Gate lock valves
These are probably the least used of all devices for feeding pneumatic conveying sys- tem pipelines. They are variously known as double flap valves, double dump valves, and double door discharge gates. They basically consist of two doors or gates that
alternately open and close to permit the passage of the material from the supply hop- per into the conveying line, as illustrated in Figure 3.20.
These gates may be motor driven, cam or air cylinder operated, or may work under gravity. The air that passes the lower gate from the conveying pipeline is vented so that it does not interfere with the material about to flow through the upper gate, in positive pressure systems. As with rotary valves, the blower should be sized to allow for this leakage, although this is not as effective in this case, as there is an order of magnitude in difference in the operating frequency.
Like the venturi feeder, care must be taken to ensure that the material is metred into the gate lock since it will cease to function correctly under a head of material, as would be the case if it was situated directly beneath the outlet of the supply hopper. A typical commercial type of gate valve feeder is shown in Figure 3.21.
To a certain extent the gate lock might be termed an intermittent feeder, since it dis- charges material between 5 and 10 times a minute. In contrast, the rotary valve has approximately 150 to 200 discharges per minute from its pocketed rotor. This reduction in the number of discharges means that the air supply, in terms of flow rate, and particularly pressure, must be correctly evaluated to prevent the possibility of line blockage. With few moving parts this type of feeder can be used to feed friable materials, and with appropriate materials of construction it is also suited to the handling of abrasive materials.
Care has to be taken if this type of feeder is used with large and hard particles. If these get trapped in the lower gate when it closes a considerable amount of conveying air could be lost from the system, in a positive pressure conveying system, which could result in pipeline blockage. Once again this would not be a serious problem when feed- ing a negative pressure conveying system.