Small Mouth: Models A, B, C – 8″ sq stand x 26-31″ high or Model D – 10″ sq stand x 42″ high
Time to do something, when Batch Weighing Accuracy Performance does not meet Accuracy Requirements. Understanding the problem should the first thing on the to do list.
Sometimes accuracy problems are design issues and sometimes maintenance issues. If it worked before, you are lucky you only have a maintenance issue. If your system has trouble maintaining itself within tolerance, you may not be so lucky after all.
Closed Loop Control and every component all affect Closed Loop Control Performance.
1) Scale accuracy and
2) the response time it takes for the control system to sense the change accurately
3) and the response time it takes to control the flow to the scale all affect performance
Material is being fed between controlling devices and scale. The shorter the distance between the two improves Closed Loop Control response time. Controlling devices can be gates, valves, rotary valves, augers, conveyors, etc. A consistent head of material feeding these is important to maintain. The system is adjusted for a given amount of material to be delivered in a certain amount of time. When that material is not there at the scale, your system has to adjust on the fly. Production rates and product quality suffers.
An unknown factor affecting accuracy is the amount of material (in the air) between the controlling device and the scale. The material in the air the flow controlling device and the scale is always questionable when flow rate is not consistent. This problem can be minimized at the cost of production rates. So the question is; At what cost do you fix the inconsistency or live with lower productions rates or trade off to less accuracy?
Production Rates and Quality Control often seems at odds with each other. Tie that in with inconsistent feed to the weighing device and meeting set point becomes more difficult to achieve.
Most readers already have a n electro-mechanical system in place. Usually, mechanical fixes are harder to implement than electrical systems.
Batching systems compare setpoint to the scale readings. Material can be weighed in one of two ways.
- Continuous Weighing (Dynamic Weighing) devices such as Bin Weigh Loss or Impact Scales or Belts Scales weigh material on the fly. Weigh Loss Systems have an advantage because material is not free falling to it but diminishes when the system takes some time to average out over time what the flow rate is. Belt scales have the same advantage but the advantage diminishes when a belt scale reaches set point and the belt is left with a half belt full of material. This half belt full is an unknown about to be fed to the next batch. Consistent feed rate helps to minimize that problem The impact scale is your best choice because it weighs at the end of a conveying device. Continuous Weighing devices take less vertical] space and are easier than Batch weighing to fit into a process. Some continuous weighing devices only take 4” of vertical space.
- Batch Weighing (Static Weighing) receives material in a container, weigh hopper. The hopper issuspended from load cells and has means to empty the material out of the bottom. When set point is reached, the filling process is stopped and the discharge device is activated. If the flow rate remains consistent from batch to batch, a Preliminary Cutoff Factor can be entered into the Batch Control to shut the filling process early to compensate for the material in air not that is yet weighed.
Whether you are weighing through the scale or to the scale, material will not be accounted for in free fall. This becomes an assumption. This assumption can be most accurate the more we keep our flow constant. A constant head of material helps.
When flow is not constant and when slower production rates are acceptable, the batch control can be set to creep up to setpoint at slower feed rates by adding an additional set point or two. Auger, rotary valve and conveyor speeds can be slowed, gates can go into a chatter mode, gates can be positioned from completely open to half open to 10% open to closed.
The human element is always a factor. Everyone seems to have their recipe for the best operating procedure. Develop your best operating procedure taking all tradeoffs you have into consideration. Train your operator to use it consistently.
Net Weights vary…
- Check for varying feed rates. Minimize varying rates.
- Check that scale holds zero rate before and after weighing material.
- Check sensitivity of batch control and actuator system. Adjust for best performance
- Check for leaking feed and discharge gates.
- Check for consistent controlling device activation-deactivation. Loose linkage is often a problem. Operate the system in Manual Mode. This puts your problem in slow motion which it makes it easier to understand the problem.
- Check that all material weighed is being captured in the container being filled.
- Check reference scale for consistency.
- Develop your best operating procedure taking all tradeoffs you have into consideration.
- Train your operator to use operating procedure consistently.
Life is all about trade-offs. Educate yourself on the trade-offs and navigate to the best set of them for the job.
Clarence Richard, Instructor
Plant Operation Workshop Web-Based Training Provider http://clarencerichard.adobeconnect.com/workshopdemo
Last summer, we offered several asphalt mix producers in an area, a site visit to train their people while auditing the status of their operation and as a bonus, fixing problems we had the resources to fix while we were there.
For Flinn Paving, we not only identified problems with their combustion, we fixed them. Whereas, others before us tried to reduce the incomplete combustion, we stuck with fixing it until we did. What made this fix difficult to pin point was the fact that they had two different things contributing to the problem. Material was falling through the flame and the air to fuel arrangement on the burner was set too rich. Both resulted in reading extremely high Carbon Monoxide readings at the stack. Once these two issues were resolved, that burner roared like never before. The combustion intensity at their normal burner position was so great, the burner position was lowered significantly. In so many word, it took less fuel to heat and dry their aggregate.
