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.