Tech Tips July 2006


July 25, 2006


Inventory tank gauging—Part 2: Contact guided microwave/radar guidelines

Inventory tank gauging (ITG) applies level-measuring technology to inventory-storage and custody-transfer applications in contrast to process control measurement. ITG systems often range their measurements over greater heights and with more accuracy than process level-measurement devices.

The previous 'Tip of the Week' offered some guidelines on non-contact microwave/radar tank gauging. Here are a few tips on applying contact guided-microwave/radar technology.

Contact guided-microwave/radar level-measurement devices also mount at the top of tanks. They transmit radio- and low-microwave-frequency signals along transmission lines or wave guides , which extend into the material to be measured. As with non-contact-type microwave/radar devices, pulsed and frequency-modulation varieties of contact-type devices are available.

The latter can measure levels of liquids, pastes, slurries, powders, and particularly granular materials. Contact-type microwave/radar gauges—some of which are referred to as RF- and time-domain-reflectometry—operate at considerably lower frequencies than the non-contact types and cost less, yet offer many of the same advantages. In addition, contact-type microwave/radar gauges typically have more installation flexibility and are less expensive to install. Their use is not subject to site licensing by the U.S. FCC or similar government regulators elsewhere.

These all-electronic units can measure over short or long distances and are impervious to dust, air movement, temperature, or pressure fluctuations. On the other hand, heavy material build-up on the transmission line or wave guide can cause false readings. Contact-type microwave/radar gauges have difficulty in sensing plastic materials with extremely low dielectric constants such as EPS beads. However, the technology works very well with plastic pellets and flakes.

Source: Control Engineering, May 2006 Inside Process article, ' Level Measurement and Inventory Tank Gauging .'

July 18, 2006


Inventory tank gauging—Part 1: Non-contact microwave/radar methods

Inventory tank gauging (ITG) applies level-measuring technology to inventory-storage and custody-transfer applications in contrast to process control measurement. ITG systems often range their measurements over greater heights and with more accuracy than process level-measurement devices.

Non-contact microwave/radar level-measurement devices mount to the top of a tank, transmitting microwave signals down toward the surface of the material in the tank. Signals reflect as an echo, which is detected by a receiver. Based on a difference between the transmitted- and received-signal, the system calculates material level in the tank. Two modulation techniques are used:

  • Pulsed systems measure the time for the echo to return from transmitted pulses. Time delay between pulse transmission and reception is a direct measurement of level of the material.

  • Continuous wave transmission with frequency modulation (FMCW) is based on altered frequency of the return echo mixed with the transmitted microwave signal. Since the two signals are at different frequencies—due to frequency modulation—the mixing results in a signal frequency proportional to distance to the material surface.

Frequencies of microwave used significantly affect the performance of radar level gauging. High frequencies (shorter wavelengths) are more sensitive to vapor, foam, and contamination in the measured material. Around 24 GHz, even a small amount of water vapor can absorb microwave signals. Lower frequencies' longer wavelengths and wider beam angles result in numerous interference echoes from tank walls and internal agitators. Optimum frequency has been found to be around 10 GHz.

Non-contact microwave/radar devices are completely or relatively immune to measurement problems associated with many other technologies. Their use avoids or minimizes:

  • Density dependence;

  • Dielectric constant dependence;

  • Dust sensitivity;

  • Maintenance intensity;

  • Non-operation under vacuum;

  • Pressure sensitivity;

  • Sensitivity to the atmosphere between the medium and sensor (foams and variable-density vapors);

  • Sensitivity to scaling; and

  • Temperature sensitivity.

The method is inherently suited to level-measurement of harsh or corrosive substances.

See the following 'Tip of the Week' for some guidelines on contact-type guided-microwave/radar tank gauging methods.

Source: Control Engineering, May 2006 Inside Process article, ' Level Measurement and Inventory Tank Gauging .'

July 11, 2006


Maybe your safety-instrumented system doesn't actually need SIL-3

Safety integrity-levels (SIL 1. 2, 3, and 4) provide a measure of system performance: the higher the number, the better the safety performance—such as lower probability of failure on demand. Revised standard ANSI/ISA 84.00.01-2004 (IEC 61511-1) is the current document that covers SIL methods, providing a rational, uniform way to assess and mitigate risks. Properly used, the standard directs where spending is necessary, allowing savings elsewhere. With this in mind, why use a higher performance safety system if it's not needed?

