Thread: Anyone good with irrigation pipe/pump calcs?

Originally Posted by Vern

love ya work, any way from all this i can tell you that it is not on level land, some would flow up to 3m head and some down 3-4m, and some on level ground. I hooked up my shed pump today (28m/head) and all drippers opened and worked, however this was hooked to my water tank, the 9m pump is built into the treatment plant. So by subs you mean a second tank and pump to pump the waste through the irrigation?, So the pump that deliver 320l/m at 11 m head wouldn't cut it either?

Thanks Damien! ... it's been (one of) my day job for the past 25yrs.

The head required is to the highest emitters in the system, so, if it's 3-4m above the supply (where the water is situated) then you'll need to ADD 3-4m to the figures I supplied ... and then a bit for good meaure ... so if we selected a pump to do the the required flow at 11m, add another 4m gets us to 15m ... plus a 10% margin (all pumps loose a little of their factory performance after the first 12 months or so). So 15-17m would be an ideal choice (17m = 24.14psi).

The level lines, and the 3/4m lower lines will get a bit more pressure, but that's OK as the drip emitters are pressure compensated ... pressure compensated drippers are great for uneven slopes/ground. Won't cause any problems.

Yeah, your 28m pump (where do you get the 28m figure from ?) does the job because the "SYSTEM" only requires 17m head to push the required flow rate thru the pipe system whilst overcoming the losses in the system. (Got makes/models of these pumps we are talking about ?) The 9m pump might appear built in, but it should be easily removed and replaced. Yes, it can do the flow, but not at the required head ... so the pipe system feathers the pump out at whatever that pump/pipe combination will allow ... make sense ?

Just to clarify, the 320l/min pump. Is that 320l/min open flow (no head) and 9m head (no flow) ? .... as an example ... a small 150watt Davey sub will deliver ...

145l/min at 2m head
100l/min at 3m head
80l/min at 4m head
50l/min at 5m head
35l/min at 6m head
ZERO l/min at 7m head

That pump has a MAX flow of 150l/min, and a MAX head of 7m .... NOT 150l/min at 9m ... make sense ?

If so, then running a 320l/min pump at 13-14l/min is not good practice. You can maybe run down to 10% of maximum rated flow (5% under ideal working conditions) ... so that's 32, or, 16l/min ... at 13l/min you're running the pump hard to the left of it's operating curve ... exactly where a pump is NOT designed to be run ... and all that aside, that pump STILL won't develop the head your system needs to operate correctly.

As I said before, these little subs (submersible pumps) aren't the right choice for this type of application ... Davey make a nice little twin stage high pressure sub', I think it goes 25m max, but it's solids handling is particle sizes of less than 2mm ... you'd be pulling the pump and cleaning it too regularly I think. It's really intended for clean clear _water_ ... unfortunatley, these subs get used for this type of work because they're cheap ... and then there's the filtration aspect ... no matter how clean the grey water effluent looks, it's full of suspened matter and fine particles (fines) ... this MUST be filtered before putting it into a drip style system ... say a 120 to 200micron filter with a LARGE filter area (I use filters rated to 500l/min for this sort of application ... not because I need the flow rate, but because I don't want to have to CLEAN the dam thing every 2-4 days ... the 500l/min filters give me 3-4 months between cleans ... on some jobs up to 6 months.

You could use a 2nd tank and a surface mounted pump (above ground pump) which would make finding a pump to do the require performance much easier, and the surface mounted pump could just be sat ontop of the tank, with a suction line and foot valve and strainer ... then that introduces another set of problems (allways does!) ... foot valves only work 100% in CLEAN CLEAR WATER ... not grey water. Strainers block up. Too fine and it blocks real quick, starving the pump for water, and in extreme cases causes a pump failure. Too coarse, and all the stuff you're trying to hold back ends up in the pump ... again, leading to eventual failure .... then the detergent loading in the water can cause the water to "foam" in the pump casing because of the vacum required to lift the water up the suction line (no where near the same issue with a submerged pump) ... foaming water, or froth ... ever tried drinking froth thru a drinking straw ??

You COULD use the sub in the treatment system to fill an above ground tank, which then supplies your second pump (gravity supply to pump inlet) which supplies the drippers .... how are you going to switch it on/off ... pump the tank dry and dry run the pump for 5min and it's usually all over! ... just another set of issues to overcome.

So, it's a loose loose situation as I see it .... because the drip product was never intended to be used in this type of application ... and sub pumps don't do the required head capacity in this type of installation

All that said ... if you go with a surface mount pump, a DAB 62m, or a Davey XJ50 ... both are shallow well jet pumps ... would be more than adequate, providing more pressure & flow than required (the DAB is cast iron casing, the Davey is plastic). Personally, I'd use the DAB.

If you go with a sub, just remember, it MUST do better than 17m head AT the 15-20l/min flow rate ... the flow will be easy to acheive ... the head is another storey (pun intended). Low flow/High head cheap drainage submersible pumps aren't the norm .... they are high flow & low head.

Now armed with the relevant info, all you can do is choose the best of a bad bunch These things are being spec'd and supplied into these applications by NON pump people who haven't got a clue. More than half of these jobs I get asked to look at all suffer from the problem we've been discussing here ... but that's what the waste water industry do, and we make money fixing these stuff ups after the fact.

Any more questions or advice needed, just ask or PM.

Cheers
Kev..

PS - Look at the Davey web site, and look for the D42A/B (i think). It's the high head sub ... might be a workable choice, but again, check the materials handling size.

Originally Posted by Vern

The 28m head came off the pump placard. I checked the specs on the Davey D42A/B, going by the flow chart it flows 85l/m at 20m head, that sounds alot, but if you think that'll do it then i'll get a price.

Hi Vern ... OK, 28m off the placard follows my explanation ... that means it's 28m MAX (at no flow - ieump can push a column of water vertically 28m).

I checked the curves for the 42A/B ... it will do fine, and the plain D42 will too ... (60l/min at 20m, or 20l/min at 25m). The D42 is a little cheaper. Spec' rates it for waste water systems running sub-surface drippers or min sprinklers ... just watch the inlet strainer around the base of the pump as it's very fine and stands a much greater chance of blocking up ... like with a Land Rover, maintenance is everything!

Don't worry about the flow being more than you need, the pump will just produce more pressure ... and even at 25m (35psi) you're OK as the dripline & poly main should have a rated max' working pressure of 50psi (35m).

PM me if you want a price for either of these units.

Cheers
Kev..