
There is no reliable universal replacement interval for the first press, later presses or tissue machines. Felt construction, nip type, loading, furnish, conditioning, contamination and operating schedule all change the useful run length.
Record the felt design, position, installation date, starting dimensions, permeability test method, moisture profile, sheet dryness, vacuum, shower settings, nip conditions and production rate. Use equivalent grades when comparing later readings.
A single reading should trigger investigation, not an automatic change, unless it exceeds an approved safety or damage limit.
Separate felt condition from changes in furnish, chemistry, loading, vacuum, showers and rolls. If conditioning capacity or machine settings are the cause, a new felt may repeat the same problem.
Use tons produced, quality loss, maintenance, downtime and failure risk to determine the economic change point. Coordinate positions only when the mill's outage plan and measured condition support it; do not remove a serviceable felt solely to match a calendar.

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