Blast Furnace Valves
Blast Furnace Valves
Blast furnace (BF) process needs large quantity and variety of valves. During ironmaking in the BF, a number of gases, utilities, and dust are to be handled. These include (i) combustion gas, combustion air, and waste flue gas in hot blast stoves, (ii) cold air blast, oxygen, and hot air blast, (iii) blast furnace top gas and its cleaning, (iv) solid materials and (v) cooling water. The handling and the control of these gases and fluids necessitate use of different types of valves which include gate valves, goggle valves, butterfly valves, chimney valves, crude gas bleeder valves, and equalizing / relief valves etc. Fig 1 shows schematics of a typical cold and hot air blast systems showing some of the important valves used in the BF process.
Fig 1 Schematics of a typical cold and hot air blast systems
Many of these valves are specialized valves and bleeders designed especially for different applications in the BF process. Some of these valves operate under the most severe operating conditions. Also, some valves are being used for high temperature application and hence they are to be water cooped and / or refractory lined. In some of the valves, critical components are water cooled. For the valves to meet and exceed the most severe operating conditions to be expected in BF operation, the valves are to be specially designed. Normally these valves are designed for long life, high reliability and ease of maintenance.
Various process functions are carried out through these valves. These functions include, control and regulation functions (control of flow and pressure etc.), sealing functions, isolation functions, and safety functions etc. Several of these valves are part of BF automation system. Many of these valves carry out normal functions which are expected from the valves. Reliable functioning of some of the valves is very critical for the stable and efficient operation of the BF. Some of the important valves whose reliable functioning is critical for BF operations are described below.
Hot blast valve
Hot blast valve is very critical equipment for the BF operation. This valve is intended for complete separation of a hot blast stove from a hot blast main under ‘on-gas’ operation of the stove. This valve is also effectively being used as a shut-off valve and back drafting valve. This valve when used for separation of a gas burner from the hot blast stove under ‘on-blast’ operation of the stove is known as burner valve or gas shut off valve.
Hot blast and shut-off valves are installed vertically in a pipeline. Hot blast valves are normally installed in the horizontal hot blast main near the stove. The pipeline is opened and closed by the movement of the valve disk with the use of electro-mechanical drive. In event of power failure, it is possible to operate the drive manually.
Hot blast valve normally consists of a water cooled disc resting on a water cooled seat, and moving vertically within a water cooled valve body and bonnet, and other supporting members and mechanisms which maintains a smooth, exact seating and non-stick release of the disc. Normally the valve is water-cooled as well as the stem, but in some design of the hot-blast valve this is not done, the valves are made solid without water-cooling. The reason for this practice is that if water-cooled valves leak they can discharge a great quantity of water into the hot blast which gets absorbed at once without showing the least sign, but the effect of this moisture is highly detrimental to the furnace. However, presently most of the valve producers have water cooled valve design and construction.
The electro-mechanical actuator is designed for heavy duty equipment and is motor protected by a built-in torque device and externally mounted limit switches. It has motor overload relays for safety and a hand or chain wheel and de-clutch lever for manual operation in case of power failures.
This valve is to pass and shut off a hot blast of 1,000 deg C to 1,400deg C, and hence has a refractory lining to the interior of its body. All the elements of the valve exposed to heat except disc contact surfaces and housing thrust rings are protected with refractory lining. Reinforced alumina castables are normally used for the refractory lining.
Housing cooling system allows effective cooling of the thrust rings, the most heat exposed elements of the valve, as well as attachment flanges and housing neck zone and prove normal working conditions for the flange packings. The valve disc has a solid shell and spiral partitions which form disc cooling system. Also, the heat exposed elements of the valve have minimum quantity of welds in the zone of high temperatures in order to provide high operate reliability of the valve.
The gland seal of the pipes which supply and discharge cooling water to the valve disk is produced from ecological clean graphite and this provides advantages under service condition of the valve. Cooling water velocity both in the housing and in the disk is normally higher than 1 m/sec which ensures removal of mechanical suspensions and eliminates the danger of cooling pipes blockage. Fig 2 shows sectional views of a typical hot blast valve.
