A Pump Buyer's Guide To Dirty Water Management
Discover the diverse world of pump technologies, unlock the full potential of pumping systems, and dive into the fascinating intricacies of fluid dynamics. Our guide is your gateway to a journey toward optimising your dirty water management!
Contents
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Pumps, like the term “dirty water”, come in all types of classifications. This guide focuses on the best pumps for dirty water often found in construction, mining, quarrying and other heavy industries.
Water used for industrial processes, or groundwater in the case of mining and quarry effluent, is typically contaminated by suspended solids. These are usually pebbles, grit, sand,clay, or other minerals, sometimes with trace metals, and can also have acidic or alkaline properties. This ‘dirty-water’ presents a variety of challenges for pumping equipment; their corrosive or abrasive properties, higher viscosity, and ability to block or jam the system.
Awareness of the different pump technologies, pumping system effectiveness and the fluid’s characteristics can help you make the right decision when improving your dirty water management. Some knowledge of these factors can also speed up the buying process when discussing your needs with your pump supplier.
When choosing a dirty water pump, factors to consider include:
- What type of pump is best for the task; submersible, dry-sited centrifugal, or positive displacement?
- What medium is being pumped; the fluid’s pH value, turbidity, slurry density, temperature, solids content, and maximum particle size?
- What flowrate is needed, and what distance and height is it to be pumped?
- Is there a specific pipe material needed for the fluid? This can affect friction losses, as can internal diameter, bends, joints and valves.
- Is there a specific pressure needed at the pipe’s terminal (i.e. process water for a washplant)?
- What power is available? Pneumatic, diesel or electric (110v, 240v or 400v)
The difference between clean water and dirty water pumps
Clean water (or clear) and dirty water (or ‘grey water’) are relative terms, but pumps designed for dirty water have a more robust construction with thicker casings, larger tolerances, and specific materials to cope with abrasive grits and solids. They cost more to manufacture, and being designed with larger tolerances (to cope with pumping solids-laden water), suffer a drop in efficiency compared to their clear water cousins. Conversely, using a clean water pump with abrasive, or otherwise aggressive, dirty-water / slurries would very quickly cause them to wear excessively until point of failure. Think of them as being either a sprinter or a fell-runner.
Whilst most pumps will operate fine within a given range, the consequences of over-sizing or under-sizing need to be understood. Either way can lead to high energy use, overheating, and excessive wear from cavitation damage. Understanding what goes into specifying a pump will help you narrow the field when shortlisting which brands to consider. Many industrial pumping situations will benefit from the involvement of a specialist pump engineer as they are often able to customise the solution to drastically reduce the lifetime cost of ownership. You can contact Atlantic Pumps technical advice team on 0808 196 4938.
As solids content and size of particles increase, so must the tolerances between the moving parts such as the impeller and the volute – preventing blockages and overload. The minimum solids passage can be increased further by using an open impeller and even reducing the number of vanes however, with this comes a drop in efficiency. Due to the drop in efficiency that comes with higher tolerances, the flow / pressure output of a pump with a closed, 5 vane impeller would be higher than that of an open 2 vane impeller given the same power input / pump running speed.
Below left (Figure 1), is a photo of a pump rig for water at the cleaner end of the dirty scale; capable of moving 150 litres per 150 litres per second (540 cubic metres an hour), at a head of 50 metres (approximately 5 bar) via the 12” outlets. To ensure 100% uptime for this deep mine, the configuration has two paired pumps and motors to allow for maintenance and service. Notice how the volute chamber is slimmer by comparison to the slurry pumps in the photo on the right. The SlurryPro pumps (Figure 2) are made for thickened slurry with a high-solids content, and can withstand abrasive particles much better than standard dewatering pumps.
When we talk about dirty water pumps in an industrial context, we are referring to fluids with a percentage of solids/small particle dirt content with the largest occassional particle size of 6-20mm, depending on model. If the fluid has a high percentage of solids, you’ll likely be wanting a slurry pump (aka mud pump) rather than a dirty water pump. So, ‘dirty water’ is subjective. What would be considered dirty water for a fine tolerance positive displacement pump for example, would be clean for the likes of a slurry pump. A heavy sludge which would be easily pumped by a slurry pump, would cause a clean water centrifugal to fail rapidly.
Volume VS Height (Pressure)
It takes 1.42 psi of pressure to raise water one metre in vertical height. Thankfully pump manufacturers make it easy for us by quoting a maximum height in metres that a given pump will provide.
There are four things to bear in mind when talking about pump volume and height:
1) Assuming other conditions (such as RPM, impeller trim etc) remain the same, then the greater the height requirement, the less the volume/flow (or a larger pump is needed). Conversely, reducing the height demand will increase the volume per min/hour capability.