Many end-users specify 'certified for use in SIL 3' redundant-logic solvers (often referred to as 'safety PLCs'). However, mere use of these certified logic solvers does not create a SIL 3 system . A system includes sensors and final-control elements. Only in very special cases can redundancy be avoided in a true SIL-3 safety system. With input sensors, the logic solver and the final-control element (or actuator) will almost certainly need to be redundant in a SIL-3 design.

Rules for how much fault tolerance must be applied to field instruments at any given SIL level is defined in the standard (Section 11.4.1); and it further describes cases where fault-tolerance requirements may be decreased by one, or where the number needs to be increased by one. Fault tolerance requirements can be relaxed where the field instrument is certified as having a particularly low level of dangerous failure modes or where detailed information exists about the hardware—such as failure rates, failure modes, and internal diagnostics levels.

Satisfying these requirements for valves, actuators and other final-control elements can be difficult and costly. A higher SIL has a significant impact on the number of valves required and the installation's complexity to allow proper maintenance and proof testing.

A true SIL-3 system typically requires triplicate transmitters, a triplicate (two-out-of-three) or 1oo2D (one-out-of-two with diagnostics) logic-solver, and either three valves in series, or dual valves in series incorporating partial-stroke testing—or frequent full-stroke testing that's often impractical.

When SIL determination methods are effectively applied, SIL 3 requirements should be extremely rare. In many cases, it's more effective to redesign the process to be less risky than it is to require a SIL-3 safety system. SIL 2 will often be the highest true requirement in most applications.

General-purpose PLCs are only suitable for use in SIL 1 applications. Triplicate—and 1oo2D—SIL-3 approved safety PLCs are over-designed and unnecessarily costly for SIL 2, in the authors' experience. So, is the sensible resolution a SIL 2 solution?

Specifying a SIL-3 logic box is not the magic key to a safer facility and does not mean the overall design conforms to requirements of industry standards. Proper determination of safety integrity levels will often result in no more than SIL 2 requirements for most process applications.

Source: Control Engineering, March 2006, Inside Process article, ' Safety Integrity Level 3 ' by Paul Gruhn, president L&M Engineering and Dave Reynolds, vice president, marketing, MTL Open System Technologies .

July 3, 2006


Reducing the impact of recall in food processing

To help quickly narrow the search for an adulterated product, the U.S. FDA looks for information in three areas of production. This allow a search process to 1) include all products that could be affected, 2) exclude products that could not be affected, and 3) prove dilution or concentration of affected products. Fast discovery is especially significant in the case of dairy and food products manufactured in environments where lots are mixed and byproducts created and reused or sold to other manufacturers. Items 2 and 3 above provide manufacturers an opportunity to reduce the impact of a potential recall.

Ability to exclude products that could not have been adulterated also allows isolation of suppliers and customers. It could be the one factor that allows your company to return to business quickly after an event. This effort should consist of information about:

  • Bulk dry or liquid materials added to a storage tank or silo after a downstream destination has changed;

  • Products made in any equipment after a full wash or CIP (clean in place) has been completed; and

  • Products that could not have physically co-mingled with any other. Mix-proof routing valves and physical pipe connections are examples of physical breaks that make cross contamination impossible.

While the most difficult for manufacturers to furnish, information about dilution or concentration of affected products will also reduce impact of an investigation and recall. Being able to say quickly and accurately which products have the highest concentration of suspected ingredient, which have lower concentrations, and which have none will give investigators confidence in handling the event. These data can be gathered by:

  • Tracking electronically all movement of materials and products in manufacturing;

  • Placing meters in strategic locations to monitor bulk materials flow; and

  • Tying metered flow to lot genealogy accurately.

If meters are in the right places, investigators can estimate percent dilution for each scenario. These products can be tested and any recall significantly reduced. A system needs to enable this investigation to occur within a few hours to give confidence that it can be trusted.

Source: Control Engineering, April 2006, ' Are you ready for a recall? ' sidebar in article, 'Bioterrorism Act: Burden or Benefit?'

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