Methods of Measuring Solids Flow

If you are like me, you started your adventure into process measurement with measuring liquid and gas flows, temperatures, and pressures.  I got very good at designing systems using the proper instrumentation to measure and control these.  However, I didn’t pay much attention to solids measurement.  This has since changed.  I work with solids control in a large percentage of my projects now.  I have come to realize that the expertise gained in liquid control does not always directly translate to solids measurement. 

There are 5 main ways to measure solids flow.  They are as follows:

  • Belt speed measurement
  • Conveyor motor amp draw
  • Belt Scale Conveyor
  • Impact Mass Flow
  • Radiometric Absorption Conveyor

Belt speed measurement is the simplest method, but it is also the most inaccurate.  This method just measures the linear speed of your conveyor or screw and estimates the mass of the dry product it conveys per minute.  Sometimes this is ok, but it assumes a steady feed rate and steady product density.  This method shouldn’t be used for control or inventory tracking.

Conveyor motor amp draw measurement assumes that increased amps is an increase in mass being conveyed.  This is a better idea than purely monitoring the conveyor’s speed, but it too has massive drawbacks.  It assumes that the conveyor itself is steady state mechanically.  Does your conveyor have a consistent friction at all speeds and temperatures?  Probably not.  Do mechanical systems wear over time causing “calibration” drift?  Probably.  Does the amp increase vary linearly with mass increase?  Probably not.  I personally don’t like these systems as they don’t work very well and give a false sense of having measured something well, when in fact you have not.

Belt scale conveyors are the gold standard in mass flow accuracy.  They consist of load cells on independently supported rollers under a conveyor belt. There can be from one to 4 of these roller/load cells in series.  As solid product rolls over these load cells, it is weighed.  This weight and an output from a speed sensor combine to give you an extremely accurate mass flow.  These systems, depending on the number of load cells, can give a mass flow reading of up to +/-0.25%.  These systems aren’t very expensive in the scheme of things, but they do require a lot of straight space in your belt to install and work properly.  They also rely on moving parts and are exposed to the dust from the solids which will require maintenance and periodic recalibration with provided reference weights.  Overall, these are the most accurate systems you can get.

Impact mass flowmeters are the next most accurate solids flow measurement device.  These require that product be lifted above this “box” and dropped through it down to a bin or secondary conveyor.  This “box” consists of a top entry chute and a bottom outlet chute.  In the middle of this box is a angled plate attached to a load cell. As solids fall into the box, they strike the load cell plate and then fall off and out of the box.  The load cell sensor assembly is sensitive enough to track the rate and mass of the solids going through it and gives you a reliable +/-0.75 to 1.00 percent accuracy spec.  These require less maintenance than the belt scales, but still need cleaned periodically as the sensor is exposed to dust and the plate is open to wear from the impact of potentially abrasive solids.  These are a great option if you have the vertical space to fit it into your process.

Lastly is relatively unknown method of radiometric absorption.  This method uses a radio isotope to emit gamma rays at a known and regular rate.  The conveyor and the material being conveyed absorb these gamma rays.  The conveyor never changes density, so it absorbs the same amount of gamma all the time.  The change in absorption over time is therefore directly related to the change in product mass flow.  The levels of gamma rays emitted are so low as to make these safe for install in the field. In fact, (only with Ronan) no special certificate or government regulation applies to most of their installs.  These systems do not make contact with the product and are installed outside of the conveyor.  This means they require no maintenance as dust and abrasion are not a factor.  They have a pretty good accuracy rating of +/-1-2% in most applications. 

Now that you know the main ways to measure solids mass flow, it’s time to include this in your plant’s mass balance calculations and inventory controls if you haven’t already.  If you have questions about how to implement this in your process or how to correct an install this isn’t living up to your expectations please contact us.

The Unplanned Outage

Oh Crap! All hands on deck! You’re in another unplanned outage and you are working 16 hour days trying to get the plant up and running again. This time it was a broken agitator shaft, but you’ve dealt with other equipment failures and operator error in the past. Regardless of what caused the outage, you are stuck working to fix it and get back up. Outages are a pain and they are expensive. You have to pay your people overtime to get the problem fixed. You have to spend big dollars to replace/repair the broken equipment. You are also not making any product and therefore not making any money. It’s a trifecta of terrible. What if we could take steps to minimize unplanned downtime? Would you do these steps or at least some of them? I think it is worth it to find the time when things are running well to prevent things from going badly.

First, you must figure out the potential failure points in the plant. On a grand scale it could be three things. Operator error, loss of utilities, or equipment failure (there could be more, but for the sake of this article, let’s go with those three).

In regards to operator error, can you predict what an operator could do to damage or shut down the process? If you can, then implement control system or SOP fixes to prevent them from being able to that damaging activity. Noone wants to shut down a plant, but accidents can happen. Take steps in the software to prevent those accidents.

Regarding the loss of utilities, I don’t know that there is much you can do. You could keep a spare transformer or other large utility items as spares, but these events are relatively rare and it would be really hard to predict which component (yours or the utilities) would fail.

Lastly you have equipment failure. This is preventable or at least predictable. We all do preventative maintenance. Do you do predictive maintenance? Changing oil in gear boxes and replacing consumable parts on a regular basis prevents unplanned down time, but what if there is something that doesn’t get these PMs that fails? What if you could predict this failure? instrumentation has a lot of diagnostic information ready to be used already. You just have to look and have I/O capable to reading and logging that data. You could also use a smart cloud service (like Siemens) to analyze that data for you. There are also SIL (Safety Integrity Level) devices and equipment. These have proven and tested and predictable mean time to failures and can be used to ensure the safe and reliable function of your process. Keeping spare parts and complete assemblies for all your critical components is worth the inventory cost, espessially if you will lose enough paying idle people, not making product, and paying expedite fees to ship an emergency part in from somewhere else.

So, while you are stuck in this unplanned shutdown, use this as an opportunity to make some changes to your inventory, SOPs, and data analysis to prevent the next one. If I could do a little work and spend a little money now to prevent spending a lot of money and doing a lot of work later, I would.

If you like this article (or not) leave a comment and let me know what you think. I will try to cator future articles to the interest of my readers.