2) In practice, the basic head pressure calculations are affected by resistance caused by pipe length and diameter, materials used, bends, connections and valves and flow rate. Dewatering pumps often need a bit of extra capacity to cover peak flows, whereas process water supplies (to a wash plant for instance) are likely to remain more stable and easier to predict.
3) A pump’s best efficiency point (BEP) will be lower than its maximum head/maximum volume. Having a pump that spends most of its working life around its BEP will save energy and extend its life. Standard pump performance figures are usually based on a specific gravity of one (pure water at 4 degrees Celsius), so allowance should be made for fluids at different densities and temperature extremes.
Identify your critical need: for volume or height pumped?
Which is most important to you, the volume of water or the distance/height pumped? In the case of site dewatering, you will most likely have the need to move the dirty water a certain height and distance. If you have a lagoon, sump or another collection area that can ebb and flow, the volume per minute is probably the 2nd priority. On a space-constrained construction site, a higher volume pump might be needed for rapid dewatering of foundation digs.
Another scenario where it is important to specify the correct volume capacity is where a steady feed of water needs moving a short distance, for instance into a treatment tank for dirty water recycling. In that case, getting a pump that’s too large will result in frequent starting and stopping, or cavitation caused by the pump’s net positive suction head needs (NPSHr) being greater than what the source can supply (NPSHa, or Net Positive Suction Head available).
Dirty water pump siting; dry and wet pumps
“Wet pumps” are submersible or semisubmersible pumps that are very popular for dirty water removal. Their advantages over the ‘dry pumps’ that sit above the reservoir include:
- Being immersed in the water means they don’t need priming. Dry pumps require an auxiliary submersible pump, or non-return valves to prevent the need to prime at every start-up.
- They are kept cool by the surrounding liquid.
- With Audex submersibles, this is true even when only the inlet end is submerged.
- Electric submersibles are very low maintenance; no regular oiling or gland packing is needed. Double mechanical seals prevent leakage.
- Stop/start automation is very simple to implement, via a float switch or water sensor.
“Dry pumps” are those installed at the side of the source reservoir or feed tank, instead of submersed. They are often used for slurries and in instances where blockages are more likely to occur, due to the ease of access and simple unbolting of the casing to access the inner parts. Dry sited pumps can also cost less to purchase than their submersible cousins as the IP rating of the electrical parts is less stringent. The motors on dry sited pumps can usually be repaired independently of the pump and vice versa too.
Typically, a smaller wet pump is used to flood the suction end of the centrifugal pump, which then does the heavy work of adding the pressure head required.
Pump Types For Dirty Water
The many styles and technologies of pumps can be broadly split into two basic working principles; Dynamic and Positive Displacement (PD). Dynamic types impel the fluid forward by exerting high-velocity energy via spinning blades (the impeller), whereas PD pumps mechanically force the fluid through by repetitive strokes of a piston, cam, or diaphragm.
Centrifugal pumps
Centrifugal pumps are the most effective pump for moving large volumes over a long distance or steep elevation climb (i.e. having a high head requirement). Also, their design principle is better suited for tolerating wear, a key factor if you need to pump abrasive liquids such as quarry slurry, construction wastewater, etc.
Centrifugal pumps have an impeller which spins inside a circular volute, putting the fluid in motion through centrifugal force. Various impeller types can be specified, from “closed” designs to “open” vanes to suit the contamination level of the dirty water being pumped, and the particle sizes of the suspended solids.
Several factors influence the performance and capabilities of the centrifugal pump, in addition to impeller design, such as gearing, engine/motor size and metal or rubber linings. High chrome alloy or hard rubber impellers are recommended depending on the type of dirt and solids contained in the water. Below is a simple overview showing where rubber or chrome metal is usually specified for different dirty water pumping.
The relatively simple design of the centrifugal makes maintenance easier, especially if the volute casing is radially split for ease of access to the wearing parts.
Positive displacement pumps
If the dirty water has a high specific gravity or is particularly viscous, a positive displacement pump such as the peristaltic type can be a good option. Whilst they typically don’t have such high head pressures, they are good for moving highly viscous fluids.
Peristaltic pumps are easy to clean as the fluid only comes into contact with the flexible pipe it’s transported in. Unlike centrifugal pumps, they can also run dry for short periods without overheating and are notably reliable pumps in the applications they are intended for.
What about progressive cavity pumps for dirty water?
As progressive cavity pumps rely on fine tolerances between the rotating parts and the cavity wall, these are not generally recommended for abrasive fluids such as water contaminated with sand or grits. However, they are good for non-abrasive fluids, sludges and pastes, especially those that require low-shear handling and consistent flow.
Fluid Properties
From a pump’s perspective, the most challenging fluids are those containing sharps that wear away the inner parts, chemicals that eat into the pump’s materials and solids that are too large to pass through the impeller.
