Five (5) Rules For Positive Displacement & Centrifugal Blower Reliability

The issues that we see in our shop when we tear down the units would have the same effect on any rotating equipment in any other plant. The five (5) items to having long lived positive displacement, rotary lobe blowers (or) centrifugal blowers are:

  1. Keep product (or) other foreign debris out of the blower
    1. For example:
      1. Abrasive materials will erode the interior components compromising performance and lock them up
      2. Polymer pellets alone can lock-up the unit due to particle size and usual higher concentration
      3. Polymer dust (or) occasional pellets will coat hot internal blower components and when shutdown the material will freeze, and lock a unit up
  2. Keep proper grades of oil at proper level
    1. Proper level is half-way up the sight glass with the blower NOT running
      1. There should only be oil “splash” visible in the sight glass while the unit is running.
      2. Almost all of the oil will be “in play” during operation
    2. Use proper oil viscosity and grade based on unit’s service and operating temperature
    3. Change as required due to service
      1. Hydrocarbon oil has a 1,500 hr. life per API
      2. Other synthetic oils have a 3,000 hr. life per API (Mobil SHC is NOT ACCEPTABLE)
      3. Roots synthetic oil has a 6,000 hr. life
      4. All above hour life is based at 180F operating temperature
        1. Oil life is cut in half for each 15F interval above 180F
          1. As an example, at 210F (two 15F intervals above 180F), the oil life is divided by one-half twice
          2. So at 210F; 6,000 hr. life ÷ 1⁄2 ÷ 1⁄2 = 1,500 hrs.
  3. Keep belts tight and aligned
    1. Typically, new belts will have a deflection of:
      1. 3V section has 3/8” deflection @ 8-9 lb. force
      2. 5V section is 3/8” deflection @ 12-15 lb. force
      3. All above will be 50-60 Hz. using a sonic belt tension instrument
    2. Any positive displacement device, air or liquid, is a constant flow, variable pressure unit.
      1. They are constant torque devices
      2. PD blowers have high WR2
      3. The result is Howden Roots mandates a 1.4SF belt drive design
    3. Ensure that the belt sheave is as close to the blower face as possible.
      1. 1/4” is Roots recommendation
  4. Do not dead head (or) starve the unit for air/gas
    1. Since it is a compression device, it generates heat due to work input
    2. Air (or) gas flow is required to take away the heat of compression
    3. Dead heading at best causes discharge through the relief valve
      1. The relief valve is only a protection device of last resort
        1. The relief hasn’t enough orifice area to allow sufficient airflow to prevent over-heating if run in this mode for extended periods
      2. A plugged inlet filter also starves the unit
        1. This makes it work harder to pull in inlet air (or) gas
        2. This only increases the temperature yet again
      3. Higher inlet vacuums AND having higher discharge pressures is a sure way to overheat a unit
  5. Do not allow rolling starts
    1. Most common source of units “slipping” gear timing
    2. Use check valves on outlet of units
      1. Prevents blower from rolling backwards when it is shut down and system decompresses

Finally, being a positive displacement device, the Roots rotary lobe blower will deliver the design air volume up to the horsepower available and/or relief valve setting and/or its own mechanical failure – period.