If there is starvation of flow in the metering stream,
the control valve will open to compensate. However
if it reaches a point at which it can no longer
properly control, a cut-back (pacing) signal is sent to
the control system to reduce the master demand rate
to equal the maximum flowrate at which the lagging
component can maintain correct ratio and accuracy.
Most blending systems, particularly those in the
petrochemical industries, operate this way and
produce very good results. This type of system also
lends itself to skid or trailer mounting, enabling it to
be towed around a harbour/terminal or mounted on
the deck of a bunkering barge.
The use of microprocessor based controllers enables
several advanced features and facilities to be
provided. For example a number of predetermined
recipes can be stored, the operator then only calls up
a recipe by name or number and the controller sets
correct ratios. Temperature compensation can be
performed electronically using the signal from a
resistance bulb or temperature transmitter to correct
measured volumes to a base temperature. The blend
controllers also provide the facility to drive a printer
to and communicate with another computer system