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Apr
19

Tracking Down Disturbances

The first flow that has significantly changed or started to oscillate is normally directly or indirectly associated with the root cause of a disturbance. If the original disturbance is temperature or concentration, the correction in the manipulated flow of a pressure, temperature, or concentration loop is the key to focusing on the right unit operation and loop. The most frequent unidentified disturbance is a change in raw material composition. Often this changes the utility or vent flow for temperature or pressure control, a reagent flow for pH control, and a recycle, reflux, or reactant flow for composition control. The deployment of wireless flow measurements can help track down disturbances, since the predominant inputs to processes are manipulated flows.

 The most common readily preventable disturbances are feed rate changes by the operator. Often these are seen at the start of a shift control room as operators go for their “sweet spot.” Undersized surge tanks, equipment performance problems, equipment constraints, manual startups, and manual actions are conducive to setting up a continual transient in plant operation. The automation of all manual and corrective actions to make them repeatable and predictable coupled with plantwide flow feedforward control can move process diagnostics from the basic to the advanced level by looking at changes in the flow ratios as indicative of changes in raw materials, operating condition, utilities, and ambient conditions.  These disturbances and front end production rate changes typically propagate downstream so the furthest upstream loop with an oscillation is normally the culprit.  However, when the flow is set to centrifuges and dryers, level control on slurry tanks or evaporators may manipulate a flow into the vessel. In this case, a change in back end production rate propagates upstream. However, since the disturbance is a readily observable flow change, the upset is easy to spot. In fact the best way of finding the source of any disturbance is the first flow with a significant change on a trend chart of all of the PFD flows, achievable with wireless measurements and an expansive trend or trajectory visibility on an improved operator interface. The time range of the trends (trajectories) of flows must be consistent and long enough to spot the initiating event.  For a list of the sources of disturbances checkout “Without Deadtime and Disturbances I would be Out of a Job.

 Perpetual oscillations are usually the result of improperly tuned level loops or limit cycles from excessive valve stick-slip or deadband.  The most common mistake with level loop tuning is a reset time that is too small. Most people don’t realize that the product of the controller gain and reset time must be greater than the inverse of the integrating process gain (level ramp rate per % change in flow). The counter intuitive situation exists where in order to use a small reset time, a large controller gain must be used. The limit to how high you can go in controller gain for stability is way beyond the comfort level so the gain limitation is amplification of measurement noise and how fast you are allowed to change a flow.  For surge tanks a velocity limited feedforward could be helpful as described in the March-April InTech article Feedforward control can enable flexible, sustainable manufacturing.” 

The amount of stick-slip and deadband in a final control element can be quantified by plant tests. The fix may be as simple and quick as a digital positioner tuned for the application or as expensive and time consuming as replacing a fundamentally poor valve choice. If plant oscillations stop or their amplitude is reduced when the controller is put in manual, the suspect controller is confirmed as the culprit. Steps are then made in the controller output in both directions starting with a minimum size, such as 0.25%. The steps are increased in size until there is a consistent, uniform, and timely process variable response. Temperature and level measurements are too slow. Many pressure loops cannot be put in manual long enough to use pressure as a measurement to judge valve response.  Many temperature and gas pressure loops have an integrating response like level loops, making identification of the size of the valve response problematic. Flow measurements provide the fastest and most direction indication of the valve response. If a flow measurement is not installed, a portable wireless flowmeter can be the solution for the tests. Since stick-slip can increase at lower production rates (operation closer to seat) and from stem corrosion and excessive tightening of the packing, periodic testing may be warranted. If the flow measurement is used to provide a flow loop, the stick-slip can monitored in closed loop operation as being simply the amplitude of the saw tooth oscillation in the flow controller output.  Noise, poor actuator-positioner performance, disturbances, and cascade control can complicate the identification of the limit cycle making tests in manual useful.