Taking a general application where an additive is to
be dosed from its bulk container or IBC into a process continuously and without
pulsation. No automatic control is required as manual stroke adjustment is
sufficient to attain the level of dosage required. Accurate calibration of the
dosage rate is however a pre-requisite and pump protection is required to
protect against pump overload should the main process plant shut down
unexpectedly and the dosing pump continue to run.
Incidentally diaphragm pumps can run dry but will
self-destruct of they pump against a closed head so fit a pressure relief valve.
Details of the additive have been received and the pump materials
selected in this case pump head and valves are Polypropylene with Viton
0-rings and PTFE-faced diaphragm. Capacity has been sized for roughly
70% normal operation and because of the size of the pump an external
pressure relief valve has been specified for pump protection as |
Installation with (external) relief valve
|
integral
pressure relief is not available on a pump of this size.
Pressure relief is set for 3 bar and pump
delivery head under normal process conditions is calculated to be close
to 2.5 bar. The supply tank is checked to ensure it is covered and not
subject to dirt ingress and that the outlet to the dosing pump is above
the floor of the tank [+-100mm] to avoid any sludge or sediment being
drawn into the pump suction. After an isolation valve fitted to the
tank, which should also have a drain valve, a dirt trap should be
considered if the solution is prone to sedimentation then install the
calibration cylinder close to the pump inlet or suction valve.
This is fitted with an isolation valve and
for calibration purposes it is filled by opening this valve and allowing
the level to rise under head pressure from the supply tank, until
sufficient fluid is in the cylinder. The supply tank is now isolated and
the dosing pump started and allowed to dose drawing out of the
calibration cylinder until a predetermined time has lapsed. This is then
checked against the dosage taken up and used to adjust the pump manual
stroke adjustment until the required capacity or dosage rate is achieved. |
Installation of injection valve |
A pulsation damper is installed in the dosing pump output or delivery
line and this will smooth the pump delivery to ensure a continuous
accurate flow. At the point of addition into the main process a non-return
valve is fitted to prevent back flow from the main line.
“What
happens if I want to dose into an open tank and it is positioned below
the additive tank and dosing pump?”
Dosing pumps are designed with a two stage action [see Note Book No.1]
and require internal pressure to get the intake and output valves to
operate. If the point of delivery is lower than the supply side the
fluid being dosed will siphon through the pump and the pump becomes
inaccurate.
To prevent this or excess delivery it is
necessary to provide a back pressure on the output line. This can be
done by repositioning the supply tank and dosing pump to below the point
of delivery.
|
Installation of a pressure keeping valve
|
Alternatively, and more commonly, a pressure
keeping or pressure retaining valve is installed in the output or
delivery line usually set at a nominal relief pressure [1.5 bar] and in
both cases this provides sufficient head on the pump output or delivery
to allow the input and output valves to operate correctly. Later on we
will discuss “valve slippage” and how this affects valve design.
“What
steps should be taken to protect the dosing pump because of it’s
positive displacement design?”
Being positive displacement dosing pumps
must be protected from damage due to over pressure as a result of
blockage in the delivery line or the closure of a down stream valve,
etc. This is best achieved by fitting a pressure relief valve into the
system and this can be either integral in the dosing pump head or as a
separate device. Smaller capacity dosing pumps are usually fitted with
integral pressure relief valves and larger capacities tend to use
separate pressure relief valves although there is no hard and fast rule. |
Installation of a pressure relief valve
|
The important issue when installing and piping a pressure relief valve
is to ensure that it is laid with a gradient that will allow it to drain
into an open storage tank or discharge outlet. Alternatively it may be
piped into the dosing pump suction line.
"How
do I prevent fluid from the main line from entering the dosing line?”
When dosing into a main line with an
elevated process pressure it is important to prevent the main line fluid
from entering the dosing line on each pulse as undesirable mixing will
take place in the dosing line and could result in early failure of the
system. This is overcome by fitting an injection valve or non-return
valve at the dosage point. |
Installation of injection valve |
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