Most of these issues encountered with dirty water have been overcome through engineering. Below are ways that pump manufacturers have innovated over the years:
Pump wear
- High chrome impellers and internal surfaces are harder than cast iron or standard steel.
- Hard rubber impellers and linings resist surface scratches and are cheaper to replace than their metal alternatives.
- Split casing allows quicker access into the volute chamber for cleaning, inspection and replacement of the ‘wet parts’.
Blockage and jamming
- The impellers on dirty water pumps are of a more open design than standard dewatering pumps. The general rule of thumb is that the pump’s inlet and outlet should be at least double the maximum particle size, but check with your pump supplier on this.
- Again, split-casing helps the process of unblocking a jammed impeller. On very heavy pumps, the pump can be mounted on a sliding base to further speed up the opening and closing of the casing.
- Preventing solids that are too large for the pump can be achieved with strainers at the intake end or by channelling the water into settlement lagoons or wedge pits and drawing off near the surface. By taking steps to rid the slurry of the largest particles early on, the remaining fluid can be pumped more efficiently, saving energy plus wear and tear.
Chemical, pH and temperature extremes
- Inform your pump supplier of any chemicals likely to be present in the pumped liquid. Whilst not likely an issue with most dirty water pumping, some chemicals, solvents and hydrocarbons might need special pump linings and compatible impeller materials.
- pH extremes such as cementitious water or highly acidic fluids can also attack pump materials, seals and valves. pH correction or specifying acid or alkaline-resistant pumps might be necessary.
- What is the safe leakage tolerance? Basic centrifugal pumps tend to leak through the shaft seal, especially if they use the cheaper gland-type packing. If leakage could be a problem for your application then speak to your supplier about expeller seals or mechanical seals which reduce or eliminate leakage. The SlurryPro pump range uses expeller seals as standard, with mechanical seals fitted where the application requires it.
- Although temperature changes can affect the density/specific gravity (and viscosity) of a fluid, and hot fluids can cause overheating, this is only likely to be a significant practical difference if it is extremely out of ambient range. In freezing conditions, keeping the water in motion (with mechanical agitation where necessary) and heated jackets can help keep a pumping system operational.
How To Select The Pump Size
Which size of pump you’ll need depends on both volume and height (head pressure).Volume is usually measured in litres per second/minute/hour or cubic metres per hour depending on the scale of the operation.
We’ve touched earlier on the correlation between head pressure and volume so once you know your capacity requirements, it’s time to consult the pump curve graphs. Whilst there are a number of graphs we can chart for a pump’s performance, the main one we will focus on is the Head/Flow correlation.
Flow / head curves for a selection of Audex pumps:
In the pump curve chart above, we can see that the larger industrial dirty water pumps have a flatter curve; these pumps can transfer significantly higher volumes with less effect on the head height than for the (relatively smaller) 3” outlet models.
When choosing the best pump for your requirements, you need to be aware of the designed duty point, or “best efficiency point” (BEP). This is generally around the centre of the curve; beyond the BEP, a steeper increase of power will be used for a diminishing rise in volume pumped. If a too-powerful pump is chosen, you will need to ensure that the supply pressure remains above the minimum NPSHr of the pump whenever it runs. In practice, this means stopping the pump before the water level gets too low, or restricting the outlet pipe to increase the resistance.
Using the example of a quarry needing to move dirty water 11 metres in elevation, with an extra head pressure of 1m due to system resistance, let’s look at the above pump curve to select possible solutions. We calculate from the water catchment area that we need to move 600 – 900 litres per minute.
Plotting it on the chart, we can see visually that the AW 3-400 (shown as the orange line) would move 650 litres per minute but wouldn’t cope with the higher flow requirement. The AW 3-370 would move 780 litres/min, whilst the AW 4-370 would be a great choice at 920 litres/min. It is important to remember that the different flow rates of the various pump models, will in themselves, increase or decrease the friction loss.
What about the AW 4-600, or the AW 6-900? At the head pressure of 12 metres, the former will pump 1,200 and therefore only be needed to operate on a 50% - 75% duty time which makes it a safe choice. The larger pump however is designed to move 2,500 litres/min at this head, it’s operating on the right of the curve and way beyond its efficiency point. It might be useful for rapid dewatering in an emergency flooding situation but not a good, everyday efficient workhorse for your dirty water processing needs.
Pump engineering can work out specific requirements for the optimum system, factoring in expected variances plus lifetime costs and initial capital expenditure. Considering a new heavy-duty dirty water or slurry pump for your site? Atlantic Pumps technical advisors can provide you with the equipment spec you need to achieve your site's dirty water management requirements, in the most cost-effective way. Contact us today about your pumping job and an experienced pump technician will work with you to provide a proposal that works for you, your team and your site.
Download the industrial dirty water pump selection guide here
