Functions of Surge Drum
The surge drum performs various important functions. They are: To separate the liquid out of the gas and allow the gas to pass to the flare line, to drain the liquid from the flare line. The third function of a surge drum is to absorb the violent liquid surging that occurs when the flare line is quickly opened.
In order to reduce the effect of flow rate variations between interconnected process units surge drums are frequently located between 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 needed.
Despite 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. It is generally more important to allow levels to "float" in order to minimize flow rate variations for surge drums.
The distinction between drums and tanks is that of size and is not sharp. Usually they are cylindrical vessels with flat or curved ends, depending on the pressure, and either horizontal or vertical. In a continuous plant, drums have a holdup of a few minutes. They are located between major equipment to supply feed or accumulate product.
Surge drums between equipment provide a measure of stability in that fluctuations are not transmitted along a chain of equipment, including those fluctuations that are characteristic of control instruments of normal sensitivity. For example, reflux drums provide surge between a condenser and its tower and downstream equipment; a drum ahead of a compressor will ensure freedom from liquid entrainment and one ahead of a fired heater will protect the tubes from running dry; a drum following a reciprocating compressor will smooth out pressure surges, etc. Tanks are larger vessels, of several hours holdup usually.
For instance, the feed tank to a batch distillation may hold a day's supply, and tanks between equipment may provide several hours holdup as protection of the main storage from possible off-specification product or as opportunity for local repair and servicing without disrupting the entire process.
When a valve is either fully or partially closed at some point downstream, the fluid will continue to flow at the original velocity (considering fluid flow). The pressure will rise significantly (pressure surge) just upstream of the control valve and may result in damage to the pipe system in order to counteract the momentum of the fluid. If a surge chamber is connected to the pipeline just upstream of the valve, on valve closure, the fluid, instead of being stopped suddenly by the valve, will flow upwards into the chamber, hence reducing the surge pressures experienced in the pipeline.
The fluid continues to flow, passing into the surge tank causing the water level in the tank to rise,upon closure of the valve. The level in the tank will continue to rise until the additional head due to the height of fluid in the tank balances the surge pressure in the pipeline. At this point the flow in the tank and pipeline will reverse causing the level in the tank to drop. This oscillation in tank height and flow will continue for some time but its magnitude will dissipate due to the effects of friction.
The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.