Pumping wastewater is not an easy job for a pump, and working in a wastewater treatment environment is not easy or safe for the operators involved. For pumps, the presence of all manner of foreign materials such as feathers, pieces of hoof, rags, wet wipes, hair and other stringy materials provide a difficult challenge. Other issues that some industrial wastewater pumps need to overcome is hot water from wash-down events and the presence of chemicals used for cleaning. For operators, there are many dangers. Working at heights, working over water, working with heavy swinging weights, and working with cranes just to name a few.
Different styles of pumps are applied to the pumping of industrial wastewater, with the most common being the submersible pump. There are numerous brands and several good quality manufacturers, but all have basically the same set-up. They are suspended [lowered and raised] by rated lifting chains, and are set on guide rails to enable them to engage with a permanently installed discharge bend inside the wet well.
Submersible pumps have been the default choice for several decades because of their lower initial cost, but as the cost of implementing safety systems and controls increase, some may look for more sustainable long-term solutions. Many submersible pumps are also adversely affected by water above 40OC, which may cause issues where high temperature hot water or steam cleaning is used by processing plants.
Standard centrifugal pumps can also be used, but the costly construction of “dry wells” to house them has reduced their use over time.
Self priming centrifugal pumps are being increasingly used by industries around the world as asset owners and engineers look at the increasing cost of maintaining wastewater pump assets and the increasing cost of safety protocols associated with working on and around these assets.
Wastewater pumps and the hierarchy of controls
There are numerous risks that operators face in maintaining pump assets. In a survey conducted by Hydro Innovations, the following risks were highlighted by industry operators:
- Working over water
- Working at heights
- Working with heavy swinging weights
- Working with cranes
- Working in confined spaces
When a manufacturing facility chooses to go with self-priming centrifugal wastewater pumps for their wastewater systems, many, if not all, of these risks are eliminated.
By eliminating these risks, the effectiveness and sustainability of the asset is increased. This in turn, reduces participation and supervision, which in turn naturally has an impact on maintenance costs.
Whenever chokes need to be removed, pump maintenance needs to be done or guide rails or discharge bends need attention in the “conventional” system, the risks require the increased participation and supervision depicted in the above diagram. Those choosing self-priming pumps have eliminated these risks.
While there is increased costs associated with increased participation and supervision, there are “hidden” costs associated with operating “conventional” systems.
What are some of the hidden costs associated with maintaining a conventional (submersible) wastewater pump?
- Power consumption. A submersible pump is losing efficiency from day one, and the only way to get “factory settings” back to worn wear rings is to replace them or adjust clearances. Until then, pumps are not delivering their published efficiency. Getting back to published efficiency requires a submersible pump to be removed from the wet well [with a small crew and a crane]. Until this is done, costs are “leaking” from the system. This is something that is not noticed immediately. Pump cycles will continue to lengthen gradually, over time, wasting energy along the way, until the pump cycles become so long, someone notices.
- Again on power consumption, all submersible pumps rely on the sealing between the pump and the discharge bend. This may be all fine on “Day 1”, but at some point in time, that metal to metal “seal” will wear, and begin leaking. After leaks start, they get larger as more material passes through that point. Also, as pumps get lowered back onto discharge bends, they may not always seat properly [because of foreign matter], again causing leaks.
- All lifting chains for these submersible pumps need to be rated/certified. They need to be inspected or replaced every single year. This may not be a huge cost for one set of chains for one pump, but the cost to replace these chains on multiple pumps every year, over several years, adds up.
- Some production plants have their own lifting apparatus to raise submersible pumps out of wet wells, but others need to rely on 3rd party contractors with crane trucks. This could be a yearly event [or more often if blockages occur], coming at a high cost.
- The number of personnel needed to provide maintenance services on submersible pumps is also a factor often overlooked when assessing how much it will cost to maintain an asset.
- And if a submersible pump needs to be accessed while still in the wet well [stuck on guide rails or broken lifting chain or replacing discharge bends, a very expensive and dangerous confined-spaces entry will be necessary.
Selecting the Right Self-Priming wastewater pump
Self priming wastewater pumps are mounted on the surface at ground level, with only the suction lines entering the wet well. Wet well lids remain closed for all routine maintenance events [such as oil changes, clearance adjustments, valve greasing], and even major overhauls are all done with the wet well lids sealed. All mechanical and electrical equipment is mounted at ground level. This does make them substantially safer to operate than submersible pump systems.
