17.1 Pumps, Blowers and Mechanical Maintenance Programs

Pump Types

Never start or run a PD pump against a closed discharge valve. Pressure climbs until something breaks, so PD pumps need pressure relief valves and should never be throttled to control flow. Centrifugal pumps can run briefly against a closed valve at shutoff head, but they overheat if left there.

Pump Curves

A centrifugal pump curve shows head, efficiency, brake horsepower and NPSH required against flow. The operating point is where the pump curve crosses the system curve (static head plus friction losses).

• Running far left of the best efficiency point (BEP), at low flow, causes recirculation, heat, vibration and seal and bearing wear.
• Running far right, at high flow, risks cavitation and motor overload.
• Parallel pumps add flow at the same head; series pumps add head at the same flow.

Affinity Laws (same pump, changed speed)

$$\frac{Q_2}{Q_1} = \frac{N_2}{N_1} \qquad \frac{H_2}{H_1} = \left(\frac{N_2}{N_1}\right)^2 \qquad \frac{P_2}{P_1} = \left(\frac{N_2}{N_1}\right)^3$$

Example. A pump delivers 1,000 gpm at 1,780 rpm. A VFD slows it to 1,500 rpm, a speed ratio of 0.843.

• Flow ≈ 1,000 × 0.843 = 843 gpm
• Head ≈ 0.843² = 71 percent of the original
• Power ≈ 0.843³ = 60 percent of the original

That cube relationship is why VFDs save so much energy on variable-demand pumps and blowers.

Shaft Sealing

When repacking: remove all old rings, check the sleeve for wear, cut new rings to fit, and stagger the joints (about 90 to 120 degrees apart). Seat each ring separately and position the lantern ring under the seal water port.

Bearings, Lubrication and Alignment

• Bearing failure causes: contamination (dirt and water), misalignment, imbalance, improper lubrication and electrical fluting from VFDs.
• Lubrication: follow the manufacturer's type, quantity and interval. Over-greasing packs the housing, raises temperature and blows seals. Keep grease guns clean and labeled, and never mix incompatible greases.
• Alignment: use dial-indicator or laser alignment. Correct angular and offset (parallel) misalignment, check for soft foot before final alignment, and recheck after the pump reaches operating temperature where required.
• Couplings, belts and chains: inspect for wear. Tension belts correctly, since a loose belt slips and a tight one destroys bearings.

Blowers and Compressors

• Positive displacement (rotary lobe) blowers deliver nearly constant air volume. They need relief valves, inlet filters and silencers, and must not be dead-headed.
• Multistage centrifugal and high-speed turbo blowers run efficiently over a narrower range and must be kept out of surge, an unstable flow reversal at low flow and high pressure.
• Air compressors: drain receiver condensate, test relief valves, change filters, and inspect pressure vessels as required.
• Monitor diffuser pressure: rising header pressure at the same airflow points to diffuser fouling.

Maintenance Strategies

A computerized maintenance management system (CMMS) schedules PM and records work orders, labor, parts and costs. It also builds equipment history, which supports asset management: knowing what you own, its condition and criticality, and when to repair or replace it. Keep spare parts for critical equipment, and keep O&M manuals, pump curves and as-built drawings where crews can use them.

Valves Worth Knowing

Gate (isolation), butterfly (isolation and throttling in large lines), plug and eccentric plug (sludge), ball, globe (throttling), check (swing, ball, silent), pressure-reducing and -sustaining, and air release and vacuum valves. Exercise isolation valves on a schedule so they work in an emergency, and record turns and condition.