Sounding pipes, Air pipes and Ventilators

To understand “Sounding pipes, Air pipes and Ventilators” we need to address questions like Why? Where? How?

In earlier chapter we have seen existence of tanks and other compartments of the ship. We may be filling tanks and/or compartments with liquid through a pipeline. If we had a lid open, we could have seen the level of the liquid. However when we are filling Tanks lids are closed. So how do we know how much liquid is inside? What is the rate of filling? When will it fill completely? On ships we use some sort of remote mechanical arrangement. It is called sounding pipe.

Look at the diagram of a tank. This shows a vertical pipe towards the after end of the tank. This pipe starts from the deck (Remote location) and goes right to the bottom of the tank/compartment. End on the deck is closed after use to avoid flooding of the compartment during bad weather. It may be flushed with the deck or raised. Closing arrangement is done by use of a cap known as “Sounding pipe cap”. In machinery spaces, this cap is in addition to the automatic closing arrangement (Required for preventing Fire due to leakage of fuel oil vapours). Pipe is welded to the deck making it a watertight arrangement. With due supports pipe travels to the bottom of the compartment and ends 3-5 cms from the bottom. It is open at the bottom to allow liquid to enter the pipe and seek same level as in tank. So it means that the height of level in the tank can be measured by measuring height of liquid in the sounding pipe. Tanks tables are calibrated to indicate volume with reference to the height of liquid column in the sounding pipe and trim correction.

To know the level, a tape called “sounding tape” is lowered in the tank through the sounding pipe from deck. Once the end has reached the bottom, it is pulled out. Liquid level will mark the tape and same is measured on scale of the tape. This reading is referred to tank calibration tables to find volume of liquid in the tank.

Sounding of the tank means height of liquid level from the BOTTOM of the tank. Since tank heights are known, it is also possible to know remaining height of the tank. This is called ULLAGE. Ullage is remaining height before tank becomes full. Normally ullage is measured for cargo tanks and fuel oil tanks.

Since sounding tape is used on daily basis and has a sharp heavy weight (Plumb bob) for making it reach the bottom, sharp metal keeps on hammering base plate (Bottom shell plate) damaging plate slowly. Over the years, we can expect serious damage by way of a hole in shell plate. To avoid this issue, a small plate (Striker plate) is welded directly under the opening of sounding pipe to take beating every day. This is sacrificial plate and is replaced when worn out. Purpose of “STRIKER PLATE” is to prevent damage to the bottom shell plating.

Compartments/Tanks have a pipe connection for filling and emptying. Arrangement is fixed at the bottom. Compartments/Tanks are enclosed from all sides by bulkheads(Walls). Even empty compartments are filled with air. What happens if you start filling a tank? Liquid will compress air inside till it reaches same pressure as liquid. Then it won’t fill further till the time air pressure is released. Means we need to make sure that the air pressure is always same as atmospheric pressure. To achieve this AIR PIPES are fixed to the ceiling of the tank. They allow free flow of air preventing pressure or vacuum, keeping pressure same as atmospheric pressure. Upper ends of Air pipes have specific fittings to prevent unintentional flooding during bad weather. This will be covered later on in ship construction class. Air pipe is term used for tanks, void spaces and cofferdam spaces. Essentially they are automatic (Since they automatically correct vacuum and pressure) small ventilators. Term Air pipe is NOT used with CARGO Compartments.

Ventilators: Ventilator ventilates a compartment. Ventilation means a system or means of providing fresh air.

Why do we need to ventilate?

To expel heat, odour, toxic gases, cold and replace compartment with fresh air. Ventilation can be done by removing all sides of the compartment to allow maximum exposure. But it is not possible as compartment sides cannot be removed. Think of a room where we have windows and door. They are nothing but small openings in walls posing limitation problem. Ventilation needs certain speed. This speed is dependent upon how much volume of air is replaced per hour. This depends on area of cross section of the opening and ventilation fan capacity.

Ship needs ventilation for living areas, stores and cargo spaces. Each has different requirements. Spaces are served by ducts, trunkings and fans. Ducts and trunkings are required for reaching all spaces. Fans are reversible for supply/exhaust modes.

Ventilation may be natural (No fans) or mechanical (With fans). Unlike Air pipes, Ventilators have Manual closing arrangements by way of FLAPS.