What should I do if I have a problem with my control valve?
What should I do if I have a problem with my control valve?
Depending on the type of valve, correcting the problem requires changing the entire valve or changing the valve trim. Over 30% of control valves have problems, many of which can be detected during normal operation using simple tests. Correcting valve problems can significantly improve bottom-line results.
Why are small changes in valve position cause poor control?
A small changes in valve position has a large effect on flow. This leads to poor control performance because any valve positioning errors, like stiction and dead band, are greatly amplified by the oversized valve. Nonlinearity. A valve with a nonlinear flow characteristic can also lead to tuning problems.
Why does the stem of a control valve stay closed?
Instead of the stem smoothly lifting immediately as pressure exceeds the bench-set value, this valve will remain fully closed until enough extra pressure has accumulated in the actuator to generate a force large enough to overcome spring tension plus valve friction.
How is the flashing of a control valve predicted?
An important parameter predicting flashing in a control valve is the valve’s pressure recovery factor, based on a comparison of the valve’s total pressure drop from inlet to outlet versus the pressure drop from inlet to the point of minimum pressure within the valve. = Pressure recovery factor (unitless)
Can a function generator cancel out the nonlinearity of a control valve?
If this is the case, a function generator (X-Y curve) can be placed in the path of the controller output to cancel out the control valve nonlinearity. Some processes react differently based on operating point, production rate, or the product being made.
What are the most common control valve problems?
The most common control valve problems causing oscillations are: Control valve stiction. Do a stiction test with the controller in manual to determine if this is the case. Positioner overshoot. Do step tests of various sizes and be on the lookout for signs of overshoot in the process variable.
Are there any problems with a control loop?
While many control loops are easy to tune and present almost no control problems, a few control loops can be very problematic and never seem to control right. Control practitioners can spend many hours or even days trying to improve the performance of these challenging control loops, but the results often remain unsatisfactory.
What to do if your purge valve doesn’t seal?
Blow air through the purge solenoid while in test mode to make sure it seals properly. If it doesn’t seal properly you can try blowing some compressed air through the ports to dislodge any crud that may be in the valve. Some of the purge valves you can disassemble and clean the valve seat and the bore it sits in.
