Hydraulic installations
Hydraulic accessories
22
Stepper motor-driven diaphragm dosing pump NXP-M and NXP-P
Operating instructions
8.7.2 Contact-type water meter
The contact-type water meter measures the throughflow in a pipe and
sends a pulse to the dosing pump, which then starts dosing. This means
that ideal proportional dosing is also possible with large throughflow
fluctuations.
The contact water meter is connected to connection port 2 (see chapter
9.2.2 “Connection socket 2“ on page 25).
The ratio of throughflow to executed strokes of the dosing pump is
determined in "Pulse input" mode (see chapter "Water meter" on page
24).
8.7.3 Pressure-relief valve
Pressure relief valves have an important safety function for protecting the
dosing pump and the associated pipes and fittings. The dosing pump can
generate a pressure that is many times the rated one. A blocked pressure
line can lead to dosing medium escaping.
An improperly high pressure can occur if:
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the shut-off valves are closed even though the dosing pump is
running,
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pipes block.
At an appropriate pressure, a pressure relief valve opens a bypass line
and protects the system in this way from damage caused by over-pres-
sure.
Notes on assembly:
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The line for returning dosing medium from the pressure relief valve
must be routed to the dosing tank or to a collecting pan.
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The pressure in the dosing tank must not be too high so that it is
possible to accommodate the returned dosing medium.
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As an alternative, the system can return dosing medium into the
suction line in front of the dosing pump. In this case, there must not be
a non-return valve or a foot valve in the suction line.
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You should install the pressure relief valve as close as possible to the
dosing head.
8.7.4 Back-pressure valve
Back-pressure control valves are necessary if:
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There are considerably fluctuating system pressures,
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the pressure on the suction side is higher than on the discharge side
or if you intend to carry out dosing into depressurized lines.
In cases like this, if you do not use a back-pressure valve, imprecise
dosing results will occur or overloading will result. The back-pressure
valve solves these problems by generating a defined, constant backpres-
sure.
In some circumstances, a back-pressure valve is unnecessary if you use
a hose injection nozzle and if the backpressure that it generates is
adequate.
8.7.5 Pulsation dampener
Pulsation dampeners have the following functions:
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Damping pulsating delivery flows for processes that require
low-pulsation dosing,
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reducing the throughflow resistance with long pipelines.
When installed on the suction side:
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Damping of acceleration mass forces and with this reduction of wear
on the dosing pump.
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Preventing cavitation (pull-off of the liquid column) due to too high
acceleration.
However, pulsation dampeners also have important safety functions,
since they prevent pressure peaks from arising that lead to piping
vibrating and cause them to snap.
This problem can occur:
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With the high amplitudes of the vibrations,
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when using long pipes (the severity of the pulsation increases with the
length of the pipe),
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when using rigid piping instead of elastic hoses.
Notes on assembly:
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You should carry out assembly in the direct vicinity of the location
where you want to damp the pressure peaks (directly in front of the
suction valve or directly behind the discharge valve).
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Pulsation dampeners should be installed with throttle valves or
back-pressure valves installed directly behind them. By setting the
valves appropriately, you can further-optimise damping of the
pulsations.
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To prevent unnecessary pipe friction losses, you should lay the
connecting line straight and in accordance with the rated width of the
pulsation dampener.
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You must separately fasten relatively large pulsation dampeners and
ones with hose connections.
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Pipelines must not transfer any mechanical tensions onto the
pulsation dampener.
8.7.6 Priming aid
Priming aids are particularly advisable:
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In the case of dosing pumps with small volumetric displacements per
stroke or with low stroke length settings,
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with high suction hights,
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with highly dense dosing media,
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at priming for the first time due to dry valves and air in the suction line
and the dosing head,
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in dosing systems with frequent downtimes.
Further advantages resulting from priming aids:
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Preventing cavitation in the suction line,
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gas removal,
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optical dosing control with small amounts,
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smoothing of the suction flow.
8.7.7 Level monitoring
Only for NXP-P: Level monitoring of suction-side feeding of the dosing
medium to prevent the tank being sucked dry and to ensure that it can be
topped up again in good time.