Shotcrete vs. Gunite vs. Pump Casting vs. Casting – Misconceptions About Refractory Installation Methods
I’m amazed at how many times we’ve met with a potential client where another refractory contractor has told them something off the cuff like “you need to shotcrete this” or “you need to pump that” … without taking the time to actually go through the proper steps to evaluate the best method for the application. This usually happens because the other contractor is limited in their offering.
That’s like telling someone who drives an SUV to buy off-road tires — without determining what they drive, where they drive and how they drive it!
There’s a lot of chatter out there in our industry, and sometimes partial explanations or incomplete evaluations can lead to the incorrect installation method, the wrong materials, or higher costs than needed.
Some of that chatter is causing plant engineering and maintenance teams to think they are behind the curve by not shotcreting or pumping their furnace lining.
Please don’t fall into this trap! The best installation method depends entirely on the specifics of the job.
The 4 Main Refractory Installation Methods
If you’re considering a refractory application that requires a monolithic castable refractory, there are essentially four main installation methods:
- Casting (or vibration-casting)
- Pump casting
- Shotcrete
- Gunite
Which one is the best for your project? Well, it depends. We wish it was simple, but the truth is, there are many factors that influence the decision for the best application method (and ultimately the best installation) including:
- Location and site conditions
- Environmental factors & installation equipment
- Volume required
- Storage conditions
- Skill of installation team
- Bake-out (curing) requirements
- Budget
Refractories are not a commodity, and the proper installation method for your job requires a careful evaluation of the details of your project.
Each Refractory Installation Method Explained
To start, let’s take a quick look at each installation method.
Casting
Casting is a fairly straightforward method of mixing and pouring (casting) wet castable into forms. The forms hold the castable in place until it’s set, and then they’re removed.
Pump Casting
Since refractory castable is fluid, most types of castable can also be pumped to the location where it’s needed. Typically, pneumatic truck or trailer-mounted line pumps equipped with pipe and hoses are used to convey large volumes of material to a specific location.
Gunite
Gunite is a dry monolithic refractory designed for use with dry gun equipment. It usually includes additives to make it stickier. Gunite is applied by a special machine that uses air to push the dry (or pre-dampened) gunite through a hose and to the target. Water (or additive) is added at the nozzle to moisten the dry mix so it sticks to the surface.
Shotcrete
Shotcrete is typically a low-cement, low-moisture refractory that is fully tempered and mixed with water (or special additives), and then applied through a machine that uses a piston pump and air to spray the wet material from a nozzle like gunite.
Choosing the Right Method
When considering each method, think of them as tools in a toolbox. Sure, all methods may work; however, one method may be better for your current application, and another method might be better for a future application.
Assuming you’ve selected the best refractory for your specific application, consider each of the following elements and how they apply to your job:
Location and site conditions
Where in the furnace is the refractory being placed? Are there obstacles to go around, over or under? How high is the location for the application? Are you going to be starting and stopping often? Are you going to need to move the equipment around?
If you can set up right at the furnace, boiler, etc, then casting may be the best method and usually provides the application with the best physical properties and the least potential equipment issues.
Installations at higher elevations have other considerations. In many applications, gunite is easier to use, especially at higher elevations, and the equipment and dry castable can usually fit in an elevator or be easily lifted to where it is needed. Pump casting and shotcrete can also be a good solution for high locations, if the pump has enough energy to get the material to the desired elevation, the pipe and hoses can reach the desired spot, and it can handle the volume of material needed.
In the right application and environment, shotcrete works extremely well. The advances in technology have increased dramatically in the last few years.
Environmental factors & installation equipment
How clean does the environment need to be around the application? Is dust a factor? Are there plant operational issues to consider? What types of safety requirements are required?
Although the general properties of each material may be similar, each castable is engineered to be installed via its respective method. That is to say, shotcrete material is engineered and manufactured to be installed via shotcrete, gunite material via gunite, etc. Some companies advertise their materials may be both cast in place or by gunite, but generally in practice most materials cannot be cross-installed with equal results.
Volume required
Pump casting and shotcrete can deliver greater volumes faster than regular casting or gunite; however, you must be prepared to accept the large volume of material, otherwise it doesn’t help you. Also, you have to consider waste. Gunite application is typically 2 to 5 tons an hour. Shotcrete is the fastest and can typically be applied at 5 tons (or more) an hour.
Storage conditions
The length of your job, quantity of materials, available storage for materials and the physical space required for the equipment and materials at the job site are also important factors.
Skill of installation team
Pump casting, gunite and shotcrete typically require more experience than casting. Is the contractor manning your project with company people or a crew of travelers?
Bake-out requirements
Many refractory materials have different bake-out (curing) schedules that can impact your overall project schedule. Don’t forget to consider this in planning your overall outage. You do not want to cut corners here!
Budget
The cost of the materials (assuming similar physical properties) is relatively equal, but the size of the project can affect the materials cost because of waste. For example, for pumping or shotcrete, waste doesn’t impact the materials cost in a job with 20 tons of material, because waste in the hose and pipe is insignificant. However, waste can be significant in a job requiring only a few tons of material – sometimes adding an additional 30% to 50% of material cost.
The main cost differentiators are equipment and labor. High-quality pump casting equipment is expensive. Shotcrete equipment is expensive, but the cost can be mitigated by the high volumes. Gunite equipment is relatively inexpensive and readily available.
In addition, pump casting and shotcrete both add the element of potential equipment issues that must be taken into consideration. An example might be turnarounds where more than one pump may be required on the job as a back-up, in case one pump goes down.
What’s the Conclusion?
As you can see, there’s no “one-size-fits-all” application. Selecting the appropriate method is like choosing the perfect tool for a job.
But don’t let the application method distract you from the most important decisions: selecting the right refractory material and selecting skilled and experienced workers to install it.
If you’re not sure which approach to take, talk to us. Our company has installed millions of pounds of material and would be glad to help in selecting the right refractory and installation method for the unique conditions of your project.