Most centrifugal and rotary pumps have steady state flow characteristics. Under normal conditions there is very little change in the velocity of the fluid pumped. That is not the case with reciprocating metering pumps, which cause the fluid to experience peak velocities and accelerations in both the suction and discharge lines. With these pumps, the fluid is initially at rest (0° of piston cam rotation), and fluid velocity and acceleration are both zero.
The fluid is accelerated during the discharge cycle through peak velocity at 90° of piston cam rotation. Similarly, the suction cycle begins at rest (180° piston cam rotation) and the fluid is accelerated through peak velocity at 270° rotation. The inertial force required to initiate flow for both pumping cycles is of major significance. Because of these unique flow conditions, common to all reciprocating metering pumps, special consideration is required of all factors that will affect pump performance.
When all variables are considered and basic hydraulic principles are followed, the installation and operation of a Reciprocating Metering Pump will be successful.
Checklist of difficult conditions:
- Piping with excessive fittings, valves, bends
- Suction lift or low suction head
- Undersized piping on either suction or discharge
- Dissolved gases in product pumped; unusually high vapor pressure
- Pumping near the required net positive suction head (NPSHR)
- Product specific gravity unusually high
- High product viscosity
- Long suction or discharge lines
- Pumping downhill—suction pressure higher than discharge pressure
- Pumping at unusually high or low temperatures
- Solids content or slurry (pipe size important to minimise settling)
- Outdoor installation in cold climate
- Changing elevations in suction or discharge lines (air traps cause problems)
Piston velocity profile for a reciprocating metering pump.
PULSA Series Hydraulic Diaphragm Metering Pump (Reciprocating).
Article courtesy of Kelair Pumps Australia. for more information, Phone: 1300 789 466 or visit kelairpumps.com.au
