I'm fitting 3 water meters to monitor and bill usage of 3 rooms which are let out. Instructions say to fit an nrv after the meter (before the pump) but in my case the meters are after the shared pump. Why is an nrv needed - is it to protect the meter from backflow ? Or spinning backwards !? 

To protect everyone from sucking dirty wash water back into supply system.  If pipe pressure fails and someone else turns a pump on it could pull from homes back into supply system.  Which is why there should not be any pumps on supply either (but there are some).

As I understand it the meter you have will go backwards once water has gone through the meter hence the NRV. 

 

If there is a meter and a check valve upstream of your branch off for the meters then you don't need one. However it would be good as a landlord to keep each tenant completely independent just in case of cross contamination.

On 3/22/2021 at 2:33 PM, VocalNeal said:

If there is a meter and a check valve upstream of your branch off for the meters then you don't need one. However it would be good as a landlord to keep each tenant completely independent just in case of cross contamination.

This makes sense if its more to protect backflow to other units, rather than to protect the meter from damage. Do they have to be installed horizontally or is there any reason not to fit an NRV vertical (with the flow upwards) ?

Most of the ones used in plumbing are spring loaded ( spring check) so are not orientation dependent. Only very simple flap or ball ones would have to be mounted "flow up". But flow up, if you can, is still the preferred orientation. 

I assume you have a pump. The Mitsubishi type pump-mounted-on-a-tank most common pumps have an integral spring check valve to stop water hammer in multi storey buildings.

Anything you add can cause flow reduction.