Since steam-powered pumps were introduced to mine dewatering in the mid-18th Century, the pumphouse has been the heart of the operation. However, this staple is quickly being replaced by innovative floating pump stations, overcoming the fundamental flaw at the heart of a static pump house: its variable Net Positive Suction Head (NPSH).
NPSH is the pressure available in a pumping system, and when this drops below a certain level the pump operation will come to a grinding halt – potentially causing significant damage in addition to halting dewatering.
What makes this difficult to manage is that several factors influence NPSH, including the distance of the water’s surface to the pump – which varies significantly throughout dewatering. Managing this usually involves capital-intensive civil work to allow you to position your pumps below the water level using vertical retaining walls.
Thankfully, pontoon-mounted pumps solve this problem entirely by floating on the water’s surface and thus maintaining a constant NPSH across their lifetime through the carefully designed draft (depth within the water) of the pontoon system. Our engineered-to-order Acqua Flow series pump pontoons, are purpose-built to meet the requirements of each application, ensuring that your pump will always be able to meet its duty.
What is NPSHA vs NPSHR?
The available net positive suction head available(NPSHA), is the static pressure you have available to the system to force the water into the pump.
The available net positive suction head required (NPSHR), is the minimum amount of energy the pump requires the entry water to possess to allow the pump to meet the required duty point. If this NPSHR is greater than the NPSHA the pump will begin to cavitate (NPSHR>NPSHA).
This can be affected by several variables, such as the static differentiation between the pump’s suction and the water level, the length of the suction pipeline, the pump’s elevation above sea level and the flow rate of the fluid through the suction pipeline.
Every pump has a minimum NPSHR requirement to ensure continuous flow, which depends on its operating duty point. A pump is capable of delivering any duty point within its curve, however all the fluid it is pushing out has to be replaced with fluid coming in, i.e. the flow rate entering has to equal the flow rate exiting the pump. If the pump is pushing out fluid at a greater rate than it can get the fluid replenished, we begin to “starve” the pump, leading to cavitation. This is where the suction side creates a lower pressure area (usually on the eye of the impeller in horizontal end suction pumps), leading the entering fluid to partly vaporise. Fundamentally, this manifests as small explosions on the surface of the pump’s impeller, damaging the metal. The extent of this effect depends on the flow rate required of the pump.
The challenge of maintaining NPSHA in static pump houses
Conventional pump houses are fixed in place and usually, for design purposes, constructed above the highest design water level. Because of this, when the water level fluctuates it both drops and moves away from the shoreline – forcing operators to introduce additional suction line length, which introduces more friction, as well as increasing the pump’s static height.
Both of these reduce the NPSHA, putting the entire pumping system at risk of failure.
Acqua Flow floating platforms maintain constant NPSHA throughout dewatering
To solve this, it’s becoming increasingly popular to simply float the entire pumping platform on the water’s surface and sidestep the challenge altogether and avoid doing the civil construction required in conventional pump houses.
Acqua Flow series pump pontoons are designed to have a specific draft (depth within the water), ensuring that the available NPSH is always constant, and the suction of the pump is always flooded. With the pontoons free to float on the water and ensure a constant NPSH, pumps can maintain better, more consistent performance than their land-based counterparts.
Acqua Flow pontoons are also mobile, so when a tailings facility has come to the end of its lifespan, all the pumping infrastructure can be relocated to another dam.
Finally, Acqua Flow pontoon systems can be combined with a wide variety of access options, including floating and pivoting walkways which allow for safe access when conducting inspections and performing routine maintenance.
Reducing the installation and operating costs of your pumping system
Changing reservoir levels don’t just impact wear – they can result in significant costs if pumps need to be relocated. While some applications will maintain a static water level, in an active mining operation water levels rise and fall regularly. Even portable land-based pumping systems are ultimately restricted by their maximum suction lift (NPSH), as reservoirs can easily rise or fall outside their operating parameters – leading to additional costs as the pumps are relocated. The cost of this relocation is influenced by some factors, including the size of the pump, the hourly cost of its flow being interrupted, equipment and labour involved and potentially the cost of installing a backup unit to maintain operations during the move.
These costs, alongside the maintenance headache of cavitation, can both be easily avoided through the use of a Pontoon Pump Barge, which simply rises and falls with the reservoir level itself.
