Flatbox vacuum varying

Note: When measured, flatbox vacuum is often different on the two sides of the machine.
1. Ensure the following:
(a) that flatbox heads are correctly set.
(b) that boxes are not filling at forming fabric changes.
(c) that applied vacuum is not varying due to malfunction at pumps or in supply piping.
(d) that level control in seal pit is functioning properly.
(e) that boxes are correctly positioned with respect to the forming fabric and are accurately positioned horizontally across the machine.
2. Determine that the basis weight is constant with time which depends on the stock supply system.
— are consistencies uniform?
— are levels on the suction side of the stock pump or above the basis weight valve steady?
3. Is there variation in the amount of fines being returned to system?
4. Check for correct functioning of headbox level control.
— malfunction here will give a varying total head, and a changing flatbox vacuum.
5. Examine level on suction side of fan pump.
— if the level varies, then flow to headbox will change and it will affect flatbox vacuum.
6. If faults in system cannot be minimized or corrected, there are only four alternatives:
(a) adjust amount of water to machine, raising or lowering slice as required to keep head constant.
(b) adjust the refining.
(c) adjust drainage elements.
— if wet suction boxes are in use, drainage before the flatboxes can be adjusted.
— with adjustable foils, adjusting foil angles may change drainage.
(d) maintain constant temperature.
Related information may be found in Consistency varying.