Without knowing specific situation I could say the following (in general);
The function of a thermal relief is to protect the line against over pressure due to external heat (e.g. solar ray). Generally this counts for liquid lines only. In case there is air present in the line a thermal relief if not required as this air operates like an expansion drum (like your central heating at home). There is no rule for the pipe length, it all comes down to the magnitude of the expansion of the liquid and the expansion of the pipe (s.s. expands approx twice as much as c.s.) which results in a certain pressure.
A thermal relief vale is only necessary when a piece of pipe is blocked in between two valves and, in general, you need two block valves only because it is a long line. Short lines don’t need two valves, but there are (of course) exceptions.
An other aspect; The valves are the weakest points in the line. On top of that, often valves are not 100% leakage proof. So, to my knowledge, you don’t have to be afraid that the pipe will burst.
Personally I find that thermal reliefs are installed too quickly, for just to be on the safe side.
Never the less the pressure should be calculated, That pressure must be lower than the allowable seat pressure of the weakest valve.
Solutions:
1. for water (and the like); a thermal relief valve to the ground or sewer.
2. for hydrocarbons, acids (and the like); A Thermal relief system jumping over the block valve into the next pipe piece.
3. Expansion drum. (expensive, and only advisable when heating and cooling all the time)
Thank you so much,guys(amit74,jm119 & Thomasjl).
Gathering from your input, you are saying there is no standard or optimum piping length by which thermal relief system can be installed, say at an interval of 30m. as put forth by our local piping guys.
It is evident then that the only way to know and be sure is to compute the thermal expansion volume of the liquid and the resulting pressure rise at a specified pipe length which in turn will determine the set pressure of the TRV (with consideration to the backpressure) as covered by a previous thread on this latter subject.
Thermal pressure relief requirements are not dependent on length of pipe segment. Pressure rise from temperature increase is independent of pipe segment length. If thermal conditions exist in an isolated segment of a line, where pressures will exceed the design pressure, install thermal pressure relief regardless of the length of pipe.
