Roots Blower on Aeration Duty: Commissioning Checks That Prevent Most Failures

A Roots blower is a simple machine that fails for unglamorous reasons. Almost every premature failure traces back to a clearance that was disturbed during reassembly, oil that was not changed on schedule, or a startup that loaded the unit faster than the discharge line could absorb. The rotor profile itself is rarely the culprit.

Five parts, and the one that matters most

The machine consists of the casing, side plates, impellers, oil tank and silencer. Side plates connect casing to impellers, support impeller rotation and provide end-face sealing. Impellers, the rotating elements, come in two-lobe and three-lobe designs; three-lobe impellers produce smaller discharge pulsation, less noise and smoother running, which is why they have largely displaced two-lobe designs on aeration duty. The oil tank stores lubricant for gears and bearings. The silencer attenuates the noise generated by flow pulsation at inlet and outlet.

Working principle is positive displacement. Two three-lobe impellers rotate in opposite directions inside the space sealed by casing and side plates. Each impeller is profiled on an involute or epicycloid envelope curve, and because the three lobes of one impeller are identical, and the two impellers are identical to each other, machining difficulty drops sharply. The impellers are cut on CNC equipment so that a constant minimal clearance is maintained at every angular position as the pair turns at fixed centre distance, holding gas leakage inside the allowed band.

As the impellers turn toward each other, the very small clearances between impeller and impeller, impeller and casing, and impeller and side plate create a vacuum at the inlet, and air enters the inlet chamber at atmospheric pressure. Two lobes of each impeller then form a sealed chamber with side plate and casing, and rotation carries that trapped air toward the discharge chamber. Because the impellers mesh at the discharge side, the air between the lobes is squeezed out. Continuous rotation delivers a steady flow from inlet to outlet.

The operating envelope, in numbers

These limits are not suggestions; violating them is how bearings and seals die.

• Inlet temperature of the conveyed medium should normally not exceed 40 degrees C.
• Particulate content in the medium should not exceed 100 mg/m3, and maximum particle size must not exceed half of the minimum working clearance.
• Bearing temperature must not exceed 95 degrees C during operation; lubricating oil temperature must not exceed 65 degrees C.
• Operating pressure must not exceed the pressure rise range on the nameplate.
• Impeller-to-casing, impeller-to-side-plate and impeller-to-impeller clearances are set at the factory and must be preserved on reassembly.
• Oil levels in main and auxiliary tanks must stay between the two red lines on the level gauge.

Ambient temperature has its own ceiling. A unit can run long term at up to 40 degrees C ambient; above that, exhaust fans or other cooling measures are needed, not to protect the process but to protect service life.

Classification tells you what the duty will demand

By working arrangement the family splits into single-stage and two-stage: one compression stage makes a single-stage blower, while two single-stage machines in series compressing twice make a two-stage series blower. Beyond that, the taxonomy runs by lobe count (two-lobe, three-lobe), by application (shaft kiln, gasification, aeration), by medium (air, coal gas, hydrogen, sulfur dioxide), by drive (direct-coupled, belt-coupled), by cooling method (air-cooled, water-cooled, counterflow-cooled), by structure (vertical, horizontal, vertical-shaft, compact grouped) and by sealing type (labyrinth, piston-ring, packing, mechanical seal).

Two of those categories decide most site problems. Aeration duty is a variable-flow, variable-pressure duty, so a belt-coupled or inverter-driven unit with a proper control scheme will outperform a direct-coupled fixed-speed machine on energy alone. And sealing type decides whether leakage becomes a safety issue: in petrochemical service, a mechanical seal is not an upgrade, it is a minimum.

Commissioning a new or overhauled unit

Before first start, confirm that the blower carries no external load from pipelines, valves or frames. Check alignment of blower and motor. Check that the base is fully shimmed all round and anchor bolts are tight. Fill the oil tank with the specified grade of machine oil to the level line — the recommended lubricant is N220 medium-duty gear oil. Confirm the motor rotates in the indicated direction. Fit a belt guard at the belt wheel or coupling. Then open inlet and outlet valves fully and turn the rotor by hand; it should rotate freely, without impact or rubbing. Only then is a trial run defensible.