Our analyzer was showing we were burning nearly all the carbon. As a result, the owner was now working on a positive cash flow that was not available to them before. Saving money is somewhat like a revenue stream.
Controls have evolved since this combustion system was put in place. Matt, the owner, wanted better mix temperature control. With an Automatic Damper control, we could maintain the best burner efficiency possible. With better safety interlocks, we could protect personnel and the equipment much better. And so, we built him a system specifically for his needs.
Over the years, older equipment gets hard to maintain because they are outdated and parts are not readily available. And as funny as it seems, Burner Controls and Damper Controls are usually not run in Automatic. The typical, justifiable excuse I hear from plant operators are that the controls over shoot the set point excessively or just don’t work.
Plant Manager, Scott Cartmill, was running his plant with controls that were not designed to be a Baghouse High Temperature Limit. The aggregate temperature was not automatically controlled. The exhaust damper would constantly hunt for set point because the damper actuator control was too sensitive and there was no way to desensitize it.
Plant Operators should not be babysitting adjustments that can be easily replaced with Automation. Plant operators should have a clear mind for other important things they do. Babysitting processes adds to their stress and adds distraction.
The trick is to set the control so it will make small adjustments so as not to over shoot the set point yet fast enough to keep up with process changes.
Many plants have a damper control that uses a Duff Norton Actuator controlled by a Dwyer Photo-Helic (sensitive vacuum switch) . The actuator moves too fast for the delayed correction to be measured by the Photo-Helic. Consequently, the system will hunt. This forces the operator to place his Damper Control Mode to Manual. The draft wanders around when the operator is not babysitting it and fuel is being wasted and this causes material temperature changes. FYI The Draft Set Point is best determined during a burner tune up. Until that has been determined, the operator should experiment with finding the best draft setting by comparing different settings with the highest material temperature. This is should be in your SOP (Standard Operational Procedure).
Our upgrade included that the Exhaust Damper and Burner Control Valves were provided with valve position indicators. During startup, this is allowing the operator to anticipate where to set these valves at without having to wait for the system to respond.
Many of the older burners had one limit switch to help determine that the mechanical firing rate control on the burner was in a low fire position. Much like making sure your foot is off the gas pedal when you start the car. We do not want High Fire Heat to be roaring down the drum to the baghouse. There are two components necessary that need to be in sync with each other, the gas valve and the burner air damper valve.
Our upgrade included that we placed a limit switch on both of those. Plaant Mechanic KevinKesselberg simply took a small sheave with a bore diameter that would fit over each valve shaft end and then tightened the set screw in a position to make the limit arm actuate the switch when the valves reached the Low Fire Position. We not only electrically tied these two limits in series to the Pilot Solenoid Valve, we wired them to an indicator light (Low Fire Position) on the Panel so we can cross check it’s proper operation every time we light up and shutdown. We used single pole limit switches to energize a double pole relay in order to accomplish this.
.Also, to protect the baghouse, we included a High Temperature Limit, UL Approved as a High Temperature Limit in the inlet. A very important improvement.
The Stack did not have a temperature readout. We added one. In the morning, it’s nice to know when your baghouse is warm enough to run. As you are running, you are expecting the baghouse to run with appropriate temperature drop. You won’t know that without Baghouse In and Baghouse Stack temperature readings. Too big of a temperature drop tells a story of why you all of a sudden lost some of your production rate. Any guesses what may have caused that?
Scott was having problems with his Aggregate Temperature reading low. After eliminating all the problems that we felt it could be, it dawned on us that much of the thermocouple body was exposed to cooler outside air. Although the Thermocouple Tip was exposed to the hot aggregate, the cooler pipe shaft was dragging the aggregate temperature reading down 80 degrees. We mounted the thermocouple so the shaft was being warmed up to temperature in the chute. Since the shaft is of softer steel material, we protected it with an angle iron to keep it from wearing out.
And finally, one of the most important things Flinn Paving benefitted from was that their people understood more about the process and the instrumentation. If you were an airline pilot, you would have to be Instrument Rated in order to fly a plane through the clouds to the airport below with nothing but your instruments. Scott Cartmill and Joe Griffin are now Instrument Rated to land that jumbo jet.
Matt Flinn has always put training for his people at the top of his list. Matt has been one of our best patrons to our training. If you want a sampling of the training they receive, please sample what we have to offer.
As you can see, I’ve been doing all the talking. If you care to contribute or debate an issue, please register. I look forward to facilitating a forum where new ideas are always forth coming.
Until we meet again, I hope the stars are aligned in your favor.