What’s In A Unit? Flow Measurement – Part 2

E-ZINE

Things are bit bumpier since leaving Philadelphia, but I will clamor away just the same. Still no major clickety-clacks though. This issue builds on Part 1 that is posted on my website.

Having discovered that there are two types of flow rates (volumetric and mass), it should not be a surprise that some flowmeters measure mass (W) while other flowmeters measure volume (Q). However, it is not quite that simple. Repeating the equations from Part 1 (for convenience), it can be seen that, assuming A is constant, Q can be determined by measuring the average fluid velocity v. Further, assuming that rho is constant, W can be determined from Q.

Q = A x v

W = rho x Q

Summarizing, some flowmeters measure volumetric flow, some flowmeters measure velocity from which the volumetric flow is determined, and some flowmeters measure mass flow. In addition, when the density is known or assumed, mass flow can be determined from the volumetric flow, and the volumetric flow can be determined from the mass flow.

So you just wanted to measure flow — did you now? It all seemed so logical and simple at the time. Stick around — it gets worse.

Some flowmeters use other principles to infer flow. The most common of these measurements measure the velocity head (1/2 rho v x v) to infer the volumetric flow. Notice that these flowmeters do NOT measure volume, do NOT measure mass, and do NOT measure velocity — but rather measure a combination of density and the square of velocity! Would it surprise you to discover that this is a description of (commonly-applied) head flowmeters, such as orifice plates, venturis, nozzles…? Further, in many applications, the inferred volumetric flow is used to determine the mass flow. Errors can enter the measurement process during each measurement and with each assumption. Is it any surprise that plant engineers often have difficulty closing material balances in their plants?

Summarizing (again), some flowmeters measure volume, some flowmeters measure mass, some flowmeters measure velocity, and some flowmeters measure inferentially. Understand the difference, but also understand that careful attention to detail can result in an inferential measurement that is better than the others.

I am almost in Baltimore and I haven’t even started talking about units yet. As Arlo Guthrie sings many times in Alice’s Restaurant, “This is a song about Alice”. It’s too late to change the title, so it will just have to wait. The plot will thicken yet again.

ISSN 1538-5280