I have a loop in a plant that involves two split range outputs. One to a small trim valve, the other to a variable speed drive on a pump. The pump and the trim valve both add a particular dense material to a tank of water in a density loop that measures the density of the solution leaving the tank. The flow from the tank is variable. The variable speed pump on the dense additive cannot change speed fast, so the trim valve is used for rapid changes based on measured density changes (due to discharge flow changes).
Both outputs are from the same controller, using a split range output. It is an old, analog controller. That controller will be replaced with a loop added to an existing DCS. Basically, the loop does not work and nobody has been able to tune it. It stays in manual, the discharge from the tank is not constant. This variation in density causes problems downstream.
Is there a better way of controlling the density? We would like to keep the trim valve about 50% open on average, going further open or closed to control short term variations in the density. Larger changes, resulting from more significant changes in the flow from the tank, will be handled by the variable speed pump, although it is slow to react.
Both the dense material from the trim valve and the material from the variable speed pump are the same material, but from different supply tanks. The trim valve does not have the rangeability to cover the necessary change in addition of the dense material.
Has anyone worked with a loop like this before?