Flow

How to measure fluid flow when solids are present

Even ordinary process fluids can present problems due to suspended solids.

By Ian Verhappen

Flow measurement is an important type of sensing, especially when it’s a custody flowmeter that is the cash register. Obviously, there are many standards related to custody-transfer flow, including all the upstream and downstream straight-run rules to assure a nice, clean profile when entering the meter.

Very few process streams are 100% single-phase, with most having either small bubbles or low levels of solids. These are normally suspended close to homogenously, so the fluid still behaves as if it were single-phase, therefore not affecting the measurement. Once they start having a noticeable number of solids—less than 200 microns primarily by accident, non-settling slurry with specific gravity less than 1.05, and less than 5% solids by weight—steams are considered a “light slurry.” Most of the time, until the second phase constitutes a significant percent of total fluid fraction, the fluid can be treated as a single phase.

Low levels of solids may be (and likely are) present in your process, even though you don’t think so. If your flowmeter is downstream of a catalyst bed or some other process vessel with solids, chances are there will be some carryover, particularly during startup. Similarly, almost every well will contain some level of entrained solids, which normally continue with the liquid stream until they’re removed in dedicated vessels. However, during different stages of plant operation, such as startup and shutdown, when the stream is at other than steady-state, even low levels of solids can have an impact.

Suspended solids at low levels that will not affect a single-phase measurement (other than perhaps higher wear) may not always stay suspended, especially if the flow is stopped. Without the fluid velocity to keep them in suspension, they will settle to the bottom of the pipe. This may affect a wide range of flowmeters, especially if they have some part of the sensor in this position. Therefore, if you suspect solids may occur, it is always a good idea to rotate your meter or impulse line by one bolt hole to keep sensitive parts of the instrument away from settling materials.

In many cases with a light slurry, some meters will no longer work and the application will require special meters to handle the solids. Typical flowmeters used for slurry streams are ideally non-intrusive or venturi, because they have less sudden change in the flow path. When using a venturi meter to measure a slurry that’s severe due to either the type or size of solids, the meter internals are often coated with a hardened material, which adds roughness at the sacrifice of accuracy, or fitted with a hardened cast lining that can be replaced when the accuracy falls below what is acceptable. Fortunately, these meters are rarely used for custody transfer, but more for plant balance and simple regulatory control.

The above examples apply for light or even medium (5 – 20 % solids) slurries. However, in mining, “real” slurries with targets of close to 40% solids are often used in hydro transport technology where the solids are mixed with water and transported via pipeline relatively long (tens of miles) distances. One of the variables that needs to be controlled with hydro transport is the solids ratio, to prevent settling while still minimizing the amount of water being recycled.

Oil sands mining takes this concept one step further by going from two-phase solid/liquid flow to four-phase flows of vapor(air), bitumen, water and solids (4-in. to micron size), where it’s necessary to maintain both the solids content as well as the air content in the line. The good news is that with the help of industry and oil sands researchers, this has been done reliably for many years. The how is a story for another day, though I did enjoy working in this challenging environment.

Like many other field sensor applications, flow may seem relatively straightforward, however, like many other parts of our industry, there are also many ways for the process to be “not as it appears,” and as a result, the opportunity arises to excel or fail miserably. Small details, such as low solids concentrations, can have a significant impact on your measurement—even though the stream may not be considered a slurry.