The positive displacement blower is often the heart of many types of pneumatic conveying and process systems. Often, such an important piece of equipment is forgotten until it fails. Once the blower has failed, the maintenance personnel must scramble to get the system back online. Some simple procedures can often prevent damage to the blower and make your life at work much better.
Low Oil Levels
Starting and running a PD Blower while low on oil will result in almost immediate failure of the gears. Once the gears begin to fail, the following can occur: Rotor lobes will clash, end clearances will change, thrusting the driven rotor into the free end plate and bearings will also fail, causing the rotor lobes to clash even further. However, if caught early, the blower is rebuildable.
Improperly Maintained Lubricant
Dirty oil: Bearings and gears will wear out prematurely. Oil should be changed every 500-1000 hours of operation; more frequently if operating in a demanding application or a dirty environment. Blowers operating with external lubrication systems with large quantities of oil and filtration can operate 3-6 months without oil changes.
Failed Lubricant: Oil will be black and beginning to turn into tar. Bearings and gears will show evidence of further damage, such as damaged rolling elements and damaged teeth. This is caused by thermal breakdown of the lubricant. Demanding applications (13-18 PSIG) may require oil changes as frequently as 100-250 hours. Oil sampling programs are very valuable for optimizing oil life and minimizing waste.
Inadequate Lubrication: Bearings and/or gears will be discolored due to heat buildup. This can be normally caused from:
- Operation of a blower at too slow of a speed. Since a blower is splash lubricated, minimum speeds are necessary to provide proper lubrication of teeth.
- Use of too low a viscosity for lubricant. Follow the guidelines for the operation and maintenance manuals for the blower. Kice typically recommends an ISO-100 synthetic oil when the ambient temperature is 90 degrees, or less. Above 90, ISO-150 synthetic oil is recommended.
Starved inlet is a condition where the airflow entering the blower is so restricted that there is insufficient air supply to provide adequate cooling of the blower. If allowed to continue, visual signs of starved inlet may include:
- Bubbling or charring of paint on the blower – most indicated near the discharge side of the blower.
- Wear on the non-gear end plate of the blower.
- Evidence of contact between rotors themselves.
- Evidence of contact between rotors and inlet.
- Seizure of blower, resulting in catastrophic failure.
Starved inlet can be prevented by:
- Monitoring an inlet filter restriction indicator
- Visually inspecting the inlet filter regularly – do not completely rely on instrumentation.
- Changing the inlet filter when necessary – when differential pressure across inlet filter reaches approximately 10” of water column.
- Installation of a vacuum switch and/or a vacuum relief valve on the inlet side of the blower.
- Installation of a temperature switch in the blower discharge stream as close to the blower as possible.
Positive displacement blowers are “workhorses.” They do not give up. They will stuff air into a pipe, or self-destruct in trying. Damage resulting from over-pressure is apparent when the discharge of the blower has been restricted. The type of damage sustained from over-pressure can result:
- Contact between rotors and inlet port of housing
- Contact between rotors and non-gear end plate
- Severe cases may include inter-lobe contact and even contact between the rotors and the gear end plate
Over-pressure can be prevented by:
- Installation of a pressure relief valve on the discharge of each blower – before any other control valves or airlocks.
- If spring type, perform regular checks on relief valve settings to assure proper setting and operation – Some include pressure relief valves in their equipment calibration schedules.
- Installation of discharge pressure switch at the blower discharge.
Damage resulting from over-temperature is not always as apparent as starved inlet or over-pressure. There may be symptoms of both in an over-temperature situation. Over-temperature can be caused by:
- Inlet air temperatures that are elevated above ambient.
- Recirculation of airflow from the blower discharge to the blower inlet.
- Throttling of blower discharge to reduce airflow.
Improper installation of valves: Check valves, pressure relief, and vacuum relief valves are unidirectional. Making sure that these valves are installed properly is vital. Be completely familiar with the specific valve and pay close attention to the direction the valve must be oriented.
Excessive overhung load: This usually results in broken drive shafts, damaged drive shaft bearings, or fretting of the drive shaft. You should tighten belts only to point necessary to transmit the motor nameplate power. Often, belts are tightened by sight. This almost always will exceed design of the drive. Make certain that you are familiar with the amount of deflection required.
Improper location of pressure or temperature sensors: This provides false reporting of what is actually over-pressure or over-temperature. Locate pressure, vacuum, and temperature instrumentation within 6 inches of the blower inlet and discharge (provides most accurate readings and best protection). Do not rely on infrared “skin temperature” thermometers. These devices will not give an accurate representation of the temperature inside the blower. These should only be used to determine trends and to stave off possible future problems.
In summary, blower life can be optimized with:
- Regular oil changes with lubricant appropriate for the application.
- Clean inlet air (or as clean as possible).
- Protection against over-pressure.
- Protection against over-heating.