As submersible pump technology has improved and become accepted, it has become more commonly used in
different applications. Nowadays, it has a small footprint and is cost effective. Additionally, suction head is not a
problem for submersible pumps. That said, some operators/owners dislike submersible pumps for different reasons, one example being that maintenance requires hoisting the pump. Submersible pumps often require factory repair because of specialized components and stringent sealing requirements.
There have been many different types, models and configurations of submersible pumps. Submersible pumps might be arranged for seating on a discharge elbow and be comprised of a duck-foot type elbow to provide support for the pump in
its operating position, as well as a seal face to allow the pump to be lifted off guide rails vertically for inspection and maintenance. The casing of the pump should be configured to suit the pump, seating arrangement and service.
Typically, the pump casing is arranged to move up and down in guide rails, and there are provisions for sealing the discharge to the discharge piping.
Great attention must be paid to the lifting systems, as there are many problems and issues reported for such systems. Proper stainless-steel or alloy-steel lifting chains are needed for the installation and removal of submersible pumps. The guide rails should ideally be round or circular sections properly sized for the application. Rectangular hollow section and square hollow section guide rails are not desired.
The guide rails should preferably be twin circular steel section and arranged to prevent binding of the pump while it is being raised or lowered. Additionally, it should be secured with appropriate brackets in such a manner as to allow complete removal of the pump without dismantling either the pump or the guide rail system. Many reliable lifting systems have been entirely fabricated from suitable grades of stainless steel.
Electrical cables should be flexible, heavy-duty, insulated, sheathed and designed specifically for continuous operation while submerged in the liquid. Generally, there is no access to the submersible pumps during operation for monitoring and inspection. As a result, a seal failure detection/probe should be fitted to all submersible pumps.
Open-Channel Flow of Liquids
Open-channel lines and networks have been widely used for applications such as process liquids, semi-finished products, different solutions, water, cooling water, wastewater, slurries, surface water management, drainage systems, sewers and more.
In fact, piping should ideally be avoided if there is a risk of blockage. For instance, liquid containing potentially pipe-blocking materials, such as rags, greases, many slurries, etc., should not be transferred in pipes. Open-channels are better options.
Although there have been textbooks and papers on the theory of open-channel fluid mechanics, there has been little published on the practical management of open-channel flows, such as on involved equipment and hardware, for instance, submersible pumps.
Wet Wells for Pumps
Determining wet well configuration and details can be complex. The wet well may seem like a simple tub for holding liquid, but poor design/sizing can cause problems for operators and damage to pumps. Considerations that influence wet well sizing and design include minimizing different risks, eliminating air/gas entrainment and avoiding solids deposition and scum entrapment.
The elevation of the maximum liquid surface is usually below the invert of the lowest incoming liquid. However, care should be taken when large fluctuations in flow occur, such as during different emergency events. Additional storage capacity is always desired, and the level of liquid should never exceed the elevation of the lowest connection.
The floor elevation and dimensions of the wet well are typically determined by site constraints and volume requirements. The minimum depth of the liquid should be sufficiently above the pump intake to avoid the vortices and other problems. Submersible pumps should also maintain sufficient submergence to provide cooling. The height of the pump intake above the floor should be high enough to avoid restrictions but low enough to minimize operational problems such as solids deposition (if applicable).
It is usually considered good practice to split the wet well in half and have pump inlets in each (or one pump in each). This allows for draining one half of the wet well for cleaning and maintenance without taking the pumping station out of service. Some pump suppliers/vendors have extensive experience and detailed design recommendations for wet well and pump station configuration. They should be consulted for guidance.
Gravity-Flow vs. Pump Stations
Some open-channel networks rely solely on gravity, but in certain cases, pumping stations are needed. Pumping stations usually form a major element of open-channel services and are required to lift liquid to a higher level to assist normal gravity-flow toward the destination. Pumping stations are usually located at low points of the open-channel system and are often designed as a safety element to avoid overflows occurring.
Overflows and spills are major risks of open-channel networks, and having pumping stations at low points is beneficial in managing this risk. Operators seek to reduce or eliminate overflows and, when they do occur, to manage the event to reduce risks of damages and operational problems.
Open-channel systems large enough to contain the entire potential liquid flow at emergencies are prohibitively expensive, particularly when uncertainties are involved, so overflow management is a necessary feature of many networks. Pumping stations that act as a safe overflow point in open-channel networks require careful design and management because of the potential risks from spills.