Hello experts and Andre Dicaire
We have a Fisher control valve calibrated for a 0-100% span corresponding to 4-20mA. However, with less than 2% valve opening, the discharge flow exceeds the downstream requirements, causing the valve to repeatedly open and close, resulting in oscillations.
The valve is a 2-inch model, recently calibrated, and the operating range of the process line should be 0-8%. I dont have much information at this time?
To address this, would adjusting the valve span to map 0-8% to 4-20mA help stabilize the system? Alternatively, should we consider replacing the valve with a 1-inch valve or installing a back-pressure regulating valve helps?
Expert in the house, which option would be most effective and reasons?
Eapen Mathew
Senior Process Automation Engineer
EQUATE Petrochemical Co
In reply to Eapen Mathew:
In reply to Jawad Waheed:
In reply to E15:
I am going to concur with the others that say, if your control valve will only open 2% for the operating range of your process, it is not correctly sized. Re-ranging the entire command span to represent the 0-8% opening is unlikely to help, as the problem likely is the valve is having difficulty modulating only a small amount, as opposed to signal issues (unless you knew of an obvious source of noise or other signal error). Also note that you initially said that you are exceeding the desired output at 2% open, so even 8% valve opening at 100% command means that the valve would be being given a maximum 25% command, if linear, under normal operation.
If you can reduce the input pressure to the valve (and that does not negatively impact the process), then you should be able to achieve a smaller change in flow relative to a larger change in valve position, which should give the valve much better control authority, and you should be able to tune the input pressure to optimize your control. Is it even practical to knock the pressure down to the point that your valve would be open at 50% under normal operation, which would give it the maximum control authority? Replacing the control valve would be optimal (don't need an additional / altered pressure control), but would likely be a more expensive solution. You mention a 1" valve. Is that mentioned because it is readily available? If your 2" valve is already over-flowing your target rate at 2% open, I'm not sure a 1" valve is nearly small enough to help. There are many additional factors, but ballparking a 1/4 reduction in the cross-sectional area is not a lot relative to the need to increase your control authority more than an order of magnitude. If there is ever a thought that you might need to increase the flow rate in the process at a later date, reducing the control valve size permanently reduces the maximum available flow (relative to the maximum available line pressure upstream of the valve).
In reply to Jeffrey Mach: