Getting Slide Gates to Work with Rotary Valves in Lean Phase Systems

Lean phase pneumatic conveying systems with rotary valves can get major operational benefits from slide gates. Read on to see how slide gates greatly improve the efficiency and functionality of this type of manufacturing plant.

Typical lean phase system

Lean phase is the most cost-effective and efficient method to transport bulk dry granular materials or powders. They are commonly used in such industries as agriculture, chemicals, food, mining, and plastics. These pneumatic conveying systems are typically used in a processing plant where the material is:

  • Stored in a buffer or silo
  • Fed using a rotary valve
  • Transferred with positive pressure

The positive pressure is provided by a blower that charges the conveying tube to no more than a couple of atmospheres. Material is fed from the silo or buffer to conveying tube using a rotary valve:

  • Provides a partial airlock to the silo
  • Maintains positive pressure in conveying tube
  • Feeds material quickly and steadily

At a glance, the setup is fairly simple and standard, yet there are some subtle flaws that stem from the rotary valve and adversely affect the system.

Rotary valve issues

A small amount of system air leakage comes from the rotary valve. How much this air leakage affects system performance depends on several factors. The rotary valves are unable to provide a complete air seal between the silo and the conveying tube. The problem is from a tiny gap that is needed between the housing and the vane. This is so there is no friction and the rotary vanes can rotate smoothly. This tiny gap makes a small air leak between the conveying tube to the silo. Air leakage can be reduced by adding more vanes to the rotary valve but it cannot be completely eliminated.

Material build-up

The minimum speed of air within a conveying tube for materials to stay airborne is called ‘saltation’ speed. Rotary valve air leakage will reduce airspeed in the conveying tube, and if it gets below saltation speed material will fall out of the airflow and form obstructions along the conveying tube. The system must be shut down so the conveying pipes can be manually decoupled and cleaned. This is costly and disruptive maintenance that also results in material loss.

Blower oversizing

Unnecessary blower oversizing can result from rotary valve air leakage. A higher than needed capacity might result from having to make up for the reduction in airspeed from the air leakage. Blowers of higher capacity use more energy so this makes the system energy inefficient. This oversizing problem would get worse when several silos are fed into a single conveying tube as the air leakage from the rotary valves would be cumulative.

Rotary valve wear

Rotary valve air leakage increases its wear and tear. Material blown back past the tiny gaps on the return side will wear valve housing and vanes. The wear and tear rate increases over time and the gaps become greater. Blown back material can also get into bearings and result in costly mechanical failure.

Slide gates help mitigate issues

Slide gates can be used to mitigate rotary valves issues and improve system functionality and efficiency.

Above the rotary valve

A slide gate (Vortex Clear Action Gate) can be fitted above the rotary valve to reduce air leakage. Its self-cleaning action on closing forms a complete air seal and barrier between the conveying tube and silo. The slide gate has a smooth bore that allows for unobstructed and unrestricted flow of material. The air-seal only occurs when the slide gate is closed and material is not required to be fed into the conveying tube. When the slide gate is closed it provides these benefits for the system:

  • Prevents material build-up in the transfer pipe from loss of positive pressure
  • Improves blower efficiency by maintaining a proper air seal
  • Prevents rotary valve wear by stopping material blow-back

The slide gate is able to isolate the rotary valve from the silo when the silo has material in it. This allows maintenance to be carried out on the rotary valve without having to empty the silo.

Below and above the rotary valve

Further reduction in rotary valve air leakage can be made by fitting a slide gate (Vortex Maintenance Gate) between the rotary valve and the conveying tube. When the slide gate is closed, it isolates the rotary valve conveying tube and stops any positive pressure loss. Installing slide gates below and above the rotary valve allows the actual rotary valve to be isolated. It allows for the rotary valve to be maintained, rebuilt or replaced while other connected silos remain fully operational and undisturbed.

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