Here at SEP, we sell a lot of specialist valves, and something which seems to cause most confusion is in relation to solenoid/automatic/actuated valves and the meaning of ‘normally open‘ or ‘normally closed‘.
You may also hear the same terms being used in relation to switches, electrical circuits, alarm panels, pressure or flow switches etc – and although principle is the same, the effect may be different.
So here it is in a nutshell – the word ‘normally’ is a specific technical term which means ‘when there is no power‘ (de-energised, switched off, power cut etc). A ‘normally open’ (NO) valve is OPEN when it has no power to it; a ‘normally closed’ (NC) valve is CLOSED when it has no power to it. This can be different to what state the valve is in ‘usually’ (meaning ‘most of the time’).
The question (confusion) arises most often in relation to Priority Demand Valves (PDVs), probably because in many cases the PDVs are sourced and installed by plumbers or general building services technicians where this sort of valve is novel, rather than by fire sprinkler engineers who have most likely come across the term before. We say this not to ‘talk down’ to anyone, but acknowledge that if you haven’t come across something before then you can’t be expected to understand it without explanation.
A PDV is a very specific type of valve, and its specification and working is defined in BS9251:2021 – it is a valve which is:
(1) installed on the domestic water supply,
(2) closes on loss of power (i.e. ‘NC’) so that it is fail-safe, and
(3) closes when a signal is received from the fire sprinkler/alarm system…
…but is otherwise open in a powered and non-fire situation.
Looking at the above, this type of valve is ‘normally closed’ but ‘usually open’ – no wonder there is confusion!
Conversely, many watermist systems (which are actually very different to fire sprinkler systems) require a valve to be installed on the domestic water supply which is ‘normally open’ and is in fact also ‘usually open’. We can help with this, although it’s not a ‘PDV’ in accordance with BS9251:2021, which is a domestic and residential fire sprinkler standard (not watermist).
We regularly supply solenoid, actuated ball and actuated butterfly valves for both PDV applications and other purposes – so remember, unless ordering a PDV under BS9251, we will need to know:
- When not powered, does the valve need to be (‘normally’) open or closed?
- Will the valve be ‘usually’ open or closed?
- When signalled (by whatever means) does the valve need to be open or closed?
- And of course, the usual things such as pipe size, connection type and control voltage.