Between the above extremes, there are many services where the need for pressure relief is not certain. In such cases, the problem should be approached by calculating the amount of liquid that must be relieved and then deciding if that much leakage can be accommodated.
The effect of some variables can be calculated; others cannot. The approximate quantity of liquid to be relieved can be calculated using the calculable variables derived at the end of this section. Generally one should begin with the simple equations that do not consider expansion of the pipe nor the compressibility of the liquid, thereby giving a conservative answer. More refined calculations can be carried out later. Having determined the quantity of liquid that must be relieved, consideration must then be given to the number and type of valves, and the amount of leakage through each (the incalculable variables), and other sources of possible relief. Personal judgment must also be used when determining whether or not a relief valve is needed.
It is difficult and perhaps dangerous to estimate and rely on leakage through valves. More and more valves in use today are high quality, tight shutoff valves that will not allow enough leakage to relieve thermal pressures (such as ball valves, Orbit valves, General twin-seal type valves and expanding-gate valves).