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.