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Choosing the right size and mounting orientation of any pollution or firewater containment valve is crucial. Getting it wrong can results ineffective sealing, or in reverse can make it impossible to open the valve against a head of water, can waste money and can even be danger to staff and the public.
It is important to remember that just one meter head of water imposes one tonne of pressure over a square meter. Hence the larger the valve the more pressure there is acting upon its face, trying to force it closed (on seat pressure) or to push it open (off seat pressure) depending upon which way round it is mounted.
Drains historically seldom have been sized for a particular peak flow rate. In the vast majority of cases most drains have a flow capacity many times that actually required and are often sized for storage capacity, ease of access for cleaning rather than peak flow rate.
Safety considerations are very important and are often overlooked. We must consider structural stability, the risks to service crews working within the drains, plus staff and members of the public at the point of discharge (pond, lake or river). Taking the last issue first, just imagine if a 900mm valve holding back perhaps a 3m head in a 900mm diameter pipe, this would equate to roughly 30 tonnes of water with a force of almost 2 tonnes of pressure. Without doubt there is a serious risk of injury or drowning due to the many tonnes of water that will come cascading down the drain the moment the valve is opened.
The question is does a valve have to be the same size as a drain? The answer is yes if the drain is small and certainly no if it is large. It all comes down to common sense backed up with some elementary fluid mechanics.
When it comes to selecting the right size valve there is no hard and fast rule and a few of the additional considerations are listed below.
a) What is the purpose of the drain?
b) What is the risk that the drain is likely to be exposed to?
c) What is the maximum possible head in the drain following an incipient?
The BHS (British Hydrological Society), the author of this article is an Associate Member also strongly supports the notion of flow attenuation and valve down sizing.
Peak UK rainfall equates to something like 50mm to 75mm per hour in the extreme 1 in 100-year event. For every 100m square surface area this equates to a capture rate of 2 litres per second, which a 6in drain is able to cope with, at a velocity of 0.2m/s. Yet many sites have 8in (200mm) or even 15in (375mm) drains, to serve just 100m square. We assume the maximum desirable flow rate was 1m/s then a 6in drain could cope with roughly 1000m square of capture area.
Based upon the above facts we suggest the following:
4in (100mm) > no reduction
6in (150mm) > no reduction
8in (200mm) > reduce to 6in (150mm)
10in (250mm) > reduce to 6in (150mm)
12in (300mm) > reduce to 6-8in (150-200mm)
18in (450mm) > reduce to 6-8in (150-200mm)
24in (600mm) > reduce to 8-10in (200-250mm)
30in (900mm) > reduce to 10-12in (250-300mm)
Except in extreme circumstances the down sizing a valve will reduce cost, lower the risk of leakage, ease opening and can help eliminate water surges and ultimately improve safety.
Please contact you local EIL Agent for further details.