They are in fact quite dangerous, with the possibility of injury to operatives adding inhibitors and biocides to the system. It is possible for such units, with the valves open in the wrong sequence, if a "blow back" occurs, to end up with all you have added in your face. Always clearly state in specifications and clearly show on the diagrammatic drawings the units complete with NRV's. Please click on the attachment and you will find the correct instructions for operating a dosing pot and the correct illustration on a diagrammatic drawing. You should be aware of the following acronyms:
If in any doubt visit the HSE website: COSHH assessment: Identifying hazard and assessing risk
All Supaflex Dosing Pots Contain a NRV as standard (view the link below). Fitting them without could make you very vulnerable. The best answer, do not do it.