Three Element Drum Level Control

I am giving below the write up for the Three element drum level control what we normally follow in our boiler. THREE ELEMENT DRUM LEVEL (FEED WATER) CONTROL

1.0 Introduction: In the boiler the steam flow fl ow changes according to the load demand from the turbine or from the process consuming the steam energy. To match the steam take off from the boiler, the feed water flow has to be increased i ncreased or decreased as required. The feed water is automatically controlled by suitably positioning Feed Water Control Valve (FCV) provided in the feed water line at boiler inlet. 2.0 Three Element Feed water Control: For automatic control of the feed water flow to t he boiler, following three (elements) primary inputs are normally being considered. 2.1 Drum level. 2.2 Main Steam flow. 2.3 Feed water flow. In addition, measurements of Drum Pressure, Main Steam Pressure and Main Steam temperature are also taken for correction/compensation correction/compensation purpose as detailed below. Following paragraphs briefly explains how the feed water flow is regulated to match the steam demand from the boiler 3.0 Drum level measurement: Two numbers of level transmitters (dP transmitters) one on left hand side and the right hand side of the Boiler Steam Drum are provided to measure drum level. 3.1 The two level signals from these t ransmitters are averaged averaged and connected to a manual three-way selector switch. The other two inputs for the three-way selector switch are directly from the two transmitters. Normally the average signal is selected for the control. In case any problem with any one of t he transmitters; the healthy transmitter is selected in the three-way selector switch for the control. 3.2 The functioning of two level transmitters is continuously monitored monitored through a deviation monitor. In case of difference between the two level transmitters is higher than the preset value, and then the control is automatically transferred to Manual with an annunciation.

3.3 The selected drum level signal i s compensated for density variation in the drum water due to pressure changes (actually due to temperature changes) in the drum, through a function generating with drum pressure signal. 4.0 Drum Level Control: The compensated drum level signal fed to a reverse acting P I controller as process variable. This measured drum level is compared with set point and a correcting signal (with proportional plus integral action) is generated in the controller according the deviation between the measured process variable (drum level) and the set point. 5.0 Main steam flow measurement: The dP Transmitter measures the dP across the ventury nozzle, provided in the main steam line (The dP measured is proportional to the square of th e flow through the ventury nozzle and hence the square root of the dP taken as proportional to flow.) The measured dP is square rooted and compensated density variations due to change of Pressure and Temperature. 6.0 Feed water flow measurement: The dP Transmitter measures the dP across the orifice fitting is provided i n the Feed water line. The measured dP is square rooted to arrive at t he feed water flow. 7.0 Feed water flow control: The compensated steam flow is added with the corrected signal generated in the drum level controller. This sum is taken as the demand of feed water flow (cascade set point) for the boiler and compared with the actual feed water flow to t he boiler. The deviation between the demand and the actual feed water flow, fed to the P & I controller. The processed out put from the controller is given to the final control element through a ' Hand selector switch' and an 'Auto – Manual station' – the Feed water control valve as positioning signal. 8.0 Single Element Control: During low load operation, three-element control is not required. Drum l evel measurement with it's set point is adequate. Hence the output of the Drum Level Controller is directly given to the Feed water Control Valve through the selector switch provided after Cascade P & I Controller. 9.0 Auto Manual Operation: An Auto Manual Control Station is provided as the last element of control system. Auto or Manual operation can be selected from the Auto Manual Selector Switch i n this Control Station. If auto selected, the positioning signal from the selected controller (through Hand selector switch) will go to the Final control Element – Feed water Control Valve. If manual is selected, the controller's outputs are isolated and the Feed water Control valve can be positioned from this control station.

Raise/Lower push buttons are provided in this control station through which F eed water Control valve can be opened or closed as desired by the operator. Bump less transfer facility shall be built in as part of the Control Station to avoid sudden jump of control valve position during change over from auto to manual or vice versa. 10.0 Indications: Normally Set point value indication, Deviation indication, Final Control element (Feed water Control valve) position indication and Process value indication are provided either part of  the Auto Manual Control Station or separately. 11.0 Alarm and Trip signals: Low and high l evel alarm signals are generated from one for the transmitters output through software switches. However very Low and very high level trip signals (software switches) from both the transmitters output and also from the averaged/compensated signals are considered through an 'OR' gate. Signals from transmitters are also checked for the healthiness of the respective transmitters before connecting to the 'OR' gate. Following are the Trip/Alarm Signals generated. 11.1 Transmitter deviation Alarm. 11.2 Drum level – Low Level Alarm 11.3 Drum level – High Level Alarm 11.4 Drum level – Very Low Level Trip 11.5 Drum level – Very High Level Trip 12.0 Hard Wired hooter Following Trip/Alarm Signals are also hardwired to the Hooter, which is to be mounted externally in control room. 12.1 Drum level – Very Low Level Trip 12.2 Drum level – Very High Level Trip