The no-load trial is where most projects cut corners incorrectly. No-load means running with inlet and outlet valves fully open. There should be no abnormal smell or smoke, no collision or rubbing sounds, and radial vibration velocity at the bearings should not exceed 6.3 mm/s. Run for about 30 minutes and, if conditions are normal, move to loading. If anything is abnormal, find it and repeat the no-load trial rather than pushing on.

Loading, running and stopping

Load gradually and slowly to rated load; never jump to rated load in one step. Rated load means the static pressure difference between inlet and outlet at the nameplate value, and with discharge pressure normal the variable to watch is inlet pressure, because that is where overload announces itself.

Several operating rules are counter-intuitive to operators used to centrifugal fans:

• Never fully close inlet and outlet valves during normal running, and never adjust flow by throttling. This is a positive displacement machine; flow and shaft power are changed by speed, not by valve position.
• Do not recirculate discharge gas directly back to the inlet for long periods. Return flow changes inlet temperature, and if return-flow regulation is required it must include cooling.
• Do not stop the unit suddenly at full load. Unload first, then stop.
• After the blower runs normally, usually about a minute, close the discharge valve slowly. Closing it quickly produces a large inrush current that can burn out the motor.
• Early in operation, noise and high current may appear because of lubricating oil viscosity; both normally clear within 10-20 minutes.

Where the wear actually happens

Gear and bearing faults present as three symptoms: gear wear producing excessive backlash, loose gear fastening that fails to keep the impellers synchronised, and bearing wear that opens clearance. The remedies are procedural rather than exotic — remove contaminants and inspect internals for damage, adjust backlash and replace gears if side clearance exceeds the average by 30-50%, and reassemble maintaining a tapered-fit contact area of 75%.

Corrosion is a separate problem class. General or uniform corrosion spreads fairly evenly over the blower surface; localised corrosion concentrates, taking the form of pitting, crevice attack, intergranular corrosion, stress corrosion cracking, corrosion fatigue, hydrogen attack, erosion corrosion or exfoliation. Both patterns are pronounced in petroleum, chemical and marine atmospheres. Replacing components with glass-lined, PTFE or titanium parts works but is expensive; organic anti-corrosion coatings based on polymer composites are usually the better trade, and bonded surface coatings suit areas exposed to abrasion, cavitation and corrosion, with simpler application and better safety than welded pressure-vessel repair.

Shaft heads and keyways wear because of the operating environment. Build-up welding or brush plating followed by machining is the traditional repair, but welding heat leaves residual stress that can bend or fracture thin sections, brush-plated layers are thickness-limited and prone to peeling, and both methods restore metal with metal without changing the hard-on-hard fit relationship, so wear returns under combined load. Polymer composites allow fast repair of bearing housing wear without machining, carry no welding heat stress, have no thickness limit, and their compliance absorbs shock and vibration while achieving full contact of the fitted surfaces.

Oil leakage is the last chronic issue. The traditional remedy is a long shutdown to replace gaskets and recondition joint faces, which is impossible while the unit is running. High-performance polymer sealants and lubricants with strong self-lubricating properties reduce clearances at meshing locations and mitigate noise and leakage at dynamic seals without contaminating the oil.

Energy, storage and the long view

Discharge pressure of a Roots blower is not limited by speed: pressure can remain constant regardless of speed change, while air volume is proportional to speed (Q = KN). The machine is a constant-torque load, so power saving is proportional to speed reduction. Because these units often run around the clock with long operating hours, the energy headroom is considerable. After a technical retrofit that replaces outlet or inlet valve throttling with speed control, labour intensity falls, response is timelier, yield improves and unit consumption drops.

Maintenance intervals rest on one number: replace lubricating oil every 1,000 hours of operation, and for new or overhauled units replace all oil after 8 hours. A major overhaul once a year, replacing bearings and wearing parts, is a reasonable default. For long-term storage, cut all power, store the unit in a ventilated dry place and oil it regularly. Overload does not always appear immediately, which is why inlet and discharge pressure, bearing temperature and motor current trends are the three readings worth logging every shift.