CONTROL VALVE Maintenance INTRODUCTION

CONTROL VALVE Maintenance INTRODUCTION

In order to perform even routine maintenance procedures on a control valve, it is important that the maintenance man have a thorough understanding of the fundamental construction and operation of the valve. Without this knowledge, the equipment could be damaged inadvertently, or could cause injury to the maintenance man and others in the area. Most valve manufacturers in their detailed instruction and operation manuals. Usually, a sectional drawing of the equipment is also furnished to help in understanding the operation of the equipment as well as to provide identification of component parts.

In all major types of control valves, the actuator provides force to position a movable valve plug, disc, or ball in relation to a stationary seat ring or sealing surface. The moveable member should respond freely to changes in actuator loading pressure. If proper operation is not being received, service is indicated. Before any maintenance procedures are started, be sure that all line pressure is shut off and released from the valve body and also that would damage the equipment or injure personnel.

Often corporate maintenance policy or existing codes require preventive maintenance on a regular schedule. Usually such programs include inspection for damaged of all major valve components and replacement of all gaskets, O-ring seals, diaphragms, and other elastomer parts. Following is a series of commonly performed maintenance procedures and some general instructions for performing each procedure instructions are normally furnished with control valve equipment and should be carefully followed.

* Replacing Actuator Diaphragm
* Replacing Stem Packing
* Replacing Threaded Seat Rings
* Grinding Metal Seats
* Lubricating Control Valve Packing

Replacing Actuator Diaphragm ^
After isolating the valve from all pressure, relieve all spring, if possible. (On some spring and diaphragm actuators for use on rotary-shaft valve bodies. spring compression is not externally adjustable. Initial spring compression is set at the factory and does not need to be released in order to change the diaphragm.) Remove the upper diaphragm case. On direct-acting actuators, the diaphragm head assembly must be dismantled to change the diaphragm.
Most pneumatic spring-and-diaphragm actuators utilize a molded diaphragm for control valve service. The molded diaphragm facilitates installation, provides a relatively uniform effective area throughout the valve’s travel range, and permits greater travel than could be possible if a flat-sheet diaphragm were used. If a flat-sheet diaphragm is used in an emergency repair situation, it should be replaced with a molded diaphragm as soon as possible.
When re-assembling the diaphragm case, tighten the cap screws around the perimeter of the case firmly and evenly to prevent leakage.