I built a model river in my garden. Part IV

By Stephen Rodgers

Garden Pipe System, Inlet Pipes & Pump ~10K GIFs

The Pump.

Parallel to all the other activity I had been tracking down a pump. Roughly calculating (by running a tap flat out and filling buckets) that I needed about 1000 litres per hour to be raised to a height of 2 metres, my first call was to an electric motor rewinding company, in the hope of finding something secondhand. They sent me on to a pump shop (logical) and in a factory unit full of impellers and beautiful brass castings I announced my need. Brows were furrowed and eventually an enormous swimming pool pump was produced. The price was only(?) twice my notional budget, but the power consumption was 250 Watts. The pump needs to run constantly - I couldn't afford that sort of electricity consumption and my heart fell.

The next pump was smaller, chunkier and cheaper. The trouble was that there was no integral motor and the man starting talking about RPM and gear ratios and I started to worry about getting over complicated. Then he had a brainwave and grabbed a specification sheet from a drawer. "There you are," he said, "you need a Grundfos Super Selectric Domestic Circulator!".

"What's that?", I said.

"A central heating pump, about 30 quid to you, and a maximum power of 75 watts".

I was overjoyed, and directed to the plumbers' merchants, which was where I met Ian and John (thanks, guys).

Pump performance tip.

A pump is judged by how much water it can raise, to what height. Most pump documentation includes a graph of rate against height, learn to read these.

Water pumps come in two flavours: submersible and non-submersible.Submersible pumps are generally the sort you find in 'pond shops', they are quite expensive, but extremely easy to use and very reliable. In general they cannot shift too much water, they don't consume much power, and they have an integral filter.

Non-submersible pumps are generally more serious and powerful, but you have to worry about filters, electricity supply, plumbing and so on. Priming is also a problem, because most water pumps are not capable of pumping air as well.

A submersible pump does not need to be primed as it is sitting in water so its impeller is already flooded, and any air bubbles are soon expelled. The instructions for my Grundfos told me to ensure that it was sited at the bottom of the pipe system, so that priming would not be required.

Placing a pump nearly one metre under my patio was obviously a silly idea, and it would probably involve making a hole in the polythene below the waterline, so any system I designed had to be accessible - and primable. This really needs to be taken seriously. If you have to start up a system like mine when the sump is nearly empty, the pump impeller could be one metre higher than the water level, which is quite a pressure differential, and there are other problems when you have managed to flood the pump and then start it up. If you have not made any provision to cleanly disconnect your priming system to start sucking from the sump, air will be introduced and the pump will have to be started all over again.

Electricity tips #1,#2,#3.......

If a patio collapses some people will get wet and perhaps injure ankles. If you get electricity and water wrong SOMEBODY WILL DIE. Fit an RCB (residual circuit breaker), even before you start messing around with pumps. Remember, you will probably be using power tools outdoors even before you fire up your pump.

I wanted three equally spaced watering points in my garden, and the top one was also intended to (eventually) deliver the water onto the reed bed, so I bought three outside taps, a lot of 'T' pieces, 90 degree bends, straight connectors and 50 metres of pipe. I had toyed with using 22mm pipe, but settled on 30mm, mostly simply because it looked beefier. In hindsight this was the right decision, it wastes more power to pump through thin pipe, and I believe my pipe offers very little resistance. At this stage I did not bury the pipe, nor make any solid joints accept all those 'downstream' of the pipe - I didn't mind leaking a little water, but (as I learned painfully) sucking in air disables the pump completely.

The sump had been filled and covered for over 2 weeks (I was 'dipping' the sump every morning) on the day that I set out to install the pump for the first time. It was later in the afternoon that I had prepared a cavity under the end paving slab, built a wooden box to contain the pump along with a couple of switches, and constructed the inlet pipe. I lifted the required slab to gain access to the water, and horror! The water level, which should have been 15cms below the slab, was nearly 60cms down - I had lost over 1000 litres of water since the morning!

I did a bit of headless chicken impersonation for half an hour, running around checking my cellar and alerting the neighbours. Re-checking revealed that the level had dropped a further 6 cms - soon it would be empty!

Obviously I had a leak, and equally obviously (the level had been fine in the morning) it was a new leak. Concrete jigsaws ensued as the patio was removed and stacked against the fence. Boots were donned, the submersible pump was pressed into service, and I set about draining the sump completely to investigate. Charlotte, my four year old, was keen to help so she was splashing around with me.

As the level fell to about 2 cms deep, one corner of the sump came into view, and there, quite obviously, was a semi-circular hole. A hole whose radius matched my 22mm pipe, lengths of which were stacked in one corner of the patio. At this point Charlotte said that she wanted to get out, and go away because "I think you're going to be all cross in a minute". It was reasonably obvious that she had tried to emulate my 'dipping' procedure with a bit of pipe, and pressed too hard. Charlotte was then informed how displeased I was with little girls who "played with Daddy's river before it is finished" : I just wish it was possible to ask a four year old how hard she had pressed, to gauge just how much pressure the sump polythene is being asked to bear.

Clearly the sump had to be re-lined. Until now I had left the edges of the polythene un-trimmed, and hidden them with the fence posts and paving slabs. I now took this opportunity to set a wooden baton edge all round the sump, to which the polythene could be nailed (the edge is nearly all well above water) and then trimmed. The concrete drain was destroyed with a few deft strokes of a hammer and then re-laid to incorporate the new liner.

So it was some days later that the pump was installed and tried for the first time. The first few times I primed it by pouring from a bucket into the 'suck' end, until water was seen trickling from the first tap at the blow end. I then switched on, and produced some curious gurgling noises (the pipe system, not me!). For a long time all I could generate was a long oscilating column of water in the garden pipe and every time I switched off to try something else, this water would all gush out of the tap, and need to be replaced at the next priming.

I don't think that I can adequately describe the knowledge that I have picked up in this field, but I do have two tips.

Pump tip#1.

Try to arrange the pump and any flat runs of pipe on the inlet side at a slight slope, otherwise an air lock can be caused at prime time which will stop the pump from working, everytime you hit the 'go' button.

Pump tip#2.

ABS pipe joints are very easy to make watertight, even at quite highish pressures, no drips will be in evidence. However, AIRTIGHT joints are ESSENTIAL on the inlet side of the pump, if they are not then a little stream of bubbles will enter the pump and water will stop flowing.

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