ABSTRACT

Positive displacement piston pump produces pulsatile flow output with significant null period in between each stroke changeover which could be visually witnessed at outlet. The adaptation of pulsatile flow producing positive displacement pumps to deep sea slurry pumping over 1000-6000m vertical head is discussed in this research paper. Clustering of three positive displacement pumps together with a fourth charge pump to nullify the effect of cumulative null period at the three pumps outlet is discussed in this research work and the effects are discussed. The fourth charge pump could possibly reduce the intermittent breaks in the flow and proved continuous outflow at the outlet which is vital for deep sea mining.

INTRODUCTION

Polymetallic nodules of size 20-80m diameter are abuntantly available on deep sea beds over depths of 3000-6000m. The collection and transportation of these nodules requires state-of-the-art engineering to survive at low temperature and elevated pressure ambience. The underwater vehicles collect the nodules either by mechanical or hydraulic collection method and transport it to the main pumping station from whence it is pumped to surface ship over greater vertical head. The transportation of these nodules to surface ship was done through airlift and multistage centrifugal pumps by few countries in 20th century. Airlift method relatively fall short in efficiency for smaller depths. The efficiency of the airlift system is increasing with increase in depth & diameter of the riser (Shulte,2013). Larger the diameter, smaller the velocity as per equation of continuity. To counteract this, higher flow rate needs to be produced to maintain velocity. This will chock the neck of power supply. On the other hand, the centrifugal pump, being hydrodynamic, requires multi stages to perform pumping to such greater depths which again adds weight to the riser column as it must hold the pumps clamped to it and also the power cables meant to run the pump. Mean while, vertical slurry pumping on land have gone to an unimaginable extend so much so that tall concrete buildings, including the tallest building in the world, as of now, the Burj Khalifa at Dubai have made use of it. Positive displacement pumps have been widely used in vertical concrete slurry pumping. Burj Khalifa, extensively used three Positive displacement pumps stationed parallel on ground for pumping up concrete to 600 m which became a world record vertical slurry pumping on 8th November 2007 (Alred, 2010). But, the advent of subsea vertical pumping ways back to few decades when engineers started mining deep sea polymetallic nodules. This paper is an outcome of research work in deep sea slurry pumping by positive displacement pumps and describes the possibility and expected pressure required for pumping when PD pumps are clustered together.

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