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How to control water level in a marine boiler ?Preventing steam losses and energy efficiency measures

Modern high-pressure watertube boiler
: A modern high-pressure, high-temperature watertube boiler holds a small quantity of water and produces large quantities of steam. Very careful control of the drum water level is therefore necessary. The reactions of steam and water in the drum are complicated and require a control system based on a number of measured elements.

When a boiler is operating the water level in the gauge glass reads higher than when the boiler is shut down. This is because of the presence of steam bubbles in the water, a situation which is accepted in normal practice. If however there occurs a sudden increase in steam demand from the boiler the pressure in the drum will fail.

Some of the water present in the drum at the higher pressure will now 'flash off and become steam. These bubbles of steam will cause the drum level to rise. The reduced mass of water in the drum will also result in more steam being produced, which will further raise the water level. This effect is known as "swell', A level control system which used only level as a measuring element would close in the feed control valve—when it should be opening it.

When the boiler load returns to normal the drum pressure will rise and steam bubble formation will reduce, causing a fall in water level. Incoming colder feed water will further reduce steam bubble formation and what is known as 'shrinkage' of the drum level will occur.

The problems associated with swell and shrinkage are removed by the use of a second measuring element, 'steam flow'. A third element, 'feed water flow', is added to avoid problems that would occur if the feed water pressure were to vary.

Boiler water level control

Fig:Boiler water level control

A three element control system is shown in Figure . The measured variables or elements are 'steam flow', 'drum level' and 'feed water flow'. Since in a balanced situation steam flow must equal feed flow, these two signals are compared in a differential relay. The relay output is fed to a two-term controller and comparator into which the measured drum level signal is also fed. Any deviation between the desired and actual drum level and any deviation between feed and steam flow will result in controller action to adjust the feed water control valve. The drum level will then be returned to its correct position.

A sudden increase in steam demand would result in a deviation signal from the differential relay and an output signal to open the feed water control valve. The swell effect would therefore not influence the correct operation of the control system. For a reduction in steam demand, an output signal to close the feedwater control valve would result, thus avoiding shrinkage effects. Any change in feed water pressure would result in feed water control valve movement to correct the change before the drum level was affected.

Boiler water level control , Steam distribution system & energy efficiency measures

Maintaining Boiler water level control at optimum level & the steam distribution system maintenance makes a significant contribution to energy efficiency in steam system. Measures to consider include:

To determine if your ship could benefit from a steam distribution system maintenance program, normally steam lines and steam traps surveys need to be done at regular intervals. The inspection activities will include steam pipes, insulation, traps, steam supply/discharge on or around heat exchange devices etc.

Fundamental to such inspections is the collection of good data. Aspects to consider include:
  1. Reduce steam leakage: As part of day routines, checks should be made for steam leaks. The steam leaks should be rectified as soon as observed.

  2. Heat loss due to inadequate insulation: The boiler and steam lines along with condensate return to the hot well must be well insulated. Over a period of time insulation is damaged or worn out. Any analysis by thermography or any other thermal measurement system could identify the hot spots. Improvement of damaged insulation due to repair work must be done. All these will reduce the heat losses from the system thus improve energy efficiency.

  3. Steam trap losses: Steam traps are used to discharge condensate once it is formed, thus the main function is to prevent live steam from escaping and to remove air and non-condensable gases from the line. However it is a largely neglected part of the steam distribution system. Steam traps that are stuck open allow live steam to escape thus resulting in loss of heat and also increasing the load of the condenser. Steam trap that is stuck shut results in reduced capacity of the equipment it is being supplied to. Overall, steam traps must be checked at planned intervals to show their good working conditions.

Steam end-use energy efficiency measures

Steam end-use could vary according to ship types. The main users of steam include: