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the suction line so that liquid can be maintained above the reservoir level. Remove the suction
valve assembly, fill the line, replace the valve, then remove the discharge valve assembly and
fill the reagent head as described in the third step above. The pump will now self-prime when
started up per step four above.
7.5 Calibration
All metering pumps must be calibrated to accurately specify stroke length settings for required flow
rates.
A typical calibration charge is shown in figure 24. Although
output is linear with respect to stroke length setting, an
increase in discharge pressure decreases output uniformly,
describing a series of parallel lines, one for each pressure
(only two are shown).
The theoretical output flow rate at atmospheric output
pressure is based on the displacement of the hydraulic
piston (the product of piston cross-sectional area and
stroke length) and the stroking rate of the pump. With
increasing the discharge pressure there is a corresponding
decrease in output flow of approximately 1% per 100 PSI
(7 BAR) increase in output pressure. Whenever possible,
calibration should be performed under actual process
conditions (i.e., the same or a similar process liquid at
system operating pressure).
To assure a sound hydraulic system, run the pump for ten to fifteen minutes prior to calibration. This will
allow the PTP valve to purge any air from the system.
To construct a calibration chart, measure the flow rate several times at three or more stroke settings
(i.e., 25, 50, 75, and 100), plot these values on linear graph paper, and draw a best fit line through the
points. For stable conditions, this line should predict settings to attain required outputs. See figure 25
for example of a four-point test plotted onto a calibration chart.
Checking the actual flow rates is especially important in pumps producing low flow rates and operating
against high discharge pressures. In this type of system, normal losses of efficiency can result in lack of
measurable flow at shorter piston stroke lengths. This is a function of the system conditions and does
not indicate a problem with the pump. Careful measurement of actual pump flow at several test points
will allow for proper calibration over the complete flow range.
Figure 25
Figure 24
