Darth Flow Rate

A customer  - I’ll call him Bob - recently asked me about his consistency transmitter. Bob said that they’ve noticed that their basis weight has been drifting, but that their consistency measurements were basically straight lines.  He was wondering if something might be up with his transmitters.

Good question, that.  The answer is probably both yes and no.

The answer is No because there is probably nothing wrong with Bob’s transmitter. 

The answer is Yes because Bob’s transmitter is likely responding to something other than variations in consistency in his process.

I can hear you asking, “What the ____?”.  That’s what Bob said, anyway.   Here’s something important to remember:

No consistency transmitter of any kind actually measures consistency directly. 

They all measure some other physical parameter which has been shown to have some kind of a relationship to consistency.  In the case of mechanical transmitters, they almost always measure force.  What kind of force?  Well, that depends on the physical design of the part that goes into the stock line.

If you’re talking about a blade style transmitter, the physical force of interest is Apparent Viscosity.  All the fancy talk aside, the apparent viscosity is a way of referring to the thickness of the slurry, and it’s easy to see how that “thickness” would relate to consistency.

The blade measures the force of the slurry as it passes the blade.  The higher the consistency, the more force which is transferred to the sensor.  There’s always a minimum amount of fiber that has to be in the line.  Less than that and you just won’t get enough force on the blade to measure.

That said, it’s just not the consistency that’s in play here.

In order for the blade to measure the apparent viscosity, it has to be moving.  If the slurry is at a standstill, the sensor will measure zero force.  It usually has to be moving at some minimum rate in order for the system to register the force, too.

Now here’s the key…

As the flow rate continues to increase, so too will the force measured by the blade, even if the consistency is constant.

Put another way, the blade doesn’t just respond to consistency.  It also responds to flow rate.  This is true for all blade style transmitters.  Ours.  Theirs.  Everybody’s.

That means that your consistency signal is really a consistency & flow rate signal.

What does flow rate look like in a consistency signal?  Well, mostly it looks like the stock is getting heavy or light as the flow rate goes up or down.  It gets worse as flow rate increases.  The higher the velocity, the larger the force component.   If you’re running really fast, you may have so much flow force that you will miss changes in consistency altogether.  This was Bob’s problem, by the way.  His flow rate was so high that the transmitter was pretty much only registering flow force.

So what do you do about that?

If your flow rates are relatively stable, you can just subtract out the average impact of flow rate when you first calibrate the transmitter.  If, on the other hand, you have variable flow rate (and you do, even if you don’t know it), then you’ll need to compensate for that variability. 

Compensating for flow rate is easy – it’s just an equation to subtract out the impact of flow rate on the consistency signal.  TECO’s C6000 Series transmitters have that function built-in.  Just land a flow signal on the transmitter and you’re good to go.

If you don’t have a TECO transmitter, you’’ need to program the equation into your DCS.  Personally, I think it’s easier to install a TECO C6000 transmitter.  Call me and we’ll talk about it.