Greg McMillan and Stan Weiner bring their wits and more than 66 years of process control experience to bear on your questions, comments, and problems. Write to them at email@example.com.
By Greg McMillan and Stan Weiner
Stan: Level measurement is essential for monitoring and controlling inventory, except when equipment is in-line and completely full, such as is the case with heat exchangers, static mixers and extruders. Level has an integrating response where any difference between total flow into and out of the equipment will cause the level to rise or fall. An out-of-service level device translates to equipment downtime, since level will continually ramp. Continual reliable level control is critical to prevent equipment from becoming empty, starving and damaging pumps, or overfilled, causing spills or activation of safety instrumentation systems (SIS) or relief devices.
Greg: The accuracy and precision requirements are exceptional for many unit operations and custody transfer. For material balance control in distillation columns where level manipulates reflux flow, high-level measurement threshold sensitivity and level controller gains are essential for composition control. For continuous reactors and crystallizers, residence time (volume/flow) is important for composition control. Here both accuracy and precision are important. As production rates are pushed higher, these vessels have to operate much closer to high-level constraints to achieve the necessary residence time. For batch operations, the vessels are operating at higher levels as well. Liquid carryover is a concern, particularly when gaseous reactants or products or boiling result in liquid swell. Recently the goal to minimize raw material, intermediates and product inventory has resulted in tanks operating close to minimum levels that can cause flashing and gas in the pump suction. Thus, levels are operating much closer to high- and low-level constraints.
Stan: Drift must be minimized or automatically compensated for to meet the increasing goals of eliminating maintenance, minimizing downtime and potential exposure to hazardous chemicals, besides reducing inventory errors and exceeding constraints and technician resources.
Greg: High accuracy and precision of raw material and product storage tank level can be used instead of weigh cells and mass flowmeters, since the density is well-defined for custody transfer. Transfer rates can be computed from the rate of change of level. A key to minimizing noise and updates is the use of a dead-time block to create the delta measurement as noted in my Modeling and Control website blog "A Calculation So Simple Yet So Powerful" (http://modelingandcontrol.com/2011/03/a_calculation_so_simple_yet_so/) Radar level detectors can meet all of the requirements noted here by me and Stan. We are fortunate to have Ram Ramachandran, the principle at Systems Research Int'l, Inc., and his extensive field experience with many types of measurements. Where have you seen a dramatic increase in the use of radar?
Ram: A big application has been oil storage tanks. Float devices got sucked by pumps, pulling them downwards, giving a lower than actual reading and possibly breaking the cable. Boiling light ends at atmospheric conditions also caused noise and wear and tear. Radar devices eliminated these problems, dramatically reducing maintenance, downtime and exposure besides increasing accuracy and precision. A 0.1% error in custody transfer can cost $200/min. Stilling wells large enough to provide a clear path and sway are used to eliminate a vortex in the sighted surface. You can dampen waves and ignore spurious effects to detect changes in level as small as 1 mm. Radar also lends itself to floating roofs.
Stan: What are the safety integrity level and redundancy required for oil storage, considering the need to prevent oil fires?
Ram: The classification was problematic due to so many legacy systems. A SIL of 1a was decided. Radar increased the reliability of the measurement, and dual installation is now the method of choice to provide the independent level measurements to meet the mean time between failure requirement.
Greg: When is guided-wave radar (GWR) used instead of non-contacting radar?
Ram: Cable or rigid GWR probes are used in smaller tanks and horizontal tanks. For taller tanks, flexible GWR probes can be used with weights and centering disks to keep the wire taut and positioned away from the stilling well walls. Single probes are less susceptible to buildup. GWR is used because the setup is easier, simpler and more repeatable than non-contacting radar.
Stan: Why wasn't radar used when first introduced?
