Boiler drum's water level control

The presentation is about the boiler drum's water level control, which is used on the ship for generating the steam. The presentation briefs about some controls used overboard to maintain the level inside the boiler for continuous steam supply.

1. Boiler drum’s water level control Mukul Kashiwal M.Tech (Marine engineering and management)

2. Why the boiler drum water level is to be maintained? Boiler steam drum water level is one of the important parameters of power plant that must be measured and controlled. If the water level is too low it may cause damage to boiler tube by overheating On the other hand too high drum water level leads to improper function of separators, difficulty in temperature controlling and damage in super-heater tubes . Turbine may also be damaged by moisture or water treatment chemicals carryover. Also the drum pressure control is adversely affected by poor level control.

3. Steam Drum water level error Under boiling condition, steam supporting field products such as bubbles exist below the water/steam level interface. There bubbles have volume and therefore displace water to create a misrepresentation of the true water level in the drum. Pressure in the drum has another effect upon drum level. Steam bubbles are compressed under pressure. Due to varying demands of load, the drum pressure changes and therefore the steam bubbles expand or contract respective to these pressure changes. A higher steam demands will cause the drum pressure to drop, and the steam bubbles to expand to give the appearance of a water level that it truly is . The fictitious higher water level cause the feed water input to be shut down at a time when more water is really required. A surge in water level as a result of the drum pressure decreasing is called swell . A water level decrease due to drum pressure increase is called shrink .

4. Measurement techniques of drum level Gauge Glass Electronic Probe Level Sensors Wet Leg System

5. Gauge Glass

6. Electronic Probe Level Sensors

7. The technology uses the well established principle of resistivity discrimination between steam and water to provide a clear indication of water level in a boiler drum The side-arm column stimulates condensate flow and circulation. This flushing effect results in the water inside the column being purer than the water in the drum. The sensing element is a Probe with an insulated tip inserted in a side-arm water column. If a voltage is applied to the tip, conduction occurs between the tip and the inside wall of the column.  An open circuit occurs when it is in steam state means water is less.

8. Wet Leg System

9. "wet leg" level measurement, where both tubes running from the drum to the level transmitter are filled with water. “wet leg"  measurement means that the high pressure port on the  DP (differential pressure) transmitter  should be connected to the top of the drum and the low pressure port connected to the bottom. The measurement is from the top down - if the vessel fills with cold water up to the top tap, the ports of the DP cell will have the same water head above them so the DP will be zero.  Falling level will raise the measured DP - the pressure on the low tap will fall relative to that of the high tap still full of liquid .

10. BOILER DRUM LEVEL CONTROL The information on boiler drum level as measured by above procedure is fed to various process control units, DCS( Data Control System), SCADA( Supervisory Control and Data Acquisition) etc. for analysis. After analysing various information from the drum level system and other associated sub-systems, the actuating signals are generated from control units to actuate the control valves of boiler feed water inlet as well as outlet and steam outlet, to make them open or close . The following controls are used to control drum level- Single Element Drum Level Control Two Element Drum Level Control Three -Elements Drum Level Control

11. Single Element Drum Level Control

12. The feed-water pump push water through feed-water control valves into the boiler drum. The water level in the drum is measured with a pressure and temperature-compensated level transmitter. The drum level controller compares the drum level measurement to the set point and modulates the feed-water control valves to keep the water level in the drum as close to set point as possible. 1 . Integrating Process From a controls point-of-view, the boiler drum is an  integrating process . This means that any mismatch between inflow (water) and outflow (steam) will cause a continuous change in the drum level. Integrating loops are difficult to tune, and can easily become unstable if the controller’s integral time is set too short (i.e. high integral gain).

13. 2. Inverse Response If the drum level is low, and more feed-water is added to increase it, the drum level tends to decrease first before increasing. This is because the cooler feed-water causes some of the steam in the evaporator to condense, causing the volume of water/steam to decrease, and hence the drop in drum level .

14. Two Element Control

15. If a feed pump is started up or shut down, the total feed-water flow rate changes. This causes a deviation in drum level, upon which the drum level controller will act and change the feed-water control valve position to compensate . A faster control action is needed for dealing with changes in feed-water flow rate. This faster action is obtained by controlling the feed-water flow rate itself, in addition to the drum level. To control both drum level and feed-water flow rate, cascade control is used. The drum level controller becomes the primary controller and its output drives the set point of the feed-water flow controller, the secondary control loop. This arrangement is also called two-element control, because both drum level and feed-water flow rate are measured and used for control.

16. Three -Elements Drum Level Control

17. Similar to feed flow, changes in steam flow can also cause large deviations in drum level, and could possibly trip the boiler. Changes in steam flow rate are measurable and this measurement can be used to improve level control very successfully by using a feedforward control strategy. For the feedforward control strategy, steam flow rate is measured and used as the set point of the feed-water flow controller. In this way the feed-water flow rate is adjusted to match the steam flow. Changes in steam flow rate will almost immediately be counteracted by similar changes in feed-water 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. The combination of drum level measurement, steam flow measurement, and feed flow measurement to control boiler drum level is called three-element control.

18. References IMPORTANCE OF THREE-ELEMENTS BOILER DRUM LEVEL CONTROL AND ITS INSTALLATION IN POWER PLANT by Sanjoy Kumar Chakraborty , Nilotpal Manna and Surodh Dey IARJSET ISSN (online) 2393-8021 ISSN (print) 2394-1588 international advanced research journal in science, engineering and technology national conference on emerging trends in engineering and technology (NCETET’16) lourdes matha college of science & technology, Thiruvananthapuram vol. 3, special issue 3, august 2016 by Shiji S.R, Anish .S , Swapna .M Boiler controls & instrumentation Presented at m/s. Magadi soda company ,magadi , Kenya by Thermodyne Technologies private Limited

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