If the decision has been made to install self priming pumps because of safety or maintenance cost savings, selecting the right one is important.
Some considerations effecting the selection include:-
- Suction lift
- Suction line size
- Suction line submergence
- Solids in the fluid
- Chemicals in the fluid
- Temperature of the fluid
- Safety features of the pump
- Maintenance features of the pump
The flow of the pump will be determined by the maximum in-flow. The pump will best be sized to exceed the maximum in-flow so that the pump can empty the contents of the wet well on each pumping cycle. This will ensure that the majority of the silt, solids, stringy materials etc is removed with each cycle to minimise the chances of the solids building up. Designing the wet well with a low point or sloping “sump within a sump”, allows the suction line to be positioned at this point for maximum solids removal.
This is determined by the static distance between the wet well water level and the discharge point, plus adding any friction losses caused by the water flow through the discharge pipe system. The longer the pipe, the higher the pressure. The smaller the pipe diameter, the higher the pressure. A good pump supplier will make recommendations on the optimum pipe diameter.
A good pump supplier will want to know what the static lift is between the pump suction and the planned “on level” [the level the pump is required to start] and the static lift between the pump suction and the planned “off level” [the level the pump is required to stop]. The distance to the on level is important because this will determine whether the pump is running fast enough to prime, and the distance to the off level will allow an “NPSH” calculation to determine whether there is enough atmospheric pressure to support the pump flow at the end of the pump cycle. There are many things that effect the NPSH available. These include the altitude of the application site, the temperature of the fluid, the diameter of the suction pipe, and of course the static lift. A good pump supplier will always do these calculations to ensure the pumps will perform without cavitating.
Suction Line Size
Big is beautiful as far as many are concerned, when applying centrifugal pumps. This is not the case with self-priming centrifugal pumps. A balance has to be struck between having a suction line large enough to keep friction losses down, and not so large that the priming process takes far too long. The pump supplier is the best source for advising the right suction line diameter.
Suction Line Submergence
It is almost always better to empty the contents of a wet well with each pumping cycle. This helps prevent the build-up of silt and solids, which often require intervention via vac trucks or similar if left to build up. To pump the contents as close to the bottom of the wet well as possible, the water level will need to get as close to the bottom of the suction line as possible. This brings the possibility of vortexing into play. Whether or not a system will vortex is a function of suction inlet velocity and suction line submergence. To reduce the suction line submergence, one must increase the suction line diameter by way of a suction bell. Another way of “attacking” this problem is by reducing the flow via a variable frequency drive [VFD]. The reduced flow, reduces velocity, which in turn, reduces the required suction line submergence. A good applier of self-priming pumps will also be able to offer even further ways of accessing the final contents of the wet well.
Solids in the Fluid
The type and size of solids in the fluid will have a huge bearing on the pump selected for an application. There could be large solids, hard gritty solids, or stringy solids such as rags, baling twine, gloves, wet wipes etc.
For large solids, one needs to pay attention to the solids handling capacity of the pump, which should be stated on the pump manufacturer’s data sheet. But one needs to be wary. A self-priming pump needs to not only pass the solids through the impeller, but also be able to pass them through all other passages inside the pump, including the pump’s recirculation port. If a solid will not pass through the recirculation port, the will not be able to prime. Quality manufacturer’s know this and ensure the recirculation port passes the same size solid as the impeller. Many “knock-off” brands will not be able to offer this important feature.
If the solids are grit, which could be generated by animals eating to close to the ground, or by something in the manufacturing process [possibly cleaning], that generates small, hard particles, these have the potential of prematurely wearing pump parts prematurely. It is in these cases that specifying a pump with hardened impellers, wear plates and seal plates will greatly lengthen the life of these parts and the length of time between major servicing events.
If the solids are stringy materials, these can be a maintenance nightmare. Stringy materials can wrap around an impeller, turning it into a baseball mitt, capturing all other solids that enter the pump. Gorman-Rupp has addressed this issue with the patented “eradicator solids management system”. This system incorporates many features to combat the presence of rags etc in the wastewater and significantly reduces or eliminates the time-consuming downtime associated with blockage removal.
Chemicals in the Fluid
If chemicals from processes or cleaning can infiltrate the wastewater system, this can cause pumps or pump components to corrode prematurely. The best way to approach this, is to use corrosion resistant materials like 316 grade stainless steel, or duplex stainless steels like CD4MCu.
