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Safe Operating Procedures

5 min read

Parent
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Safety Considerations
Centrifuge failures are violent and lethal
Current
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Safe Operating Procedures
Pre-flight checks and emergency shutdown

Run a centrifuge from a written checklist: inspect the rotor and fastenings, confirm balance and the shield, start at 10-20% of working speed and step up while watching vibration, and after every run let it stop completely before opening it. Time the coast-down on each run, because if it falls to half of its normal value the friction in the bearings has doubled and the machine needs attention before it is used again.

Part of Centrifuge Building

Safe operating procedures are the pre-run checks, staged start-up, shutdown steps and records that turn the safety rules of a centrifuge into the same routine at every run.

Most Failures Announce Themselves Before The Run

A cracked rotor, a loose nut, an unmatched pair of tubes and a dry bearing are all found by looking, not by running. The carrier gives the three failure modes (burst, bearing seizure, imbalance); the checks below are aimed at each of them.

Pre-Flight Checklist

  1. Rotor. Look over the whole rotor for cracks, dents, deep scratches, corrosion pits and bent discs. Any crack means the rotor is retired. Check it against the speed limit in Speed Limits and Material Stress, and check the maximum RPM written on the frame.
  2. Fastenings. Nuts, clamps and trial weights are tight, in their marked positions, and the nut thread direction tightens in operation.
  3. Balance. Tubes in matched pairs, loads even, no sludge or cake unevenly built. If anything changed since the last balance, rebalance; see Balancing and Vibration Control.
  4. Bearings and drive. Spin the rotor by hand: smooth, quiet, no catching. Check lubrication; see Lubrication. Belts and gears are guarded and in good condition.
  5. Shield and lid. In place, bolted, lid latched; see Containment Shield.
  6. The stop. Know where it is and that it works; test the brake or the belt-lift with the rotor at rest.
  7. People. The area is clear and nobody stands in the plane of the rotor.

Run Procedure

Step 1: Start at 10-20% of working speed and listen and feel for vibration, as in Dynamic Balancing. If there is any, stop.

Step 2: Step up in stages (for example to 50%, then 75%, then full speed), checking vibration at each.

Step 3: Hold the working speed for the time needed and no longer. Time is not free: every minute at speed is wear.

Step 4: Slow down by reducing the drive, then use the brake only if one is fitted and the rotor’s design allows.

Step 5: Wait until the rotor has stopped completely before touching the lid. Time the coast-down.

Step 6: Unload carefully and clean.

Emergency Shutdown

If vibration rises, a noise changes, a smell of burning or a crack of metal appears:

  1. Stop the drive by the nearest means: lift the belt, release the clutch, stop the water wheel’s feed. Do not step into the plane of the rotor.
  2. Apply the brake if one is fitted and if it is safe to reach.
  3. Move away from the plane and wait. Do not approach until the rotor has stopped, even if it takes the full coast-down time.
  4. Do not restart until the cause is found.

Fire or a chemical spill: let the rotor run down before putting anyone near it, then see Industrial Centrifuge.

Worked Example: The Coast-Down As A Bearing Gauge

A rotor of stored energy E loses it through friction power P, so the time to stop is about E ÷ P if the friction is roughly constant. The bowl in Safety Considerations holds up to 5.9 kJ, and at 10 W of friction coasts for about 590 s, about 10 minutes.

  • Record the coast-down after every run, in the log, alongside the date, the speed and the load.
  • If the time falls steadily or suddenly to half, the friction has doubled (to about 20 W in the example): the bearing is drying out, dirty or failing. Stop using the machine, inspect, regrease or replace; see Roller Bearings and Lubrication.
  • If the time suddenly rises, check the brake or the belt: something that should have held the rotor is slipping.

This reading is a trend, not a measurement of friction power. The 10 W is an assumption, so the meaning of the number is in the change, which only a log can show.

What You Need

  • A written checklist fixed to the machine
  • A logbook, a clock, and a marked maximum RPM
  • A second person within call for every run

Common Mistakes

  • Skipping the checklist because the last ten runs were fine. The eleventh is the one with the crack.
  • Opening the lid to look. Wait for a full stop.
  • Raising the speed of a machine that has had a repair. Rebalance and step up again.
  • No log. Without it, a trend cannot be seen.

Safety

Treat every centrifuge as running until it has stopped

The rotor turns for minutes after the drive stops, and a back-driven crank turns with it. Never stand in the plane of the rotor, never touch a rotor or crank that is still moving, and never open a lid on a spinning machine. Blood, milk and chemical samples need their own cleaning and disinfection after each run; see Cleaning and Sanitizing Without Chemicals.

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