Single pull macgregor type hatch cover.pdf - Cochin University of ...

(vii) It should retain all the above properties throughout a long service life,
exposed to climate extremes.
3.3.2.1 Gaskets material
Gaskets material must be of a suitable quality; it should not harden excessively when
subjected to neither sub-zero temperature nor soften in tropical conditions.
Particularly important in this respect are gaskets fitted to refrigerated vessels.
Most gaskets used for seals on dry cargo ships are of natural, synthetic or
neoprene rubber, while combination carriers requires a nitrile composition, resistant to
chemical attack by oil. In general neoprene synthetics have good heat, ageing, weather
and flame resistance, but only moderate oil and chemical resistance but worse cold
temperature properties. General construction of seal steel work, welding and painting
needs careful attention.
3.3.2.2 Compression bars
The usual arrangement of compression bars is shown. It is of rectangular section mild
steel welded to the coaming bar. After several years of service, these bars can become
badly corroded and require replacing. Moreover, they may then have sharp corners
which press into the gaskets so that the rubber takes on a permanent set earlier in its
life than would be the case for a sealing arrangement employing either a round
compression bar, as shown or rectangular one with rounded corners. The latter
produces an element of ‘knife edge’ loading which gives the good seal with the
rubber. When such compression bars are fitted, they are often of stainless steel, which,
although initially expensive, often worthwhile as it is not usually necessary to replace
the bar during the life of the corners
3.4 CONSTRUCTION MATERIALS
In general, Grade A mild steel is used for the construction of all cargo access
equipment. Grade A is the ‘ordinary’ mild steel used for most ship building purposes.
Certain application requires Grades D and E steel. Grade D is notch-tough steel with a
chemical composition largely chosen by the steel producer provided that it retains
good weldability. Grade E steel is the highest grade and is also notch-tough but its
manufacture and composition are strictly controlled. Grades D and E steel are
specified for coaming bars on refrigerated ships where low temperature brittle fracture
35