Appendix F, Valve Control, Rosemount 848L

Reference Manual

00809-0100-4696, Rev AA September 2004

Rosemount 848L

Appendix F

Valve Control

INTRODUCTION TO VALVE CONTROL

Introduction to Valve Control	page F-1

Industrial valves have two general classifications, regulating and block. A regulating valve is designed to be stable at any one of a nearly infinite set of positions between open and closed. They are mostly used in control loops so that nonlinearity and friction are corrected by feedback control. A block valve is designed to be either tight shut or wide open. They are mostly used to change the configuration of process equipment, such as a heat exchanger that can be used to heat or cool, but not both at the same time. Block valves configure steam in and condensate out for heating or chilled brine in and return for cooling. Regulating valves are being used as block valves when the actual position of the valve must be known, but analog outputs are used.

Block valves generally have some kind of switch that is closed in the open position and another switch for the closed position. These are called confirm contacts even if they are proximity switches. The valve position is unknown when neither switch is closed. If the valve actuator has adequate power then it is rare to find both switches open, except for a period of time known as the travel time when the valve is moving from one position to the other. Actuators can be hydraulic pistons, pneumatic pistons or diaphragms, or motor driven screws, in order of increasing travel time. More than 80% of the actuators use compressed, oil and water free air for power. Valves are referred to as air to open or air to close.

A block valve may be controlled by push buttons or by a toggle switch. There is no contactor as there is for a motor. Permissive and interlock circuits may be applied. The actuator may require power to be applied to open it, with a spring to return it to the closed position, or vice-versa. A block valve may be required to stay in its last position on air or power failure, so there is one pilot actuator to open it and another pilot actuator to close it. The pilot actuator is not usually designed for continuous power, so a few second pulse may be all that is required. The actuator is called a pilot because it just directs the flow of fluid power, as by pushing a spool valve from one side to the other. The spool valve directs the main flow to one side of the main actuator or the other, like the pilot valve in a power steering system. Two pilot solenoid valves are required if the spool latches in position, or one if the spool has a spring return.