Why is Level Control so Difficult?
Level seems simple to monitor, but it s often just one of many process variables on an HMI display. Compounding the problem, incorrect level often doesn t affect unit operation or cause any other significant process variable disturbance until the safe fill level is exceeded. In many processes, level doesn t have direct significance to plant production or product quality, and absolute level often varies over a large range around what is considered normal. In tank farms, the operating level is simply inventory to be managed and normally varies across a large range. High level is often an indirect hazard, with the direct hazard being too much mass or volume. Some overfills challenge the tank or vessel, causing it to overpressure or to collapse when the retained mass exceeds the equipment structural design limits. Many overfills occur because of loss of containment when liquid passes to downstream equipment that is not designed to receive it. The key problem is that, even though a tank may
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