In-situ density measurements have long been useful in process control applications from recycling nuclear fuel and refining oil to measuring beverage proof or degrees Brix (°Bx) sugar content of an aqueous solution. Density measurements can provide real-time determinations of concentration, blending ratio, fermentation, or oil API (American Petroleum Institute) gravity, which measures oil heaviness or lightness, etc. These can be used to adjust process variables and optimize throughput, quality, equipment and material utilization, and energy efficiency.
So it’s no surprise that densities of liquids, slurries and mixtures in vessels and pipes have been measured for many decades, and over those many years, density measurement technologies have been improved and refined. Today, in-process density measurements are commonly performed in vessels using servo gauges, differential pressure (DP), tuning fork instruments and sometimes radiometry. Flowing measurements can be done with radiometry and with Coriolis flowmeters.
Here’s an overview of real-time, in-process density measurement methods, with recent developments that might inspire additional or improved instrumentation.
Density fundamentals bubble up
Density is defined as mass per unit volume, typically grams per cubic centimeter (g/cc), but is often expressed in specialized units such as specific gravity (as a ratio to a standard such as water), API gravity (where 10 is the same as water), degrees Brix (specific gravity converted to sugar content), proof (calculated to reflect the percentage of ethanol), etc. The fact that the fundamental units of density—mass and volume—are simple (if not always easy) to measure makes density attractive as a direct and indirect measurement of attributes that might otherwise require complex and expensive analyzers.
Since most fluids expand and contract significantly with temperature changes—independent of the parameter of interest—the temperature of the fluid must often be taken into account when calculating properties. Measurements taken under significant pressure or vacuum may also require compensation for pressure-related density changes.
For example, for years, concentrations of acids in nuclear fuel recycling were measured by bubble tubes (Figure 1). By immersing a glass tube to a known distance below the surface of the acid solution in a vessel, providing a very small flow of pressurized gas to form intermittent bubbles, and measuring the gas pressure, engineers could determine the pressure exerted by the liquid above the bubble. With temperature and, if necessary, ambient pressure correction, they could measure the fluid density and, hence, acid concentration. Only the bubbler tube was exposed to the corrosive and possibly radioactive process—the gas pump/supply and pressure instrumentation could be placed at a distance.
Another way to weigh the volume of a liquid is by using a displacer—a float that doesn’t float. The displacer can be lowered using a servomotor and cable into and through the fluid, and the force required to support the float and cable can be measured to determine the weight of the displaced fluid.
“A servo gauge and displacer is often used to detect density interfaces in tank farm and crude oil applications,” says Gene Henry, business manager, U.S. level products, Endress+Hauser (www.us.endress.com). The displacer can be lowered to detect the liquid level, then lowered through the liquid to detect interfaces between layers, including the level of any water in the bottom of the tank. “The gauge can report the density of each layer. If needed, the density can be corrected for temperature with the input from a multi-point temperature probe,” Henry says.
Hydrostatic and differential pressure
Pressures at known fluid depths are commonly used to measure level and/or density, either by a single measurement (hydrostatic) or by the difference between two measurements (differential pressure, or DP). Bear in mind that a pressure sensor at a depth can’t tell whether a pressure change is due to a level change or to a change in density—to determine one, the other must be held constant or compensated using an additional instrument.
“Hydrostatic level, using one pressure sensor, requires the fluid have a known density or specific gravity, so density changes introduce error,” says Jeff Brand, product manager, pressure, Vega Americas. Adding a second sensor in the fluid at a known height above the first provides a DP measurement, and allows density to be calculated.
A common practice has been to use a single sensor (DP cell) that measures a difference in pressure, connected at two heights using impulse lines. These lines may be in the form of capillary tubes, sealed at the process end, and filled with special fluid. One variant is a directly-mounted seal at the bottom tap and a capillary to a single remote seal above it.
The required distance between the sensors depends on the density measurement range and the height of fluid above the sensors. “It’s complicated, but the results are very good once it’s set up,” Brand says. “As density changes due to the product or temperature, you can calculate that density and use it to get an accurate overall level.
“DP for density is usually used on open vessel applications such as flotation cells, mud pits and shakers on mud processing units where a vapor pressure would not interfere. Of course, both sensors must be immersed. In a pressurized tank, you can still calculate density but not level, as the instrument can’t tell the difference between a pressure and a level change.”
Instruments are available to mount on the outside of a tank, or through the top. “They’re immune to effects from agitation, foam or obstructions,” Brand adds.
The relatively low cost, simplicity and proven technology of DP has led to widespread use. “Everybody knows how to calibrate a pressure sensor,” says Nathan Stokes, product manager, DP level, Emerson Automation Solutions. “However, the span of DP for density is on the lower side. And if you use a DP cell, there are the challenges of impulse piping (leaks, plugging, fluid evaporation) and with remote seals, and error due to temperature changes affecting the density of the fill fluid.”
More recent developments include electronic systems to measure DP (Figure 2), with built-in calculations for density and density-compensated level outputs. “Now, electronic remote sensors eliminate the need for capillary tubes. Two separate sensors are mounted top and bottom to get a single 4-20 mA signal for DP,” says Stokes. “This system is accurate despite temperature changes. Its precision still varies with the application—the separation of the sensors—but it’s simple and cost-effective as a replacement for troublesome capillary systems. Eliminating the mechanical system increases reliability.”
Advances in sensors, seals, electronics, computing power and diagnostics are increasing precision, improving reliability and reducing maintenance. “Ultra performance class” transmitters offer precision to 0.025% of span, says Stokes. “Sensitive equipment can be fine when the process is running at status quo, but getting there can cause damage—an instrument can be out of spec from the beginning.” Overpressure protection allows significant overpressures without drift to withstand startup surges, slugs and spikes.
DP cell transmitters are being fitted with advanced diagnostics that can detect plugged impulse lines. “This is really helpful for those wet- and dry-leg applications where you get sediment buildup in the wet legs that can solidify, or get liquid buildup in the dry legs so sensitively is reduced,” says Wally Baker, content management, pressure, Emerson.
“Diagnostics also monitor transducers for overpressure, saturation or over-temperature conditions, so you can know when these conditions are present,” Baker says. “Advanced diagnostics can detect low voltages or brownouts, and diagnose power supply or water-in-conduit problems. That can be built right in via HART or wireless to give an alarm or notification.”
Stokes adds, “With wireless and simplicity of DP, you’re not cutting into lines—just the two taps—and not running wires, just using a battery that can last 10 years, so it’s easy to add a meter and get a density measurement.”