##What is valve sequencing?
Sequencing of two valves at different bench set, making each one respond differently to the same air pressure signal. Either the electrical signal or pneumatic signals will be common, control action reach each valve with split ranging.
##Valve sequencing in Field bus system:
Digital adaptation of the dual-output controller sequencing method is seen in FOUNDATION Fieldbus systems, where a special software function block called “SPLT” exists to provide split-ranged sequencing to two valves. The “SPLT” function block takes in a single control signal and outputs two signals, one output signal for each valve in a split-ranged pair
In this Fieldbus system, a single PID control block outputs a signal to the SPLT block, which is programmed to drive two unique positioning signals to the two valves’ AO (analog output) blocks. It should be noted that while each AO block is unique to its own control valve.
In control systems where valves are split-ranged in either complementary or exclusive fashion, one control valve will be fully closed and the other will be fully open at each extreme end of the signal range (e.g. at 4 mA and at 20 mA). If the sequencing for a set of complementary or exclusive split ranged control valves happen after the controller the valves must fail in opposite modes upon loss of controller signal.
Dual controllers are an option only for specialized applications requiring different degrees of responsiveness for each valve, usually for exclusive or progressive split-ranging applications only. Care must be taken to ensure the controllers’ output signals do not wander outside of their intended ranges, or that the controllers do not begin to “fight” each other in trying to control the same process variable.
An important consideration – and one that is easily overlooked – in split-range valve systems is fail-safe mode. the basis of fail-safe control system design is that the control valve must be chosen to fail in the mode that is safest for the process in the event of actuating power loss or control signal loss. The actions of all other instruments in the loop should then be selected to complement the valves’ natural operating mode