Take care of water! In this way You will maintain a high efficiency of Your Rolling Mill!
Take care of water!?In this way You will maintain a high efficiency of Your Rolling Mill!
Introduction
The Italian group PERT SRL, together with its enviromental department PERTECO , designs and supplies Rolling Mills forlong products, completed with cooling water treatment plants.
In these plants an efficient water cooling process is essential in order to grant a regular production.
A case history for a plant with a productivity of 300.000 t/y of rebar the circulating cooling water could be of approx. 1000 m3/h and 90% of this flow rate is dedicated to the direct circuits, where the water gets in contact with the production material and with different parts of the working machines.
The philosophy which is on the base of the supply of these WTP (Water Treatment Plants) is to provide the cooling process required in a plant with sufficient quantity of water and suitable quality of water. The circulating water must respect certain limits, in order to avoid any problem of incrustation and/or corrosion. After its use the water is typically treated in order to separate the solids and to be cooled before its reuse in the same circuit.
In order to grant a suitable quality of water in the circuits it is necessary to properly manage the drains flow rate from the cooling towers used as cooling equipment.
A suitable water treatment before its use in the plant as make up water is often necessary.For each plant a suitable raw water treatment is suggested for the water available in site (industrial water, natural water from wells, rivers etc.). The treatment depends on the water’s origin and in almost all cases its purpose is to reduce the salt content, the hardness (softening) and to correct the pH. The target is to produce make up water of suitable quality in order to respect the limits imposed for the cooling circuits (for the correct function on the cooling towers) and to reduce the water consumption. In the more modern applications, reverse osmosis is often used.
Generally the consumption is made up by the raw water flow rate, which, to comply with the capacity above mentioned, could be of approximately 30 m3/h.?
Fig. 2 – Process scheme for Rolling Mill WTP
It is often difficult to convince the Customer about the importance of the water quality!
This is especially difficult working in countries where the scarcity of water is a limiting problem for the plant realization.
Particularly, we refer to the areas of the North and central Africa and to the Middle East, where PERT & PERTECO work more frequently!
We now describe a real example which has recently confirmed the importance of the water quality in order to grant the production continuity!
From its start-up the Rolling Mill has been cooled using the raw water as make up water, without treatments or additives, both refused by the Customer to cut the costs.
In spite of the correct functioning of the equipment for the solids separation (scale separator, clarifier, sand filters) and for the water cooling (cooling towers), after few months serious damages have been highlighted, due to the corrosion in different parts of the plant which caused a long production stop for the plant re-establishment.
In order to clarify the cause of damages, PERTECO analyzed the raw water used as make up water.
Analysis
The analysis was made in order to verify the quality of the water and to comply with the values for a correct functioning of the machineries composing the rolling line.
Here following (Tab. 1) the analytical data, determined by a recognized laboratory, and the prescribed limits for the plant in object are reported
Tab. 1 – Chemical analysis of a raw water sample (used as make up water) and limits for the cooling waters
Based on the data at our disposal, according to the above mentioned analysis, we have calculated the indices to evaluate the aggressiveness (tendency to dissolve the incrustations of calcium carbonate) and the water corrosivity (attack to the metallic materials).
Agressiveness Index : Langelier, Ryznar, EPA
Each of these indices shows the encrusting (scaling) and dissolving characteristics relative to calcium carbonate.
In the industrial cooling waters it is suitable to have values near the balance value, or rather water which should be neither too encrusting nor too dissolvent.
In fact incrustations with large thickness reduces the thermal exchange, while its total absence could be an element which allows the metallic corrosion.
Observations:
The indices show that the water has the characteristics of balance (encrustant – dissolvent). It has not encrustant characteristics.
Index of corrosivity: Leroy, Larson
These indices permit to evaluate the corrosive tendency of the water.
The Leroy index takes into consideration the relation between the hardness and the alkalinity of the water, which must be near to 1, in order to avoid corrosion.
Larson index takes into consideration the presence of the chlorides and sulphates, present in corrosive water.
Observations:
Both the indices show a tendency highly corrosive.
Conclusions
The analyzed water does not respect the prescribed limits for the cooling waters and also has a highly corrosive tendency.
The maximum limits have not been respected, particularly regarding the water quality in direct circuits (total hardness, conductivity). The concentration of sulphates and chlorides are at the limit.
As an aggravating element, there is an increase in the concentration suffered by the water inside the circuits due to the evaporation in cooling towers, which takes these parameters to much higher values as the number of concentration cycles increases in working environments.
Consequently there have been clear damages on the machines, like for example in the fast finishing block, made in steel suitable for this type of use, with the condition that the water respects the prescribed characteristics and all the instructions given by the supplier for the correct practice are observed.
In the analyzed case, on the contrary, it has been observed the corrosion in short time, both on carbon steels and on alloy steels, which had wrongly got in contact with the water, also on parts which are not mechanically stressed or in movement (Fig. 3),
?Fig. 3 – Carbon steels and alloy steel in the fast finishing block with corrosion.
As a final support to the Customer, in order to solve the problem, where the stop times for the re-establishment of the plant must be reduced to a minimum, we suggested the use of a multi-purpose chemical additive (antiscalant, anti-corrosion and dispersant) produced by Nalco, which protects the pipes and the machineries. These additives are typically added in very low quantities in the industrial cooling circuits, they do not alter the macroscopic composition of the water, but they practice an efficient protective action on the plants.
The dosing can be done in the make up piping, with an indicative concentration of about 10 ppm to obtain an approx. concentration of 1,5 times superior in the circuit. For this intervention it is necessary to add the correct management of the cooling towers for which it is suggested to work with maximum 1,5 cycles of concentration (water conductivity inside the circuit/ water conductivity of the make up water or make up flowrate/drains flowrate).
The proposed solution is evidently of limited plant impact, since it simply foresees the adoption of a dosing pump which withdraws the additive from the supplying box and a flowmeter installed on the conduit where the additive is added. Nevertheless, it will cause a higher consumption of water in spite of the situation typically reached in a plant where the water, suitably treated, has the proper characteristics to work with 3 cycles of concentration. (Fig. 4)
Fig. 4 – Water consumptions (make up water) with the variations of the cycles of concentration
Furthermore there will be a great cost for the reagents consumption estimable in 100.000 €/y, whose real value will be evaluated on the base of the effective dosing in the plant.
In the light of our evaluation, it could be taken into consideration the possibility of adopting a raw water treatment, which was refused at the beginning!