which type of plastic insert?

[Boxerman]

I'm fitting a 22mm-15mm reducing service/check valve after a RCM 3 negative head kit on a salamander pump. The reducing side of the valve has a strange sort of compression fitting that does not require an olive but the 15mm plastic pipe will need an insert.

Does it matter which type on plastic insert I use, with our without 'o' ring seals?

Thanks

 

[spacethegas]

whoever makes the pipe use their inserts.