Introduction
The dust collected in a baghouse falls into a hopper below the bags. This hopper must have a discharge device that not only releases the accumulated dust but also ensures a seal between the baghouse and outside air to maintain negative pressure within the unit. Common hopper waste discharge devices available for baghouses are the rotary airlock valve, double dump valve (flap valve), tilt valve, and vacuum valve (dribble valve). Each of these is readily available in various designs from several manufacturers.
Rotary Airlock Valves
A rotary airlock consists of a rotating shaft with several vanes attached to it. That assembly rotates slowly inside a housing, allowing each compartment between vanes to fill and discharge material. Rotary airlocks are a very common baghouse discharge control device. However, they can pose maintenance challenges with abrasive materials in that the vanes of the airlock can wear without any external indication of a problem. This can lead to poor performance, and maintenance costs can be high in abrasive applications.
Double Dump Valves
Double dump valves are dual flapper valves mounted in tandem off the discharge of the baghouse and can be either automated or weight-based. For automated double dump valves, an automated rotating cam or pneumatic cylinder briefly opens the normally closed spring-loaded door to each valve. The dual valves are opened individually in a sequence that allows the top door to open, dump, and close before the lower door follows the same sequence.
This allows negative pressure to be maintained in the discharge hopper. The weight-based valve is very similar except that the valve opens based upon the weight of the product and not on an automated basis. Both types of double dump valves require sufficient vertical room below the baghouse hopper. Double dumps can be wear-protected well for long life with abrasive materials; however, the mechanical components of the rotating cam tend to require more maintenance.
Tilt Valves
Tilt valves are similar to double dump valves and are normally used in pairs. These valves commonly have a near horizontal and counterweighted flap door that is adjustable for closed tension. The function of the valve is to allow material to build up on the near horizontal flap gate until it overcomes the pull of the negative pressure in the baghouse. The material then dumps to the next valve as the gate shuts behind it. Typically, a divider chute 24–30 inches long is used between the two valves to allow one valve to close completely before the other opens. Tilt valves are less expensive than double dump and rotary valves, but can be hard to adjust. They have been known to stick open after being in service for a period of time, and they require four feet of clearance below the baghouse for installation.
Vacuum or Dribble Valves
A vacuum valve basically consists of a soft gum rubber tube mounted in a steel housing, which attaches to the bottom of a baghouse hopper discharge. Manufacturers have various designs available for purchase, some using a “fishtail” dribble bladder mounted inside a flanged pipe. The valve uses the negative pressure of the hopper to collapse the rubber tube and seal the discharge until sufficient material builds up and “dribbles” through. This valve normally has low initial cost, high reliability, and low maintenance, and it can fit in a short discharge space.
Conclusion
In summary, baghouse hopper discharge devices are essential for maintaining negative pressure while efficiently releasing accumulated dust. Common options include rotary airlock valves, double dump valves, tilt valves, and vacuum valves, each offering distinct advantages and challenges. While rotary airlocks and double dump valves handle abrasive materials effectively but require more maintenance, tilt valves are cost-effective but demand careful adjustments. Vacuum valves stand out for their simplicity, reliability, and low maintenance needs, particularly in confined spaces. Choosing the right device depends on specific operational requirements, material characteristics, and available space.
Reference
NIOSH Mining Program Report of Investigations, « Dust Control Handbook for Industrial Minerals Mining and Processing», Second Edition
