Safety tuyere protects against refractory damage, runout.
The most critical cupola operating period--when slag/metal overflow is most likely to occur--is from the initial blast until iron is flowing over the dam. In that event, slag/metal spills through the safety tuyere into a sump, melts an aluminum plug or disc and empties into a hopper.
Overflow, however, can occur at any time and provisions must be made to keep the safety tuyere open, dry and adequately sized to handle the overflow.
As pictured in Fig. 1 on a nonwater-cooled cupola, one of the tuyeres functions as a safety tuyere. A through-the-wall refractory opening exits into a shielded sump fitted with a low-melting-point metal plug. The plug melts and empties the metal or slag through a spout into a proper receptacle.
The design of a safety tuyere for a water-cooled cupola is more involved as illustrated in Fig. 2-3. The placement of the refractory pipe and the ability to monitor and service the tuyere are critical.
A refractory pipe with an inside diameter of 4 in. and a 2-in. wall are minimum dimensions to service the average cupola. The tuyere refractory should have the same general composition as the front slagger and be dry for potential use at first tapout.
The horizontal channel of the safety spout in Fig. 2 is visible through a sight glass in a removable cover. The cover must be removed routinely to ensure the aluminum plate at the base of the downleg is not covered by coke-ash or refractory spray.
The design of Fig. 3 lends itself to repair when using extended bottom campaigns. No attempt should be made to look into the safety tuyere through the mica lens while the blast is on.
A small (1/4-1/2 in.) air line connects to a tuyere as shown in Fig. 2-3 to use hot blast air to keep the safety tuyere warm and free of condensation and debris. Sufficient air flow is important to ensure the tuyere is kept open. If, however, carbon or carbonaceous refractories are used in the safety tuyere and the well, oxygen in the blast air can consume the carbon and cause a potential problem.
At the first sight of fluid entering the safety tuyere, the blast should be turned to relief (spill) and the taphole should be opened. If attempts to open the taphole fail and the slag/iron is entering the safety tuyere, the blast should be shut off. Some foundries have chosen to leave the safety tuyere open during this period so a safe, hot passage exists.
The safety tuyere, however, is not trouble-free. The first fluid into the opening during normal cupola operation usually is relatively cold slag that tends to run a short distance and solidify. If the flow continues, the slag builds until the tuyere becomes blocked. If that happens, the tuyere taphole should be rodded to ensure that it is open and the iron dam may need to be lowered.
If iron and slag both are flowing freely at the taphole and into the safety tuyere, the melting operation should stop and the tuyere access opened and rodded. If slag in the safety tuyere continues during normal furnace operation, the cupola's operating parameters should be reviewed, especially dam height and taphole diameter.
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|Date:||Jun 1, 1993|
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