Raw Sewage Pumping: How To Select The Right Sewage Pump
Pumping raw sewage is not an easy job for a sewage pump, and working in the sewage pumping environment is not easy or safe for the operators involved. For pumps, the presence of all manner of foreign materials such as cans, bottles, rags wet wipes, hair and other stringy materials provide a difficult challenge. 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 raw sewage, 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.
This style of sewage pump has been the industry standard for several decades, but as the cost of implementing safety systems and controls increase, some may look for more sustainable long term solutions.
Standard centrifugal pumps can also be used, but these tend to only be in the very large stations.
Self priming centrifugal pumps are being increasingly used in Europe and USA as asset planners look at the increasing cost of maintaining sewage pump station assets and the increasing cost of safety associated with working on and around these assets.
Sewage Pump Stations and the Hierarchy of Controls
There are numerous risks that operators face in maintaining sewage pump station assets. In a survey conducted by Hydro Innovations, the following risks were highlighted by water industry operators:
- Working over water
- Working at heights
- Working with heavy swinging weights
- Working with cranes
- Slips and falls in damp valve vaults
- Working in confined spaces
When a Water Authority elects to go with self-priming centrifugal sewage pumps for their wastewater pump stations, 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 he “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 sewage pump station?
- Power consumption is one. A sewage 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 the sewage 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 not usually picked up by Net Present Value [NPV] calculations.
- 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. Again, this is never factored into NPV calculations.
- All lifting chains for these submersible pumps need to be rated/certified. They need to be inspected or replaced every single year. This may appear in a general tools ledger or civil maintenance costs, but again, would probably not get factored into an NPV analysis.
- Some Councils or Water Authorities require a permanent personnel anchor point to be installed adjacent to sewage pump station wet wells. These are part and parcel of ownership of the conventional pump station, but costs associated with these, along with harnesses, winches etc are not often [if ever] factored into calculations when comparing alternative pump technologies [like comparing them to self priming pumps that don’t require anywhere near the same level of access to the wet well].
- Ownership of vehicles fitted with lifting apparatus is also part of the ownership cost of operating submersible pump stations. Again, when comparing other technologies, this cost burden is almost always overlooked.
- The number of personnel to provide maintenance services on sewage pump stations is also a factor often overlooked.
- The cost of carrying support stock is also a large burden for submersible pump station asset owners. These pumps have narrow performance envelopes [when compared to self priming pumps], and can require 5-10 times more stock holdings to support them.
Selecting the Right Self-Priming Sewage Pump
Self priming sewage 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 conventional 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.
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.
- The better self priming pumps will have a shielded fill port cover.
- 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 sewage 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 sewage 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.
- 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.
- 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™”.
Having the right safety and maintenance features is extremely important, but pumps need to be able to deliver the required service and have the required after market support.
Delivering on Performance
Safety and maintenance features are very important, but so is delivering the right flow and pressure for a given application. The pump also needs to have a Net Positive Suction Head Required [NPSHr] that is lower than the NPSH available. These are all aspects of the pump selection that need to be considered by the pump supplier. The chosen sewage pump manufacturer’s literature in this area therefore needs to be trusted. The market leaders in self priming pump technology have years of R&D put into the development of pump curves and NPSHr curves, so can be trusted to provide pumps that will deliver published performance. Getting references from other Water Authorities or Councils is a good approach in determining those brands that can be trusted to deliver on performance.
A long service life is also an important factor in determining the right pump. Pumps that are safe and easy to work on are an excellent start. 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 sewage pump is a poor investment.