It is imperative that the water level in a steam boiler be carefully managed in order to guarantee the production of high-quality steam in a manner that is risk-free, resource-friendly, and at the appropriate pressure.
Heat is produced by the combustion of fuel in a furnace, while waste heat from the main engine can also be used to produce steam. The heat is transferred to the water that is contained within the shell of the boiler, which subsequently evaporates to produce steam that is subjected to pressure.
In order to properly discharge steam from a boiler, there must be a particular amount of water surface area available. A certain height should also be allowed above the normal working level in order to allow the water level to rise with increasing load while still allowing sufficient area for the steam to be released without any carryover of water taking place. This should be done by allowing a certain height above the normal working level.
Because it is obvious that when steam is produced, the water in the boiler evaporates, and in order to keep the level consistent, the boiler has to receive a supply of water and it is imperative that the water be kept at the appropriate level at all times.
If the boiler is allowed to function with insufficient water, there is a possibility that severe damage may occur, and finally there is a chance of explosion.
For this reason, means of controls are required which will monitor and control the water level and detect if a low water level point is reached, and take appropriate action, like sounding an alarm, shutting down the feed water supply and shutting down the burner.
There are numerous standards that mandate the availability of two separated water gauges, which are made from a screen of tempered glass, which is typically attached to the front and sides of the water gauge glass that is attached to the steam or water drum or the boiler shell. Normally the high-pressure boilers will need water gauge glass that is made up of either flat or prismatic glass. The gauge glass device, which has withstood the test of time, is utilized on the overwhelming majority of boilers. This device is often designed to provide a visible range of water level that is both above and below the normal water level.
It is absolutely necessary to have a solid comprehension of what may be observed in a boiler gauge glass. Because the water surface in a steaming boiler is made up of a dense population of bubbles and has a robust horizontal circulation, it is not possible to precisely determine the height of the water within the drum. So, the gauge glass is filled with water, which does not experience current or agitation because of its location, does not have any steam bubbles within it and has a temperature that is lower than the actual water in the boiler. This indicates that the water found in the gauge glass (together with the water found in other external fittings) is of a higher density than the water found inside the boiler drum and as a consequence of this, the level gauge glass will display a level that is lower than the typical water surface level in the boiler drum.
When a boiler is operating at a high load, the vigorous circulation of the water within the boiler will generate differences in the water level at various points along the length of the boiler. There is also the chance that waves will form inside the boiler whenever there is a sudden change in the load and these waves, which can frequently be seen in the level gauge glass, are usually ignored by the water level controls.
There are three obvious applications for level monitoring sensors on a steam raising boiler, and they are as follows:
- The purpose of the level control is to make sure that the boiler receives the appropriate quantity of water at the appropriate moment.
- Alarm for low water level. If the water level in the boiler has decreased to or below a predetermined level, the alarm for low water level will sound, preventing the burning of fuel and ensuring that the boiler continues to function in a safe manner. In order to guarantee the user’s safety, rules and regulations require two separate low level alarms to be installed in steam boilers that are automatically controlled. In marine boilers, the burner will be “shut down and blocked” if the two low level alarms (low level and low low level) goes off, and it will be necessary to reset it manually in order to get the boiler back online.
- Alarm for high water level. This alarm goes off if the water level gets too high, signaling to the boiler operator that they need to turn off the feed water supply.
There are different methods used to detect the water level in the marine steam boilers and these usually are:
- floating sensors – This is a straightforward method of determining the level, where the boiler is equipped with a float of some kind, which might be in a chamber located outside the boiler, or it could be directly inside the boiler drum. As the water level in the boiler fluctuates, the float will move in an upward and downward motion.
The buoyancy of the float causes it to move up and down in response to changes in the water level. The opposite end of the float rod has a magnet that rotates inside of a stainless steel cap. This magnet is located at the opposite end of the rod. The fact that the cap is made of stainless steel makes it (almost) non-magnetic and enables the lines of magnetism to travel through it without being disrupted. In its most basic manifestation, the magnetic force is responsible for the operation of the magnetic switches in the following manner:
- The feed pump can be activated by using the switch located at the bottom.
- The feed pump can be turned off using the switch on the top.
However, in most situations, a single switch will be sufficient to regulate the on/off status of the pump, and the second switch will be used for an alarm. Alarms for different levels can be generated with the help of the same setup. A more advanced method of providing modulating control will make use of a coil that is coiled around a yoke that is located inside the cap. As the magnet is moved up and down, there will be a change in the inductance of the coil. This change in inductance is then used to produce an analog signal to a controller, which is subsequently sent to the feed water level control valve.
