Thermal relief valve is required for liquif filled service pipe, and for the blocked in piping fluid of up to 500 l and exposed to hotter fluid operating condition (either to sun or to heat tracing system). Normally people tend to install TRV instead of valve with cavities (check valve also can do the job with drilled check valve). or even small bypass check valve is also acceptable.
unless TRV is not installed i n the system, then small bore bypass valve is acceptable.
Hope this help.