Surge drums are frequently located between process units to help reduce the effect of flow rate variations between interconnected process units. A low surge drum level can result in reduced capacity while a high level can cause liquid carryover. In an application characterized by alternating inertia and turbulence, stable level output is highly desirable.
Contrary to the normal control objective of keeping a measurement at set point, the purpose of a surge drum level control is to dampen the changes in controlled flow while keeping the liquid level in the vessel between limits. For surge drums it is generally more important to allow levels to "float" in order to minimize flow rate variations.