Useful information for concrete plant design and maintenance
Getting proper information from water and admix meters
Pre-wired admix controls are usually wired so that the power for the meter comes from the filling valve solenoid output on the computer. As a result, when the filling is complete and the valve is turned off, the meter stops counting. This does not mean that the flow of admix has stopped, however. If the flow was fast, it probably continues for a fraction of a second before the valve closes and this can translate into a few ounces or many millilitres in every batch. Even worse, consider the valve that sticks in the OPEN condition, allowing admix to flow, possibly at a much reduced rate, during bottle discharge and all the time up to the start of the next batch. This will never be recorded and will take an alert operator before it is spotted.
The solution is to re-wire the admix controller, which we do on almost every installation, to power the meter from a solid power source (see diagram). As long as the batching computer continues to monitor the meter after the valve is shut off, the errors above will be caught.
Water valves and meters should be wired in the same manner.
P.S. – Master Builders assures us that their admix systems are wired correctly; the above change is not necessary! Thanks for the feedback.
Improve jogging performance with flow control valves
Concrete plant batching controllers usually control the “open time” of the gate in steps of around 1/10 second; since it takes 1/10 to 2/10 second to start the gate moving, and it can be full open in 3/10 second, a setting of 2/10 may drop no material while a setting of 3/10 can drop 50lb/20kg – too much for accurate batching. The computer’s automatic tuning can try to adjust the jog time but it will jump from 2/10 to 3/10 second and back again in successive batches. This is futile and will result in lost time and tolerance errors.
The solution is to add a flow control valve in the opening direction only. When adjusted to a speed of 1 second (approximately) from closed to full open, the open time plus the size of the opening can be adjusted in small steps. Since the volume of material dropped in each jog is roughly proportional to the area under the curve (see diagram), the amount of each drop can be accurately controlled by the same 1/10 second steps. This allows the automatic tuning (assuming that your system has it) to easily adjust to the best setting. Update, 2016: Most PLCs now have timers with 1/100 second settability, so a faster closing rate is possible – but the principle still applies!
Checking your scale calibration
Understandably, our customer was very upset. The fact is, however, that the customer is responsible for checking his equipment, and that includes scales. This was an extreme case, but it shows very clearly what can happen when we blindly believe our instruments. Even if the scales are checked every 6 months, things like this can happen.
We recommend that batching scales be checked every week by having someone apply his body weight to each scale, while someone else reads the display. This is rough and ready, but will find gross errors quickly before much damage is done. A better way is to use our SHUNT CALIBRATOR unit, which connects to each scale in turn. It simulates a known weight, checking the complete load cell circuit as well as the indicator. This avoids the need for test weights during weekly tests, but a six monthly calibration test should still be done to meet quality plan requirements.
Improve concrete plant performance
A piece of 6″ angle, welded across the hopper directly under each gate, will reduce the momentum of the falling material and spread the pile of aggregate, allowing a little more to be batched into the hopper before it touches the gate. More important, however, it allows the scale to react instantly to the falling material. This removes the bounce effect while shortening the response time of the scale.
This is also very effective when batching small quantities of material. Without the angle, it has to fall to the bottom of the hopper before the scale can react, increasing the possibility for overshoot before the gate can close. The angle allows the system to react properly regardless of the transit distance; the gate closes normally and can also be jogged to obtain greater accuracy. Size of the angle is not critical, but it should intercept at least 50% of the flow to be effective.
Choice of load cells can be crucial
To do a proper job, all load cells on the same scale must have equal sensitivity or be equalized by means of a “summing box”. Equal sensitivity cells are called “standardized” and have outputs balanced to within 1/4% or better. On a concrete batching scale, there is no need to balance the cells further, eliminating the need for a summing box and the lengthy adjustment procedure that accompanies it. Most models of the “S” cell are are low in cost and hence are not standardized. This makes it imperative to use a summing box and go through the “corner adjustment” procedure with test weights before the scale is accurate enough to be used. Consider this fact carefully before making your choice of load cells; the corner test might be difficult or impossible to carry out because of lack of room, danger to personnel or insufficient time.
We know which load cell types are standardized and can be used on batching scales of all types. Contact us for further information.
Can you run out of admix without knowing it?
