5
The second batch application is the two valve approach that uses both internal
relays of the batcher. Each relay controls a valve, as illustrated below:
Dual Valve Batch Control
Analog Signal In
Flowmeter
Main Valve
Flow
PGM
View
Edit
Enter
Start
Reset
Stop
PGM
BATCH
Durant
Relay 2 Out
Dribble Valve
Relay 1 Out
The cosmetics plant made the single valve installation and soon 55 gallon drums
were being shipped out to customers everywhere. The installer stayed at the site to
observe the operation of the system and insure that it met his high standards.
Before long, the look of concern on the installer’s face made it clear that he was not
satisfied. Two things were troubling him. First, at the end of each batch, some extra
perfume was always delivered. The installer recognized that this was a clear case of
overrun. When the batcher delivered 55 gallons and turned OFF the valve, it took a
certain amount of time for the valve to actually close. During this time some flow
occurred. The amount of flow that occurs from the time that the batcher reaches the
batch final count until flow actually stops is called
overrun
.
The second observation that concerned the installer was the pounding that his
delicate plumbing took each time the valve closed. It was obvious that the shock of
going from a full flow state to a no flow state in less than one tenth of a second would
eventually cause an unauthorized exit in the system - a leak.
The installer knew that the solution for both problems was to use the dual valve
approach. This meant installing a small pipe around the valve. This pipe, the dribble
pipe, would also be equipped with a valve, known as a dribble valve. The dribble
valve is controlled by the batch final relay (relay 1), and the main valve is controlled by
relay 2, which is programmed to be the batch prewarn output. This fiendishly clever
setup operates in a simple manner. When a batch is started, both relays turn ON,
both valves open, and flow commences at a full flow rate. At a set value before the
batch final preset is reached, the prewarn relay turns OFF, closing the main valve,
and flow is reduced to the dribble rate. When the batch final preset is reached, the
batch final relay turns OFF, shutting the dribble valve and stopping the flow.
The “set value before the batch final preset” is the
batch prewarn preset
. It will
probably be set by the installer. The batcher does the arithmetic internally to deter-
DESCRIPTION cont.
