EP2244848A2

EP2244848A2 - Method for cleaning a heat exchanger

Info

Publication number: EP2244848A2
Application number: EP08871095A
Authority: EP
Inventors: Ursula Hollwedel; Franz Ammann
Original assignee: Areva NP GmbH
Current assignee: Areva GmbH
Priority date: 2008-01-18
Filing date: 2008-12-23
Publication date: 2010-11-03
Anticipated expiration: 2028-12-23
Also published as: JP5627468B2; CA2706303A1; ES2537807T3; AR070183A1; JP2011511918A; EP2244848B1; DE102008005199B4; DE102008005199A1; WO2009089991A2; WO2009089991A3; KR20100123696A; US20100313913A1

Abstract

The invention relates to a physico-chemical method for cleaning the secondary chamber of a heat exchanger in a nuclear plant, wherein the secondary chamber (6) is dried. A cleaning solution is introduced into the chamber to treat the deposits present in the secondary chamber (6).

Description

description

Process for cleaning a heat exchanger

The invention relates to a method for cleaning the secondary space of a heat exchanger, in particular a steam generator of a nuclear installation. With such a method, which results, for example, from EP 0 198 340 A1, deposits which are present on the secondary side in a steam generator and which have formed there during operation are removed.

A heat exchanger has a primary and a secondary chamber, each of which flows through a primary or secondary coolant during operation. The primary coolant heated while releasing a portion of its heat flowing through the secondary chamber secondary coolant. A special heat exchanger is the steam generator of a nuclear installation. In a pressurized water reactor, the primary coolant heated in the reactor core is supplied to a steam generator. With the help of the steam generator, a secondary coolant is heated or evaporated, with which in turn a generator is operated to generate electricity.

While the heat exchanger tubes themselves are usually made of corrosion resistant alloys, the container and support of the heat exchanger tubes are usually made of carbon steel or other low alloy steels. During operation of the nuclear power plant, these parts are subject to corrosion.

Corrosion products, mainly magnetite (Fe 3 O ₄ ), settle as deposits on the surfaces of the secondary space of the heat exchange. These deposits and deposits mainly consist of magnetite, but also contain copper, nickel, zinc, chromium and other elements as well as their compounds.

The primary or pipe side of a heat exchanger, so the

Inside the heat exchanger tubes, is relatively easy to achieve on the primary side water chamber, and therefore relatively easy to clean from any existing deposits. The secondary space of a heat exchanger is relatively poorly accessible and therefore more difficult to clean.

Usually, a tube bundle of heat exchanger tubes extends into the secondary space. In such a tube bundle, the outer or shell sides of the heat exchanger tubes cover each other. On the shell side existing deposits are therefore difficult to remove. In addition to the tube bundle are other fixtures and brackets for mounting the heat exchanger tubes in the secondary chamber. Between the heat exchanger tubes and such support there are a number of hard to reach cracks and crevices in which deposits can accumulate.

The deposits present in the secondary space involve various technical problems. The deposits present on the surface of the heat exchanger tubes lead to a deterioration of the heat transfer between the primary and the secondary coolant. In addition, the deposits cause various damage mechanisms. For example, these can accelerate the corrosion of the affected components. To counteract these technical problems, the secondary chamber of the heat exchanger is cleaned and freed as far as possible from the deposits. In steam generators in nuclear installations, a so-called maintenance cleaning can be carried out in addition to a complete cleaning. In such a maintenance cleaning only a part of the existing coverings is removed. The aim of the maintenance cleaning is to remove the coverings to the extent that approximately the amount is removed from the steam generator, which has formed there since the last maintenance cleaning. Thus, the state of the steam generator can be maintained or possibly slightly improved.

Mechanical cleaning methods for removing the deposits, such as, for example, flushing the tube bottom, have only limited effectiveness or can only be used to a limited extent due to the poor accessibility of the interior of the steam generator. To remove the deposits and deposits, therefore, primarily chemical cleaning methods are used.

DE 102 38 730 A1 discloses such a chemical cleaning method. The steam generator is filled with a cleaning solution containing a complexing agent for the dissolution of iron-containing deposits, and at pressures between 6 and 10 bar, and at temperatures when treated at about 140 ⁰ C. For mixing of the cleaning solution of the steam generator is subjected to sudden pressure reliefs. After the iron-containing deposits have been dissolved, the cleaning solution is drained from the steam generator. If the deposits additionally contain copper or copper compounds, these are then treated with an alkaline cleaning solution. dissolved in the presence of an oxidizing agent and a complexing agent.

Another cleaning method is evident from EP 0 198 340 Al. In contrast to the cleaning method described above, first the copper compounds and then the iron-containing deposits (magnetite) are dissolved in this.

There are also known methods in which both magnetite and copper are mixed with a cleaning solution, i. without being temporarily drained and refilling the steam generator. The existing in the steam generator cleaning solution is changed after dissolution of the magnetite, so that then copper and copper compounds can be dissolved. Such a process is evident, for example, from DE 198 54 342 A1.

A disadvantage of the mentioned chemical processes is above all the high consumption of cleaning chemicals.

The object of the present invention is to provide an alternative purification process which operates with improved efficiency and, accordingly, with reduced use of chemicals.

The object is achieved by a method according to claim 1.

The method according to the invention for cleaning the secondary space of a heat exchanger of the type mentioned at the outset comprises the following steps: deposits which are present in the secondary space are dried, the secondary space being dried by the sewer. customer coolant is mostly emptied. Subsequently, a cleaning solution is introduced into the secondary chamber.

