Hydrostatic Relief Valve

I need to change the hydrostatic relieve valves in my pool.  How do I remove them since I do not know what size they are?  Which tool should I use?

We have a removal tool for Hayward hydrostatic relief valves. I’m not sure if it will work on all brands. The part number for the tool is 3821-0.

Thanks but I do not wnt to purchase the incorrect tool.  Is there any other tool I could use?

What is the make and model of your hydrostatic relief valves? We would need to know that in order to recommend a tool.