I have heard that for every 900 litres of liquid volume in a line, a thermal relief valve needs to be installed.Is there any standard which will indicate when to put the thermal relief valve on the line?

As far as I'm aware there is no such a standard. For example, API 521, 4th ed., section 3.14 does not mention a minimum amount of liquid. From this section in API 521 you may conclude that all pipes that can be "blocked in while they are filled with liquid at near-ambient temperatures and are heated by direct solar radiation" require a thermal relief valve (in most cases a 3/4 inch x 1 inch TRV will be sufficient).

An article in Chemical Engineering World of February 1998 contained a number of items that need to be considered:

- Is the pipe continuous in operation and thus not routinely isolated without being depressurised and drained? Most continuous process plant piping unit falls under this category.

- Is liquid highly toxic, corrosive or liquified gas?

- Is there a weak point (e.g. flanges) in the network where expansion could relieve without serious consequences?

As a rule of thumb, pipe containing more than 500 lts. of liquid or more than 45 m length (whichever is lower) which could be normally blocked in should be provided with TRV.

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