A merchant ship is built to carry tonnes of cargo from one port to another. Along with cargo it has to carry oil which includes Fuel oil, Diesel oil and different grades of lubricating oil in bulk quantities to run the propulsion plant and other auxiliary machinery.
Ship also has several of tanks for ballast water which are strategically located on both the sides of the ship to stabilize the ship when it is loaded or unloaded with oil or cargo.
Some tanks are also provided for storing fresh water for provision purpose or for the feed water system. Sewage holding tank may be constructed depending upon the size, type and owners demand.
Storage tanks, bunker tanks, fresh water and ballast tanks are constructed as per the class, size and type of the ship. To maintain ship’s stability, equilibrium, and hence the safety it is necessary that the level of the fluids is checked at regular interval of time. This is known as taking “sounding” of the tanks. In this article we will learn about the sounding of the tanks and process of taking the same.
What is sounding?
As discussed earlier, sounding is the process for calculating the total quantity of fluid (oil, bilge, sludge, or water) inside a ship’s tank. First the depth of the fluid from the surface to the bottom of the tank is derived corresponding to which volumetric quantity is calculated using the sounding table.
What is sounding table?
Sounding table is a chart describing the construction, capacity and most importantly the volumetric content of the tank at given depth of sounding or ullage. All vessels have there own sounding table documents for each tank containing fluid in bulk.
We will now have a look at the different types of sounding methods used on a ship.
Methods for taking sounding on a ship
In this method, a sounding tape is used with a heavy weight bob attached to one end of the tape using a strap hook. It is the most commonly used methods used for calculation of tank capacity. If the capacity inside a tank is more, free space of the tank is measured to calculate total capacity of the tank. This method is called ullage measurement.
In electronic sounding, a sensor is used which senses the pressure inside the sounding pipe or by sensing the tank pressure and sends a signal to the receiver. Here the signal is translated to the tank’s content value with the help of a PLC circuit. The value is displayed using electrical operated servo gauge or electrical capacitance gauge.
Mechanical provisions are made inside the tank so that the quantity of tank can directly be read through a level marker or an indicator or a float level sensor. In the tank a float can be attached to a pointer through a pulley. As the level varies pointer reading will change accordingly. A level gauge glass is also attached to the tank to read the quantity of the fluid inside the tank. The gauge may also be a pneumatic/hydraulic operated gauge or differential pressure gauge.