DANIELTM MODEL 788DVC DIGITAL CONTROL VALVES | D E C 2012 |
2.0THEORY OF OPERATION
The Model 700B Series Control Valve operates on a balanced-piston principle. When pressures on both sides of the piston are equalized, a spring located on top of the piston acts as a differential force and closes the piston. When the pressure against the bottom of the piston exceeds the pressure plus the force of the spring exerted against the top of the piston, spring tension is overcome, and the valve opens. See Figure 3-1 for more information.
These valves are normally closed (N.C.) and they will open when both solenoids are energized. The valves are fail-safe as they close upon loss of power. They use the line product as the source of hydraulic power to open and close the main valve piston. An electrical supply controlled by an electronic preset is the source of power for energizing the two solenoids.
These valves are used mainly for batching and they provide a means of reducing the rate of flow on on startup and before final shut-off of a predetermined delivery. This minimizes surges of pressure and line shock and assures ± 1/4 gallon shut-off (sizes 2 inch - 8 inch) of the preset volume.
The total system generally consists of three pieces of equipment: (1) a flowmeter, (2) electronic preset with digital control, and (3) a digital electric control valve. The electronic preset is the device used to set the predetermined volume of liquid that is to be delivered by the valve.
2.1Operational Sequence
With both solenoids de-energized, the main valve is closed as shown in Figure 2-1. The main valve can be infinitely positioned anywhere between 0 - 100% open by digital control of the solenoids. With both solenoids energized, as shown in Figure 2-3, the valve begins to open. It will only open to the programmed flow rate set in the electronic preset. Normally, the electronic preset is programmed to digitally control low flow start-up, maximum flow rate, low flow rate before shut-off and no flow. The electronic preset will automatically energize and de-energize the solenoids to position the main valve to limit the required flow rate. When the required flow rate is reached, the solenoids will be as shown in Figure 2-2. This hydraulically locks the main valve piston in position. Should flow increase, the valve will close slightly to adjust to the required flow rate. All of the positioning is done by digitally controlling the two solenoids as shown in Figure 2-1, 2-2 and 2-3.