A continuous blending system is like a very long batch in that they both have a startup, a run time and a shutdown. Benefits over batch blending include elimination of sequential addition, in-line dilution and a smaller footprint for reactions.
Less intermediate and final product storage is required, reducing in-process inventories. Higher-concentration raw materials can be diluted as part of the continuous blending process, resulting in smaller storage and delivery systems for those materials.
Quick changeovers and just-in-time product manufacturing are additional benefits. However, these benefits will be difficult to realize or keep consistent without proper design and maintenance of the continuous blending system.
This article addresses maintenance issues for the supply, metering, mixing, analysis, general layout and control components of continuous blending systems. See the figure.
Raw material delivery systems can vary greatly and must be addressed for each material. Recirculation-type supply systems use pumping and backpressure control, introducing mechanical energy into a system. This energy can turn into heat. Some materials can be heat sensitive and lose potency. Other materials might require heating to keep from freezing.
Positive displacement pumping is used when a raw material should not be agitated or mixed too much. Some materials require low flow rates to keep particles in suspension or solutions from settling out.
Material can precipitate and build up at recirculation points in storage tanks. Chunks then can fall back into the flow stream and cause blockage in meters and valves. Periodic cleaning could be required. Strainers can prevent the blockage.
More than one liquid blending system can share these delivery systems. The supply systems should be sized larger than the anticipated demand of the connected systems. The delivery system can absorb flow and pressure disturbances more easily as connected blending systems are started up and shut down.
A good rule of thumb for a recirculation-type supply is four times the demand. The idea here for a supply system is very much like a kitchen faucet: Open the valve and raw material is available. Starting flow to a blending system should not cause a large bump in delivery system operation. Fluctuations in the supply will cause variability in the metering to all connected running systems and will adversely affect product quality.
Delivery system flow or pressure control should be tuned relatively slow so compensatory control moves are slow and do not upset the metering loops. Leave the higher-speed control to the metering loops.
Production demands for formulations can over-draw from a supply system, causing delivery problems. Systems and formulations should be timed to avoid these situations. The alternative to scheduling is to increase the capacity of the system to meet the demands.
Liquid metering typically involves a flowmeter and flow control valve combination. This step is the "make" or "break" for continuous blending. Meter calibration can drift not only as a result of wear and age, but also as a result of raw material buildup. Very small metering valves can become partially blocked by contaminants or raw material precipitates,creating annoying control problems and product variability.
Valve performance can deteriorate if the equipment is not properly serviced and maintained. A sticky valve stem can cause the controller to cycle around its setpoint because the valve cannot achieve the position required for the desired setpoint.
This valve stiction can cause the desired flow to be a very difficult target to hit. It can make the problem look like loop tuning. A tuner will probably slow the loop down to an unacceptable level to try to minimize overshooting problems, lengthening the loop cycle time while still not resolving the problem.
To identify this situation, place the control loop in manual mode, and move the valve in small (1 percent or less) increments. If the valve does not move with each small increment but does with one larger movement after enough adjustments are made, this valve should be serviced. An acceptable level of valve stiction typically should be 1 percent or less.
The flexible operation of continuous blending systems comes with a price. Formulations can show up after a system has been running that require metering or some other part of the system to run outside of its design range. Valves can be required to support too large of an operating dynamic range for the various formulations. Former good quality might not be met.
Many processes, including blending, are installed with equal percentage valves as a default trim type. An equal percentage valve has an initially slow opening characteristic, and the further it opens the faster it opens. This feature provides good control under increasing backpressure conditions as a result of the increased flow. But it also gives equal percentage valves a large gain change over its range that can affect loop tuning.
Loop tuning of product addition should be optimized for operational conditions. Lambda tuning techniques are a good fit here because they make many loops with dissimilar physical response characteristics behave the same. If all metering runs respond the same to process upsets, the benefits to product quality are obvious. See the sidebar for more information on this tuning method.
