In process industries, boiling water to make steam is an important process. The boiling drum is where the boiling water is converted to steam, so that maintaining the level of the drum is an important process.
Controlling the level is critical because if the level becomes too low, the drum heats up in dry results in damaging the mechanical components. And if the level becomes too high, water can be carried over into the steam pipework, possibly damaging downstream equipment.
There are types of Drum level control:
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Single element drum level control
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Two element drum level control
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Three element drum level control
Single element Drum level control:
In single element level control, only the process variable (PV) is measured, that is level is measured to control the level itself.
The differential pressure level transmitters are used measure the level, a wet leg system is implemented to measure the level. Low-pressure side of DP is connected to the head of the Drum and high-pressure side is connected to the bottom of the tank.
As the level is measured using the level transmitter, the level controller gives an appropriate control action to the valve. Valve adjust the inlet flow of the water to the drum.The Auto/manual transfer of feedwater control valve is accomplished via Fk-1.
Two-element drum level control:
In two-level control, cascade control system is used. Not only the inlet flow, the outlet flow can also affect the level of the drum.
In this cascade control system, the level control becomes the primary and the flow control the secondary
An additional steam flow element is added to the single element. Any imbalance between steam mass flow out and feedwater mass flow into the drum is corrected by the level controller. This imbalance can arise from leakage in stem circuit, variation in feedwater supply pressure.
Three-element level control:
The three-element level control measures the inlet flow of the water to the drum takes. So that the rate of flow from the drum and rate of flow to the drum, then the level of the measure and put into controller action.
The three-element drum level control is ideally suited where a boiler plant consists of multiple boilers and multiple feedwater pumps or where the feedwater has variations in pressure or flow. The three-elements are made up of the following.
The three-element control is basically a type of feedforward control system.The steam flow rate is measured and used as the set point of the feedwater flow controller. In this way, the feedwater flow rate is adjusted to match the steam flow. Changes in stream flow rate will almost immediately be counteracted by similar changes in feedwater flow rate. To ensure that deviations in drum level are also used for control, the output of the drum level controller is added to the feedforward from steam flow.