Ram: The software has gotten a lot more intelligent, taking into account a lot more of the installation geometry and conditions that can make or break an installation. Suppliers and users have gotten a lot smarter about proper design and input of vessel geometry. The location must avoid falling and spraying streams and slanted surfaces. The stilling wells are used to eliminate vortexes and swirling. Very little calibration is required once properly installed and set up. Software intelligence has dramatically increased for self-checking, reporting drift, coating and signal integrity. The choices materials of construction have greatly increased, so that chemical attack is much less of a concern.
Greg: The Chemical Processing July 2011 article "Making the Most of Radar" provides excellent discussion of the do's and don'ts of a successful application and new software features. The article notes that software can now provide overfill protection, increase the signal-to-noise ratio and monitor the dielectric constant. The analysis of signal changes by the software can warn that the tank is full despite the loss of signal. Dual-port and direct-switch technology can improve the signal strength for the whole measurement range. Echologics offers the ability to ignore false echoes. Software can estimate the dielectric constant from the known length of the probe.
Stan: When are nuclear devices used?
Ram: Nuclear devices are used if the level device needs to be completely external to the process due to extremely high pressures and temperatures or exceptionally hazardous materials. Nuclear is also used for difficult applications where radar is not possible because the dielectric constant is too low and other devices, such differential pressure (DP) transmitters, are not suitable.
Greg: What are the application considerations?
Ram: The use of nuclear point sources instead of strip sources creates a nonlinearity that requires compensation by signal characterizations. The measurement is highly repeatable with an accuracy of about 1% if the effect of density changes on characterization is negligible, and the location of the source relative to the detector is correct. The beam path must avoid agitators, eductors, spargers and dip tubes. Vessels jackets complicate the installation in order to avoid the effect of varying jacket fluid. The measurement may not be possible when coolant coils are present.
Stan: Why are plants reluctant to use nuclear?
Ram: Besides the burden of paper work, a radiation officer must do periodic wipe and shutter tests. A record of background radiation survey should be maintained for NRC audit. Also, the plant cannot do maintenance on the vessel until the radiation officer shuts off the source and authorizes the work.
Greg: Bubblers and diaphragm seals offer a way of isolating the DP transmitter from the process. What are some of the considerations in their application?
Ram: For bubblers, an accurate purge flow regulator and unlimited purge source is needed to insure the purge rate and hence, the purge pressure drop is constant. The end of the tube may be cut at a 60º angle or raised off the bottom when stratification of the fluid causes buildup. For diaphragm seals, larger seals diameters and better evacuation of any bubbles from the fill have reduced the 95% response time to less than 16 seconds. The use of fill with a low-volume temperature effect has reduced the effect of unequal capillary lengths and uneven temperatures (e.g., shade versus sun) on accuracy. The level measurement accuracy for a well-designed bubbler or capillary installation with no density changes, shrink or swell is about ½%.
Stan: Larry Stichweh at Monsanto documented how pressure transients may cause a temporary rise or fall in the DP transmitter signal due to the compressibility of the purge gas. Larry advocated the consideration of a liquid purge. In some applications, the drying effect of a purge gas has caused caking at the tip of the bubbler. A small amount of water or solvent introduced into the purge gas solved the problem.
Greg: The use of Coriolis mass flowmeters on a vessel discharge stream can provide the density measurement needed to correct level measurements affected by density. The totalization of mass flow from these meters can also provide an on-line check of the inventory of process mass important for material balances and process modeling. Accurate level and flow measurements greatly enhance process knowledge.
Stan: There are some heavy reasons to use floats instead of radar, such as heavy vapors, heavy foam and heavy emulsions. Greg has his own lighter "Top Ten Reasons"
Top Ten Reasons to Use Floats
10. There is something to be said for tradition.
9. It works in my toilet.
8. I want a signal I can see and feel.
7. It is kind of like a tape measure.
6. Mechanical engineering rules!
5. These systems are my legacy.
4. I am into float trips.
3. It has its ups and downs, but always ends on top.
2. Cheap, cheap!
1. I don't want any newfangled software.