tailieunhanh - CONTROL VALVE HANDBOOK Episode 1 Part 3

Tham khảo tài liệu 'control valve handbook episode 1 part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Chapter 2. Control Valve Performance nal changes as great as 5 before it begins responding faithfully to each of the input signal steps. Valve C is considerably worse requiring signal changes as great as 10 before it begins to respond faithfully to each of the input signal steps. The ability of either Valve B or C to improve process variability is very poor. Friction is a major cause of dead band in control valves. Rotary valves are often very susceptible to friction caused by the high seat loads required to obtain shut-off with some seal designs. Because of the high seal friction and poor drive train stiffness the valve shaft winds up and does not translate motion to the control element. As a result an improperly designed rotary valve can exhibit significant dead band that clearly has a detrimental effect on process variability. Manufacturers usually lubricate rotary valve seals during manufacture but after only a few hundred cycles this lubrication wears off. In addition pressure-induced loads also cause seal wear. As a result the valve friction can increase by 400 or more for some valve designs. This illustrates the misleading performance conclusions that can result from evaluating products using bench type data before the torque has stabilized. Valves B and C figure 2-3 show the devastating effect these higher friction torque factors can have on a valve s performance. Packing friction is the primary source of friction in sliding-stem valves. In these types of valves the measured friction can vary significantly between valve styles and packing arrangements. Actuator style also has a profound impact on control valve assembly friction. Generally spring-and-diaphragm actuators contribute less friction to the control valve assembly than piston actuators. An additional advantage of spring-and-diaphragm actuators is that their frictional characteristics are more uniform with age. Piston actuator friction probably will increase significantly with use as guide surfaces