Non Return Devices in Inert Gas (IG) Plant used on Ships – Design Considerations

The non return devices in the Inert Gas (IG) system handle sensitive operation of backflow prevention, hence they should be designed to provide maximum possible safety for the sake of the ship and its crew members.

Introduction

In the previous topic we studied about the three main types of deck water seals and their operation as well as the non return valve. We will proceed to take a look at the design consideration of these components before studying the distribution system of the IG setup.

Design Considerations

○ The first and foremost factor to be kept in mind is the resistance of the material used for the construction of these devices against corrosion and fire. This is certainly very important because if the material fails in the face of a fire or corrosive attack, it would defeat the very purpose of having the non return device. There are several materials which could be used for this purpose such as low carbon steel having a protective layer of rubber lining and so forth.

○ You must have noticed from the study of the previous article that water is an important component in all types of deck seals even in a dry type seal since it prevents backflow of the cargo gases to the safe zone of the IG system. Therefore adequate provision must be made to have sufficient quantity of water available at all times and there should be a provision for observing that quantity of water in the form of sight glasses.

○ The main purpose of the deck seal is to prevent backflow hence there needs to be a maximum value upto which it can withstand back pressure before giving in. This capacity is determined based on the exact operating conditions on the ship and the seal should be able to handle the worst possible scenario conditions.

After taking a look at the design considerations of the non return devices we will now study the part of the IG system which is responsible for actually delivering the inert gas at the required places and that is the IG distribution system. If you have forgotten about it, I suggest you read this article which explains the two main sub-systems of the IG system namely the IG plant and the IG distribution system (Just keep in mind though that even the deck water seal forms a part of the IG distribution system).

Basically the distribution system serves the purpose of delivering inert gas to required locations, protecting the cargo tanks from developing either too much or too little pressure, isolates tanks if and when required from other tanks and provides for the venting of tanks when required.