Guidlines for Sizing a Anti-Pollution or Firewater Valve

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?

  1. Spills
  2. Fire water
  3. Combination of spills and fire water.
  4. Rain and stormwater.
  5. A combination of any of the above.

c)      What is the maximum possible head in the drain following an incipient?

Notes:

  1. Roof drains – these are normally small to medium size as they are unlikely to block and calculating the peak flow rate is relatively easy. Seldom is it necessary to fit valve in line with roof drains as the risk of pollution and firewater entering is minimal.
  2. Car/lorry park and yard drains: These can be large generally to enable cleaning, removing of silt etc. These are the high risk drains from the perspective of spills and / or firewater.
  3. Foul sewers: These are surprisingly small diameter often nothing greater than 8in (200mm) diameter. There peak flow rate is generally very low and predictable as toilet use is staggered. The only time capacity becomes an issues is when there is surface water intrusion due to illegal connections or broken and damaged pipes. As sewer systems are sealed seldom is it required to fit anti pollution valves.
  4. Storm drains- these are generally a large drain that collect the water from a number of smaller drains (car-park or gutters etc). Normally these drains are dry and only run when there is precipitation above a certain level.
  5. Combined Sewers – something that we, the Environment Agency and the Surfers Against Sewage (SAS) all dislike and constantly campaign against. They are a legacy from Victorian times when it was though beneficial to speed the flow of excrement using rainwater. Nowadays with the advent of modern living, we have washing machines, baths, shows, dishwashers, etc which in combination produce more than enough waste water to flush the system. The rainwater that enters the combined sewer often is the route cause of pollution incidents where the combination of the sewage plus the rainfall is simply to much for the combined sewer to cope with, necessitating the alleviation of pressure through an overflow called a Combined Sewer Overflow. SUDS is the long term answer to the elimination of combined sewers and the need for CSO overflows – but it will be many generations before they will be universally installed.

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)

Conclusion:

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.