Thread: Water Pump for gardening

Just bought a 3000l slimline tank. Water will not be plumbed to the house but only used to water veggie garden, lawn, wash car etc. Looking at attaching 1 or 2 taps but only 1 will ever be in use at any time. It's a suburban garden, the furtherest watering from the tank will be by hose (30m?) or a hose with sprinkler attached.

I phoned the local pump store and husband went in there (& played dumb). We each spoke to a different person. Each has advised a different sized pump. They are either a DAB 82 (60l/min, 462kPa) or a DAB 102 (60l/min, 531kPa). Also given 2 different sizes in Davey being XJ50 (50l/min, 46m head) or a XJ70 (70l/min, 48m head).

I don't know which to go for. Will the smaller pump be enough (either DAB or Davey) or is the bigger pump overkill? Any advice would be appreciated. Thanks

From a small tank 50l per minute will be fine for garden use.

A few things to consider here.

Firstly, you have not stated the size (Internal Diameter) of your hose. Most people use 12 mm (I.D.) but there are also 18 mm hoses available. More about that later! For now, I will assume that you intend using a standard 12 mm I.D. hose.

Let's forget about pumps for the moment as the following is the important thing to understand.

A 12 mm garden hose has a lot of friction loss. The longer and smaller the hose is, the greater the friction (energy) losses will be. Also, the faster the flow rate, the greater the loss; for example, at 10 litres per minute, the friction loss of 12 mm hose is about 1.75 kPa per metre but at 15 litres per minute the friction loss per metre is about 3 kPa per metre. At 20 litres per metre the friction loss is about 6 kPa per metre. In simple terms, doubling the flow rate requires X4 the energy!

A garden sprinkler fed from a 20 metre X 12 mm I.D. hose and attached to a mains pressure tap will flow at 18-20 litres per minute. For you to have a garden sprinkler attached to a 30 metre 12 mm I.D. hose flowing at 20 lpm and connected to a pump, the friction losses would be 30 (metres) X 6 kPa = 180 kPa friction loss PLUS additional losses in the pipework between the pump and the tap plus other hose losses because the hose will not be straight plus the losses incurred by the sprinkler itself. A total friction loss of +250 kPa (25.5 m head) could easily be realised! This determines the pump's minimum pressure requirement.

This loss is deducted from the pumps pressure to arrive at what is called dynamic head pressure. If the pumping system is unable to deliver the head needed to service the required flow rate, the required flow rate will not be realised. In the case of our example, a pump with a pressure rating of around +350 kPa would be adequate.

There is no need to buy the pumps that have been recommended to you or even as per our example as an 18 mm hose flowing at 20 lpm has about 8 times less friction loss than a 12 mm hose. The following tips will help.

Do not use small hoses or copper pipe to connect the pump to the tap as 90 degree fittings and small pipe add to friction loss. Rainwater is also naturally acidic and can chemically react with copper pipe.

Consider having two garden taps at different locations connected to the line from the pump.

Use a top quality hose connector to connect to the tap. A connector that draws air into the hose (venturi effect) reduces efficiency as air bubbles reduce volume and act as physical barriers, increasing friction loss. A quality connector also allows you to use the tank's head to maximum effect when using gravity fed drip hoses for drip watering. Unless you are watering an uphill area, a moderate head pressure will supply a 30 metre gravity fed drip hose with the standard 4 lpm it requires as water will still flow through the pump to the tap when the pump is turned off. Not many people realise this! NOTE: Gravity fed and standard drip hoses are two different animals!

Buy an 18 mm garden hose and use a (poly) reducer connector to connect it to a (much) shorter 12 mm hose so that you can connect to standard hose fittings.

Make sure you buy a pump with run dry protection. A 3,000 litre tank might seem large but a sprinkler will use 1,000 litres or more an hour.

One other important thing, have a look at the tank and see where the pump's draw off valve is. If it is fitted at the very bottom of the tank, the pump will eventually draw sludge which will shorten the pump's life. The draw off valve should not be less than 80-100 mm above the bottom of the tank. If it has been fitted to the (near) bottom of the tank, I strongly advise having another one correctly located. It is also best if the draw off valve is not fitted directly under the tank's top meshed inlet or in line with the the fall of water from the tank's infeed pipe (another common mistake).

You can also prolong the pump's life by making sure that the usual 90 degree elbow fitted to the pump's infeed pipe (suction side) is more than X10 the elbow's I.D. away from the pump.

