Hopper Scales – Left Coast Scales

Booting Your Hopper

I was recently called out to a customers because their hopper scale kept filling after the filling auger was turned off. I performed a thorough inspection and couldn’t find anything mechanically wrong with the scale. I had the customer run through the batching process so that I could observe the problem from the customer’s perspective. I watched the indicator slowly fill the hopper and after the filling equipment turned off, the scale slowly and erratically added an additional 200 to 300 pounds to the batch. I then went out and watched the batching equipment fill the hopper. As I watched, I saw the boot on the left slowly start to suck in until it was displacing only half the volume it had when the batch started. Somehow or another the boot was being put under vacuum and the boot was pulling on the scale. When the batch was done the boot slowly relaxed and returned to normal. With the customer’s consent I put a pipe into the top of the boot to relieve the vacuum. We ran another batch and the problem went away. I told the customer to figure out where the vacuum was coming from and fix it, but the problem was fixed for now. Rubber boots can cause some weird and perplexing problems.

If you have a tank or hopper that is being filled with powder or something similar, you have seen boots. Boots can be made of many different materials from cotton fabric to silicone rubber. They often are the culprit in weighing issues on hoppers and tanks that had been working fine and all of a sudden aren’t. They are used when you have materials that need to be transferred into a hopper or tank but the material will cause a lot of dust or want to flow out of the filling aperture. Since hard piping to a scale is a no-no, boots were created to allow the scale to move freely but still confine the material being transferred.

All of the issues that will occur with boots come down to some kind of bind. Here are some of the problems that you will see:
1. Material builds up inside of the boot and bridges across the gap between the fill pipe and the pipe on the hopper.
2. The boot is installed without any give, or too tightly.
3. Material works its way between the receiving pipe and the boot.
4. The material sticks to the boot and hardens.
5. The boot stretches like a rubber band as the hopper fills and moves.
These issues all prevent the hopper from moving freely. Without free movement the scale will often not zero correctly or weigh non-linearly.

These problems can be very difficult to find and diagnose. A full inspection on tanks and hoppers is not complete without examining the boots and piping. When problems are found it is always best to have your customer clean, repair, or replace suspect piping and/or boots to prevent problems from developing.

Examples of Impacted Boots on Various Hopper Scales

Fixing An Inventory Control Problem

A large customer of Left Coast Scales is a leader in packaging technologies, manufacturing materials from bubble wrap to bio-degradable foam packing. This customer approached Left Coast Scales looking for a solution to an inventory problem at their Southern California location. I had been working with them for several years doing scale calibration and maintenance, and they wanted to know if I could help them weigh their product since they were having a problem with inventory on the resin they used in their bubble wrap line. The material was supplied by rail car, and stored in silos on site. The material is a dry pelletized resin that is transferred directly to the production line with a vacuum conveyor. The amount of material that is in a rail car is known, and should result in a specific amount of product with a specific amount of spoilage, however the numbers weren’t adding up and 40 tons of material was missing in the last year. Where was it going? Was it not being recieved? Were they using too much of it? Were they losing some of it to theft or conversion?

Several easy solutions presented themselves for resolution of this problem. The rail cars could be weighed on-site, the storage silos could be weighed after filling, and the material could be weighed as it was used. The first two options were impractical in this instance due to the site location for a rail scale and construction of the silo didn’t allow for modification. Because the material is being transferred via vacuum conveyor directly to the production line, into a bulk storage hopper that is integrated into the forming machine, it wasn’t practical to weigh it at this point. An intermediate weigh hopper was needed. The material would be batched into the weigh hopper until a set-point was met, the conveyor would be stopped, the material weighed and accumulated and then dumped into the forming machines bulk storage hopper. A series of bindicators in the bulk storage hopper would be used to prevent the weigh hopper from dumping and also tell the weigh hopper when to start loading.

I chose Rinstrum to supply the controller for our project. Rinstrum’s R420 digital weight indicator comes in several different configurations, from a basic Gross, Tare, Net indicator (R420-K401) to a six ingredient 10 stage batch controller (R420-K411). I chose the R420-K410-A, a panel mounted single ingredient batch controller to control the process, with a Rinstrum D320 remote display for displaying the accumulated totals. The R420 was the perfect controller for this job, with a three stage batching engine that allowed me to check for batching conditions with interlocks such as the dump enable, fill enable and batch interlocks.

