Pulp Paper & Logistics
SOLENI 21SANDRITZ
Figure 4: NaPO4 ratio in the box results before and after the application of Knowledge Based Control at recovery boiler #4.
Figure 3 – Operating configuration and control through OnGuard iController
Initially it was developed as what is called Coordinated Phosphate, which established only the sodium phosphate molar ratio below 3.0, as a means to prevent free caustic and consequently reduce caustic corrosion possibility and other related problems. This was firstly described by Purcell and Whirl in 1943 (refs 1 and 2). In the years after that, the
new boilers were designed with higher heat transfer flux through the tubes. This demanded some changes on the phosphate based programmes. In 1964, Marcy and
Halsted created the congruent phosphate programme by setting up limits some of the previously described variables. However was only in 1975 that Rosemer and Dale upgraded the widely known pH and PO4 ratio according the boiler operating pressure. Figure 1 illustrates the typical pH and PO4 operating limits depending upon the operating pressure. Certainly after the elaboration of this programme many others where developed: Equilibrium Phosphate, Continuum Phosphate, etc. However, the Congruent Phosphate programme
still is by far the best for systems where the risk of contamination is high and there is also the need of better pH buffering.
The OnGuard iController The OnGuard iController provides a complete solution for the needs of dosage and control associated to pH/PO4 programmes. OnGuard iController automates the dosage and simplifies the control of any programme that demands a pH and phosphate ratio. The main objective is to maintain
water specifications in the pH/ PO4 treatment, automatically, maximising reliability and safety for the boiler. Figure 2 shows a visualisation of the many screens available on the
controller. On this specific screen, it is possible to configure the steam generation system.
Utilised configuration On most installations, OnGuard iController is adjusted to control the automatic dosage of the low and high pH phosphate blend, and also other regular products in chemical programmes (dispersant, oxygen scavenger and neutralizing amines). Figure 3 shows how most installations with a controller such as this are configured.
Case study: pulp plant, high pressure recovery boilers This plant is equipped with two high-pressure (85 bar) recovery boilers. The boiler water treatment has always been congruent phosphate control programmes. Since this was an old site, many variables interfere directly in the box control results, such as: ● very long and large chemical feed lines
● feedwater quality upsets and disturbances
● steam production variability ● manual adjustment of chemical treatment products Due to the above reasons and other occasional difficulties, the time in the box for the sodium phosphate was considered very poor and far from the ideal.
4 January/February 2018
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