Unit
- Centrifugal pumps: These use an impeller to impart kinetic energy to the fluid, converting it to pressure as the fluid exits the volute or diffuser. They are classified by impeller type, number of stages, and casing orientation.
- Positive displacement pumps: These move fluid by trapping a fixed volume and forcing it into the discharge line. Common types include gear pumps, lobe pumps, piston pumps, and diaphragm pumps.
- Operating principle: Pump performance is characterized by head (pressure), flow rate, efficiency, and net positive suction head required. The pump curve plots these relationships and is essential for sizing and selection.
- Affinity laws: For centrifugal pumps, flow varies directly with impeller speed, head varies with the square of speed, and power varies with the cube of speed. These relationships govern speed control adjustments.
- Pump applications: Centrifugal pumps are preferred for high-flow, low-viscosity services while positive displacement pumps excel at high-pressure, high-viscosity, or metering applications.
- Pump standards: API 610 covers centrifugal pumps for refinery service, ANSI B73.1 covers horizontal end-suction pumps, and ISO 2858 provides international metric dimensions. Each standard defines design features and tolerances.
- Material selection: Wetted materials must resist corrosion and erosion based on fluid properties. Common materials include cast iron, ductile iron, stainless steels (304, 316, duplex), and non-metallics like PTFE and polypropylene.
- Performance testing: Hydraulic Institute standards define test methods for measuring head, flow, power, and vibration. Acceptance criteria are specified in purchase contracts and must be verified during commissioning.
- Pump seals: Mechanical seals prevent leakage between the rotating shaft and stationary casing. Common types include balanced, unbalanced, cartridge, and double seals. Selection depends on pressure, temperature, and fluid compatibility. Proper installation and flush plans are critical for maximizing seal life and preventing premature failure.
- Bearings and lubrication: Rolling element bearings support the pump shaft and handle radial and axial thrust loads. Grease or oil lubrication is selected based on operating speed and temperature. Bearing life follows L10 reliability theory, and precise shaft alignment significantly reduces the risk of premature bearing wear or failure.
- Net Positive Suction Head (NPSH): NPSH available must exceed NPSH required to avoid cavitation, which causes pitting, vibration, and performance degradation. System design must account for elevation, friction losses, and fluid vapor pressure to ensure adequate suction conditions throughout the operating range.
- Pump drivers and couplings: Electric motors are the most common drivers, though steam turbines and engines are used where appropriate. Flexible couplings transmit torque while accommodating minor misalignment. Common coupling types include grid, gear, and elastomeric designs, each with specific torque and misalignment capabilities.
- Installation and commissioning: Proper foundation, grouting, piping stress analysis, and temporary strainer installation are essential. Commissioning steps include verifying rotation direction, monitoring vibration levels, checking seal flush systems, and confirming performance against the pump curve and test certificate.
Equipment Specification
Equipment: Centrifugal Pump (ANSI B73.1)
- Maximum Working Pressure
- 285 psi (19.6 bar) at 100°F (38°C)
- Impeller Diameter
- 6 to 14 inches (152 to 356 mm)
- Temperature Range
- -20°F to 500°F (-29°C to 260°C)
- Standard
- ASME B73.1 / ANSI
Quick Revision
- Centrifugal and positive displacement pumps operate on fundamentally different principles.
- Pump curves and affinity laws govern selection and speed control.
- International standards define dimensions, materials, and test procedures.
- Material selection depends on fluid corrosivity and temperature.