Expanding the Sweet Spot: Measuring Pulp & Paper Stock Consistency Properly

If you want to measure consistency properly, it’s important to remember that all consistency transmitters have their so-called “application window”.  The application window consists of all those process parameters that have to be within a certain range in order for a particular transmitter to work properly. Which ones are relevant to you depend on the technology behind the consistency transmitter in question.

As long as you’re within a particular range for each of these parameters, you have a good chance that the instrument is reporting stock consistency reasonably well. Of course, if you get outside of that range – and unfortunately, it’s not always obvious that you have exceeded the limits – then the transmitter output can start to deviate from reality, sometimes in a really big way.

So, it pays to pay attention to the application window for an instrument - the “Sweet Spot” - if you hope to get the most out of your measurements. 

When it comes to mechanical transmitters, the process conditions you need to consider include production flow rate, furnish types and, oddly enough, stock consistency itself. 

We’ll start this discussion by asking the question:  Why is production flow rate important? 

Simple passive mechanical transmitters like blades respond to the “apparent viscosity” of the process.  Apparent viscosity is just a fancy way of referring to how thick the process slurry is.  As you would expect, the higher the consistency, the “thicker” the process is. 

Blades, however, don’t really measure the thickness of the stock directly.  Instead, they respond to changes in force as stock moves past the blade (that’s why, incidentally, they are called shear force systems).  The stock imparts a force to the blade as it moves – or shears – across the blade surface.  Stock motion, however, is the key point – the stock has to be moving past the blade.  A blade transmitter immersed in stationary stock would register zero, irrespective of what the consistency is. 

What isn’t always obvious, however, is that the force that the blade is responding to isn’t merely a function of consistency.  It has a flow-rate component to it as well.  As the flow-rate goes up, the force imparted to the blade will also go up.  This is perhaps one of the most important aspects of blade systems and it is also, one of the things that is most often overlooked by mills.  Simply put,

Blade Force =  Consistency Force + Flow-Rate Force

So how do you deal with the flow rate component?  Some manufacturers will publish flow velocity-consistency graphs for their designs.  The implication here is that if your process stays within the valid region as defined by the manufacturer, the measured force will be consistent with changes in consistency.  This is a reasonable approach if flow-rate variability is kept to a minimum,  but it is not suitable for applications with highly variable flow regimes.  Under these circumstances, you must compensate for variable flow-rates if you hope to get a useful consistency measurement.

That said, there are two ways you can compensate for highly variable flow rates.

You can measure the flow rate and mathematically subtract out the flow rate component from the force signal and/or you can select a sensor geometry which has a flow rate response which minimizes the impact of flow rate for your application.

When it comes to TECO’s StockRite® line of consistency transmitters, you can get both.

TECO’s C6000 consistency transmitters are shipped with automatic flow-rate compensation built-in.  All you need to do is to land a flow-rate signal on the transmitter and the flow-rate component is automatically removed from the consistency signal in real time.  You can drop our C9700 blade into your existing blade application – our systems fit our competitors process connections, by the way – and automatically compensate for flow rates which vary from 0.5 to 12.0 fps.

That’s what I call expanding the sweet spot.

Of course, wouldn’t it be nice if you had a sensor design which was immune to variability in flow rate in the first place? I’m happy to say that there is one available:  Our C3000 Probe design has a flat flow-rate response for production flow rates up to 3.0 fps.  That means that the C3000 has a zero flow-rate component for all flow rates below 3.0 fps.  Put another way, you could have production rates of over 1000 GPM in a 12” line and never have to worry about flow rates disrupting your consistency signals ever again.

If you’re having trouble with your consistency measurements, give us a call.  We’ll really good at helping our customers get the most out of their consistency measurements.