vertical expansion vessel

[catfordbags]

i'm still building me rain harveting system. i've got a pump which will maintain a constant pressure in my pipes and i've also bought a vertical expansion vessel to help keep the pressure steady(which came without a bracket to fit it to the wall)

It'll sound like a daft question this but does this vertical expansion vessel have to be mounted vertically ? If so, why ?

It's just that it would fit perfect under the sink lying on its side......

[Justme]

Does it have a bag / bladder? If not then yes it needs to be mounted vertically (and the right way up as well) or all the air will get out & then you just have a larger volume of water & no motive force to smooth the flow.