Stop installing 1?” pipe! By Jimmy Lamb FISPE & John Cheek FISPE Dip MRSPH

Jimmy Lamb: Why is that time and time again, I see images on social media of pool companies installing two main drains which is good practice, but?they’re plumbed up together with 1?” diameter pipe! Wait, why am I talking imperial…. this is the 21st century, shouldn’t we all be working in metric by now?

Despite having two drains fitted at the recommended minimum of one metre apart the 1?” inch pipe or to be contemporary, 50mm can still present an entrapment issue.

In supplement 8 of the SPATA standards, it’s stated that: At least two floor outlets should be coupled together and hydraulically balanced in a pipe size that each will take the full design flow of the pump, at a water velocity of 0.5m/s (or less) at the grille. This is good advice and to help prevent entrapment and/or hair entanglement should one of the drains become covered when the skimmers or overflow suction is closed. This supplement goes on to say; The velocity through the pipes should not exceed 1.5m/s, to prevent a vortex forming. The maximum flow through a 50mm pipe (1?”) @ 1.5m/s is only 7m3/h.

If you apply the maximum turnover period deemed acceptable by SPATA standards of 8 hours, then you have a maximum pool volume of 7m3 x 8 = 56m3. The 7m3/h is what’s described above as ‘full design flow’.

In my opinion, 8 hours is too long on any domestic pool, but if a pool company wants to install a domestic pool with a volume over 56m3 to SPATA standards at the max. 8 hours turnover, they can still use 50mm pipe on the pool main drains by installing a third drain if space permits for example; let’s take an 80m3 (17,600gallons) with an 8 hour turnover = 10m3/h. at a velocity of 1.5m/s you would need 63mm pipe to be able to accept the full design flow. With two main drains here on 50mm pipe we have a problem should one get covered. If we bring in a third main drain and each of them are on 50mm pipe and hydraulically balanced, you would have 3.33m3/h being drawn through each when in full flow. Should one of the drains become covered, the full flow would divert to the remaining two each now drawing 5m3/h. Where all three of the 50mm suction pipes connect, they will have to go up to 63mm pipe via a manifold where the velocity should be no more than 0.5m/s.

The same velocity should be applied to manifolds (Sometimes referred to as ‘headers’) in the plantroom where all suction lines meet before the pump. In our example above, 10m3/h @ a velocity of 0.5m/s a manifold of 90mm is required! I often see 2 x 1?” coming from two main drains and the a further 2 x 1?” coming from two skimmers and they’re plumbed down to a single 1?“?to the suction side of the pump all the way through the system before splitting off post filter into 2 x 1?” on the returns. This means 10m3/h through 1?” has a suction & delivery velocity of 2.2m/s. For flow efficiencies, the suction should be no more that 1.5m/s and delivery 2.0 – 2.5m/s

Coming back to our 56m3 pool, what if we want to work at a more suitable turnover (in my opinion) of say 5 hours? 56m3 ÷ 5 = 11.2m3/h. Now two main drains can still work, but consideration should be paid to the pipe diameter. 11.2m3/h at a velocity of 1.5m/s requires a pipe diameter of 63mm (2” in old money). The flow through both drains whilst in full design flow would be 5.6m3/h each drain. Should one become covered during full flow, the open drain would take the full flow of 11.2m3/h, but the velocity would still be max. @ 1.5m/s as so not to cause a vacuum on the closed drain. The 63mm pipe should be routed all the way without reduction to the suction manifold in the plant room.

John Cheek: Most manufactures main drains are designed for 13m3/hr in the pot and over the cover. So, with both main drains in operation (as it would normally be) it would be a safe area even with everything else closed off, and would still remain safe with one main drain covered.

The reason for this is simple, without the second main drain a person can get entrapped on to a single main drain, therefore the second main drain is a relief of the suction so neither drain becomes a hazard.

If a pool is shallow, then 3 or even 4 main drains should be installed. It is though preferred that outlets on a very shallow, say kids paddling pool that no main drains should be in use whilst the pool is in operation. All the items connected to the pump should be either skimmers of overflow channels only. The return fittings are floor inlets that can be cross manifolded to suction if draining or backwashing is required, this being only when the pool is out of use.

One other note, most pumps have a 2” suction fitting that should be connected with 2” pipe and increased in size where applicable, certainly not reduced in size! If and isolation valve is required before the pump then it should be sized to the increased pipe size and not reduced. This way the pump becomes so much more efficient and so much quieter.