Filter vessel - MAWP is 20Barg, I have a normal relief valve sized for when the pressure goes above the MAWP pressure, but my client wants a thermal relief valve. I'm a little confused, does that mean the pressure inside the vessel would never actually exceed the MAWP pressure without thermal effects? I read that thermal relief valves normally have smaller orifice size. Is it possible to have a relief valve that will relieve due to thermal effects as well as when the pressure exceeds the MAWP?

Upstream of the inlet is a pump, but it's outside of our scope, so I'm not sure exactly what else is there.

"Thermal" relief valves are not thermal at all but pressure relief valves. The industry calls them "thermal" because of the fact that the scenario is a liquid filled vessel that is heated (thermal effect). When the liquid heats it expands and puts enormous pressure on the vessel. The relief valve relieves this pressure. Again, it is really a pressure relief valve.

Since the relieving rate for this scenario is typically very small, there are relief valves on the marker with very small orifices, smaller than the typical API D orifice, but they are still pressure relief valves. So no, they are not special and they are still code valves.

You can't have the client dictate to you, the process engineer, what the controlling relieving scenario is, this is your responsibility. You must check all credible relieving scenarios and come up with the one that produces the larges relief valve.

That pump is NOT outside of your scope if it is part of the system that is being protected by the relief valve. You must consider excessive head due to a possible blocked-in scenario. If the pump can't overpressure the filter, then that is not credible, but you still must check it. What about fire?

Since this is a filter, the main reason why you would need a thermal relief valve is if the minimum operating temperature is below the ambient temperature. In this case if the filter is blocked in, then heat from the atmosphere can increase the temperature of the fluid inside the filter which increases its volume, and therefore the expansion needs to be relieved. Many times "thermal relief" valves are used to protect piping, and are small "D" orifices.  If you already have a "G" orifice i am pretty sure that is big enough to also handle a thermal relief case. The "G" orifice is likely there for protection against fire, blocked discharge, or some other contingency.

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