Pulp & Paper Stock Consistency Transmitters True Cost of Ownership

One of the objections I hear regarding passive mechanical consistency transmitters is the high cost of ownership that these systems purportedly have.

The thinking goes something like this.  Mechanical transmitters typically have a sensor in the line that protrudes into the flow in the line.  Sooner or later, that sensor will get hit and damaged and will need to be replaced.  Thus, to keep a a mechanical sensor operational requires that the sensor be replaced and represents an ongoing expense.  The alternative, a rotary transmitter is typically installed such that its sensor is wholly contained within a stilling chamber and is thus not likely to be hit and damaged.  Its cost of operation must be lower, right?

While there is some truth to this, it’s not the whole story – not by a long shot.
It’s true that passive mechanical systems do get hit from time to time and their sensors will need to be replaced.  It’s also true that rotary systems don’t often get damaged because their sensors are offset from the flow.   That said, what is not true is the notion that the cost of ownership for a rotary is far less than that for a mechanical. It isn’t.

Let me illustrate this with an example using my company’s C3000 sensor:

The TECO C3000 Consistency Sensor

A rotary system will cost you somewhere in the neighborhood of $30,000. Let’s assume that it will last five years before it will need to be replaced.  A complete TECO C3000 mechanical system, on the other hand,  will typically cost you somewhere under $7k. Let’s say you have to replace the C3000 sensor once per year.  Your annual cost, including the trade-in credit for the original sensor core, is under $2k per sensor.

Over five years you’ll pay less than half of what you’d paid for the rotary initially.  Let me say that again – you’d pay less than half of what you’d pay for the rotary.

Don’t get me wrong, rotary consistency transmitters are cool devices and they certainly have their positive points, but they ain’t cheap.  Passive mechanicals are way, way less expensive and you can use them to measure mostly the same consistency range that you would use a rotary to measure.  Properly applied, the TECO C3000 sensor will give you way more bang for the buck than any other system available in the world today. 

Sampling v2.0

I want to take another look at proper sampling because it so key to a good calibration.   While there are statistical tricks to get the most out of anything you produce calibration-wise, if you don't have good sampling, you are, in the best case, creating big problems for yourself.

We want to collect samples such that they are representative of the process.  Samples that are representative have an average that is very close to the average of the whole process at that moment in time. Samples that are not representative will have averages that are not at all similar to the process. 

Collecting representative samples isn't difficult, but you do have to follow certain rules. 

1) Collect samples from lines where the flow characteristic is known to be stable, i.e., in plug flow.  Stable flow means that you will likely not have any turbulence in the line that might de-water your stock or otherwise introduce non-representative sampling.   The easiest way to ensure this is to find a straight length of pipe that is at least seven pipe diameters long, and without any bends or obstructions in it.  

2) Make sure the pipe is full.  No, really, make sure the pipe is full.  Choose lines that are horizontal, or vertical lines with flow going up.  Choosing a vertical lines with flow going down is asking for trouble.  Do not take samples from chests if you can avoid it.

3)  If you are planning to use your data to build a calibration for an instrument, you should make sure that the sample port is close to the instrument in question.  There is no point in running analyses if the instrument is in another line or on the other side of the mill.

4) The sample port should have an internal extension that protrudes roughly to the center of the stock line.Use proper sampling valves, if you can, and avoid ball valves that have been installed on the side of a pipe.  The image below illustrates how variable things can get as they move through your stock line.  As you can see, it can sometimes be a challenge to get that "representative" sample.  That said, your best chance will be to take samples from the center of the pipe as opposed to the sides.

Variability in a stock line

5) Open the valve and let the stock run freely for a few seconds to ensure that all the stock from the last sample is fully discharged from the sampling line.

6) Collect a large quantity of stock (i.e, a gallon or two at minimum - five gallons is better).

7)  When back in the lab, agitate your large volume of stock and take at least two small samples.  Analyze each according to your favorite method and average the results.  This will yield you one data point.

6) If you haven't done so before, run a Total Error Variance (TEV) to estimate the quality of your sampling and analytical technique.  TEV's are sort of a poor man's six sigma.  They will provide you an estimate of how much variability in your analyses is attributable to your sampling and how much is due to your technique.

If you don't have a TEV in hand, send me an email and I'll send you a copy of our spreadsheet that you can use.