Need for water level gauges
We have been studying about boilers and various types of boilers in the past few articles. The most important aspect of a boiler starting procedure and operation process is to maintain an appropriate water level inside the boiler. Changes in the level of water inside the boiler causes detrimental effects on the boiler's internal parts and structure.For e.g., if the water level inside the boiler is too high, the water might overflow and cause harm to the equipments specifically designed for accepting only steam. If the water level is too low , the excessive heat inside the boiler may damage the heat transfer surfaces, rendering permanent damage. This could be very dangerous situation not only for the ship's engine room but for the entire ship as well which could be carrying lots of passengers if it is a cruise liner, or any other type of cargo. It is for this reason , that boiler water level transmitters and gauges are used to keep a constant watch on the water level inside the boiler drum.
Water gauges gives an exact indication of the water level inside the boiler and even provide a visible signal if the water level deviates from the normal working level. Moreover, it is important that water gauges are fixed in each corner of the boiler as there might be incorrect indication due to fluctuating motion of the ship.
The type of the water gauge fitted on a boiler depends on the type and working pressure of the boiler. Depending on the operating pressure of the boiler there are mainly two types of water gauges.
Round glass tube type
This type of gauges are used for boilers having a maximum working pressure of 17 bar.
The gauge consists of a glass tube, which is connected to the boiler shell with the help of cocks and pipes. The glass tube is contained in a protective cover known as a guard, in order to prevent accidental damage. Packing rings are fitted at the tube ends to prevent leaking and ensuring tight sealing arrangement. Isolating cocks are fitted at both steam and water lines and a drain cock is also provided at the bottom. The water gauge is usually connected directly to the boiler without any attachment in between. A ball valve at the bottom of the tube ensures shutting off of the tube in case of tube damage or water leakage.
Plate glass type
This gauge is used for boilers having working pressure above 17 bar. The gauge consists of glass plates fitted inside a metal housing. A sandwich kind of arrangement is made with glass plates and a central metal plate in between the two metal plates. Sealing is done with the help of joints placed in between the glass and metal plates. Moreover, to prevent any kind of damage to the glass and to ensure additional insulation, mica sheets are placed over the glass surface. The mica sheets also prevents the glass from breaking due to high temperature.
As the glass plates are soft, extra care should be taken during tightening of the assembly. Even a slight amount of excess pressure can lead to cracked glass plates and leaking assembly.
Nowadays, all the gauges are remotely monitored from the engine control room.
Periodic "blowing through" of the gauges should be done to prevent any kind of blockage due to steam or impurity .
Water gauge blowing through procedure
For the blowing through procedure, as shown in the figure, the following steps should be performed :
- Open steam cock A
- Close water cock B, and open drain cock C
- The pressurized steam will blow through the drain cock C, forcing any kind of impurities or blockage out.
- Now Cock A should be closed and B should be opened, with C remaining in the open position.
The jet of water will force out any kind of remaining blockages. If the flow of the jet of water is uneven, this means that the gauge is still chocked.The whole procedure should be repeated in such cases.
Sometimes the gauge might now show correct reading when the boiler is running hot or just shut down. These effects are known as swell and shrinkage and have been dealt in another article.