Boiler Valves

Dear all, I am little bit confused about some Boiler globe valves/Angle valves/ Gate Valves constructions.
I have seen that some Steam stop valves have an integrated gland warm-up line DN15 from upstream of the pipe and some valves have gland warm-up line from downstream of valve. In our plant all angle Boiler stop check valves has the integrated gland warm-up line from downstream of valve. Some stop valve has integrated gland warm-up line with an Isolation valve. I understand some steam/Feed water valves has integrated valve By-pass to warm-up downstream pipes prior to open the valve.
I would like to know the function of gland warm-up line with and without isolation valves. In addition to that, I am keen to know the reason for this inconsistency.