Fig 2 Sectional views of a typical hot blast valve
Burner valve or gas shut off valve
This valve is used for separation of a gas burner from the hot blast stove under ‘on-blast’ operation of the stove. It facilitates fast and safe changing of stove from ‘gas’ to ‘blast’. It is normally a water cooled valve which has replaced the conventional burner door on many blast furnace stoves. The valve opens or closes quite fast (in around 10 seconds) to complete the stove changing cycle, and incorporates electro-mechanical actuator. The valve has normally steel fabricated construction, is self-supporting, and needs small space.
Burner valve consists of a water-cooled disc, moving vertically within a water cooled valve body and bonnet, and other supporting members and mechanisms which maintain a smooth, exact seating and non-stick release of the disc. The disc is flange-mounted to a hollow ground stainless steel stem which is attached to a manifold at the other end. The top of the stem is also connected by a lever and link system, complete with bearings, to the output shaft of the actuator. The actuator is designed for heavy-duty equipment and is motor protected by a built-in torque device and externally mounted limit switches. It has motor overload relays for safety, and a hand or chain wheel and de-clutch lever for manual operation in case of a power failure. Pneumatic or hydraulic drive can also be used.
The disc and the seats of the valve body are water cooled. Particular attention is paid to the accurate and positive seating of the disc. Both the disc and valve body have welded stainless steel seats, polished ground for positive shut-off. An exclusive position seating is accomplished through externally adjustable eccentrics, located at the centre line of the valve body. In closing, wedges on the disc make contact with the eccentrics at the extended point of disc travel, positioning the disc on the body seat. Stove pressure is utilized for additional seating pressure.
A reverse action occurs for the opening cycle. At no time, the disc is wiping the body seat, assuring extended seat life. A silicone rubber seal is incorporated in the disc for an absolute tight shut-off when the valve is used. Burner valve provides an extra large clean-out arrangement. This collecting area helps isolate the deposited dirt and moisture entrainment from the BF gas, away from the valve seating surfaces which is a major cause of seat deterioration or improper seating.
Cold blast valve
Cold blast valves are installed at the hot blast stove end of the hot blast main. These valves are normally not lined since the temperature of the cold blast is generally in the range of 150 deg C to 250 deg C, which is the temperature resulting from the heat of compression at the blower.
Cold blast valve is intended for the complete separation of the blast furnace stove from the cold blast main. It is installed on the horizontal cold blast main near the stove. The cold blast valve is the type which is held closed by the pressure of the cold blast main.
A standard design for this valve has gradually been evolved, and while the individual valves used by different stove builders differ in details, the general principle of all is the same. They are all flat slide valves designed to withstand pressure from one direction only, since this is all that is necessary for this service. Fig 3 shows cold blast valve designs.
Fig 3 Cold blast valve designs
There is one point which is important in the case of cold blast valve. When the valve is to be opened to put the stove on blast the unbalanced pressure on it is very great, and in such condition if the valve is opened then in spite of the friction produced by the pressure, the rush of air into the stove can be so fast that it can cause damage to stove refractory checker work. Hence, it is necessary that before the valve can be opened, the small ports in the valve disc are to be opened to pressurize the stove and equalize the pressure on each side of the valve.
The equalization of pressure in some design of cold blast valve is carried out in a different manner (Fig 3). The rack on which the pinion works to operate the valve is not rigidly attached to the latter, but is free to slide between the guides within the limits set by the stops, these come against the guides and through them move the valve proper. The end of the rack is widened out to a ‘T’ shape and this covers a slot through the main valve. With this arrangement, the first movement of the pinion slides the rack proper on the back of the main valve and thus uncovers the port in the latter. Through this, the air flows until in around a minute, the pressure within the stove is the same as that in the blast main. This relieves the friction on the valve, which can then be moved back into the valve housing without further difficulty.
Snort valve is a butterfly valve which installed in the cold blast line main before the stove normally located near the blast furnace. It is used for reducing or completely stopping the air blast to the BF without stopping the operation of the blower.
