PM4 Downtime Analysis - Paper Breaks
Strength analysis has been simplified in the following way; the data sets have been split between "Low Strength grades" and "High Strength grades" to mitigate the starch influence when trying to compare all strength. Within the two categories the strength analysis has been conducted using index values for each strength parameter. The is a known correlation between basis weight and the strength parameters. Removing this influence means any deviation or special cause deviation can be attributed to other events.
Using Strength index will remove the "Over", "Under" weight running we have been doing over the last year.
For Reference - Index strength is calculated as (Strength/ Basis weight)
Cumulative sum graphs have been created using the following formulae, Each value has been compared to the overall sample mean (i - mean(i)). this value is then added together sequencly to produce the cumulative sum values. The graph increase shows that the quality in this portion has been higher than the sample mean. a decreasing trend line shows that the quality values have been below the sample mean. The data reference is the 01 Jan 2021
Quality parameters - CUSUM Charts (low quality Grades - DU/ Medium)
There is a very significant Peak in May where both SCT and CMT Index drops. Burst has remained fairly stable. there is a drop after the 12/06/2021 but later returns to the same level as in the beginning of the year. ~The most significant change to the paper machine happened on the April shut (27/04/2021), the peak however is after this date, which suggests that the new vacuum fopild had no, or continues to have a positive impact on the Quality.
Change point is between 08/05/2021 and 09/05/2021, quality has declined since. Quality looks to have stabilised since 26/ 07/2021
Quality parameters - CUSUM Charts (High quality Grades - HP+ and DU+)
There is a step change in backwater solids on the 27/04/2021 (this is around the April shut)
Significant work carried out around the shut is as follows; Change of Low vacuum box tops with lower angled foils
Cationic Starch addition
cumulative sum of the cat ionic starch addition, shows a "reversed" shaped graph compared to quality cumulative sum graphs. This coroborates the Quality decline. HP+ grades are more challenging to achieve the quality specifications, cat ionic starch is the variable operators use to help achieve the quality.
PM Steam flow (8 Bar Steam flow to Dryer groups)
Steam flow to the Drying groups (excluding 25 bar line, and Starch kitchen) behaves in a similar pattern as Cat ionic starch/ quality. There are two trains of thoughts that could help to explain the shift in steam usage. firstly, it is believed that cat ionic starch addition to the paper reduces the drainage ability of the sheet. however the paper "The influence of starch on drainage and retention in paperboard mill systems" suggests that cat ionic starch addition around 1% (6-7Kg/T) improves Drainage, but with the addition of cpam (Polymer additive) there is a negative affect in vacuum drainage response. Flocculation with a polymeric additive (cpam) is associated with some air leakage through the sheet, this inhibits vacuum dewatering. Simple put addition of cat ionic starch in combination with retention additives hinder vacuum de-watering resulting in higher couch moisture content.