Insights

Submersible pumps versus surface pumps. What’s the difference and which should I buy?

Surface pumps or suction pumps are suited to lifting surface water from rivers, ponds, tanks, lakes etc… upwards, or “sucking” water upwards from shallow wells in which the water level is less than about 7 metres below the pump level.  Beyond this, suction is no longer possible.    

In some cases, where water is deeper than 7m below the surface, surface pumps can be lowered into the well and placed on a stationary or floating platform. This is generally only possible in hand-dug wells (rather than drilled boreholes). Nonetheless, surface pumps operated in this way are vulnerable to flooding or tipping over. Air locks can easily form causing problems requiring re-priming. Self-priming surface pumps avoid this, but these generally cost more and are less efficient. 

Raising water from more than 7 metres below ground – and often considerably less – usually requires “submersible” pumps* that “push” water up from below. Until now, this has generally meant submerging electric motors, impellers and sliding seals below the waterline, often in hostile conditions: dirty water and varying water levels regularly cause pumps to wear out “run dry” and burn out. 

There are broadly two categories of submersible pumps: centrifugal pumps and helical rotor pumps.

Centrifugal pumps deliver higher flow rates, but are generally less efficient than helicals and a given pump is only suited to a narrow range of water depths and power inputs – this makes them less well-suited to variable power sources such as solar power than helicals. Furthermore, centrifugals are sensitive to a phenomenon known as “cavitation”, which occurs when pressures become low at the inlet due to volume of water being sucked into narrow pump inlet. This requires them to be situated a long way below the waterline, or risk irreparable impeller damage above a certain flow rate.  

Conversely, helical pumps can deliver efficiently at high heads and are much less sensitive to cavitation, but flow rates are generally low. Both types of submersibles are sensitive to sand and silt, as well as dissolved metals such as manganese and iron. Neither can run dry for more than a few seconds without damage and both require additional level sensors to prevent this from happening if water levels drop too far.  

How is the FlexExtend different to other submersible pumps? 

The FlexExtend changes all of this: it’s insensitive to cavitation and can run dry – for days – without incurring any damage at all. In fact, it’s almost as happy pumping air as it is water. Furthermore, it can deliver high flow rates with very high efficiency across a wide range of heads. It’s tolerant of dissolved metals and handles sand, silt and grit with ease.  

The FlexExtend is powered by a surface pump – which pumps water down to it from surface level, providing water power to lift more water up. This means that motors, electronics and sliding seals are all brought to the surface level, where they’re less likely to fail and easier to fix if they do. 

Like for like, correctly installed surface pumps are more efficient, more reliable and cheaper than their submersible pump equivalents. 

So, Impact Pumps’ FlexExtend range brings motors, electronics and sliding seals to the surface. Why is this better?

There are many advantages of bringing fault-prone components to the surface, particularly if all can be brought up at once. Some of these are obvious. Others are not so apparent. Here are a few examples: 

• Motors are less likely to fail but if they do, any problems can be diagnosed and repaired quickly without the difficulty of removing and re-installing submerged equipment.  
• The surface pump can be used for high pressure/flow distribution from surface storage and easily transported for use in other applications.  
• In a FlexExtend installation, all surface components are pressurised, so air-lock issues that can trouble surface pumps no longer occur. 
• There are no long cables trailing the height of the well, which are easily damaged and can even attract lightning strikes. 
• Electronics, including solar power controllers, motor controllers, remote monitoring and Pay-As-You-Go functionality can all be integrated into the pump casing. This improves reliability and further brings down costs. 
• Conventional solar-powered submersible pumps cannot lift water when they’re broken or the sun doesn’t shine. With a FlexExtend, the surface solar pump can be swapped for a replacement or a petrol, grid, or human powered backup in less than 2 minutes. 

How does the FlexExtend achieve these advantages? What’s new and innovative about it?

The FlexExtend is based on a little-known phenomenon called the Joukowski effect, which has been at the core of one of the most efficient and reliable pump technologies in the world – the “hydraulic ram pump” – for more than 230 years.

There are known examples of individual hydraulic ram pumps that have operated continuously for over 100 years. However, despite their high efficiency and reliability, traditional hydraulic ram pumps are only suited to niche applications with a very particular set of conditions and requirements – otherwise, they would be everywhere!  

Traditional hydraulic ram pumps are powered by an existing flow of surface water over land, which must also be on a slope, like a mountain stream. The pump uses the energy in this flow to lift a small proportion of that water up to a higher level. This scenario is uncommon for most pump users!  

Instead of harnessing the power of a flowing stream, power is provided to the FlexExtend by a surface pump, which counterintuitively pumps water down the well from the surface at high pressure. The FlexExtend returns this water to the surface, with further water drawn in from the well. The combined water flows are separated at the surface in a tank (along with any air and sediment drawn up) and a proportion is then drawn into the surface pump again. 

The FlexExtend was developed off the back of ground-breaking research at the University of Oxford, leveraging deep understanding of the physics at the core of the Joukowski effect to bring the efficiency and reliability advantages of hydraulic ram pumps to lifting groundwater. Combined with modern materials and know-how, it has been possible to engineer a FlexExtend range that brings all the advantages of traditional hydraulic rams to groundwater lifting at a competitive cost.   

