Understanding, troubleshooting Hydroheater autopilot

Amplification of process upsets are avoided when Hydro-Thermal’s Hydroheater autopilot is used correctly, writes Alan Strauss. This article is titled "Understanding your autopilot" in the November issue of EPM.
By Alan Strauss | October 15, 2015

The industrial Hydroheater is used to heat slurries over a wide range of viscosities and solid contents to a precise temperature. Many corn ethanol plants use Hydro-Thermal’s Autopilot to help control their operations automatically.  What exactly happens in auto operation, and how can you tell if yours is operational?

First and foremost, the autopilot is designed to maintain a differential pressure (DP) set point of choice—nothing more and nothing less. This is done with the actual, current DP signal, which is calculated between the incoming pressure and the discharge pressure from the distributed control system (DCS).  Establish the actual DP signal and the desired set point DP, and the combining tube (CT) moves accordingly—CT opens to lower DP or closes to raise it. This will achieve the desired set point in the auto setting, regardless of flow, temperature, percent solids or anything else.

Process imbalances can lead customers to believe that the autopilot isn’t functioning correctly. However, what is occurring is that the process is upset and the autopilot is making it worse.  For instance, if the flow drops, the CT will close in order to raise the DP. It will close it all the way down if left in auto mode and really restrict flow, thus making a bad situation worse, although it is functioning as it is designed to do. If there is a process upset, switch it into manual mode until the situation is under control, and put it back into auto once stabilized.  Auto is similar to cruise control in your car.  You don’t start, stop or accelerate in cruise control. You turn on cruise when you are going down the road at a steady speed for a while and it will then maintain your speed going up and down hills.
In the K5 Autopilot manual, there is a description of each mode. Look in the operation section for an explanation of the Automatic Run-Mode and Manual Operation Mode, for further explanation. Some plants have recently upgraded to the K600 and while the mechanics are different, the function on auto pilot is the same.

Troubleshooting
For troubleshooting, field service technicians first establish if the customer has the DCS hardwired and programmed to operate the Hydroheater in auto, plus verify that the function is turned on in the DCS. This is usually done during installation and startup. The autopilot needs DP, the DP set point, actual DP and enable signals in order to function in auto mode.

The enable signal wire is the easiest of the three to find and to figure out if the Hydroheater is wired for auto mode. Open the autopilot electrical panel (following all OSHA rules and electrical safety practices) and check to see if there is a wire coming from the bottom of the panel, connected to the lower part of the blue terminal block, on terminal Input No.10.  If there is no wire connected the K5/K600 will not operate in auto mode.

If there is a wire in that terminal location, the programming must also have been done to turn the unit off or on.  A quick check can be done to see if the circuit is active.  Have the DCS operator put the Hydroheater in auto from the control panel.  While the K5/K600 electrical panel is open, watch the PLC for Input No. 10 to light up.  With the DCS in auto and the Input No. 10 light on, turn the manual/auto switch to auto mode. 

Other considerations to know: Many customers have the PLC program called “pulse program,” which creates a pause in operation when the DP is within 5 PSI of the set point—the air motor on the CTA will run for 3 seconds and pause for 30 seconds, continuing until the set point is achieved. This prevents hunting, where the CT constantly moves above and below the set point. The very latest PLC program works with the K5/K600 and has a 2 PSI dead band. This change in program incurs less flow upsets, which assists in reducing wear. For your reference, the CTA digital display shows the percentage the combining tube is open. The pressure differential digital display shows the current and actual DP.

Author: Alan Strauss
Field Technician, Hydro-Thermal Corp.
astrauss@hydro-thermal.com
262-548-8900

Editor’s note:  Ethanol Producer Magazine is introducing a new column that will appear periodically. It will offer industry-written best practices and troubleshooting tips for ethanol producers, diving into the details as this first one does, or covering broader topics from the frontlines of plant operations.

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