FR2501357A1 - Water treatment to de-foul heat exchanger - involves using additives to remove deposits, suspend particles, inhibit corrosion and/or improve filterability - Google Patents

Abstract

Treatment of water used as a heat-transfer medium in heat-exchange equipment is effected by adding additives to the water, successively or simultaneously, during the normal operation of the equipment, and opt. filtering the water to remove suspended solids. The additives must each have one or more of the following properties: (a) removal of deposits; (b) suspension of particles resulting from (a); (c) protection of surfaces uncovered by (a) against corrosion; (d) improving the filterability of the water on washable porous media. Additives of type (a) are NaOH, Na3PO4, Na borate and Na benzoate. Additives of type (b) are Na tannates, Na lignosulphonates and polymethacrylates. Additives of type (c) are (i) reducing agents, e.g. Na tannates or hydrazine hydrate, (ii) pH regulators, e.g. NaOH, Na2CO3, Na3PO4, Na benzoate, Na borate, cyclohexylamine and ethanolamines, and/or (iii) film formers, e.g. Na tannates, Na3PO4, Na silicate, NaNO2 and carbazole derivs. Additives of type (d) are Na tannates, Na lignosulphonates, alginates and synthetic polymers.

Description

Installations for heating premises of any kind,
such as apartment buildings, offices, hotels and hospitals, are almost always
a few months of operation, despite the precautions taken to avoid corrosion and scaling. These failures occur
as a result of the formation of muddy deposits in boilers, pipes, radiators, regulators, etc.
rehabilitation of the facilities then requires many inter
on the part of the maintenance staff and causes an increase in
sensible cost of heating costs.

The muddy deposits that cause the above disturbances are caused by the presence in the heating water of
solid particles of various sizes and natures.

 Among these, mention may be made of iron oxides originating, on the one hand, from the oxidation of the steel during the preparation of the sheets and the pipes in the factory (calamines), and on the other hand, an oxidation of these materials during their storage on site, debris of plaster and cement accidentally introduced into the pipes during the assembly of the heating circuits. Other solids may also be present, but those mentioned above usually occupy first place.

 At the time of mounting the various circuits and heating elements, these solids adhere to the walls of the apparatus.

After the commissioning of the installation, as a result of temperature variations, expansion of the metal, modification of the physicochemical composition of the water, at least a portion of these solids fragment, detach from the metal and are thus hindered by the water circulating in the installation. Once helped by the water
particulate matter settles in places where water velocity is low or zero at the low points of the pipe
the formation of concretions makes that the difficulties mentioned above are not slow to appear.

We have already tried several ways of remedying these disadvantages and among the proposed methods, we can distinguish the following three main techniques
filtration, carried out using screen filters which eliminate the coarser particles, the mesh size being chosen so as to limit the pressure drop in the circuits.

However, the solids suspended in the water of the heating circuit can be too fine (0.5 p> to 10> rn) to be retained by these filters, this is the case in particular calamines. In this case, if the sieves are adapted to retain these particles, one is faced with the problems of a quick clogging of the filters, requiring frequent interventions for their cleaning, and a high pressure loss modifying the flow characteristics of the pumps. water recycling
the addition of dispersants which are stable at the operating temperatures of the installation.

These products keep the particles in addition, provided that the water circulates with sufficient speed (greater than 0, 2 m / s). The use of these products is limited due to the fact that many devices (radiators, hot batteries, etc.) are equipped with devices for regulating the flow of hot water, so that, even during the heating period, water circulates only at speeds too low for these dispersants to be effective.In addition, during periods when the plant is shut down, there is no circulation of water and dispersants, whatever If they are, become totally ineffective, then forms muddy deposits that do not recover much more in suspension after restarting the installation. It is therefore necessary to provide sludge evacuation devices at all low points.
- the treatment of networks in which water circulates, by acid leaching. This treatment consists of dissolving undesirable solids by introducing acid products into the circuit (hydrochloric, phosphoric, sulphamic, citric acids, etc.).

