Main oil pump #1
Main oil pump #2
Hydraulic cylinders
Oil tank
Hydraulic concept of pulsation free DHT
controlled air vessel. These alternatives
operate in a similar way to the pulsation
dampener and although they have some other
advantages/disadvantages they will not be
further described in this paper.
Pulsation free DHC (VZ system)
The most effective way of dealing with the
phenomenon of pressure pulsations is to avoid
generating them. This is done by creating a
discharge flow that is continuous despite the
reciprocating motion of the pistons. With a two
cylinder pump this can be achieved if both
pistons are driven independently of each other.
Using this concept, the pump is divided into
two single-cylinder pumps each driven by its
own hydraulic system. The individual systems
are controlled by a PLC that controls the
respective piston speed profiles so that the
total net discharge flow is constant, resulting
in a constant discharge pressure.
The speed profile of the individual pistons
can only be controlled if their respective
positions and speeds are monitored
continuously during the complete length of the
stroke. This is achieved with a linear
transducer system that is integrated in the
hydraulic side of each cylinder. The timing of
the piston speeds and the corresponding
opening and closing of the valves is critical for
the correct functioning of the system. As
described above, the VZ system requires both
the cylinders to be driven independently from
each other; each cylinder is driven by its own
Paste
cylinders
dedicated oil pump. Note that depending on
the total required power, the two main oil
pumps can be driven by the same electric
motor.
The VZ system with its pre-compression of
the paste in combination with the timed
takeover of the discharge stroke only works on
a piston pump with hydraulically actuated
valves. In regards to a swing tube pump pre-
compression is not possible.
Pulsation free DHT
In the case of a traditional swing tube pump,
there is no pre-compression of the suction
cylinder because the swing tube is not
connected to the suction cylinder. Therefore,
the flow compensation of the pump has to be
done differently. Furthermore, the power drawn
from the main HPU motors is also subject to
these fluctuations. This can result in a higher
motor power and motor control centre (MCC)
rating, increasing the system investment costs.
Possibly, the (existing) power supply system
cannot handle these electrical power demand
fluctuations.
A third cylinder has to be introduced to
compensate for the flow variation during the
switch-over. However, rather than allowing the
cylinder to discharge a certain volume into the
main pipeline over a predetermined time
period, with limited beneficial results, the
following unique working principle has been
applied: during each discharge stroke of one of
the main cylinders, an additional flow above
the nominal slurry flow is delivered. This
additional flow is fed into an additional third
Min
100%
0
2
4
8
6
time(s)
P12 International Mining | MARCH 2018 Supplement
10
12
0%
14
Max
cylinder 1
cylinder 2
cylinder 3
power
consumption
Schematic concept of pulsation free DHT
cylinder. The hydraulic flow from this third
cylinder is transferred back to a hydraulic
pump, and mounted on the same motor shaft
as the main hydraulic pumps. The additional
energy required to move the third cylinder is
thus fed back to the same motor shaft, apart
from minor power losses resulting from piston
friction and efficiency losses. During switch-
over, the third cylinder discharges into the
main pipeline allowing the pump to operate at
nominal flow. The third cylinder starts to ramp
up when the discharge cylinder is ramping
down to generate a smooth take over from one
cylinder to another. Due to the constant flow,
pressure fluctuations in the pump discharge
line will be reduced to a minimum.
As a result of the energy recovery of the
third cylinder during its fill stroke, fluctuations
in HPU power consumption are minimised to
the level of friction and efficiency of the
hydraulics for the third cylinder. A slurry non-
return valve is installed in the slurry line
downstream of the pump to prevent back flow
from the main pipeline and the third cylinder to
the pump.
For a recent project a pulsation free DHT
pump was built and commissioned. The pump
design flow is 200 m 3 /h against a design
pressure of 120 bar. The pulsation free DHT
concept allowed the system to have the full
pressure available for continuous pumping and
required no excess pressure capacity for
acceleration losses. It was understood that for
the same project this could be estimated as
being 50-100% of the average operating
pressure in case traditional DHT technology
would be used. The pulsation free DHT
technology has many benefits including:
n reduced power requirements in the range of
25%, allowing for additional operational
design margins
n smaller sized PD pumps and related
investments costs in the range of 10-15%
n reduced costs for pipeline and pipeline
supports in the range of 10-15%
Conclusion
While with DHC hydraulic valve style pumps a
constant flow/pulsation free option has been
available for years, the same fully integrated
concept is now also available for DHT pumps.
Through the introduction of a third cylinder,
combined with energy recovery, this concept can
now be applied for DHT swing tube pumps.
Pressure pulsations are minimised, power
requirements are reduced by 25% and investment
costs in the PD pump system by 10%. The concept
is patent pending and offers a unique solution to
customers in the mining industry.
*The full Pulsation free DHT white paper will
be presented at Paste 2018