley is lifted from below the silo. A method for attaching and detaching a quantitative feeder, characterized by moving it. When removing the quantitative feeder from the silo, a first step is to place the trolley below the quantitative feeder with the lifting mechanism lowered , and a step of raising the lifting mechanism to remove the pair of structures . a second step of supporting the connection flange at the upper end of the quantitative feeder; and a third step of removing bolts and nuts connecting the connection flange and the other flange while the support is maintained. 4. The quantitative feeder attachment/detachment method according to claim 3, further comprising a fourth step of lowering the elevating mechanism. When a quantitative feeder is attached to the silo, the trolley with the replacement quantitative feeder supported by the pair of structures is placed below the silo with the lifting mechanism lowered . a fifth step, a sixth step of raising the elevating mechanism to bring the connection flange located at the upper end of the quantitative feeder into contact with a mating flange; 4. The method for attaching and detaching a quantitative feeder according to claim 3, comprising a seventh step of fastening with bolts and nuts, and an eighth step of lowering the elevating mechanism.

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