Air in the system

1. As a general rule, avoid air entrainment into stock or white water systems by:
(a) preventing stock or white water pumps from leaking at glands and sucking air.
— a common device is to tap a one half-inch line from fan pump to the wire pit or silo so that
machine tender can check for excess air.
(b) using a vortex breaker on fan pump suction down spout from wire pit or silo.
(c) avoiding cascading or waterfalls in any part of system.
(d) ensuring that air is not taken into system by too low a head in a stuffbox, or by overflow
from a stuffbox to a machine chest.
(e) making sure that air is not entrained at seal pit.
(f) replacing open machine screens.
— if this is not possible, maintain correct level.
2. Incline “horizontal” piping up 8 – 12 cm per meter of run.
3. Avoid air entrainment below jet at breast roll.
4. Keep lower surface of converging top slice lip clean and polished.
— stock hang-up here collects air and gives flying saucers on the forming fabric.
5. Foam the breaker showers with fog nozzles and a surface-active agent.
— steam the showers following fifth or sixth drainage element.
6. Install a de-aeration system (as a last resort).
7. Monitor system for entrained air.
— determine where buildup occurs and correct.
8. Air can get entrained in centrifugal pumps causing flow variations due to cavitation.
9. Air in stock can disturb the operation of the basis weight valve.
10. Entrained air from cascading contributes to dirt and foam throughout the system.
11. Low stock velocity lets entrained air come out of system and create problems.
— stock velocity in approach system should be 2.0 – 4.5 m/s (higher velocities simply create higher loads on fan pump and related equipment).
— silo velocities should be less than about 0.3 m/s.
See Foam control.