 To carry out this treatment, it is inevitable to stop the installation, to completely empty the circuits after the leaching, and to practice an abundant rinsing followed by a careful neutralization.

Apart from the many operations to be performed that this treatment includes, it has the disadvantage of deteriorating the joints and some metal alloys and destroying the protective layers applied to ensure the inhibition of corrosion of metals.

 These prior treatments to prevent the formation of muddy deposits all have the disadvantage or to be only partially effective, or to require relatively expensive interventions, with interruption of the operation of the installation.

To overcome these drawbacks of the prior art, the subject of the invention is a process for the treatment of water in installations for heat exchange in which this liquid provides thermal transport in the circuit provided for this purpose, characterized in that without interrupting the normal operation of the plant, active or active agents, each having one or more of the following properties, are added to the water successively or simultaneously
a) disaggregation of concretions and / or deposits previously formed,
(b) suspending the particles resulting from the action under (a),
(c) protection against corrosion of surfaces exposed by action under (a),
d) improving the filterability of the water on washable porous substrates, and in that the optionally water is subjected to filtration to remove the solid matter in suspension.

 It has indeed been discovered that it is possible, by a judicious combination of known products, to obtain the effects mentioned under a) to d) above.

 In practice, in principle, the treatment is initially applied to an installation, new or old, embouched, - cleaning treatment - and thereafter spot treatments are applied depending on the introduction of make-up water, - maintenance treatment -.

 The cleaning of the walls consists, as mentioned above, in breaking up the incrustations and deposits that formed during the previous use. The first two activities, mentioned under a) and b) can be carried out by different chemical agents whose choice depends not only on their specific properties, but also on the manufacturing materials of the circuits to be treated with which the water is in contact. In the context of the present invention, one distinguishes first ferrous metal facilities optionally equipped with brass elements, or bronze, and secondly facilities in which the water is in contact with aluminum alloys.

 For the disintegration of the deposits and the dispersion of the released particles, a mixture of alkaline reagents and dispersants is used.

In the case of ferrous metal installations used for heating and operating in hot spots at temperatures above 50 ° C, caustic soda is preferably chosen as the alkaline reagent and sodium tannates as dispersing reagents. In order to improve the deposition attack conditions by the active substances, a surfactant is added, which is advantageously of the fatty amide type, but it can be used well. Other dispersants are sodium lignosulfates and polymethacrylates, lignosulphates, although having better dispersing activity, are discouraged when the water comes into contact with copper alloys.

 Other alkaline reagents are phosphatetrisodique, sodium borate and sodium benzoate.

 The protection against corrosion is advantageously provided by means of sodium tannates in the case mentioned above, that is to say, in the case of installations made of ferrous metals used for heating and operating at hot spots at high temperatures. temperatures above 50 "C.

 It should be remembered that sodium tannates have very diverse activities namely dispersant, reducing oxygen and oxides rich in oxygen, and film-forming, and they are effective filtering agents on porous layers.

 Other corrosion inhibitors may be used, for example hydrazine hydrate. The latter, however, is toxic and should be avoided in installations comprising a circuit for the supply of hot water, and in which an interconnection could be established accidentally between the two circuits.

 Sodium tannates also have the advantage of coloring the water, making the control of the excess of this reagent extremely easy.

A disadvantage of tannates is their low cold-forming activity, which makes it necessary to choose other reagents in cases where the temperature in the circuit is too low.

 Other corrosion inhibitors which may be mentioned are those which at the same time have a pH-regulating action: sodium hydroxide or carbonate, trisodium phosphate, sodium benzoate or borate, cyclohexylamine and ethanol-amines, and than those having a film-forming action trisodium phosphate, sodium silicate sodium nitrite and carbazole derivatives.

 For the protection of circuit elements made of copper alloy, dithiocarbazole is especially used.

 According to the invention it is thus possible to carry out at the same time the cleaning and the corrosion treatment of installations made of ferrous metals used for heating and possibly for cooling operating at hot spots at temperatures above 50 ° C., by introducing during operation a solution containing for example caustic soda, sodium tannate, a surfactant and dithiocarbazole.

