With a Little Help from Your Global Peer-to-Peer Community

by , | Jan 29, 2016 | Control & Safety Systems, Services, Consulting & Training | 0 comments

Social networks have made peer-to-peer knowledge sharing easier and more accessible than ever. Given the challenge of running process manufacturing and production facilities safely, reliably and efficiently, this ability to tap into others’ expertise is critical.

A great example of where this knowledge sharing occurs is in the Emerson Exchange 365 community. For example, in the DeltaV group, here are five recent posts with either verified answers or suggested answers:

I’ll share an example of this knowledge sharing in post started yesterday, Insight On Demand Tuning, Error, Level Control. The person starting this thread asks:

I need to tune a level control loop output from oscillating to big. The level is used to control level in a column and simple feedback loop, normally in AUTO.

The problem is the pv tracks the sp ok, but the output (valve) swings too much which things downstream. I have been tasked with reducing how much the valve opening and close just to control sp…

Is there away to tune the loop so the valve output does not oscillate so much? Is Insight On Demand Tuning giving error because my loop is oscillating too much?

Emerson’s Mark Coughran responded [hyperlink added]:

Emerson's Mark Coughran

Yes, there is a systematic way to stop the oscillation but still make the loop respond in the time you require. It is called Lambda tuning and is taught in Emerson Educational Services Course 9030, 9032, or 9035. Emerson consultants and our local business partners (including some people available to your plant) have used Lambda tuning to fix thousands of problems like yours.Your DeltaV Insight Tune has some capability for Lambda tuning. However, as you asked, there may be a problem that prevents Insight from getting a good process model. It could be a problem in the control valve, or just getting the process to a stable condition before starting the test. This may not be a trivial task–that’s why I suggest you gain a little knowledge before proceeding.

A good article for you to read was published in controlglobal.com

Andre Dicaire with Emerson local business partner, Spartan Controls added:

Sounds like this might be a surge tank, and rather than control level at setpoint, it should be absorbing swings in the process to stabilize flow down stream. This is a common problem in industry where level control is applied incorrectly. If stabilizing the flow downstream is your main criteria, you should not be attempting to maintain a tight level control. There are several options available on the PID loop to create a gap control or other behavior to meet the requirements of the process as a whole. You should be looking at the retention time of the tank and what its intended purpose in the process is. But you description appears to indicate that stable flow downstream is more important than tight level control.

I shared two blog posts where we discussed surge tank level tuning, Tuning Considerations in Surge Tank Level Control and How Good is Your Level Control?

Emerson’s Lou Heavner offered:

Emerson's Lou Heavner

In truth, every tank level application is a surge control problem. In the case of distillation, usually both the base level and reflux accumulator level need to be held within fairly narrow limits. Lambda tuning, as Mark Coughran noted above and as is mentioned in your links, provides a nice first order response to a load disturbance. Lambda, the closed loop time constant, should be selected as large as possible in most level control applications, but short enough that the level will remain within allowable limits in the face of the largest possible load disturbance. In the overhead, this becomes particularly important if the level is controlled by manipulating the reflux flow. This arrangement isn’t that common, but becomes necessary in distillations with a high reflux/distillate ratio. The bottom product can usually be fairly loose unless the bottom flow is feeding a downstream unit or is being used to preheat some other stream. Then it should be as stable as possible. The important thing is to tune for gradual non-oscillatory flow.

The best part is that this knowledge sharing is not buried in email, but available for anyone else with similar issues via a quick Google search. Check out the full list of Emerson Exchange 365 groups you can join, once log into or create and log into your account.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.

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