Pulsation free DHT
Erik Vlot and Ron Keijers of Weir Minerals Netherlands review the new GEHO ® DHT pump constant flow / pulsation free option *
Ahydraulic driven piston pump is typically a two cylinder, single acting, positive displacement pump, consisting of a pump unit and a hydraulic power unit( HPU). The HPU drives one or more hydraulic pistons that are connected to the slurry pistons. When a piston moves backwards( suction stroke), the cylinder space in front of the slurry piston is filled with slurry. When the piston moves in the opposite direction( discharge stroke), the slurry is discharged from the cylinder into the main slurry line.
The pump cylinders can either be connected to a valve box( DHC) or a swing tube box( DHT), also referred to as the pump liquid end. These pump types are utilised for handling slurries that are highly viscous. A valve operated pump allows for smaller particles to be pumped compared to a swing tube pump. In many paste backfill installations, DHC type pumps are used when particles are no larger than 10 mm. When dealing with larger particles, up to 1”, DHT style pumps need to be used as larger particles can get jammed between the cone valve and seat.
All pump operations are sequence controlled by a Programmable Logic Controller( PLC).
Most pumps are equipped with two slurry cylinders, where one is in discharge mode and the other in suction mode. For a“ standard” pump the two hydraulic cylinders are interconnected ensuring that the two pistons always move in opposite directions at the same speed: when one cylinder is performing a suction stroke, the other cylinder is performing a discharge stroke at exactly the same speed. Typically the pump is driven by a hydraulic power pack containing one or more main oil pumps. Alternatively, the flow can be reversed or controlled by the hydraulic pump( s), so the two cylinders are operating independently.
Due to the reciprocating motion of the pistons the discharge flow is discontinuous. During the discharge, as well as the suction stroke, the flow is proportional to the piston speed. At the reversal point of the pistons, the piston speed and resulting discharge flow is zero. During the standstill of the pistons the valves or swing tube switch from suction to discharge mode( and vice versa for the second cylinder). This is called the“ switch-over time”.
These flow direction changes cause significant pressure spikes in the system. This typical hydraulic pump phenomenon had to be dealt with and addressed in the system design.
Reducing flow and pressure pulsations
A discontinuous flow results in pressure pulsations. The effect of the pressure pulsations on the piping depends on the design of the pipeline system downstream of the pump and is influenced by a number of factors including the pipe length, diameter, and bends. Pressure pulsations are the result of a change in flow; every start and stop of slurry in the discharge pipeline causes pressure pulsations. Pressure pulsations can create enormous forces on the pipeline and the pumping system in general.
Therefore, pressure pulsations are an undesired phenomenon in pumping installations for a number of reasons: n Possible damage to the pipeline system: typically high pulsation levels result in high fluctuating forces on the pipeline system. If the design does not account for the pulsations, parts of the system can get damaged. Sheared-off pipeline supports are the most common and most visible result of this. However inline equipment such as control and isolation valves or measuring equipment can also get damaged. n There is also the potential risk of injury if equipment fails, for example when pipe supports shear-off. n High noise levels and safety risk: the noise level combined with the related frequency spectrum can be very disturbing for personnel and possibly exceed acceptable limits. n Unplanned interruptions of production: if a piece of equipment or instrumentation is damaged by hammering of the discharge pipeline, for example a flow meter, a density meter or the rupture of a pipe, the pumping system must be shut down for repair Besides the additional, unplanned, repair costs involved, there are the costs of production downtime which can be substantial in loss of minerals mining production. In relation to pumping cemented paste for mine backfill applications, the pipeline can fill with hardening paste causing further problems.
A way to reduce pressure pulsations is to use a pulsation dampener in the discharge pipeline.
A pulsation dampener is a vessel that is prepressurised with gas( usually nitrogen) and has a rubber diaphragm that separates the gas from the slurry. The dampener is placed in the discharge line close to the pump for optimal performance.
The gas is pre-compressed to a certain value that is determined by the operating pressure of the system. Any sudden change in the pump flow will be compensated by the dampener, consequently reducing the pulsation. The level of dampening depends mainly on the size of the dampener.
Besides the fact that the pulsations are only reduced in part, a dampener only works in a specific operating range. Any deviation from the operating pressure affects the correct functioning of the damper. Alternatively, a controlled pulsation dampener can be used.
When pumping a cemented paste for mine backfill, a dampener cannot be used because the hardening slurry will block the dampener over time and stop functioning as required.
In some cases, other equipment can be used for pulsation dampening such as a level
Typical cross section of valve pump
Typical cross section of swing tube pump
MARCH 2018 Supplement | International Mining P11