PRINCIPLES OF OPERATION
The Digital Control Valve basically consists of a diaphragm operated main valve & two solenoid valves. Normally open (NO) solenoid connects the valve cover chamber to the upstream pressure, whereas normally closed (NC) solenoid connects the valve cover chamber to the down stream pressure. The operation of the valve is simple & is explained below. The total batch quantity delivered & controlled through the valve is divided into 7 stages for the purpose of convenience.
Initially both NO & NC solenoid valves are in de-energised condition. The NO valve applies high upstream pressure on the diaphragm, whereas the NC valve prevents this pressure from getting drained to the downstream side. The valve remains closed as a result of this.
Stage1
Both NO & NC solenoids are energised. NO valve now restricts the high upstream pressure from entering into the cover chamber. NC valve permits the pressure above the diaphragm to vent to the low downstream pressure. This creates a differential pressure across the diaphragm, the high upstream pressure acting from below the diaphragm opens the main valve & allows the flow to start through the valve.
Stage2
As the flow rate reaches a predefined value (set in the batch controller as "Low Flow Start") the NC valve is de-energised. This prevents further draining of the chamber & locks the valve at this flow rate. Initial delivery at slow speed avoids splashing of product & also avoids the generation of static charge.
Stage3
When a sufficient quantity is delivered at the slow speed, the NC solenoid is energised again. This allows further draining of the chamber & hence increases the flow.
Stage4
When the flow reaches to the maximum level (sent in the batch controller), the NC solenoid is de-energised. This maintains the constant high flow rate for the remaining batch. During this stage, the flow rate control is done by the batch controller. When a number of Flow Meters are connected to the same pump, stopping (or starting) one or more Meters increases (or decreases) the flow rate at the remaining Meters. When the flow rate increases, the solenoid is energised momentarily. This allows slight draining of the chamber & allows the valve to open further in order to maintain the set flow rate.
Stage5
At the end of the batch, the NO solenoid is de-energised. This injects high upstream pressure into the cover chamber & throttles the valve.
Stage6
When the valve throttles sufficiently to achieve a predefined slow closing flow rate (set in the batch controller), the NO solenoid is energised. This maintains uniform flow rate.
Stage7
When the batch quantity is delivered, the NO solenoid is de-energised. (NC solenoid is already de-energised). This applies high upstream pressure into the cover chamber, which makes the main valve to close completely to achieve bubble-tight shut-off.
Note The number of stages during opening & closing are programmed in the batch controller & can be as many as desired. One stage during opening & two during closing are recommended.
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