I am also a big fan of using a large (+100 litres) stand alone pressure tank connected to the pump. Check Ebay for these at good prices and make sure that the pump you buy can be used with one. Most pumps you buy with an attached (usually very small) pressure tank have limited benefit. A large pressure tank will substantially reduce pump start ups, have the pump operating in its 'sweet spot', save on energy costs and substantially prolong the pump's life.

Buying a smaller but suitable pump will not only save on the purchase price, it will also save ongoing energy costs. Many good pumps are only 400 W, the ones that you have been recommended are around the +800 W mark. Think of the energy difference between pumps as switching on 6-8 60 W light globes every time the bigger pump is in use!

Edited by Bedford at Danny's request, changed 300 kpa to 350 kpa.

Hey Danny that sort of information deserves a "Sticky' Thanx

Thanks Moondog.

It's not all good though as my quoted friction loss of 6 kPa per metre @ 20 lpm for the 30 m X 12 mm garden hose in the example I gave is actually for 13 mm LDPE (poly pipe). Oops!!!

A revised kPa/m friction loss would push the example's friction loss guesstimate from +250 kPa to possibly 300 kPa as 13 mm poly has 17% more volume and lower friction loss than 12 mm garden hose. This would then require a pump with a minimum of +400 kPa but this would be determined by looking at the pump's flow chart and perhaps accepting a lower worst case scenario flow rate of 18 lpm. The DAB82 with 462 kPa doesn't look so silly now after all.

Nevertheless, the point I was trying to get across was that using a larger 18 mm hose is a heck of a lot better than outlaying the hard earned on a big pump. The stated X8 less friction loss when substituting an 18 mm hose for a 12 mm hose also varies with the flow rate and hose length but is a good rule of thumb. The X8 less friction loss however does not mean 8 times more flow. 18 mm hose has a 50% greater diameter than 12 mm hose but has 125% greater volume. Doubling a pipes diameter increases the volume X4 but maintaining the same pressure roughly increases the flow rate X6.

Thanks very much Danny.

Not only for the knowledge you've shared, but the logical explanation ... and taking the time to write it all ... Legend.

We'll be using 18mm for sure.

Hi Destorman,

You will also save money by buying online and most online traders give advice over the phone. When I Googled the DAB 82 pump, I found a price difference of over $300 between traders! I have also noticed that there is an ad for pumps that appears on the top of the page for these type of threads that states they will beat any advertised price. Some dealers even offer free postage.

Pump designs have not changed much over a very long time but for added interest and to see the future, click the link below.

http://www.supastelthpumps.com/

Danny, again many thanks for the info and the PM ... really appreciate.

For everyone else, here goes:

How stupid are we ... forgot the FIL, a retired 'old school' plumber put in a tank a while back. Spoke to him today and guess what? He used 18mm poly pipe and bought at pump from Stratco for about $250. It gives his boat a good bath and plenty of water for the garden too (although his veggie patch needs some TLC, but that's another story).

Between reading Danny's info and talking to the FIL, I went to the another local pump shop today. Gave the bloke the same info. and he recommended a pump for $220, but a brand I had not heard of ('orange'), anyway, also got details of a 'new' davey pump (sj25-03cp) which he can supply for $240. He also gave me a price on a 'pumpmaster' TT45 for $264, again, haven't heard of them.

Prefer to stick with the brands we've heard of, and can get repaired down the track. For us, locally that's mainly Davey. Thinking that's the way to go ... and we also get to support the aussie assembly industry

Again, Danny, thanks for the info. Legend.

I think the Orange might be a cheaper version made by Davey but you will have to check this. I saw some pump performance figures for the Orange HT100M on the Orange site; it stated 20 lpm @ 10 m head and 13 lpm @ 15 m head. Is this the model recommended to you?

9.8 kPa = 1 metre head. You will have at least four 90 degree elbows between the pump and the tap. Friction loss depends on the elbow's I.D. size but allowing .75 m for each, this gives 3 m friction loss. Let's also say that there is an additional 1 m head loss through the (suitably sized PEX) pipe from the pump to the tap and straight away you have lost a minimum of 4 m head.

