1. Shutoff operation: the most dangerous heating
Shutoff operation means running the pump with discharge valve fully closed — zero flow. In an FBCN centrifugal pump, this may happen intentionally (during startup procedure, moments before opening discharge) or accidentally (block valve mistakenly closed, check valve stuck, automatic flow control at unfeasible point). The danger is that the motor keeps delivering energy to the impeller, but as there is no useful flow, all that energy is dissipated as heat in the liquid trapped inside the casing.
Heating rate in shutoff is frightening: for a typical 75 kW pump operating with water, internal casing temperature rises ~10 to 15 °C per minute. In 5 to 10 minutes, water reaches local boiling, generates vapor and the pump starts to cavitate internally. In 15 minutes, mechanical seal fails from temperature. In 30 minutes, there is real risk of shaft warping and permanent impeller damage.
Solution: never operate centrifugal pump in shutoff for more than seconds during startup maneuver; always install minimum bypass line (calibrated orifice or automatic valve) that ensures minimum flow of 10% to 15% of BEP even with main discharge closed.
Excessive internal recirculation through loose wear rings is a chronic partial shutoff: a significant portion of pumped liquid returns from discharge to suction within the pump itself, never exiting through the discharge nozzle. The thermodynamic effect is the same as shutoff, but at smaller scale — the pump heats progressively in apparently normal operation. Diagnosis: measure axial clearance between impeller and casing and radial clearance of wear rings; replace when they exceed double the original clearance.
2. Bearings without lubrication or with aged grease
Bearings — ball or tapered roller bearings supporting the shaft and absorbing axial and radial loads — are the most temperature-sensitive components in any pump. Adequate lubrication (high-performance NLGI 2 grease with EP additives, or mineral oil in oil-bath systems) creates film between rolling elements and races, preventing metal-on-metal contact. Without lubrication or with grease contaminated by moisture/solids/chemicals, direct friction generates uncontrolled heat and the bearing reaches catastrophic failure in hours.
FB Bombas establishes a preventive lubrication plan in the manual of each series: for FBCN with grease bearing, regreasing every 2,000 operating hours or 6 months (whichever comes first), with the exact quantity indicated on the bearing nameplate. Grease excess also causes heating — the bearing dissipates heat through the moving grease itself, and too much grease hinders this effect. Correct lubrication = neither too little nor too much.
In oil-bath systems, check level through sight glass each shift and change oil every 4,000 hours or 1 year.
3. Pump-motor misalignment and ISO 10816 vibration
The coupling between pump and motor shafts must be aligned within very tight tolerances — typically 0.05 to 0.10 mm total parallel and angular misalignment. When misalignment exceeds these limits, cyclic radial and axial loads emerge that overload bearings every shaft rotation. Direct result: bearing heating, mechanical vibration above tolerable, and premature mechanical seal failure (which has its own axial and radial movement tolerances).
ISO 10816 establishes four vibration severity zones for rotating machinery: zone A (newly commissioned, vrms < 2.3 mm/s), zone B (acceptable long-term operation, 2.3 to 4.5 mm/s), zone C (restricted operation, 4.5 to 7.1 mm/s) and zone D (unacceptable vibration, > 7.1 mm/s) for 15 to 300 kW machines on rigid foundation.
Pump operating in zone C/D almost always has misalignment as root cause, and confirmation is made by stopping the pump and measuring alignment with dial indicator or laser alignment. Solution: retighten base bolts, reshim the pump, and redo cold alignment considering thermal growth predicted by manufacturer.
4. Over-tightened packing and mechanical seal without cooling
Pumps with traditional packing have a delicate balance: the gland nut must be tight enough to contain leakage but loose enough to allow controlled drip of 30 to 60 drops per minute. This drip serves as lubricating film between packing and shaft, and as a conductive heat removal system. Inexperienced operators often tighten packing until leakage stops — the result is dry packing, direct friction against the shaft, uncontrolled heating and premature wear of both packing and shaft sleeve.
Mechanical seal, in turn, depends on the liquid film between sealing faces for lubrication and heat removal. In clean process fluids at moderate temperatures (up to 80 °C), the pumped fluid itself lubricates the seal. In hot fluids, suspended solids or aggressive chemicals, the seal needs external flush plan (pressurized clean water per API 682 plan 32) or barrier plan (double seal with clean barrier fluid, plans 53A/53B/54).
Operation without the correct plan causes heating of the faces, vaporization of the film between them and failure by dry contact. Diagnosis: check external seal chamber temperature with infrared thermometer — above 80 °C indicates problem with sealing plan.
5. Temperature limits and FB Bombas startup procedure
FB Bombas establishes temperature limits for each critical component, based on international standards and expected material life. Bearings with standard NLGI 2 grease: operating limit 70 °C, critical limit 90 °C — FBEI manual MAN001-10 reference, startup procedure. FBCN casing for water: operating limit 100 °C with standard seal; for higher temperatures, high-temp seal and cooling water chamber are required.
FBOT pumps for thermal oil normally operate at 250-350 °C — but with bearing thermally isolated from hot end by air chamber and dedicated seal cooling, precisely to keep the bearing below 90 °C despite the hot fluid.
FB Bombas-recommended startup procedure with thermal monitoring: (1) with pump stopped, verify bearing lubrication per plan (new grease or correct oil level); (2) record pump house ambient temperature; (3) start pump following standard procedure (see "pump not drawing water"); (4) during first operating hour, measure external bearing temperature every 15 minutes with infrared thermometer — expected curve is asymptotic rise to a plateau between 50 and 70 °C; (5) if temperature crosses 80 °C still rising, stop pump and investigate; (6) record stable operating temperature as baseline for comparison in future reviews.
Gradual rise of stable temperature over months indicates progressive bearing wear — schedule preventive maintenance before failure.
6. When to stop and when to call FB engineering
Field decision rules of thumb: stop the pump immediately if bearing temperature exceeds 90 °C, if there is visible vibration in the casing accompanied by heating, if smoke or burning smell appears, or if the motor enters thermal protection repeatedly. Do not stop and try to diagnose with the pump still hot — wait for natural cooling before any mechanical intervention to avoid burns and biased measurements from thermal shaft growth.
Engage FB Bombas application engineering when: (a) temperature exceeds 90 °C repeatedly after grease change and alignment check; (b) pump serves critical service that does not allow prolonged shutdowns for trial-and-error; (c) there is suspicion of sealing problem in special fluids (thermal oil, acids, volatile hydrocarbons); (d) vibration remains in zone C/D after realignment. FB can offer route-based vibration analysis, endoscopic inspection of pump interior, and documented technical opinion. Contact: comercial@fbbombas.com.br or WhatsApp +55 11 97287-4837.