These are not inexpensive options, but can substantially add to the life of a pump asset.
Another way to approach some corrosive applications is to provide cathodic protection for the pumps. Hydro Innovations does this by providing cast iron pumps with 316SS internals [impeller, shaft, wear plate and seal plate], but also fitting a replaceable sacrificial zinc anode to the pump casing. The anode is positioned away from high flow areas, but where it is always in contact with the pump materials and the corrosive fluid. The zinc rod takes the corrosive “heat” away from the cast iron components of the pump, greatly reducing corrosion to these parts.
Temperature of the Fluid
High temperature wastewater can negatively effect seals and elastomers in pumps, but fluid temperature also effects the available suction lift. For example, if wastewater is 40OC it will effect the available suction lift by about 0.8m, but if it is 70OC, it will effect the available suction lift by over 3 metres. A good pump supplier will do a thorough NPSH calculation, factoring the vapour pressure of the high temperature liquid into the equation.
Safety Features of the Pump
If you have selected a self priming pump for safety reasons, make sure your self priming pump offers its operators the right level of protection. The better self priming pumps will have some or all of the following safety features:-
- A priming fill port retained by a clamp bar with a finely threaded screw, and the cover plate sealed with a Teflon gasket. This is important because if operators attempt to open the port while the pump is pressurised, pressure is released horizontally, not vertically. If released vertically, corrosive, hot or toxic fluid could shower down on operators.
- The better self priming pumps will have a shielded fill port cover to direct unexpected water pressure towards the ground. See below images.
- A good quality pressure relief valve to vent excessive pressure from the casing if the pump is accidentally operated against a closed head [and your pump supplier should know the pressure these valves will start to “vent”].
- A non-return suction flap valve with a safety burst disc centre to vent excessive pressure in case of a water hammer spike.
- The pump should have external adjustment of the internal impeller/wear plate clearances, and for convenience and safety, operators should be able to do this from the non-drive side of the pump.
The pump should also be equipped with maintenance features that reduce the cost of ownership for the life of the installation.
It is very easy to think that all self priming pumps will have the same features, because they look the same, have the same shape, and are marketed as “equivalent”. But appearances can be very deceiving. A good quality self priming wastewater pump will have some or all of the following maintenance features:-
- Good self-priming pumps will have shim-less adjustable clearances. The better ones will have their adjustment systems on the suction side of the pump [away from the drive end], to make it easier and safer for operators. The best ones will have their clearance settings easy to index and lockable so that they remain in place during other maintenance events.
- Good self priming pumps will have at least 3mm of wear built into the impeller/ wear plate clearance system. The best ones will have up to 6mm of wear built into their clearance adjustment system.
- A good self priming industrial wastewater pump will have a replaceable suction flap valve that can be accessed for cleaning and/or replacing without the need for disconnecting suction piping. The best ones will enable access to the flap valve without the need to open the pump. See below images.
- Good quality self priming pumps will be able to pass solids [in sizes as advertised on curves and data sheets] through all internal passages inside the pump. Not only the impeller, but advertised solids size spheres should be able to pass through the recirculation port, discharge nozzle and all other passages. Quality manufacturers can guarantee this and even have certified drawings to support this claim.
- The better self priming pumps will have pusher-bolt capability on both the cover-plate and rotating assembly to make it easier for operators to remove these items for service even if they’ve been operating for “years” without being removed. See below images.
- Non-clogging/blocking ability is a very important feature if operator intervention is to be kept to a minimum. Besides having through-let sizes big enough to pass published sphere sizes, pumps need to be able to handle stringy materials such as rags. Pumps with standard wear plates will not measure up in this area. Only select pumps with “self-cleaning” wear plates or with specialist rag handling systems like Gorman-Rupp’s “eradicator solids management system™”.
- Pumps that have features that protect the pump as it operates year in-year out will deliver the desired asset longevity. Features like a double lip seal and atmospheric vent protecting the bearings and shaft will add years to the replacement intervals for these parts. Anti-rotation ribs in the seal area will also add to mechanical seal life by slowing velocities down within the seal area.
- “After Market” performance is also a measure of a good supplier. Parts and replacement pumps should be readily available for ex-stock delivery. An unsupported pump is a poor investment.