You make batch after batch with no admix; this can be a disaster. Fortunately, there is an easy fix. A simple proximity detector can be attached to the outside of the plastic admix tank, allowing early warning of low admix level. When connected to BatchTron inputs, alarm messages can be generated.
Even better is to use our SiloWeigh with liquid tank sensors, to give you a display of the amount in each tank and programmable alarms too.
We have installed these sensors in concrete plants. Call us for details.
Cure a common cement scale problem
The reason is almost always an air pressure difference between the interior of the hopper and the vessel which it feeds (typically the mixer). This can be caused by a dust extractor attached to the mixer, aeration of the hopper (to aid in discharge) or merely by the flow of materials in the system, which displaces air.
The solution is to add a bypass pipe. This can be as simple as a length of flexible plastic hose 3″ or 4″ in diameter, connected to the top of the cement hopper at one end and the mixer or vessel being fed at the other. This equalizes the pressure at both sides of the discharge valve as well as the feed point (see diagram), which in turn eliminates any vertical force on the load cells or scale. An important side benefit is that it provides a path for air displaced during both filling and discharge, speeding operation and eliminating another source of weight error during feeding. Any dust in this displaced air is returned to the mixer which in turn is evacuated by the dust collector, if used.
Be careful to use flexible hose (the corrugated type is best) and to secure it to the building structure to prevent it affecting normal weight readings. Note also that for larger hoppers, and some mixers that can plug up this pipe with cement and water, a filter is preferable.
Butterfly valve saves cement dollars
After the motor is stopped, the overfeed depends on the screw position and will vary; assuming that the batching computer has accurately compensated for the “in-flight” material, the end weight will be over or under the target weight because of this variation. This forces the concrete plant operator to add a little more cement than is needed on every batch, which costs a huge amount over a year’s production. Also, excessive aeration of the cement silo, needed to improve flow, can cause fluidization of the cement in the screw feeder which then creates an overweight situation.
Both these problems can be eliminated by adding a fast acting butterfly valve under the screw feeder exit. If the exit tube is long, it should be as close to the cement scale as possible to minimize the in-flight amount. Ensure that a very flexible sock is fitted between this valve and the scale to minimize the effect on the weight reading.
The valve operating cylinder should be actuated from a solenoid valve which is connected to the motor starter for the screw feeder. If you are worried about the valve failing to open, mount a limit switch on the valve and use it to close the circuit to the screw feeder motor starter.
The resultant material saving on most feeders averages 1/2 to 1%. Add this up over a year’s production; you can probably justify a new BatchTron system with the amount you save!
Protect batching system from power surges
This is caused by a high voltage appearing on the case of the controller, which then appears on everything inside it, eventually finding the shortest path to ground through the offending device. This occurs when the path from the controller to earth ground is long. In one case, the phone line into the concrete plant had its lightning arrester ground point connected to the controller cabinet but there was only a thin 16 gauge ground wire from the cabinet to power ground about 50 ft away. Static discharges (due to nearby electrical storms and induced voltages from power lines) traveled down the phone lines, through the cabinet ground, the moisture sensor and load cells to the plant frame, frying the sensor and load cell in the process. A European-style power system with power line neutral coming in through overhead cables can be just as bad.
The path for the incoming surge is through the power line, power neutral, telephone line or other overhead wire to the control cabinet. The analog inputs for the moisture sensors and load cells are grounded to the cabinet and conduct this surge to the moisture sensor or load cell, where it jumps the insulation and grounds itself through the concrete plant’s frame.
The solution is simple and effective. Connect the plant frame to the controller cabinet with a thick (#10 gauge or 5 mm) ground cable as short as possible. This minimizes the voltage that can appear between the two and reduces the risk of striking through the insulation of the sensor or load cell. We recommend that this is done on all plants, regardless of whether they have had a problem or not. It’s just good insurance. If the problem appears to be severe, it also helps to put an isolating transformer (equal voltage input and output) in the power line before the controller (and before the UPS if used, since most UPSs do not provide isolation).
Much as we love the business that lightning, fires and floods bring, we appreciate that it costs your business a lot of money when it happens. You might also want to consult us on the best grounding methods if you’re not sure. Most of the time we can do it without a site visit.