The valve is opened when it is necessary to decrease the blast pressure rapidly. This discharges the cold blast air to the atmosphere and keeps a positive pressure on the cold blast line so that the gas from the furnace cannot travel back to the blower. Because of rapid discharge of the air when the valve is opened, it is to be equipped with a muffler to control the noise level. The excess air is blown away through a blow off device which is mechanically interlinked with the main valve for proportionate opening / closing. Fig 4 shows sections of a typical snort valve.
Fig 4 Sections of a typical snort valve
Bleeder valves are also referred to as pressure relief valves or explosion prevention valves. They are normally installed in bleeder pipe of furnace top pressure control systems. These valves are needed for controlling high top pressure at the BF top. These valves provide nearly constant pressure during progressive flap opening against springs and there is no pressure ’overshoot’. The valves are simple, and efficient with little maintenance. They have high reliability and availability.
Bleeder valve comprises a valve seat associated with the exhaust conduit, a movable closure member having a central closure surface, and a peripheral sealing surface matching with the valve seat, and the closure surface comprising a convex surface at least in proximity to the sealing surface. The valve further has an actuating mechanism, which is connected to the closure member for moving the closure member between a closed position on the valve seat and an open position distant from the valve seat.
The valve is of fabricated construction with valve actuation by hydraulic cylinder. The valve seat is made of a combination of wear resistant metal and heat resistant rubber which provides complete sealing.
Bleeder valves provide safety from BF over-pressure. They protect the top of the furnace from sudden gas pressure surges. Typical implementation involves installation of two to three raw gas bleeder valves at the top of the blast furnace uptakes and one on the semi-clean gas-side of the first scrubbing stage. Fig 5 shows a typical bleeder valve.
Fig 5 A typical bleeder valve
Sealing valves are the part of the bell-less top charging system. They play an important role during the charging of the raw materials in the BF. These valves are meant for sealing the bin from the BF gas leakage. They are mainly used for equalizing pressure and releasing gas of the BF charging system. These valves are located one at top of the bin and the other at the bottom of the bin.
The sealing valve consists of valve body, valve seat, a flap, and an actuating cylinder. Flap closes against the seat during closing. The surfacing alloy on the sealing surface provides it with good wear resistance. The seat has a silicone rubber ring. While the metal sealing of the valve is hard sealing, the provision of rubber ring makes it soft sealing. Once the valve is closed, it does not allow any leakage through the valve.
The sealing valve is hydraulically driven. The hydraulic cylinder drives the valve flap through curved arm to control the opening of the sealing valve. It can also be operated through remote automatic control through output of signal by stroke switch. The normal position of the sealing valve is the closed position. Fig 6 shows the sealing valve.
Fig 6 Sealing valve
It is intended to separate the BF stove from the chimney. In the plenum chamber of the stove below the grid which supports the checkers are openings to the chimney and to the cold blast main. In many of the modern stoves, there are two chimney valves, which are opened when the stove is being heated so that the product of combustion is drawn out to the stove stack. When the stove is on blast (heating the blast air), the chimney valve is closed. The seats of the valve are so arranged that when the stove is on blast, the pressure in the stove holds the seat together to prevent leakage. Fig 7 shows a chimney valve.
Fig 7 Chimney valve
Blow off valve
There is also an air blow-off valve provided in the stove, the function of which is to blow off air left in the stove when it is taken off the furnace and put on heating. Blow-off valve is to be opened to relieve the pressure. Because of the need to de-pressurize the stove rapidly, the air is to exit at a very high velocity. Hence, the blow off valve is to be equipped with mufflers for keeping the noise level within tolerable limits. The blow-off valve is placed close to the bottom and is sometimes arranged to open quickly with a lever instead of with a screw, so that the air is blown out fast. The violent air blows out through the valve and carries as much dirt as it can from the passages around the base of the stove. The typical construction of this valve is shown in Fig 8. The elbow in which the valve is mounted is arranged to swing as a whole so that this opening serves as a cleaning door as well as a blow-off.
The blow off valve is also located in the bustle pipe of a BF to blow off counter-flow gas to the atmosphere from the BF when the furnace is at a stop. The body and the disc are both made of cast steel for high temperature and constructed so as to be water-cooled, because the BF gas of a high temperature flows through the valve. The body is refractory-lined in its upstream side interior to protect it from a high temperature.