Technical Paper 4.1: Standard drying and firing schedules
Practice learns that the time gained by fast methods of installation with monolithic refractory materials like guniting, is not found again in the subsequent aftermath of getting started, although that is generally the expectation of the end-user.
It is a mistake to think that drying and firing schedules can be performed before or following after operational start-up of the equipment. Early planning of refractory dry-out and firing can result in a well-balanced, time saving and combined procedure.
It is also a mistake to think that no drying and firing should be required for small repairs or small refractory applications. As a general rule refractory repairs should be treated in the same manner as newly installed linings.
Figure 6: Harmonised curing, drying and firing schedule insulating aggregates do not differ much under these conditions.
Upon firing the cement has to pass through various stages of volume changes, whilst the aggregate follow their natural thermal expansion behaviour. A typical thermal expansion for hydraulically setting refractories is shown in figure 4:
Clearly is visible that due to the shrinkage of the cement (C) at around 200 °C a permanent shrinkage results in the castable (A). It is much less then the cement itself because of the dedicated granulometry of the mixes. With use of lower quality castables, shrinkage will generally be higher as also lower quality cements will be used.
But the most important phenomenon during drying and firing is the evaporation of the encapsulated water.
Trapped water exerts already a pressure of 1.3 MPa at 200 °C, over 8.5 MPa at 300 °C and until water passes the critical steam temperature 22.5 MPa (!). Hence, 'hold periods' in order to release this pressure built-up are mandatory, but will depend of course on the type and thickness of the refractory material to be fired.
Tests have shown that 30 - 70 %w of the remaining water after air-drying will disappear between 200 °C and 350 °C, whilst at the same time the major changes of the hydraulic cements take place as can be observed in figure 4.
However, the hot face area or only a thin front layer will follow the prescribed heating schedule. By natural law water migrates always towards the colder area due to re-condensation at the lower temperature. Here it meets ultimately an impermeable membrane: the casing.
On the contrary, water vapour removal has to be achieved through the internally circulating heated air, hence at the wrong side. An additional factor is that drying of the refractory material will initially proceed simultaneously with an almost twofold decrease of the thermal conductance. (figure 5)
In conclusion, careful reviewing of the required procedures in relation to the design and the construction is of utmost importance in order to result in a balanced start of the drying and firing schedules.
Schedules from manufacturers do vary considerably from fastest till most conservative ones.
In general, the less porous a refractory material is and the faster a schedule will be, the more loss of ultimate strength and 'life time' will result or shall be taken into account.
May 2018 Issue
In the latter case it will be important to establish the location of such repairs relative to the position of the heat source, e.g. burners or flow of heated combustion/process gases. The ‘further away’ from the heat source the better chance to get dried and fired in the appropriate way.
The lengths of schedules differ amongst manufacturers, but they are all in agreement with respect to the temperature levels where 'hold times' are required. From experience and practice in many refineries dedicated procedures have been developed in the course of time. (figure 6).
Although the schedules are generally applicable and shall be used in combination with possible specific
material requirements, sometimes
special requirements prevail. For example, linings of burners which are also used for start-up, or linings of catalytic crackers which require different heat treatment, should be treated firstly but they can still be treated simultaneously.
RULE OF THUMB:
! All procedures ought to be considered as fastest allowable procedures !
In very special cases a specific drying and firing schedule can be calculated based on material properties, evaporation rates, strength developments, etc. These are however cumbersome operations and generally do not result in faster procedures.
Also very important is that, a once started drying and firing procedure shall never be interrupted before reaching operating temperature and holding the temperature steady for a dedicated period thereafter. This accounts as well for inspections after drying and firing of refractory linings. Only when specifically needed the temperature shall be brought to ambient again for inspections.
Generally sufficient information about the lining's condition at the end of the procedure is obtained via peep-holes and other openings present in the lined equipment.
In case interruptions do occur, the procedure shall be restarted at the lower point of intercept of the actual temperature on the schedule. Even 'hold times' shall not be omitted, though considerations may be given to reduce the length of such period.
Manufacturers have developed special materials which allow faster firing or recommend insertion of organic threads/fibres, which mainly result in a more open, capillary-type structure allowing the water to escape more rapidly. The use of these materials and other types of 'porosity creators' shall of course depend on the operational conditions, i.e. gas atmosphere, corrosion resistance, etc. and shall already be taken into account during the design stage.
ENGINEER THE REFRACTORIES 13
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