Pumps

This lesson covers pump selection and specification for process plants. Learn centrifugal vs positive-displacement choice, API 610 pump types, NPSH available calculations, seals (single, double, sealless), materials, datasheets, spare philosophy, and minimum flow and protection.

A process plant has hundreds of pumps moving everything from water to hot oil, acids and slurries. Choosing the right type, sealing, materials and protection is key to safe, reliable operation — pump failures are among the top causes of process leaks and fires.

Hydraulic theory (head, flow, efficiency) is covered in the Fluid Mechanics module; here we focus on engineering selection.

An analogy

Selecting a pump is like hiring for a job: the right skills (type), protective gear (seals, materials), and a backup person (spare) keep the plant running.

Centrifugal or positive displacement?

Centrifugal: most services — moderate head, high flow, low viscosity. Positive displacement (gear, screw, lobe, diaphragm, plunger): high viscosity, high pressure/low flow, metering, shear-sensitive fluids. PD pumps need relief valves because flow doesn't drop when blocked.

NPSH and suction design

Hot or volatile liquids have high vapour pressure, reducing NPSH_a. Raise the suction vessel, cool the liquid, reduce suction losses, or choose low-NPSH_r pumps (inducers, vertical can pumps).

Sealing

Mechanical seals (API 682) replace packing. Single seals for benign fluids; double pressurised seals with barrier fluid for toxic/flammable; sealless magnetic-drive or canned pumps eliminate leaks entirely.

Specification and reliability

API 610 for refinery pumps (OH, BB, VS types); datasheets define duty, fluid, materials, seals, driver, testing. Spare philosophy (1 + 1 installed spare for critical services), minimum-flow recirculation, vibration monitoring and condition-based maintenance improve reliability.

Formulas

• NPSH available: NPSH_a = (P_surface − P_v)/(ρg) + H_s − h_f
• Margin: NPSH_a ≥ NPSH_r + 1 m (or × 1.1–1.3)
• Power: P = ρ g Q H / η

Worked example: NPSH available

Given: Open tank at 101.3 kPa, water at 60 °C (P_v = 19.9 kPa, ρ = 983); Liquid 2 m above pump, suction losses 1 m

Find: NPSH_a.

• (101 300 − 19 900)/(983 × 9.81) = 8.44 m
• NPSH_a = 8.44 + 2 − 1 = 9.4 m

Answer: 9.4 m — fine for a pump needing 4 m

Worked example: Boiling liquid in a vessel

Given: Liquid at its boiling point in a vessel (P = P_v); Level 3 m above pump, losses 0.5 m

Find: NPSH_a.

• Pressure term = 0
• NPSH_a = 3 − 0.5 = 2.5 m

Answer: Only 2.5 m — elevation is the only source; many column bottoms pumps are elevated for this reason

Common mistakes

• Wrong: Using gauge pressure in NPSH. — Right: Use absolute pressures for surface and vapour pressure.
• Wrong: No relief on PD pumps. — Right: Always protect PD pumps against blocked discharge.

Applications

• Refinery services: Hot hydrocarbon pumps with double seals (API 610/682).
• Chemical transfer: Mag-drive pumps for toxic or corrosive liquids.
• Dosing & metering: Diaphragm and plunger pumps for precise chemical injection.

Key takeaways

• Centrifugal for most; PD for viscous/metering/high pressure.
• NPSH_a = (P − P_v)/ρg + H_s − h_f.
• Seals per hazard: single, double, sealless.
• Specify to API 610/682; plan spares and protection.

Going further

• Suction specific speed: High values indicate risk of recirculation and cavitation.
• Pump reliability programs: MTBF tracking, seal failure analysis and condition monitoring.
• Viscosity corrections: HI charts derate centrifugal pump head and efficiency.