Hiten Dalal’s Question #1
I have been trying to get a handle on small ripples in one of the pipelines by using a rule of thumb to successively reduce proportional action by 20 percent and integral action by 50 percent. Using the same rule, I could stabilize the ripples on Friday. On Sunday, the product changed in the pipeline and with that back came those 4 percent ripples. There is one control valve that impacts line pressure. I could stretch ripples a bit but could not eliminate them. Output going to zero is natural scheduled shutdown of pipeline. I know it is a lot of information that I am providing but perhaps you can glance through and pinpoint something that stands out. I am learning since I started tuning the control valve that it is product sensitive as well.
Greg McMillan’s Answer #1
Since I don’t know if there is a trend of valve signal and valve flow, I am not sure what is happening. If the considerable decrease in gain does not help or makes it worse, I am wondering if there is some valve stiction or backlash, respectively. Is the valve the same for both products? Could a product be causing more stiction due to buildup or coating on valve seating or sealing surfaces or stem? Could the Sunday valve be closer to the shutoff where friction is greatest?
It sure looks like you have too much proportional (P) action for the new product. The integral action is already greatly reduced and most of the overcorrection is occurring very quickly due to proportional action. I would try decreasing the proportional mode action (proportional mode gain) by 50 percent (cut gain in half). If this helps, reduce the proportional gain again. Based on the very small integral (I) action, you may be able to increase integral action once you decrease proportional action. However, I reiterate that if decreasing the gain simply increases the period of the oscillation, you have backlash or stiction. If amplitude stays the same, you have stiction.
Please make sure there is no integral action in the digital valve controller.
Hiten Dalal’s Question #2
When you say no integral action, do you mean in valve positioner or in controller? I don’t think our positioner has any PID setup. Only PID action is in controller. Since it is liquid pressure and flow, we use P&I. Are you suggesting we use only P action in my controller?
Greg McMillan’s Answer #2
I meant no integral in the valve positioner that for Fisher is called a digital valve controller (DVC). You should use integral action in most process controllers (e.g., flow and pressure). Integral action in the process controllers is essential for the PID control of many processes. So far as tuning the process controller for pipeline control, the integral time also known as reset time (seconds per repeat) should generally be greater than four times the deadtime for an ISA Standard Form. You must be careful about what PID form, structure and tuning setting units are being used. If the integral setting is an integral gain, such as what is used in the “parallel” PID form depicted in textbooks and used in some PLCs, the integral setting may not just be a simple factor of the deadtime (e.g., four times deadtime) but will also depend upon other dynamics. Also, some integral settings are in repeats per minute instead of seconds.
Please make sure you extensively test any tuning settings by making small changes in the setpoint with the controller in automatic or in the controller output by momentarily putting the controller in manual. There should be little to no oscillation. The tests should be done at different valve positions particularly if the valve installed flow characteristic is nonlinear. Oscillations may be most prone near the shutoff positioner where stiction is greatest from seat/seal friction.
If there is interaction between loops, the least important loop must be made slower or decoupling used by means of a feedforward signal. If you are going to do some optimization via a controller that seeks to minimize or maximize a valve position, the proportional gain divided by the reset time for this controller doing optimization must be an order of magnitude smaller than process controller to prevent interaction. These PID controllers used for optimizing a valve position are called “valve position controllers” (VPC). I hesitated to mention this to avoid confusion because these are not valve positioners and are only used for optimization. Also, nonlinear or notch gains and directional move suppression via external reset feedback are used to keep the VPC from responding too much or too little so the process controller does not oscillate or run out of valve.
Many newer smart positioners have added integral action to positioners in the last two decades. In some cases, integral action is enabled as the default. This prompted me to write the Control Talk blog post “Getting the Most Out of Positioners.” This blog does not address setting integral action in process controllers (e.g., flow and pressure controllers).
Hiten Dalal’s Question #3
Do you teach a control valve tuning class? Is there a specific method you recommend for a pipeline control valve?
Greg McMillan’s Answer #3
I do not offer a class on tuning positioners. Supplier courses on tuning positioners are good but you will need to insist on turning off integral action. You can have them talk to me if they disagree. In general you should make sure you do not use integral action and that you use the highest valve positioner gain that does not cause oscillation since for pipeline flow and pressure control, oscillations are not filtered. If you have an Emerson Digital Valve Controller (DVC), I recommend “travel control” with no integral action and with the highest gain that still gives an overdamped response. The valve must be a true throttling valve and not an on-off valve posing as a throttling valve as discussed in the Control Talk blog “Getting the Most out of Valve Positioners”. Note that in this blog we are going for a more aggressive response than what you need. Because of the lack of a significant process time constant in a pipeline, you need a smooth valve response. In the blog, the valve positioner gain is described to be set high enough to cause a slight overshoot and oscillation that quickly settles out. Oscillations in the valve response are useful to get a faster response for vessels and columns since there is a a large process time constant to filter out oscillations. You want to still use a high gain and no integral action in the positioner but seek an overdamped (non-oscillatory) response of valve position.
Hiten Dalal’s Follow-up Reply
I have bought Tuning and Control Loop Performance Fourth Edition. I reference tables from there for suggested PID values. I have removed derivative from several pressure and flow loops and observed them to be equally efficient. In the process of tuning I have learned that operations installations have impact on loop tuning. I have made the following types of corrections,
(1) As installed, the logic had the PID getting initiated as soon as block valve #1 was fully opened but block valve #2 was getting commanded to open after #1 causing PID output to ramp off to high output limit since the control valve was not seeing full flow. We solved this by setting temporary upper clamp in PID output at safe limit to avoid overshoot until block valve #2 was fully opened.
(2) Transmitter range was high and margin of error was not acceptable by operations. Re-ranged transmitter to suitable range and brought error within acceptable margin.
(3) EIM Controls Electric and REXA electrohydraulic actuators have a limit on number of actuations. I added an acceptable dead band to reduce number of actuations.
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