I defined the sequence of operation for the system as follows:
1. The bulk storage hopper would reach the low level alarm and request more material.
2. (Stage 1) The system would check to ensure the weigh hopper’s dump gate was closed then start filling it to 750lbs.
3. When the hopper exceeded the set-point it would tell the system to stop sending material. (I couldn’t get the weight to meet the set-point exactly due to the volumetric filler on the pneumatic conveyor system.)
4. After the hoppers weight stabilized the weight in the hopper would be added to the accumulator, and the accumulated weight would be shown on the D320.
5. (Stage 2) The scale controller checks to see if there is any room in the bulk hopper through the use of the high level alarm. If there is room the weigh hopper opens the dump gate and empties all of the material till the scale is empty, then closes the gate. If there is no room, the hopper waits till the bulk storage hopper requests more material before emptying. (This stage is where the scale spends the most time, waiting to dump.)
6. The scale returns to Stage 1 in a continuous cycle.

After completing the installation, testing the equipment and ironing out the bugs the system worked flawlessly. I trained the operators, turned the system over to production and the system was up and running. Everything worked for several weeks until I found that power at the site was not always reliable. The plant would occasionally lose power. I was suddenly hit with the question, “What happens if the system is batching material into the weigh hopper and all of sudden the power goes out?” I lost a couple of loaded hoppers worth of material when this happened. The solution took a little creative thinking about how the system needed to operate. I ended up swapping stages 1 and 2. Now if the hopper was at zero when started it would immediately go to stage 2 and start filling. If it wasn’t at zero it would wait till there was room and dump. I set it up this way under the assumption that there would be material in the hopper when power was lost and I needed to start at zero for the accumulations to work correctly. I then installed a UPS to power the controller when the power went out. When power is lost the R420 will receive a signal from a relay that I installed on line power causing the instrument to stop filling, accumulate the batch and then abort the process leaving the material in the hopper. Now when the customer loses power the operator only needs to press start and the system takes over.

With the success of this installation, I have installed a second system at this location with two more in the process for installation at their plant in New England.

The Load Cell Went Out and Blew Up Your Indicator

I picked up a customer recently who was tired of the high cost of repairs with their old scale company. It seemed that whenever one of their scales would go down it would have multiple components fail. The load cell, a j-box and the indicator would all go at once. I have only had multi component failures in very few instances and only in severe environmental conditions, like high pressure wash down and immersion or where the scale was abused. I find that when a technician has found multiple failures it is more likely that he is not sure where the problem lies and is throwing everything at it until the problem is fixed. He arrives on site and the scale is drifting. Aha! He thinks it has a bad load cell. He replaces the load cell and it is still drifting. Ok, well it must be the indicator. Now the indicator gets replaced, oh no, the scale is still drifting. Well, it must be the j-card; ah shoot the home run cable is smashed next to the j-box. Well, he got the ok to replace the j-box, might as well replace it as well as put in a new cable. Then he can look good by telling the customer that he’s not going to charge them for the cable. This seems to be a fairly common scenario, because I have heard it many times. I don’t think it is dishonesty that makes it happen; just the technician’s desire to look good in front of his customer and not understanding proper troubleshooting practices. When proper troubleshooting practices are followed the above scenario would not and could not happen.

A technician should always have the tools necessary to trouble shoot the scales he will be working on. To be properly equipped he should have a good load cell simulator, a multimeter capable of reading up to 5000 mΩ or displaying nanoSiemens (nS), a measure of conductance (mΩ = 1000 ÷ nS), a 9 volt battery, and the necessary technical manuals for the equipment. He should also have a thorough grounding in how to use them.

By following the steps outlined below a technician should be able to trouble shoot most scales in a relatively efficient manner. The steps should be followed until the problem is resolved or the problem can’t be found. Many times an intermittent problem will not repeat while you are troubleshooting the scale and everything will appear normal.

It is dangerous for the novice troubleshooter to fall into the trap of familiarity. Just because a problem looks like something you have seen before doesn’t mean the cause of the problem is the same. After a while familiarity will help your troubleshooting but learn to do it correctly first before taking short cuts.