The mixing manifold sets the order of addition. Pipe lengths and flow rates can be set to allow residence times or reactions to occur before the next materials are added.
Once the materials are in the same pipe, the introduction of turbulence by static mixers or powered dynamic mixers can homogenize the ingredients or promote reactions. Mixers typically are sized for minimum effective flow rates. Running below this range can cause quality problems. Also, maintenance of these mechanical components can be a challenge if the product is prone to buildup or the components cannot be cleaned efficiently.
Product sampling also should be considered. Regular sampling and testing typically are required to monitor and adjust the process for quality. The use of quarterturn cocks on open-sampling systems can create a personnel exposure problem as a result of difficult-to-control sampling rates (quick-opening valves can splash material out of a sampling container), as well as flow and pressure disturbances to a running system. Valves designed for this service should be used. However, opening a valve creates product egress and contamination ingress points.
Inline analysis can prove beneficial here. The continuous feedback regarding process quality can provide immediate control benefits. For example, by monitoring a parameter such as product density, plants can determine whether an ingredient flow is drifting or is out of calibration.
General layout and other considerations
Liquid blending systems should be laid out for maintainability and expandability (change is inevitable). Clearances should be provided for draining, and mixing equipment and instrumentation should be easy to remove or replace.
Consideration should be given to equipment such as coriolis meters, which are harder to drain. System flushout and cleaning should be considered, in addition to the expansion characteristics of piping from these procedures. Outdoor installations have environmental issues such as humidity and freezing that create operational and maintainability issues such as heat tracing and insulation.
Turndown of all system components should be considered. Turndown is the minimum flow for a component that guarantees a minimum accuracy. For metering runs, a flow meter might be capable of 200:1, but the valve might be able to supply only 100:1, which sets the limit for that run.
The plant should set minimum and maximum production rates for each formulation by looking at these parameters for the system as a whole. Low production rates, which equate to low-flow conditions, might not be optimal if heating or cooling is involved. Energy usage might be lower at these lower rates but less efficient.
Electrical design should address ease of access for calibration and troubleshooting, as well as protection in corrosive or hazardous areas. The National Electrical Code (NEC) allows flexible wiring methods in hazardous locations to make maintenance or changes easier. For new installations, blending modules that are pre-assembled and inspected are a good choice.
This example shows Lambda-tuned multiple loops that have the same effective settling time. Two setpoint changes are shown. The desired ratios are held relatively constant and good product is made during the transition to the new system setpoints.
Depending on the amount of running data required and the running speed of the blending system, various control system considerations are available, including I/O and processor speeds and the combined accuracies of the metering equipment. System interlocks of raw materials are important to keep ratios in spec and are useful for operational issues such as leak detection. The right amount of "smarts" and sensing designed into a system can aid in troubleshooting and minimize off-spec material.
Newer digital communication instrumentation can give more process and troubleshooting information. For example, coriolis-type flow meters not only for new installations give flow in volumetric and mass units. But also can provide the temperature and density. If the material is out of spec, an alarm, shut-down control or automatic adjustment can keep the blending running. This benefit can be significant if the consistency of the raw material is prone to drift. Digital control valves can give maintenance alerts based on calculated or internal measurements.
Continuous blending system maintenance should include standard good practices. Goals for running the system ," such as production requirements, system changeover, cleaning and data reporting ," should be defined. System constraints and disturbance sources should be identified and used to set minimum and maximum operating rates for each formulation.
Tune for best response, and optimize energy usage where possible. Log recurring issues to focus corrective action. Take the time to understand each application and perform what-if analysis to predict and correct problems. A well-maintained continuous liquid blending system will reward your facility with dependable operation and quality product for the consumer. CP
McCormack is a principal engineer for Emerson Process Management's PC&E division in St. Louis. PC&E is a systems integrator specializing in automation projects for chemical, petroleum and other process industries. He can be reached at (314) 872-9058, or via e-mail at email@example.com.