Friction loss through every metre of 19 mm LDPE Poly pipe @ 20 lpm is .8 kPa. I don't have the figures for 18 mm garden hose but also allowing for kinks, terrain undulations and bends, let's say 1 kPa per metre or 1 metre head loss per 10 m of 18 mm garden hose. If you had 30 m of 18 mm hose, you would already have a guesstimated total of 7 m of system head loss PLUS the losses for the attached short length of 12 mm garden hose and sprinkler to come. Getting the pump's 20 lpm @ a 10 m head could be borderline but this flow rate is very good for a sprinkler, many and perhaps most sprinklers connected to mains get about 18 lpm.

I actually use 19 mm poly pipe to connect to the tap as it has very high UV stability and is heaps cheaper than a quality 18 mm garden hose. You need good hose connectors though as the Outside Diameter (O.D.) is smaller than 18 mm garden hose because poly pipe is thinner walled. Just don't have the tap too low or otherwise the poly might kink. I am also considering joining a longer 25 mm (25.4 I.D.) LDPE poly pipe between two end lengths of 19 mm poly as the friction loss of 25 mm poly @ 20 lpm is only 0.2 kPa per metre, i.e., over 30 times less than 12 mm garden hose! My pump is near the front corner of the house but my vehicle wash area is at the opposite back corner and I need to have a 10 m length of 12 mm hose attached to the end of the poly for convenience of use.

For what it's worth, a tank manufacturer near me had a special (available to the public) last year on 2,000 litre round poly tanks for $360 that also came with the option of buying a 35 m head pump for $75. I told my customers about this and as we generally install two tanks, I just said that there was a 1 year warranty and if the pump packed up after this time, just fit the other one. So far, none have failed. We religiously have the pump's draw off valve a minimum 80 mm above the bottom of the tank and advocate the use of a settling tank to feed decanted water to the pump supply tank whenever possible. This way, the pump only draws clean water. We also use various methods to cut down on pump use and friction loss when connecting the tank to a cistern (high number of pump starts of short duration) and I am confident that those $75 specials will outlast most $700 to $1,000 pumps that have been installed by the average rain harvesting 'expert'.

The link below is the Onga pump manual and is interesting in that a page 8 diagram indicates that the 90 degree elbow on the pump's suction line must be a minimum 150 mm from the pump and the pump's draw off valve should be a minimum of 100 mm above the bottom of the tank. If all installers knew the basics and read the instructions, so many (unnecessary) expensive pumps wouldn't be failing!

http://www.onga.com.au/objectlibrary...ual%202006.pdf

Hi there,

I didn't want to start another thread re water pumps so sorry to hijack. We don't have a proper water tank installed yet so at the moment there's a few makeshift tanks (think tubs) in the backyard collecting rainwater. The purpose is purely for watering the garden and I want to make things easier by getting a pump. I want the pump to work like a high pressure hose where you can just use water from a bucket via a hose. Are there water pumps out there that can do the same thing and provide enough pressure to water the garden?

Thanks,
Trev

You would probably need a submersible pump and a power source.

For what it's worth, I was in Bunnings during the week and they had Silverstorm WP5630 300W submersible pumps marked down to $39. They have a maximum head of 11 metres and operate with a minimum depth of 25cm. It can be easily connected to a 18mm hose and it is deemed suitable for 25mm and 32mm hoses but one outlet connecting pipe and the attached hose adaptor are both 12mm. Go figure! Problem is that it is low pressure and not designed to operate for more than a few minutes at a time. It also does not have a float low level cut out, hence the low minimum water operating level, which means that if they operated while dry due to no cut out provision, you will ruin the pump real quick. You would also be limited to watering within a short distance of the tub. A cheap submersible however is handy to have but wheeling around wheelie bins full of water to water the garden is unrealistic...despite what some ads and the photo on the box suggests. For intermittent watering in close proximity to the tubs, it might do.

To have high pressure flow would require a larger submersible pump but that also means having the standard float cut off which also means a higher minimum water level and greater cost. Having it feeding from a bucket topped up by a hose is unrealistic.

Until you get a tank, you may have to consider raising one of your tubs to increase the head and prioritise water harvesting to that tub with any overflow going into the other tubs by some means. This would then allow you to run an inexpensive 19mm poly pipe to a gravity fed drip hose (GFDH) that you could move around to water selective garden areas. Water could be transferred from the lower tubs when needed.

GFDHs are a lot different to the standard drip hoses. The quality Holman gravity fed 'Green Hose' for example has large double emitters that are designed to each emit 2 litres per hour for maximum hydraulic conductivity efficiency. This is much better (and cheaper) than high volume watering via a pump which also demands your time.

Holman Industries