Dolphin strainers

Dolphin Strainers – The Floating Pump Intake That Protects Your Pump

What Are Dolphin Strainers?

Looking at a Dolphin strainer basking on the surface of a lagoon or lake, you can see how they got their name. It certainly sounds better than a “floating mine lookalike”! 

Although we haven’t found a direct application for Dolphins yet, they could reduce the chance of fish and eels getting drawn into the suction line, as they do filter out objects over about 5mm diameter.

These industrial grade floating filter intakes are used extensively in dewatering lagoons and water abstraction duties from open water bodies, which typically have silt buildup at lower depths and floating debris and scum on the surface.

A simple but robust design makes dolphin strainers a cost-effective, low-maintenance way to protect your high-capacity pump system from blockages and impact damage from solids.

Floating Strainer Sizes 

Made available in popular pipeline diameters (1.5”, 2”, 3”, 4” and 6”), Dolphin strainers have 4.75mm holes around their detachable side plates. 

The pipe connection sizes are shown below, along with their maximum flow rate design:

1.5 inch (38mm) - 140l/min
2 inch  (51mm) - 280l/min
3 inch (76mm) - 560l/min
4 inch (102mm) - 1,150l/min
6 inch (152mm) - 3,000l/min

Dolphin Strainer Installation And Features 

These strainers attach to pump’s suction pipework via a length of flexible hosing to allow the inlet to rise and fall with the water/fluid level. They keep the suction inlet just underneath the surface where the water is generally at its clearest, away from floating matter and the bottom sludge and settling silt.

A rotating joint prevents hose twisting and keeps the unit on an even keel.

The damage-resilient polyurethane foam buoyancy keeps the strainer in an upright position, with the inlet always below the surface. This buoyancy head is protected by a rounded steel cap (that looks like a dolphin head, with a bit of imagination). The solid bottom (no strainer holes) helps to prevent it from burying itself in shallow water or turning into a dredger if it grounds.

In fast-flowing waters or deeper, choppier waters, weights or a mooring chain can be easily added, thanks to the recovery hook eye. 

The optimal length of unsupported flexible hosing is around 3m. If your levels fluctuate more than this then we’d look at:

1)    Extending the rigid pipework into the lagoon, so that its joint with the flexible hose sits between high and low levels.
2)    Consequences of the floating strainer being wholly underwater. When the depth rises beyond the available pipe length, the strainer is held underwater. This is not usually a problem as the contamination at this level in deeper water tends to be a lot less.

Can I Use A Dolphin Strainer On A Pipeline With A Non-Return Valve (NRV)?

Non-return valves are often needed to prevent backflow, especially where the pump isn’t self-priming. If needed, these NVRs are best fitted at the lowest point in the submerged suction pipe to prevent the formation of air locks.

Does A Dolphin Strainer Affect Flow Resistance/NPSH?

The strainer plates and diameter of the strainer unit get bigger in scale with the outlet size. There’s plenty of redundancy with the holes totalling 3 – 4 times the cross-sectional area of the outlet pipe. 

If you do get a drop in NPSH (net positive suction head), this will be because the strainer has become blocked and needs clearing. If it is often getting blocked, speak to a pump specialist about alternative arrangements.

Dolphin Strainer Price

Where these strainers excel is in reducing the cost of maintenance and wear through the subsequent pumping system. By preventing blockages and downtime, and extending your asset lifetime, the cost savings can quickly recover the initial investment costs.

The cost of a 2” (280 litres/min) dolphin strainer and its associated fittings has averaged £350 delivered to UK sites. 

As you’d guess, the price goes up with size and capacity. We supply dolphin strainers that handle up to 3,000 litres per minute with a 6-inch pipe outlet, for which you can expect to pay up to ten times the cost of a 140 litre per minute model. A good guide is that if the strainer extends the pump’s lifetime by 50% or more, it is a quick payback. The ROI can also be figured out using the forecasted increase in system uptime, reduced H&S incident risks from blockage clearing, and other factors that are site dependent. 

If you think a dolphin strainer sounds like a potential solution for preventing excessive damage, wear or blockages at your site, discuss your situation with our pumping experts for application-specific advice.