Gunning patch

Hot-gunning refractory can be an effective stop-gap for numerous types of heat processing equipment. Boilers can be tricky, depending on the ability to access to surface. If you have concerns over a possible hot spot, or a problem area in your high temperature equipment, get a second opinion from F.

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Cleveland 19 News Federal monitor's report criticizes Cleveland police internal affairs unit over investigations. Rebound is expended or improperly hydrated material which bounces off the guniting surface. Guniting over rebound in high temperature applications ultimately leads to failure. Hence care is to be taken to ensure that any rebound which collects is properly cleaned out.

This can be easily achieved by shutting off the material at the nozzle and turning up the air pressure air to blow out the corner or area where the rebound has collected. While guniting of refractory material, it is important to bring the refractory material out the full thickness in one pass rather than in layers. A monolithic lining with no laminations provides a superior result. When the refractory is cold gunited to the full thickness in a furnace, vent holes are to be poked in the green material with a straight steel rod to the full depth of the lining, on around at mm centres.

Venting the refractory is extremely important since it provides a place for the steam to escape during the heat up. During a heat up, it is not uncommon to see steam coming out of all vent holes. Entrapped steam has a great amount of power and if the unit is heated too quickly, it can cause an explosion.

As a furnace is heated, the chemically combined moisture in the new refractory material migrates away from the hot face toward the steel shell, and when it reaches the steel, the steam escapes back toward the heat source through the vent holes.

Hence, taking the time to properly vent the gunited refractory is essential. Quick fire fibres are now common in refractory installations. The quick fire fibres are typically made of polypropylene and burn out quickly when the temperature increases, creating fissures and avenues for the steam to escape. Even with quick fire fibres, it remains a good practice to vent the gunited refractory.

The dry mix process consists of five steps namely i all dry ingredients, except water, are thoroughly mixed usually done by the supplier of the guniting refractories , ii the guniting refractory material is fed into a special mechanical feeder or gun which is called the delivery equipment, iii the mixture is normally introduced into the delivery hose either through a metering device or air pressure alone orifice feed for delivering of the material into the hose, iv the guniting material is carried by the compressed air through the delivery hose to a nozzle body which is fitted inside with a water ring through which water is introduced under pressure and thoroughly mixes with the guniting material, v the material is jetted from the nozzle at high velocity on the damaged refractory surface to be gunited.

Since in the dry mix process an intimate mixing of the water and dry materials does not occur at the nozzle, dry-process relies heavily on the skills of the guniting operator who manipulates the nozzle in order to produce an effective mixing on the application surface. The amount of water added at the nozzle is critical, as insufficient water increases both the material rebound and dust, while excessive water causes the mix to slough off. Only fine refractory materials are used in dry guniting process, since higher particle size of the guniting material tend to rebound from the application surface.

The guniting refractory material is required to generally have high effective cohesiveness to reduce rebound and produce a certain refractory build-up of the damaged surface. The wet mix process also consists of five steps namely i all the ingredients of the guniting material and mixing water is thoroughly mix, ii the wet guniting mixture is introduced into the delivery of the equipment, iii the mixture is metered into the delivery hose and moved by positive displacement, iv compressed air is injected at the nozzle to increase the velocity and improve the shooting pattern, and v the wet guniting mixture is jetted from the nozzle at high velocity on the damaged refractory surface to be gunited.

Wet guniting process consists of a mixing chamber fed by a pneumatically-driven stream of dry solid particles and by jets of high pressure water.

After in-line mixing, the wet solids pass out through a nozzle and the air stream eventually disengages. Nozzle design largely controls the dispersion of the spray. The liquid feed rate, hence ratio of liquid to solids, is controlled by the liquid pressure and by valves.

In the wet mix process, due to the reduced possibility of rebound, higher size refractory materials can be used in the mix. Since the guniting operator has no control over the mixture proportioning of the final product, the quality of wet-mix guniting is less dependent upon the skills of the operator.

In the dry mix process i there is instantaneous control is available over mixture water and consistency of the mixture at the nozzle to meet variable field conditions, ii the process is better suited for finer refractory guniting materials, iii the delivery hoses are easy to handle, iv the process is well suited to condition where the timing of pacing the guniting material cannot be predicted or is intermittent, and v there is lower volume to be handled per hose size.

In the wet mix process i the mixture water is controlled at the mixing equipment and hence can be accurately measured, ii the process provides better assurance that the mixture water is thoroughly mixed with the refractory guniting materials, iii there is lesser dust and lesser loss of refractory guniting material during the shooting operation, iv there is normally lesser rebound resulting in lesser waste, and v there is higher volume to be handled per hose size.

While comparing the two processes, it can be seen that either of the process can be suitable for the guniting repairs of the damaged refractories surface. However, differences in the capital and the maintenance cost of the equipment, operational features, suitability of the available guniting refractory materials, and placement characteristics, can make one or other method more attractive for a particular application.

The main equipment for guniting of the refractory material is a guniting machine. The guniting machines are mounted on wheels for portability. A valve is provided at the nozzle for the regulated flow of water which ensures very efficient and controlled hydration of the guniting refractory mix. For avoiding manual work, one lever operation is generally provided for charging the guniting material into working chamber. This also makes the operation easy and safe.

Auxiliary equipment upstream includes the necessary pumps and pressure regulators, pre-wetting equipment, and the solid feed regulating system where the solids flow rate is determined.

Manifolds are provided in airline to ensure minimum air pressure loss for the optimum utilization of air. This results into saving in air consumption. The upper and bottom vessels in the guniting machine can be unbolted for inspection and cleaning.

A check valve is normally provided before manifold to prevent back flow of material into valves and air motor. The guniting machines are simple and safe to operate and can be operated even by an inexperienced person. Some accessories are needed for the guniting machines.

These are standard hoses for air, water and material discharge with end connections. For nozzle body the accessories needed are light weight mixing nozzle complete with water regulating valve, water ring and nozzle tip with wear resistant liner. The nozzle assemblies are for the guniting of the refractory materials. The guniting machine can be with manual or automatic operation.

Guniting refractories are granular refractory mixtures designed for application with air placement guns. A variety of air guns are used to spray the mixes at high velocity and pressure to form homogeneous compact lining essentially free from lamination and cracks.