A Word on Managing Calibrations

I’ll often hear my customers say something along the lines of “I really wish we could get a more accurate consistency transmitter”.  When I dig a little deeper, I usually find that the problem isn’t necessarily so much an issue with the transmitter’s accuracy, but rather, with the way that the calibration for that transmitter is built and managed.

What do I mean?

When somebody installs a consistency transmitter, what they usually want is for that transmitter to generate a consistency number that agrees with whatever their lab says.  If the lab analyzes a stock sample and says its 3.4%, they want the transmitter to say 3.4%, too.

Of course, the reality is that the transmitter will never actually read 3.4%. It will always be different.  Always.

What most people don’t fully understand is that this behavior is absolutely normal.  The question isn’t if the transmitter and the lab disagree, because they will.  The real question is how much can the lab and transmitter disagree before you start to think that you’ve really got a problem.

And that’s a question that far too few people ask.  The consequences of not asking that question can be severe.  Take this scenario, for example.

The operators get a consistency reading, X, from the lab.  The value of X doesn’t agree with what the transmitter is saying, which is reading Y.  The operators call the E&I shop and tell them that they need to go and service the transmitter so that it reads X instead of Y. 

In a couple of hours, or maybe the next day, an E&I tech goes to the transmitter in question and adjusts the output so that it now reads X.  When the operators get the next lab analysis, they note that while the transmitter is now reading X, the lab now says it's actually Z.  So, again, they call up the E&I shop and tell them that transmitter has drifted again, and now they need to make it read Z instead of X. 

The E&I shop dutifully dispatches another tech, probably not the same one as before, who goes to the transmitter and readjusts it to now read Z.  It’s a good bet, by the way, that the first and second tech don’t use the same method to adjust the transmitter.  The next lab sample evaluation comes, and now, it’s W, instead of X, Y, or Z.  By this time, everybody starts calling the transmitter “that damn transmitter” and walks around cussing out the vendor for his piece of s**t instrument.

Does that sound familiar?

This is a situation that can be avoided if one knows the statistics behind a calibration.  When you build a calibration, there is something that you have accept as if it were the Gospel truth:  the instrument is better than your lab. 

What I mean by that is that your instrument will likely respond pretty much the same way given the same process conditions with very little variation.  Most undamaged and properly functioning instrumentation will respond with less than 1% of variation.

The same can’t be said of the lab.

This doesn’t mean that your lab is bad, or that your lab techs are lazy slobs.  It’s just the nature of the beast when it comes to manual analyses.

The TAPPI T-240 method for establishing consistency, for example, reports a repeatability standard of only 10%, and that presumes that your sampling techniques isn’t adding a whole lot more error.

In real terms, that means that you would expect that if the first lab analysis said 3.0%, then 95% of the time, you would expect a second analysis of the same stock to not deviate by more than 0.3%, or in other words, to come in somewhere between 2.7% and 3.3%.

That’s a pretty big range.  And you get that if you’re doing it properly. 

Now, what does this mean, exactly?

It doesn’t mean that your consistency readings are worthless.  Quite the contrary, it means that you now have a benchmark by which you can gauge the health of your consistency readings.

Remember that the instrument is probably responding to real changes in consistency with a repeatability variability of less than 1%.  If your calibration was built properly in the first place – a topic for another column, by the way – then if the lab typically reports X when the meter is reading Y, you shouldn’t get excited until the lab value exceeds X +/- 10% of X.

Remember, 95% of the time, you expect that lab to get within that 10% of X.  If it is within that 10%, then, everything is working OK and you shouldn’t mess with your transmitter or the calibration.

I wouldn’t even get excited if you occasionally get a reading in excess of 10% of X.  Remember, 95% of the time you expect it to be less, but that also means that 5% of the time, or about once every twenty times, it could be more than 10% and everything would still be OK.

So when should you get excited?

If your readings are consistently deviating more than that 10% of X, then it’s time to review the situation.

If your variation is always positive, or always negative, then it’s time to review the situation.  A valid calibration should see both positive and negative variation.

I stop here for now, but I’ll leave you with one more thought.  You can do a lot better than that 10%.  More on that in a future column.

Epiphany

E-piph-a-ny - [ih-pif-uh-nee] - a sudden,intuitive perceptionof or insight intothe reality or essential meaning of something, usuallyinitiated by some simple, homely, or commonplace occurrenceor experience.

I had an epiphany the other day, which is rather rare for me.  I was thinking about paper machines when it hit me…

A paper machine is nothing more than a giant water removal device.  I mean, wet stuff goes in one end and dry stuff comes out the other end.   Sure, there’s a lot more to it than that, but when you boil it down to its simplest, most fundamental function, that’s what a paper machine does – it removes water from paper stock.  And if that’s true, then how water drains from paper stock is probably the most fundamental aspect of the paper making process, right?  Any change to the drainage quality of your stock will impact the function of the paper machine.

You’ll see it in how the wet line moves.

You’ll see it in the couch vac.

You’ll see it in the press section.

You’ll see it in the dryers.  And so on…

If the drainage quality of the stock will impact the fundamental function of the paper machine, then controlling the drainage quality of the stock is probably the most fundamental aspect of the paper making process.  And before you can control it, you have to measure it.

Gee, I wonder how you can do that?

TECO Drainac 

Startups can be Absolute Hell on Inline Instrumentation

Here’s something you may have experienced before. 

You’ve been down for your scheduled shut down and it’s time to start the process back up.  As the pumps start to circulate, you hear all of that awful knocking as the lines fill and stock starts to move again.

