Air Receiver Capacity, Copal Varnish

Define the considerations governing main air receiver capacity.

The rule used by government bodies and classification societies which states that the total capacity of reservoirs or receivers must be sufficient to make 12 or more starts without refilling the reservoirs.

If the ship has twin screws, the size of reservoirs must allow not less than 12 starts on each engine.

When ship has CPP (Controllable Pitch Propellers) which are used to for supplying astern power, it is usual to have the air reservoir capacity such that six starts can be made.

Ships are usually supplied with air reservoir capacity well in excess of these requirements. Actual number of starts that can be made without refilling the air reservoirs is dependant on engine temperature, lubricating oil temperature, the skill of engineer manoeuvring the engine and the minimum air pressure that will start the engine.

Usually most engines will start when air pressure has fallen to half or even less than half of designed max starting maximum pressure.

Explain why internal surfaces of air receivers are coated with clean copal varnish

The internal surfaces of an air receiver are subject to pitting and corrosion caused by the water and oil (which becomes oxidised and turns acidic) carried over in the compressed air. If this becomes excessive, the strength of air receiver is compromised and may necessitate expensive replacement.

To prevent this from happening the internal surfaces are given a protective coating which may be derived from organic products or synthetic

The properties required are:- 

1. The coating is clear, so that the surface of the receiver can be inspected.

2. Coating is flexible, so that it can expand and contract with the receiver.

3. Coating has good adhesive properties so that it adheres to the surface of eceiver.

4. coating is impervious to oil and water.

Explain why air receiver drain valve should be overhauled at every available oppurtunity.

Air receiver drain valaves must be operated reliably to ensure that water and oil are drained from air reservoir. If they become blocked then oil and water accumulation at the bottom of receiver is likely to cause corrosion and there is a chance that accumulated oil carried over by air compressors could be carried over with compressed air into the air start lines, where it poses an explosion risk.

If the drain valve do not seat correctly after operation, then this will lead to excessive compressor running hours, with resulting increased maintenance and cost.