One of the consequences of pumping sand seems to have been that the Grundfos SQ1-65 pump no longer wants to pump above about 5 bar (~72 psi). I was operating this system with a turn off pressure of 5 bar and a turn on pressure of 4 bar (~58 psi) with two 300 litre bladder-type pressure tanks pre-charged to 2.5 bar (~36 psi). This combination worked really well to give me enough flow rate and volume to backwash the iron filter despite me having a flow restrictor valve fitted in the pump line to keep the pump flow rate down below 10 litres/minute (~2.6 US gallons/minute). It also gave a good back up of stored water should the pump go off line, as it has been during the recent work (luckily I'm still building the house so the water is only for the toilet, hand washing and making cups of tea).
There's going to be a dispute over the warranty repair of the Grundfos pump, I know, but the drilling company that supplied it told me, in writing, to run it continuously to waste for two weeks to clear the sand from the well. My view is that I shouldn't pay for the repairs, as it seems likely that they are due to wear caused by pumping all that sand that I finally got out last week, but in the meantime I need a pump to run the house for a while until the Grindfos gets fixed, just a cheap back up pump that I can keep as a spare.
There's a Polish company I've bought other stuff from who are really good value here, and the products look to be well-engineered (I worked with some Southern Polish engineers years ago and found they were damned good, so I tend to trust Polish made stuff).
I was amazed to find that this Polish company (Dambat) manufactured a couple of 3" diameter stainless steel borehole pumps (they even make a 2" diameter pump) What was more amazing was the price. I don't know the price of pumps in the USA, but here we pay the equivalent of at least $500 for a cheap, no-name (probably Chinese-made) pump, up to way over the equivalent of $1000 for a decent brand name pump (the Grundfos I have was around $1000).
The Polish pumps were amazingly cheap, they do a small 20 litre/minute (~5.2 US gal/min) 7.5 bar 3" pump for just about $105 (delivered) or a more powerful (1hp) 11 bar, 40l/min pump for around $170 delivered.
As a spare pump for emergency use that I could use whilst the Grundfos was being fixed, and to keep as an emergency spare the larger of the two looked to be a really terrific bargain. I trust the company, having bought a nicely made pressure set surface pump and pressure tank from them (which were also very good value) so I didn't feel it was taking a big risk to buy one of their well pumps as a spare.
I've looked at the pump curve and spec and it seems fine, at 10 litres/min the bigger of the two 3" pumps they sell can still deliver 8.8 bar (~127 psi), so even allowing for my resting water level in the well (standing head effectively) and the pipe losses from the pump to the pump shed, I should still be able to get around 7.5 bar or more at the pump shed. There is some limited info on these pumps, with curves, on their web site, here:
http://www.dambat.com/sqibo-deep-well-pump.html
My question is this. Having looked at the pump, I've found it uses a screw displacement pumping method, rather than staged impellers (usefully they sell the screws and stators as spares pretty cheaply, as in a bit over $50 delivered:
http://www.ebay.co.uk/itm/spare-hydraulic-kit-for-IBO-3-SQIBO-0-75-borehole-pump-/171820480373 ). Now I know nothing about positive displacement pumps like this, except what I've ready on this Wikipedia entry:
https://en.wikipedia.org/wiki/Progressive_cavity_pump
So, how is a pump like this going to behave when fitted with the flow restrictor I'm pretty sure I still need? My first thoughts are that, from the pump curve, the pump side pressure will rise to around 8.8 bar with the restrictor in place, and that there must be some leakage back past the rubber stator moulding in the pump that means the pump isn't a "true" displacement pump, but has a progressive leak back past the screw as the pressure increases. My second thought is that it probably needs a check valve at the well head (no problem to fit) as the top of the rotor shaft and the stator opening can be seen just below the 1" BSP pipe connection and this type of pump probably doesn't have, or need, a foot valve. The down side on it being Polish is that the instructions are not exactly clear!
If anyone has any knowledge of positive displacement screw-type pumps and could confirm or deny what I've assumed above I would, as ever, be very grateful.
(BTW, they sell these pumps in the UK from their UK warehouse in London, via Ebay of all places. Their Ebay store is stores.ebay.co.uk/division303pump in the UK, if anyone is interested at looking at their stuff. I hope mentioning this isn't breaking forum rules - I just thought it might be of interest to see what's available from other parts of the world).