- differential pressure cells – on one side of the differential pressure cell there is always going to be a certain amount of water pressure. On the opposite side, there is a head that is adjustable in accordance with the amount of water in the boiler.
An electrical level signal is generated by the measurement of a diaphragm’s deflection using one of three methods: variable capacitance, strain gauge, or inductive. These methods measure the deflection of the diaphragm in different ways.
The differential pressure cells are used in systems with water of a high quality that has been demineralized, where the conductivity of the water is extremely low, which may indicate that the conductivity and capacitance probes will not function in a dependable manner.
- conductivity probes control – a point measurement can be obtained through the use of the conductivity principle. When the water level reaches the tip of the probe, it will cause an action to be taken by a controller that is associated with it.
It’s possible that this action is to turn on or turn off a pump, open or close a valve, raise the alarm level and switch on or turn off a relay.
However, a single tip can only offer a single action, often known as a point action. Therefore, in order to turn on and off a pump at specific levels, a conductivity probe needs to have two tips attached to it. As soon as the water level drops and the tip at point “pump on” becomes visible, the pump will start operating. When the water level reaches the second tip close to point HW, the pump will be turned off because it has reached its maximum capacity.
- capacitance probes – they consists of a conducting, cylindrical probe, which acts as the first capacitor plate. This probe is covered by a suitable dielectric material, typically PTFE. The second capacitor plate is formed by the chamber wall (in the case of a boiler, the boiler shell) together with the water contained in the chamber. Therefore, by changing the water level, the area of the second capacitor plate changes, which affects the overall capacitance of the system.
Onboard vessels, usually the boiler water level control is a modulating system which uses PID controllers (you can find and learn more about PID controllers, if you follow this link) .
In some cases, for measuring and control of the water level, the boiler is equipped with a differential pressure (DP) water level transmitter unit. The unit comprises a level electrode, mounted in a protection tube, and the level transmitter. The unit works by a capacitance measurement, with the electrode and protection tube forming a capacitor. If the level of the boiler water located between the two capacitor plates changes, the current flowing through the plates changes. The level transmitter produces a standard analogue output of 4-20mA, which is sent to the control system. The control system processes the signal from the DP transmitter and provides level alarms/shutdowns, and the control of the regulating feed water valve.
The boiler normally operates with two different set points for normal water level. This increases the volume available for shrink and swell during start and stop of the exhaust gas economizers. When the main engine is running and the exhaust gas economizer is in operation, the highest set point for normal water level will be active (NW2). When the main engine is stopped, then there will be a shrink in the auxiliary boiler water level, and the set point for the normal water level will switch to be at NW1.
A second independent safety device is fitted for the ‘too low water level’ shutdown function. The safety device consists of an electrode and level switch; when activated, the switch will cause the control system to shut down the burner. The electrode operates on the conductive measuring principle using the electrical conductivity of the boiler water for level signaling. When the electrode tip is submerged in the water, the imbalance of the level switch bridge circuit is positive. If the water level falls below the electrode tip, the electrode produces a negative imbalance of the bridge circuit. This causes a ‘too low water level’ shutdown signal to be generated and consequent shutdown of the burner.
The feed water is normally supplied to the boiler through the feed water automatic regulating valve, but it can also be supplied using a separate auxiliary line. The regulating valve is controlled by the level of water in the boiler, and will open and close to adjust the feed rate to maintain the correct level in the boiler. The auxiliary feed line is used if the automatic level control system is inoperative. The auxiliary feed water system requires manual control of the boiler inlet valves to maintain the correct level.
The automatic feed water valve operates on the boiler’s main feed line. The valve has a plug of parabolic form and the fluid flow direction is against the closing direction. The valve is operated by a pneumatic actuator which is mounted above the valve; the actuator (read more about this by following this link) is controlled by a signal from the water level transmitter.
On other cases feed water supply to the boiler is handled by a single element control system, which is designed to maintain the boiler water level and provide an alarm and safety shutdown should the level not stay within set limits. A transmitter is mounted on the boiler, which sends a signal to the controller, which in turn regulates the opening of the feed water control valve.
The feed water control valve has a valve positioner for automatic operation, with
a handwheel for manual operation. The duty feed pump operates continuously and the feed control valve regulates the amount of water directed to the boiler, depending upon the current water level.
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