GENERAL

Thermal Ceramics Australia provides a wide range of products designed for application by pneumatic gun. This method involves refractory castables being pneumatically conveyed through suitably designed flexible hoses to the installation position, whereupon they are projected through a handheld nozzle at the target area. This type of installation is well suited for:

where large volumes of refractory are involved, gunning may reduce installation time and costs. there is no need for shuttering, further reducing the cost of the job. where access is difficult, provided a nozzleman and hose can be accommodated, a lining can be gunned. for elevated positions, materials can be conveyed vertically for reasonable distances, solving logistic problems. where multi-layered walls are required, it is often easier and more cost effective to gun two layers rather than use two sets of formwork. thin linings are easier to place.

There are two types of gunning wet and dry.

Wet (or slurry) gunning involves mixing all of the required water with the mix in a paddle mixer prior to conveying to the nozzle. This seldom used method requires a higher water content and thus density of the placed material will be lower. However, there is little or no dust caused by this method and low rebound, lamination-free linings result.

Dry gunning is the much more popular of the two methods. It allows the nozzleman to operate up to 300 metres horizontally and 150 metres vertically away from the feed station. Desired air pressures at the nozzle are 250 – 330 kPa for dense castables, 180 – 240 kPa for medium weights and 100 – 170 kPa for lightweights, depending on the product. For more information, contact your nearest Thermal Ceramics Australia representative or office.

Due to the pressure drop between the nozzle and the gun, the gun operator should as a guide add 35 kPa for every 15 metres of hose length more than the standard two lengths, and 35 kPa for every 15 metres of elevation above the gunning machine. Fore lightweights add 25 kPa. If the pressure is too high, material density and rebound will increase; low pressure results in low strength and lack of consolidation.

Since the amount of water added is determined by the nozzle operator, the person gunning should have enough experience with the particular refractory being used to know how much water to add to get the correct consistency on the wall. Too much water and the material will have low density and strength and could slump of the wall, too little, and the rebound and dusting losses are very high.

PRE-INSTALLATION

Storage.

Castables should be stored in a dry, well-ventilated area and held off the floor on pallets. If stores outside, the bags must be protected from rain or dripping water by a fixed cover. If they are further protected by plastic sheeting, ensure that there is sufficient ventilation underneath the plastic sheet to prevent water condensing on the bags. Avoid storing in areas of high humidity.

Pallets should not be stacked more than 3-4 pallets (6 tonne) high when in storage. This should be reduced to 2-3 pallets (2 tonne) when storing lightweight castables. This is to prevent the consolidation and caking of material on the bottom rows of the pallet.

Most Thermal Ceramics Australia castables have a finite shelf life called a “nominal shelf life”. The nominal shelf life is half of the maximum expected shelf life of the material. When quoting shelf life, a nominal shelf life of 6 months is used. Product that is older than the nominal shelf life should be checked for setting properties prior to use.

Signs of aging are longer setting times and reduced strengths. All bags have the date of manufacture printed on them, to aid in determining the age of the material. Always use the oldest materials first and if aged beyond the nominal shelf life, have the material tested before commencing use.

Preparation Prior to Refractory Placement

The work surface shall be substantially free of dust, scale, oil, water, dirt and loose foreign material and shall be cleaned by sandblasting if necessary.

Anchors.

Both ceramic and metallic anchors are appropriate for use with gunning grade castable refractories. This subject is treated in a separate procedure (IP/008).

INSTALLATION

Temperature.

Ambient, material and gunning water temperatures can significantly affect the ease of gunning and initial setting time of the castable. In very cold conditions, the bags should be kept in warm storage – not less than 15(C for at least 48 hours before use.

Cold material retards the activation of the cement binder thereby increasing rebound during gunning and may also cause slumping.

The ideal mix temperature lies between 15(C and 25(C. When gunned in place until thoroughly dried, the castable and environment should preferably not fall below 10(C. and certainly not below 5(C. If the castable freezes before the hydraulic set is completed, the ultimate strength of the material can be reduced by as much as 60-70%.

In very hot weather, the bags should not be kept in cool storage and cool water used for gunning. The steel shell may also need cooling prior to gunning. This can be done by spraying the shell with water from the gun, ensuring that the water does not pool where the refractory is to be installed.

Joints

There are two types of joints used in gunning, although sometimes linings are gunned without any joints:

Construction Joints

These joints are designed to break the job up into conveniently sized panels. Adjoining panels are gunned against each other without leaving any gaps using the set face of previously gunned sections as the formwork edge. Panels are generally 1.0 – 1.5m². This size can generally absorb stressed without cracking. The construction joint can be regarded as placing a crack where you require it, rather than leaving the castable to stress relieve itself in a random check pattern.

Expansion Joints

For these joints adjacent panels are separated during installation by materials such as cardboard or plastic that will burn out during firing and leave a gap, or by high temperature materials such as ceramic fibre which remain in the gap, but have some degree of recovery after compression.

Equipment

Equipment used is conventional concrete guniting equipment. This involves an air operated gun forcing material through a flexible hose and a nozzle where water is added to it. Most Thermal Ceramics Australia gunite refractories typically use a Lova ( Reed Gun with 20 of 21 pockets, or a Sova( Reed Gun with an 18 pocket feed wheel, although other makes of rotary gun are also suitable.

Material is placed in the gun hopper where it feeds into a wheel with pockets at the base of the hopper. The wheel turns by motor driven gears powered by compressed air.

When the feed wheel rotates, the loaded pockets pass under an air inlet section which forces the material to the outlet neck and into the feed hose in a steady flow. The material passes along the hose to the nozzle, where it is mixed with water (if needed) and sprayed onto the wall. Generally two 15 metre lengths of 38 mm I.D. hose are used. If more lengths are required air pressure at the gun must be increased as previously noted. The quantity of material able to be conveyed is only limited by the inside diameter of the hose and the quantity of air available.

Multi-layer Linings

Where an insulating lining has previously been installed, it should be coated with an impermeable membrane to prevent premature moisture loss from the fresh castable into the insulating layer. The membrane used should burn out at low temperature when the drying cycle is started. Alternatively, the backup layer may be dampened prior to casting by lightly spraying with water.

GUNNING

Nozzle Water Control

The appearance of the gunned refractory surface is the best indicator of correct water/mix ratio. Freshly placed material should have a wet, silky sheen which rapidly disappears when the nozzle is moved away from the area. The coarse aggregate should make small craters on the surface on impact. A sandy, gritty surface indicates too little water is being used. Slumping, ripples of a washboard surface indicates too much water.

NOTE: Water pressure at the nozzle must be at least 70kPa higher than the air pressure to ensure that the water is able to completely wet the material as it passes the water ring. A water booster pump may be required to increase plant water pressure.

Apparent inconsistent mixing of water is often caused by either pressure being too low or blocked holes in the ring. This should be periodically checked and cleaned.

Gunning

The nozzle is held between 45( and 90( to the wall approximately 0.5 metre away from the work surface. It is moved in a circular motion building up the full thickness over the whole of the designated panel starting from the bottom of the wall and working up, then from the top down and filling the centre section last. Lower sections and corners are generally gunned first so that rebound does not become entrapped there. Care needs to be taken to ensure that the rebound falls clear of the target area and does not become entrapped. This causes laminations or spots of low density in the panel.

When gunning overhead, water addition should be reduced slightly to prevent slumping prior to initial set of material. An increase in rebound should also be expected when gunning overhead. The nozzle should be at an angle of about 45( when gunning overhead.

The nozzleman should break up the installation into conveniently sized panels, which allow construction joints to be put in. Upon completion of a panel the edges are cut back at right angles to the shell, removing all waste and rebound before proceeding to the next panel.

If, for any reason, an interruption delays application in excess of 10 minutes, any area which has been partially gunned shall have the edges cut back at right angles to work face where full thickness has been achieved. Any areas not built up to full thickness shall be removed and replaced.

To control the thickness of the lining, one of three methods are normally used. When ceramic anchors are used they act as a guide since the end face of the anchor brick is required to finish flush with the lining.

When metallic anchors are used, they are generally set back 25-30 mm from the hot face. In this case, the nozzleman’s trowel-hand uses a rod of 2mm wire, bent at right angles to the required depth. This is inserted in the lining at right angles to the working face and turned full circle. If the lining is too thick, a circle remains on the wall, indicated the correct thickness and giving the troweller a level to cut back to. If too thin, then there is no mark and the wall needs further building.

The third method involves using gauge boards set at the lining thickness. The lining is gunned then cut back with a leveling board held on the gauge boards. This method is used where accurate thicknesses are a must.

When gunning around anchors, avoid “shadowing” as this is detrimental to the lining, causing laminations. When this occurs, the material should be removed and re-gunned. Similarly, any overspray on anchors or steelwork should be removed on a continual basis.

If the lining consists of two layers, e.g. an insulating castable backup lining and a hot face, the hot face can be gunned onto the first after the backup has set. Generally, the backup layer is given a coating of a concrete curing compound to prevent loss of water of hydration from the hot face material to the backup. Preferably, the backup lining should be cured before the hot face lining is installed.

Finishing

The as gunned finish of the refractory is quite acceptable, but if a smoother surface is required, it can be scraped with the edge of a trowel or wooden board as soon as possible after the completion of gunning. The rough open texture should be left; smooth trowelling or floating to slick the surface is unacceptable as this draws a fine finish of cement rich segregated material to the surface, which can spall of during heating.

Venting

In very dense gunning materials such as MORAL( CAST 180 AR GUN RFT, the surface of the gunned refractory should be vented with a welding rod to at lease 75% of the lining thickness. This is to facilitate moisture removal from the refractory and to prevent steam spalling during heat-up.

Curing

The purpose of curing is to prevent the premature loss of moisture from freshly gunned refractory concrete during the chemical changes associated with hydration of the calcium aluminate binder. When mixed with water, an exothermic hydration reaction takes place which drives off water at an early stage. Loss of water from the surface of the castable before the cement is fully hydrated results in a weaker lining. To prevent this, various methods of moist curing should be used.

Exposed refractory can be lightly sprayed with water, covered with plastic sheeting or sprayed with a concrete curing compound. This is called moist curing. The concrete curing compound forms an impermeable membrane to prevent moisture loss during curing but will burn out at a low temperatures during firing. Moist curing needs to be performed for a minimum of 24 hours.

After the initial 24-hour moist cure a further 24 hours air-curing should be allowed before the lining is fired to further increase the final strength.

Curing is essential to the achievement of maximum strength in the green unfired state and this will effect the ultimate fire strength.

FIRING

Detailed firing schedules are separate from this procedure. They are available from any Thermal Ceramics Australia office or representative.

BACK

TABLE OF CONTENTS

Combustion & Chemical Engineering.
Copyright © 1998 [CCE]. All rights reserved.
Information in this document is subject to change without notice.
Other products and companies referred to herein are trademarks
or registered trademarks of their respective companies or mark
holders.