The resulting FlexExtend range from Impact Pumps is designed to fit wells with internal diameters of 4” and larger, delivering flows in excess of 6m³/h and lifting water over total heads of up to 60m. Its efficiency peaks at over 85% (>90% with steel pipes) and remains well above 80% for a wide range of water depths and flow rates. If the well runs dry – no problem – with no motors or electronics down the well to keep cool, and no sliding seals to seize up, there’s not much left to go wrong.  

Impact Pumps has carried out more than five years of field testing. The first prototype ever deployed is still running just as well as the day it was installed, delivering water for the farmer, his family and neighbours, day-in, day-out, year after year. The FlexExtend range has undergone a further 3 stages of improvements since, and extensive full-load testing under hostile conditions for over 10,000hrs at a time. However long we test it, and in whatever water quality, the FlexExtend can deliver again and again.

*The only exception is the deep-well or “double-drop” jet pump which allows water to be lifted from above by powerful surface pumps. Jet pumps are extremely inefficient and can only operate at a narrow range of depths and flow rates, making them unsuited to variable power sources such as solar, wind or human. For this reason, they have almost entirely been superseded by submersible pumps and are now only used in applications where the advantage of being able to access the surface pump apparatus for repair and maintenance vastly outweighs other concerns. They are therefore not discussed in this article.

Can submersible pumps like the FlexExtend be used in shallow wells with water depth of less than 7m?

Yes, although in most cases the benefits of a surface pump alone outweigh the benefits of a submersible pump or additional FlexExtend if your water source is less than 7m deep. There are however some exceptions where a submersible or FlexExtend might be considered:  

1. Variable water depth: If the water level varies and could drop below the suction limit, a submersible pump solution should be considered. The FlexExtend is particularly well suited to variable water depths.  

2. Altitude: In areas significantly above sea level, the suction lift limit of 7m drops considerably, and can be as low as 4 or 5m. Cavitation is also more likely to occur, even when drawing from surface water sources. A suitably-specified submersible such as an FlexExtend can solve this problem. 

3. Re-priming: Most pumps, including surface pumps, allow for a small amount of leakage around their “thrust bearing” or “shaft seal” (rotary pumps) or “gland packing” (piston pumps) to provide lubrication, aid sealing and prevent overheating. Except for applications, including a FlexExtend installation, in which the surface pump is left connected to the bottom of a delivery tank, this can lead to air being drawn into the pump chamber when the pump is not in use. Submersible pumps avoid this problem provided their inlet level remains below the waterline. 

Do surface pumps have any disadvantages compared with submersible pumps?

Yes. Despite their advantages over submersible pumps, surface pumps do have some disadvantages in applications where either could be used. However, operated in conjunction with a FlexExtend, these can be eliminated too: 

1. Air ingression: Surface pumps generally suck water from above. If there are any leaks in the suction hose/pipe or fittings, they can draw in air, which gets “locked” at high points in the system and can cause the pump to stall. In extreme cases, it can even lead to seal failures (though motors are generally safe). When working in conjunction with a FlexExtend, the surface pump actually draws in water at the positive discharge pressure of the overall system. Once installed, this prevents air ingression from occurring, even if leaks are present. 

2. Cavitation: Similarly, to air-ingression, surface pumps are prone to cavitation at high flow rates, even with positive pressure at their inlet. Cavitation significantly reduces efficiency and causes irreparable damage over time. By raising the inlet pressure to the discharge pressure of the system, cavitation is similarly avoided in the surface pump. In the FlexExtend itself, there are no moving parts in the vicinity of the inlet flow. This allows the inlet region to be constructed entirely from cavitation resistant materials.   

3. Particulate damage: The FlexExtend itself is highly resistant to particulate damage, having no sliding seals or bearings in which particles can become trapped between moving surfaces. However, the surface pump used to drive it will still in general be sensitive to particulates. Nonetheless, this is largely avoided in a FlexExtend installation, particularly if a storage tank or pond is used, as the surface pump draws water from the tank or pond rather than the well itself. This allows most particulates – particularly larger ones – to settle out for later removal. In extreme cases, or when desired, a filter can be added at the inlet to the surface pump, without leading to cavitation and access issues for cleaning that would normally render this unfeasible.   

4. Overheating: Surface pumps are air-cooled. Air is a less effective cooling medium than water.  Most surface pumps have fans integrated into the motor casing to guarantee an air-flow over the cooling fins. Overheating is rarely a problem with centrifugal pumps, but does occur with surface-level displacement pumps such as progressive cavity (helical rotor), rotating vane and piston pumps in which electrical currents can be high. In a FlexExtend installation, overheating is generally avoided as the surface pump is operated at relatively higher voltage and lower current. Despite the superior cooling properties of water, submersible pumps often suffer motor burnouts due to unplanned dry-running and insufficient water flow over their motor casings – particularly in larger wells, whereas air flow over surface pumps is more-or-less guaranteed by design. 

The FlexExtend range combines all of the benefits of a submersible pump and a surface pump, whilst avoiding the problems commonly associated with each too. For further information, contact one of our distributors or visit our website at www.impactpumps.com.