4 replies to this topic

[daure.tejaswini]

In boiler 3 element control, why we take feedwater flow rate in cascade with drum level and steam flow rate in feedforward control?
How connection for lead-lad realy is done to avoid shink and swell?

According to my knowledge, inlet flow rate is taken in feedforward and outlet flow rate in feedback to control level in tank. Can some please explain why this is not followed in boiler

[Bobby Strain]

Daure,
The function of the loop is to maintain the drum level. Since most drums are small in comparison to holdup, the steam flow is a feedforward measurement to modulate BFW flow without waiting for a level change. It's that simple.
Bobby

[daure.tejaswini]

Thank you Bobby.That will be like material balance. Will you please tell me significance of level measurement?

Edited by daure.tejaswini, 26 September 2012 - 02:43 AM.

[vijji]

Daure,
The function of the loop is to maintain the drum level. Since most drums are small in comparison to holdup, the steam flow is a feedforward measurement to modulate BFW flow without waiting for a level change. It's that simple.
Bobby

For steam drum level control, the modern 3-element system steam flow, feedwater flow and drum level should be selected. Steam and feedwater flows are compared, with feedwater being requested accordingly and trimmed by the drum level signal. This system is better than having the drum level directly control feedwater, because foaming or changing steam drum conditions can cause a misleading level indication. Also, the 3-element controller responds faster to changes in demand.

[Bobby Strain]

Daure,
It's the level that you want to control. The three-element control accomplishes this most reliably.

Bobby