Silo isolation can be a challenge if you do not use the correct type of valve for the top of the cement silos. The cost of selecting a wrong valve can be huge: you may end up contaminating a full silo of valuable cement due to a leaking valve.
In many cases companies are stuck with the process equipment that was originally supplied when the plant was built. However, today there are many new and better technologies available. Concerning shut-off and control solutions pinch valves are one of the best technologies for the isolation of pneumatically conveyed cement. For example, many Asian cement plants have recognized the benefits of a pinch valve and have not only replaced old valve technologies with pinch valves, but are also using pinch valve as standard for their new terminal projects across the Asia.
Diverter and Knife Gate Valves in Silo Isolation
Diverter valve is a commonly used valve type. It has been used for many years and it works reasonably well. However this type of valve often has problems over time. The mechanical flapper in these diverter valves over time usually begins to cause problems. The problems typically experienced are 1) increased force required to switch the flapper from one side to another, 2) no longer tight shut-off when switched and finally 3) these types of valve typically need to be repaired every year.
The other type of valve that is widely used for silo isolation is a knife gate valve. These valves are good for pneumatically conveyed materials and they also are one of the least expensive alternatives. Unfortunately, they are the one type of valve that will be replaced more frequently than any other type of valve used in these systems. What will typically happen to a knife gate is that a small pinhole leak occurs in the seating area. The cement and air blow through this tiny hole at high velocity and slowly gouge a larger hole. The entire knife gate and body are typically destroyed in a short time span.
Pinch Valve is the Best Solution
The best available technology today for the silo isolation at cement plants is a pinch valve. A rubber tube or sleeve is pinched by steel bars on the centerline of the valve to close. Self-cleaning pinch valves provide 100% tight shut-off even if solids have built up on the sleeve wall. When compressed, any crystallized particles flake off the sleeve surface and are washed downstream.
The operating principle of a pinch valve is simple. In the open position, the valve is at full bore with no flow restrictions thus making the valve an integral part of the pipeline. During closing, two pinch bars squeeze the sleeve shut on the centerline.
The core of the pinch valve is the sleeve. It has to be made from three layers: wear resistant inter layer, fiber reinforcement layer, and protecting outer rubber layer as shown in picture below. This construction guarantees long lifetime of the sleeve. There can be found some copies with molded one layer sleeves, but they are gaining only 10% of quality sleeve lifetime.
Pinch valve itself is rather simple to build; however, the sleeve is not. Making quality sleeve demands wide knowledge of elastomer technology to mix right rubber for each application. In today’s markets there are only three pinch valve companies that are manufacturing layered and reinforced sleeves. Buying pinch valves from other than these manufacturers is comparable to assembling bicycle tires under 18 wheel truck. Tires are cheap but you need to stop the vehicle to repair the tires all the time.
Why Elastomers are Better Against Abrasion than Hard Materials?
The actual reason why pinch valve is better for abrasive duties than metal or ceramic valves is behind the following fact. Energy from particle impacts is absorbed to different materials in different ways. Metal, for instance, absorbs the impact energy of particle to itself. Every time when particles hit to metal they take a small piece of metal surface away with the flow. That eventually causes a hole or a sealing leak. Softer materials allow the abrasive particles to bounce off the surface without destroying it. So the energy of the particle impacts is used to maintain the flow instead of wearing the valve.
For this reason, pinch valves are constantly taking more market share from conventional valves in mining and cement applications where very coarse slurries and abrasive pneumatically conveyed materials are handled. A pinch valve also offers protection against clogging and jamming that can occur with other valves in cement service. Many valves such as knife gates or diverter valves with stellite or harder coatings might be able to withstand the abrasiveness of pneumatically conveyed cement. However, they are subject to jamming or clogging because they have cavities that allow for material collection.
In conclusion, many cement companies agree that high quality pinch valves are the best valve solution for silo isolation because they have long service interval, they are easy to operate, are easy to repair and in general they have very low cost of ownership. Finally, even if the pinch valve sleeve is beginning to fail, you will typically still be able to close the valve sleeve and prevent silo contamination.