All of a sudden, some of your inline consistency instrumentation goes dead as a doornail.  You pull the sensor out of the line and notice that the sensing element has been bent, or has been completely sheared off.

So what happened? 

Most likely, your inline consistency sensing element just got nailed with a slug of stock that had been dewatering while sitting in the line during your downtime.  At start up, that dewatered slug – or log - started flying down the line as the process was coming up and slammed into your sensor at light speed.

It’s as if you just had a sledge hammer hit your instrument.  No wonder the sensor got damaged, huh?

So, now that you know that, how do you prevent it from happening again?

Here are a couple of ideas:

Install breaker bars both upstream and downstream of your sensor body.  Breaker bars are pieces of dumb metal – ours are cylindrical – that will hopefully break up those dewatered slugs of stock before they slam into your sensor.  It doesn’t work all of the time, of course, but it will work most of the time.  Breaker bars are cheap insurance that should be installed in front and back of every inline sensor in your mill.

By the way, we keep a good supply of these in stock ready to go at a moment’s notice.

A better idea is to install one of our C5000 retractable consistency sensors instead of the one you are currently using.  The C5000 sensor body was designed to be pulled out of the line so that it could be hot swapped while the process is active.  It can also be pulled out of the line during a start up so that dewatered slugs of stock will pass by harmlessly.  The sensing element can then be reinserted in the line once things settle down.

Nifty idea, that.

It Ain’t the Transmitter, Man… well, mostly

A customer of mine recently showed me a competitor’s consistency transmitter that had been installed in one of his pulp lines.  The blade of that transmitter had been snapped clean off and the attachment point was bent about 30 degrees from its normal position.

“Look at that”, he said.  “That’s what I’m talking about when I say I need a more durable sensor.  That’s the third transmitter I’ve had to replace in the last four months and I gotta tell you, I’m getting damn tired of it.  What can you do for me?”.

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Well, the short answer to that is “it depends”.  It depends on the details of the application because our transmitters, just like everybody elses' transmitters, are designed for a specific range of process conditions.  As long as you’re within the acceptable application window for an instrument, you’re going to get a dependable and durable measurement.  If, however, you stray outside of that range, you’re likely going to get a noisy or bad measurement, or, in extreme cases, a destroyed instrument.

I asked my customer what the conditions were in his line.  He told me that he was running about 4% at 4,000 gpm in a 14” line. Well, that works out to a stock velocity of about 8.337 fps.  Put another way, that’s about 5.6 mph.  While that doesn’t sound very fast, when it comes to instrumentation in a line, 5.6 mph is screamingfast. 

It doesn’t take much in the way of a de-watered stock slug to wreak a lot of damage at that rate.  The reality is I think just about anybody’s transmitter would have been destroyed under those conditions, mine included.

High stock velocities are bad for other reasons, too.  Apart from damage, higher stock velocities can make a transmitter indicate a change in consistency that really isn’t there. 

Mechanical sensors respond to various conditions in the line, of which a change in consistency is only one.  In addition to consistency, transmitter readings are typically a function of flow rate and furnish type, among other parameters.  This means that the reading you’re getting isn’t just giving you information about consistency, but other things as well.

Blade style sensors, in particular, are sensitive to changes in stock velocity.  At high enough speeds, those variations in velocity will look just like shifts in consistency, and woe unto your control loop when that happens.  Your dilution control will try to accommodate for non-existent changes in consistency, and then, the non-existent changes become very real.  The next thing you know, your stock has become really light, or really heavy, all the while there doesn’t seem to be any change on the consistency reading at all.   And if you get really, really heavy, you might lose another transmitter.

You can compensate for higher flow rates – to a point, that is - provided your transmitter accepts a flow input (ours do).  You use the flow information to calculate both the the current velocity and also a correction to the consistency measurement.  Our transmitters are pretty unique in this regard as we are the only manufacturer to incorporate this as a standard feature in our line.

There is a limit to flow compensation, of course.  At very high flow rates, the flow component of the measurement becomes so large that the consistency component is swallowed up in the noise.  Under those conditions, what you really have is a flow indicator and not a consistency transmitter.

The best solution for all of this, of course, is to slow the stock velocity down.  Slower stock velocities will impart less “velocity noise” that you have to compensate for.  In some cases, you can eliminate the flow induced component altogether.  Secondly, slower velocities will also decrease the likelihood of sensor damage. 

There are basically only two ways to reduce stock velocity.  You either slow your production rate down, or you increase the size of the line where you want to make a consistency measurement.  Most mills are in the business of making pulp and paper, so reductions in production rates are usually not an option.  This leaves increasing the line size as the best way to slow the stock velocity down. 

So how much do you want to slow your velocity down?  Well, that depends on the transmitter you want to install.  If you’re looking at TECO’s C3000 or C5000 series sensor, you’ll only need to slow your velocity down to about 2.5 fps or so.  At that rate, the C3000 & C5000 sensor become immune to changes in flow velocity.  In other words, the flow-velocity-induced component of the measurement drops off to zero at rates of 2.5 fps or less.  This value is somewhat variable and dependent on the furnish you’re working with.  For shorter fiber stocks, for example, you could exceed 3.0 fps and not worry too much about velocity noise.

What if you can’t get your velocities below 2.5 fps?  The C3000 & C5000 series sensors are fully compensatable for line velocities up to about 6 fps.  Our C6000 transmitter, don’t forget, accepts a flow input to make this adjustment automatically. 

That said, you’ll get the best results when you keep your velocities below 2.5 fps.

So what did I tell my customer?

I told him that I could provide him with a rugged and durable transmitter that would give him a reliable measurement, provided we made a few changes to his line.  

www.teco-inc.com