 In practice, however, one can still distinguish two cases the installation may contain soft or softened water, or hard water (raw). In the second case, the quantities of alkaline product required are relatively greater than those of sodium tannate and it will be preferable to add different volumes of two solutions, one containing, for example, mainly sodium tannates and the other of sodium tannates and caustic soda in high proportion, which will allow a more flexible assay of the reagents In the case of fresh or softened water (TH 4. 5 "f) a single solution is suitable in most cases and makes the treatment is particularly simple, while being very effective.If nevertheless analyzes reveal that one of the active products is exhausted, it must be completed.Many cases may occur in practice.

 A high consumption of tannates can occur for example when the circuit is very long or very dirty or also when the water frequently comes into contact with air. It can then be completed with a solution of tannate with a low surfactant content.

 Another case may occur in case of the introduction into the circuit of a large amount of clean water. On this occasion the water of the circuit is enriched in acidity (carbonic acid) and oxygen, and it may be necessary to add an additional quantity of caustic soda and tannate to compensate for the losses caused respectively by neutralization and by reduction.

 And finally, in the case where the circuit has relatively large surfaces of copper alloys exposed to water, a complement of dithiocarbazole may be necessary.

 Once this initial treatment has been carried out, periodical maintenance treatments will be carried out according to the amount of make-up water introduced into the circuit. These quantities vary greatly depending on the facility. The additions are determined according to the control analyzes relating to the contents of dispersing agent and corrosion inhibiting agent, and as a function of the measured pH.

 Insofar as the treatments described above cause the presence of solids in the water, which is inevitably the case during the initial treatment, filtration of the circuit water is carried out. This filtration is advantageously carried out on porous support filters, washable without disassembly. This filter is installed as a bypass of the heating water return manifold. The efficiency of this filtration is ensured for a filtered water flow rate of between approximately 5 and 20% of the overall recycling rate (water flow rate on the filter: 10 to 20 m3 / h per m2 of filtering surface).

 As indicated above, the treatment which has just been described applies to ferrous metal heating system circuits operating at temperatures above 50.degree.

 Since the film forming activity of the tannates is very low at low temperatures and cold temperatures, they are not suitable for circuits whose operating temperature is less than 50 ° C, either permanently or periodically as they are. This is the case of mixed circuits used for heating in winter and for cooling in summer, so these circuits should be treated with products having sufficient activity at lower temperatures.

There are, moreover, aluminum alloy circuits in which to protect this metal, the pH must be between 8 and 9. However, in this pH range, the film forming activity of the tannates is also very low, even at high temperatures.

 For the treatments of these two types of circuit, solutions containing sodium nitrite as film-forming agent and sodium borate are advantageously used, both for the regulation of the pH and for its dispersing activity. The addition of sodium fluosilicate further enhances the film forming activity. For the protection of elements made of copper alloys, as indicated above, a carbazole derivative, such as dithiocarbazole, is added.

 Since nitrites and fluosilicates are only compatible in dilute solution, separate concentrated solutions are used in practice, so that these reagents only mix after dilution in the water to be treated.

 The dosage of the reagents for the first treatment is determined according to the alkalinity and hardness of the circuit water. For the steel circuits, the various reagents are dosed so as to maintain the pH between 9 and 10, the simple alkalimetric titer (TA) between 5 and 10% and the water hardness (TH) less than 2%. . The tannate content is either expressed in oxidizable organic matter measured in equivalents of 20 mg of oxygen per liter of water, or simply estimated according to the color of the liquid to be "cognac" or "dark tea". In the first case the content must be between 10 and 20.

 In plants made of aluminum alloys, the limit values to be respected are: pH between 8 and 9 and TH of filtered water <5 "f, the concentration of nitrite, preferably used as film-forming agent, must be understood between 0, 8 and 1, 2 g / l, expressed in NaNO2 The TA is here unimportant.

 The introduction of the reagents is preferably carried out in the expansion tank - supply of the installation.

 To illustrate the process of the invention, a non-limiting example of practical implementation is given below.

Example
The treatment described below was applied to a heating system that had already worked, but which had to be stopped mainly because of a large formation of sludge, both in the circuit and in the boilers.

 The circuit is steel, its capacity is about 80 m3, the operating pressure between 4 and 5 bar. The maximum temperature is preset at 90 ° C.

 The installation is fed with softened city water. The single figure of the accompanying drawing shows schematically how are carried out in the present example the introduction of the various treatment agents and the intermittent penetration of the circuit water.

 In this figure, the tap water with which the installation is fed, passes through the duct 1 in a softener before being driven to the tank 4 expansion and supply. In this tarpaulin is carried out mixing with the various treatment agents. These are added as ready-to-use solutions in tanks, only one of which is shown (ref.3). The boiler 5 with the pumps 6 and 7 is mounted quite conventionally.

The filter 8 equipped with a pressure drop gauge 10 and a flow meter 11 is connected to the main circuit so that it can be switched on when the analyzes show the need. A corrosion measuring probe and a control tube are also arranged in the filter circuit. The pipes 14 and 15 make it possible to carry out the washing of the filter, with raw and softened water respectively. The pipes 13 conduct the water to the heating circuit.

 The filter has a diameter of 1.25 m and a filter surface of 1.2 m 2. The washing is carried out when the pressure drop is 0.3 bar higher than that of the clean filter, for 10 to 15 minutes at a flow rate of 25 m3 / h / m2 (= 5 to 8 m3).

For the actual treatment, two solutions are used
solution A, which is strongly reducing and weakly alkaline, contains 300 g / l of sodium tannate and 10 g / l of a fatty amide (surfactant)
- Solution B, strongly alkaline and weakly reducing contains 100 g / l of sodium tannate, 150 g / l of NaOH and 10 g / l of a fatty amide (surfactant).

The attached single table gathers the results of the control analyzes carried out at one-month intervals from the initial treatment, for 6 months, as well as the quantities of the solutions.
A and B added to correct these results, so that the different quantities are included in the limits indicated below.

At the bottom of each column is further indicated whether filtration has been necessary and for how long.

 Limit values to be respected are as follows: pH between 9 and 10, TH (2 "f, TA between 5 and 10" f, the color must be yellow-brown.

