- Automation Basics
Readings can be taken from the top down or bottom up. Examine the application to determine the right instrument and technique
By Lydia Miller
In most applications, level measurements need to be quantified and sent electronically to an automation system, which requires some type of instrument. Sorting through the instrument selection process begins by determining what data is needed for the application and how it can be obtained as simply as possible. Level measurements are generally concerned with these points:
- Level of liquid-How close is the vessel to being full or empty?
- Volume of liquid-How many liters or gallons is in the vessel?
- Level has reached a high limit-Will the vessel overflow?
- Level has reached a low limit-Will the vessel run dry?
The first two points require a continuous measurement, which tracks the liquid level in real time as it moves throughout the tank. The third and fourth points may only be of concern when the level has crossed a specific point, but in many situations knowing when the level is both too high and too low will be necessary.
Frequently, all these readings may be used with the continuous measurements provided to control room operators, while the high- and low-limit measurements are tied to alarms to avoid either extreme. Both continuous and point measurements can be used in safety-instrumented functions to prevent overfill, typically tied into a separate control system specifically for safety.
Point versus continuous level
If the objective is to determine if liquid has moved to or beyond a given point, a point-level measuring device, also known as a level switch, can be inserted through a vessel wall or inserted from above. Older switch designs used floats. Newer designs might use a vibrating fork (figure 1) that vibrates at a different frequency when immersed in liquid than when exposed to air.
Because the vibrating forks are not mechanical devices, they are more suitable when a switch is needed for a safety application. As the name implies, a level switch can only indicate if liquid is present or not. If it is immersed, there is no way to tell if it is just below the surface or under many feet of liquid.
If continuous measurement is necessary, as it is when an application requires knowing where the liquid is inside the tank at all times, there are many instrumentation options, but the majority fall into two categories: measuring from the top or bottom.
The bottom approach uses one technology for all practical purposes: static pressure. A pressure instrument reads through a penetration in the vessel wall and registers pressure created by the weight of the liquid. If the vessel contains water and is vented to atmosphere, a pressure reading of 4.34 pounds per square inch indicates there is 10 feet of water above the instrument.
This concept is straightforward in theory but can be complex in practice for three reasons:
- The position of the pressure instrument relative to the vessel penetration will change the reading. That means it is critical to know where the actual sensing point is if the instrument is mounted.
- Liquid density affects the reading, so the density characteristics of the process fluid must be understood to determine its effect on the measurement.
- A single pressure reading works only if the vessel is vented to atmosphere. If the system is closed and above or below atmospheric pressure, a differential pressure (DP) reading is necessary. The high side of the reading is the weight of the liquid, and the low side is connected to a second penetration at the top of the vessel to sense the head space pressure.
Newer differential pressure level options-including tuned-system assemblies or electronic remote sensor systems-significantly improve the performance of DP level systems and make specification less complex. Using DP for level is an excellent approach, because it is unaffected by equipment or structures inside the vessel, or by turbulence and foam, with minimal effects related to liquid characteristics outside of density.
Figure 1. A vibrating-fork level switch can identify the presence of a liquid, which is sufficient for many level measurement applications. (Shown is Emerson’s Rosemont 2140 Level Detector.)
Take it from the top
When the level instrument is mounted on top of the vessel, there are multiple technology choices. Older approaches are more mechanical in nature, for example, a float connected to a tape.
Over the past decade or so, many more nonmechanical methods have emerged. Radar level measurement options in particular have increased, because of improvements in cost and their ability to measure easily in many conditions.
For all radar options, the common denominator is bouncing a microwave radar signal off the liquid surface and measuring the time necessary for it to go down and come back to a sensor. This can be accomplished by measuring time of flight for a microwave pulse, or the degree of frequency shift with a frequency-modulated continuous wave (FMCW) signal. In any case, top-down techniques determine the distance from the instrument to the liquid surface.
Radar can measure the distance very accurately regardless of the liquid characteristics, with no compensation necessary for changes in density, dielectric constant, or conductivity.
The two main types of radar instruments are guided-wave radar (GWR) and noncontact radar (NCR). With GWR instruments (figure 2), a metal probe extends down through the air or vapor space and into the process medium. This helps concentrate the pulse, so the reflection is less affected by reflections from vessel walls, internal structures, or agitators. On the other hand, if there are moving agitators, the probe could get wrapped around them, so a noncontacting method might be better.
NCR level transmitters provide continuous level measurements, but without touching the process medium. Some models use a microwave pulse, while others send an FMCW signal to perform the measurement. With pulse radar, the same time-of-flight technique used by GWR determines distance.
With FMCW instruments, the transmitter sends microwaves in a continuous signal sweep (figure 3) with a constantly changing frequency. The frequency of the reflected signal is compared with the frequency of the signal transmitted at that moment, and the difference between these frequencies is proportional to the distance from the radar to the surface, providing the data required to determine level.
Figure 2. A guided wave radar (GWR) instrument uses a metallic probe to guide the pulse to the surface and back. (Shown is a Rosemount 5300 Level Transmitter.)
Figure 3. A frequency-modulated continuous wave (FMCW) instrument delivers more powerful reflections to provide a higher degree of accuracy than most pulsebased radar transmitters. (Shown is a Rosemount 5408 Level Transmitter.)
Level versus volume
When a volume measurement is needed, accuracy is typically important, because inventory value or product custody transfer could be at stake. Level instruments do not measure volume. If a volume value is necessary, it has to be calculated based on the vessel dimensions, which must be fully understood. A radar instrument often has an accuracy of ±0.12 inch (3 millimeters). But if the vessel diameter measurement is off by several inches, the calculated volume will not be accurate. Higher-accuracy applications may call for radar instruments with ±0.02-inch (0.5 millimeter) accuracy, but additional methods are needed to get an accurate volume calculation.
For example, situations where the volume measurement must be very precise require "strapping" where the tank diameter is measured critically at multiple points, with the values incorporated into a look-up table so a change at any level will reflect the correct volume change. This is particularly important with distorted or odd-shaped vessels, such as spherical, conical, and horizontal cylinders.
A change of 10 inches near the top of the vessel may represent a much different volume than the same change near the bottom. Additionally, temperature and pressure measurements may also be required to get a full picture of the actual volume inside a very large vessel. Such situations are rare outside of custody transfer applications where money changes hands based on product volume measurements.
In many real-world applications, repeatability alone is sufficient, and the level measurement by DP or radar level instruments can certainly deliver. A company will have to examine the needs of each application to determine which type of measurement and instrument is appropriate for its needs. Fortunately, there is no shortage of options.
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