Capacity variation via stroke length adjustment (either manual or via electrical actuator) allows metering pump to keep the same strokes per minute, regardless of adjustment percentage being set to 0 or 100%……….. while using a variable speed drive motor, strokes/minute may vary from over 100 to less than 10, dramatically decreasing pump volumetric efficiency and make it suction starving (unable to prime in the worst case scenario).
There are several factors that makes the metering pumps capacity adjustment via electrical actuator (manufactured by OBL) much more: reliable, trouble-free and economical than installing a frequency inverter and using variable speed drive motors.
| OBL electrical actuator advantages | VSD motor comparison |
| The adjustment percentage ranges from 0 to 100% | Capacity range is much more reduced. Motors cannot be stopped. |
| Proportional dosing throughout 0-100% capacity range | Capacity variation is not fully proportional (below certain percentage VSD motor cannot run) |
| Keeping same strokes/minute, makes pump operate trouble free when dosing: viscous products, high SG chemicals (98% H2SO4, Ferric chloride) and other chemicals that easily release gases (NaOCl2) | Increasing strokes/minute to achieve higher flow rates with chemicals, makes pump: inaccurate, unreliable, not proportional, not linear and often suction starving |
| Keeping same strokes/minute guarantees, throughout the capacity range (0-100%), accuracy, linearity and repeatability. | Accuracy and repeatability are very much influenced by strokes/minute variation. Linearity is not even applicable. |
| Once max operating capacity is set, pump dosing rate can be easily adjusted, and electrical actuator needs no installation calculation | Proper VSD installations need:
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| OBL electrical actuator can be installed on each and every OBL metering pump and are extremely competitive. Please compare OBL prices with left-indicated list. | Proper VSD installation need:
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| With ZC OBL electrical actuator is possible to change on site the pilot signal (4-20 mA, to 0-20 mA, to 20-4 mA, to pulses, to RS485), by adjusting dip switch. | Frequency inverter can, but requires some knowledge to naviagte through the menus. |
| With OBL electrical actuator a 4-20 mA feedback is always available | With inverter there is no feedback available, therefore, in order to have a feedback, a stroke counter should be installed on the pumps and the proper logic on control panel shall be prepared. In any case, if stroke length is manually changed by local knob, the feedback signal shall be re-calculated. |
| OBL electrical actuator always have local visual adjustment reading | By inverter, pump capacity cannot be easily checked since there’s not any local reading (the manual stroke control indicates stroke length only, not speed). The only way to check the actual capacity is to install a flowmeter on discharge line. |
In case electrical actuator has problem on either electronics or driving motor:
| If a motor operated through inverter burns (likely when not specifically engineered for VSD application) pump cannot pump nor be adjusted |
| Z type electrical actuator are IP 66 std and with anticondensation heater on demand | Inverter have rarely IP rating higher than 55, and therefore have to be mounted in a appropriate enclosure. All forced ventilation motor are IP 54 or even 44 |
Pump with electrical actuator can be automatically operated by two pilot signals
| Pumps with VSD have one and one only pilot signal |
| In case of capacity over selection (e.g. actual max flow rate is 60 l/h and pump has 100 l/h max capacity), all OBL electrical actuator have “Q max” facility that allows dosing range reduction, keeping the same pilot range adjustment | In case of pump oversize,manual stroke adjustment is required to adjust the required max capacity which results in an even more reduced range of dosing range. |