Troubleshooting Steps:

Determine from the operator and/or observation what the problem is. Without a thorough understanding of what is going wrong the technician can spend fruitless time searching for the solution to an imaginary problem. Sometimes the problem lies with customer expectations, and the equipment is incapable of performing the expected task or the customer is not properly trained in the use of the scale.
Thoroughly examine all of the components of the scale noting any discrepancies and deficiencies. For example debris under the scale, the hole in the keypad, the abraded cable, any of these could be the problem or an additional problem that could be confusing the issue further. Next, correct these deficiencies where possible and re-examine the symptoms.
Check the scale setup, dip switches, jumpers and program to ensure that the indicator is set up correctly. The following are real samples of problems that I have seen in the field.

The GSE 250/255 has a program soft switch to select 4 or 6 wire load cells. If the unit is set for 6 wire and a 4 wire load cell is used, the scale does not respond to weight.
On the old Fairbanks H90-5200, the one that has dip switches. If the dip switches are set incorrectly the scale can display underload/overload or even drift.
Several manufacturers have indicators that have a 2mV/V – 3mV/V jumper that can prevent an indicator from going to full scale if set incorrectly. I had one that would weigh perfectly to 72,380 lbs but then stop going up over that weight. I spent hours hunting for that problem.
The Mettler Toledo Jaguar/JagXtreme relies on an internal backup battery and when it dies the setup can be completely lost.

Scale controllers have too many variations to list the possible problems in set up, however if the instrument is set up correctly the problem most likely exists on the electrical side.

If the problem is not in the set up of the scale then remove all of the accessories, printers, scoreboards, computers, chart recorders, set point boards, etc…. Any of these accessories could be causing the problem with the scale, through feedback, shorts, miscommunication or numerous other possible ways. If the problem goes away it was in the accessories. Re-connect the accessories one at a time until the problem starts again; once it does you have found your problem component.
If the problem does not go away; put the indicator on a simulator. Check for stability and linearity. The indicator should be stable and by clicking the simulator up and down should repeat and increase and decrease in a linear fashion. If the indicator does not perform correctly then it should be repaired or replaced.
If the indicator passes, the home-run cable should be re-connected, and the simulator should be connected at the j-box. This test is to determine if the home run cable is good. The cable can look fine but still be damaged inside the jacket. If the scale fails and has transient voltage protection (surge voltage protection, SVP) in line with the indicator, bypass the transient voltage protection (SVP), if this works then replace it. If it doesn’t, replace the home run cable and retest the scale.

For Single Load Cell Systems:

Check the wiring at the load cell, perform load cell tests or if possible replace the load cell. If this fixes the problem then it was the load cell. If it doesn’t work go to step number 11.

For Multi Load Cell Systems:

Remove all of the load cells from the J-Box and wire the Load Cell simulator to the J-card and test the test the Indicator for stability and linearity. Does it work? If the indicator doesn’t work replace the J-card and retest it.
If the j-card is good, test the load cells by using the following tests:

Tie the signal, excitation, and sense wires together and test their resistance to shield. The resistance should be at least 1000 megohms, or less than 1 nanoSiemen. If it isn’t registering in the correct range then the load cell should be replaced.
If the load cell passes the first test, then wire the load cell to an excitation source (A 9 volt battery works fine.) The output of the load cell with no load should be approximately zero. If the load cell can’t be unloaded then you need to know the maximum output of the load cell, for example, a 3 mV/V load cell with 9 volts of excitation would be 27mV. Now a determination must be made to see if the output is above what you would expect and if it is then it needs to be replaced.

** An alternative to the above tests is to re-install the load cells one at a time until the bad load cell is found. This method does not always work due to indicator issues.

After the load cells have been tested and if necessary replaced, the cells need to be re-wired to the j-box and the scale re-evaluated.
Does the scale now work? If it doesn’t work re-evaluate the above tests to make sure they were done correctly. If everything was done correctly and the problem was not resolved call technical support.

Many manufacturers are willing to help you troubleshoot their equipment, because if the equipment can be maintained and repaired it doesn’t get replaced with their competitors. Some of the larger manufacturers will not help because they are trying to protect proprietary knowledge. If you are in a situation where the manufacturer can’t or won’t help, there are several companies that will help if your company has developed a working relationship with them. These companies are the after market repair companies such as National Scale & Control, Debac, RL Ziemba and others. They not only can provide support but often can provide the repair parts for the equipment, which is why they help. Contact them before you need them and find out what they need from you before calling for help. Nobody likes a freeloader and the time they work with you is an investment that they want a payback on.

By following a troubleshooting technique, whether mine or another, your technicians will find the problem faster and more efficiently. This makes for happier customers and less frustrated technicians.