BOARD

<tb><SEP> and <SEP><SEP> Intervals <SEP> months
<tb> treatment <SEP> 0 <SEP> 2 <SEP> Z <SEP><SEP> 3 <SEP> 4 <SEP> 5 <SEP> 6
<tb> TH (f) <SEP> 2 <SEP> 0, <SEP> 1 <SEP> 0. <SEP> 6 <SEP> 4 <SEP> 0, <SEP> 5
<tb> TA <SEP> (f) <SEP> 6 <SEP> 0.3 <SEP> 6 <SEP> 18, <SEP> 5 <SEP> li <SEP> 1 <SEP> 4.5 <SEP>
<tb> pH <SEP> 9.4 <SEP> 10.5 <SEP> 10 <SEP> 8.45 <SEP> 9.1
<tb> Fel <SEP> (mg / l) <SEP> 9 <SEP> 0, <SEP><SEP> 6 <SEP> 0, <SEP> 3 <SEP> 1 <SEP> 0, <SEP> 6
<tb> Fe <SEP> (mg / l) <SEP> - <SEP> 0.25, <SEP> 0.1 <SEP> 0.06 <SEP> 0.3 <SEP> 0.06
<tb><SEP> 2
<tb> Hardware <SEP> in
<tb> suspension <SEP> 120 <SEP> 13, <SEP> 5 <SEP> 2 <SEP> 1
<tb> (mg / l)
<tb> Quickly <SEP> Géné
<tb> se <SEP> of <SEP> rali- <SEP> 10 <SEP> 2.5 <SEP> 7
<tb> corro- <SEP> se <SEP> 65 <SEP> 2
<tb> sion <SEP> loca
<tb> loca- <SEP>
<tb> an) <SEP> read <SEP> 5 <SEP> 3
<tb> an)
<tb><SEP> brown- <SEP> yellow <SEP> yellow <SEP> yellow <SEP> brown- <SEP> yellow <SEP> brown
<tb><SEP> oir <SEP> pale <SEP> mild <SEP> yellow <SEP> pale <SEP> yellow
<tb> Aspect <SEP> om- <SEP> ment <SEP> slight- <SEP> slight- <SEP> I <SEP> practic <SEP>
<tb><SEP> many <SEP> ruble <SEP> ment <SEP> ment <SEP> quemen <SEP> t <SEP>
<tb><SEP> particulu <SEP> disorder <SEP> disorder <SEP> limpid
<tb><SEP>
<tb> Reaction <SEP> sol <SEP> 0, <SEP> 5 <SEP> 0, <SEP> 05 <SEP> 0, <SEP> 19 <SEP> 6 <SEP> 0 <SEP> 0, <SEP> 06 <SEP> 0
<tb> tifs <SEP> A
<tb> ajou
<tb> tees <SEP> 3 <SEP> sol <SEP>0;<SEP> 5 <SEP> 0, <SEP> 18 <SEP> 0, <SEP> 06 <SEP> 0 <SEP> 0 <SEP> 0.12 <SEP> 0, <SEP> 06
<tb> (1 / m <SEP>) <SEP> B
<tb> Mise <SEP> in <SEP> cir
<tb> cooked <SEP> of <SEP> filter
<tb> Duration <SEP> - <SEP> 4 <SEP> 8 <SEP> 20 <SEP> 30 <SEP> 30 <SEP> 45
<Tb>
Fel = Total iron
Fe2 = Iron dissolved after filtration 0.45 m

Claims (1)

CLAIMS I - Process for the treatment of water in installations for heat exchange in which this liquid provides thermal transport in the circuit provided for this purpose, characterized in that during normal operation of the plant is added to water, successively or simultaneously, active agents each having one or more of the following properties: a) deagreosation of the concretions and / or deposits previously formed, b) suspending the particles resulting from the action under a),  (c) protection against corrosion of surfaces exposed by action under (a),  d) improving the filterability of the water on washable porous substrates, and in that the optionally water is subjected to filtration to remove the solid matter in suspension.  2 - Process according to claim 1, characterized in that one uses for the accelerated disintegration of previously formed deposits solutions of caustic soda, trisodium phosphate, sodium borate or sodium benzoate.  3 - Process according to claim 1, characterized in that solutions of sodium tannates, sodium lignosulphonates or polymethacrylates are used for the dispersion of solid materials.  4 - Process according to claim 1, characterized in that one uses as reducing corrosion inhibitors solutions of sodium tannates or hydrazine hydrate.  5 - Process according to claim 1, characterized in that the following are used as corrosion inhibitors having a pH regulating action: sodium hydroxide or carbonate, trisodium phosphate, sodium benzoate or borate, cyclohexylamine, or ethanolamines.  6 - Process according to claim 1, characterized in that used as corrosion inhibitors having a film-forming action sodium tannates, trisodium phosphate, sodium silicate, sodium nitrite, carbazole derivatives.  7 - Process according to claim 1, characterized in that it uses as filtering aids: sodium tannates and lignosulfonates, alginates, synthetic polymers.  8 - Process according to any one of claims 1 to 7, characterized in that the active agents are added in one or more times in amounts sufficient to meet in the water to be treated in a steel installation the following limits: pH between 9 and 10, the simple alkali titer (TA) of between 5 and 10 to f, the hardness of water (TH) less than 2 ", and the content of oxidizable organic material of between 10 and 20 aeg of 20 mg of oxygen / ml.  9 - Process according to any one of claims 1 to 7 characterized in that the active agents are added in one or more times in amounts sufficient to meet in the water to be treated in an aluminum alloy installation the limits following pH between 8 and 9, water hardness (TH) less than 5 f, nitrite content between 0.8 and 1.2 g / l expressed in NaNO2.