The method according to the invention is based on the following considerations: It has been found that the deposits present in the secondary space of the heat exchanger are mechanically destabilized by drying. As a result, they at least partially burst from the surfaces of the secondary space. The existing on the shell side of the heat exchanger tubes deposits are mainly solved and fall on the tubesheet. In this way, at least a portion of the deposits present on the surfaces of the secondary space can be removed without the use of chemicals. The deposits removed in this way accumulate on the tubesheet of the heat exchanger. The deposits still present on the surfaces are then at least partially removed with the aid of the cleaning solution introduced into the secondary chamber. The method according to the invention is thus a combined physicochemical cleaning method.

Compared to conventional cleaning methods, the chemicals used to dissolve the deposits can be dispensed more sparingly for the following reasons. In particular, the cleaning chemicals can be metered in substoichiometrically based on the mass of impurities present in the secondary space. The accumulated on the tubesheet of the heat exchanger deposits offer, based on their mass, the cleaning solution to a relatively small surface. By contrast, the deposits still present on the surfaces of the secondary space have a comparatively large surface area, based on their mass. Also in In absolute comparison, the total surface area of the deposits present on the surfaces of the secondary space will generally be many times greater than the surface area of the deposits piled up on the tubesheet. The deposits still present on the surfaces of the secondary space, in particular on the shell sides of the heat exchanger tubes, thus provide the cleaning solution with a comparatively large attack surface. For this reason, the deposits still present on the surfaces of the secondary space of the heat exchanger are dissolved many times faster than those deposits piled on the tubesheet.

The cleaning solution used for cleaning the secondary space of the heat exchanger therefore does not have to completely dissolve the deposits and impurities present in the secondary space, and can therefore be metered under the stoichiometric amount, based on the total mass of the deposits. In the cleaning method according to the invention, it is merely waited until the deposits which are still present on the surfaces of the secondary space of the heat exchanger are dissolved. The accumulated on the tube plate deposits are removed after draining the cleaning solution, for example by means of a mechanical cleaning process from the secondary chamber of the heat exchanger. To remove the deposits lying on the tubesheet of the heat exchanger, it can, for example, be rinsed (tube sheet lancing).

Physical drying of the deposits also causes them to crack. These cracks enlarge the

Surface of the deposits, therefore, these provide the cleaning solution a larger attack surface. Also allow the cracks facilitate access of the cleaning solution to the interior of the deposits. Contaminations or pores that may be present within the deposits become accessible through the cracks for the cleaning solution. Compared to conventional cleaning processes, the deposits are more effectively attacked by the cleaning solution.

The physical drying step upstream of the dry cleaning, which can be done, for example, by introducing hot air or inert gas, also causes the water contained in the surface pores and channels of the deposits to be removed. In conventional methods, pores present in the deposits may still be filled with water, so that, on the one hand, the penetration of cleaning solution is severely hindered, and, on the other hand, due to the presence of water, a local dilution which reduces the cleaning action takes place. Due to the upstream physical drying, the cleaning solution can penetrate practically undiluted into the pores and channels of the deposits. The cleaning solution is thus used more effectively than with conventional

Procedure is possible. In a cost-saving manner, the cleaning can thus go faster and with less use of cleaning chemicals.

In a particularly preferred variant of the method, the drying of the deposits present in the secondary space is effected by evacuating the secondary space. To support the evaporation of water according to a further embodiment, the drying takes place both by negative pressure, as well as at elevated temperatures, for example by utilizing an operational residual heat. It has now surprisingly been found that the cleaning effect of a cleaning solution is particularly high, if it is maintained in the secondary chamber vacuum preferably during the entire filling phase. One possible explanation is that the cleaning solution can penetrate more easily under vacuum into the evacuated cracks and pores than is possible under atmospheric pressure. As a result of the evacuation, the cracks and pores contain virtually no gas, which otherwise has to be displaced by the cleaning liquid. The cleaning solution can thus more easily penetrate into the pores and cracks.

Another advantageous effect is that a part of the cleaning solution evaporates when it is introduced into the still hot and additionally subjected to negative pressure secondary chamber. The gaseous cleaning solution condenses on the coverings and precipitates preferentially in the pores and cracks (capillary condensation).

As already mentioned, the drying of the deposits causes them to be mechanically destabilized and at least partially flake off the surface of the secondary space. This effect can be enhanced by boiling the cleaning solution introduced into the secondary chamber according to another embodiment. The cleaning solution present in the pores and cracks of the deposits also begins to boil. The resulting in the pores and cracks, so in the interior of the deposits, overpressure leads to a mechanical destabilization of the same. The heating of the cleaning solution can be effected or assisted by introducing superheated steam into the secondary chamber. The introduced into the cleaning solution hot steam causes in addition to the heating that it is mixed. Thus, unused Cleaning solution to those areas where there are increasingly deposits that can now be resolved.

The deposits, which form in operation on the surfaces of the secondary space of a heat exchanger or steam generator, contain mainly iron oxide (magnetite), but partly also metallic copper and copper compounds. Cleaning solutions can be used to dissolve these deposits, as can be seen from the above-mentioned DE 102 38 730 A1, EP 0 198 340 A1, DE 198 57 342 A1 or EP 0 273 182 A1.

Depending on which combination of chemicals is used for the cleaning solution, the drying step according to the invention is carried out at least once, namely before the steam generator is filled with the cleaning solution. Such an approach is for example when using cleaning chemicals according to DE 198 57 342 Al, in which the steam generator between the magnetite and the copper removal is not emptied attached. In a cleaning process in which the cleaning solution is drained between the magnetite and the copper removal, as provided, for example, in DE 102 38 730 A1, optionally a further drying step can take place after draining off the first cleaning solution. Of course, such an intermediate drying step can also be used a process are carried out in which first the copper and then the magnetite is removed, as is apparent, for example from EP 0 198 340 Al.

The cleaning solutions used are particularly effective at a temperature between 40 ⁰ C and 160 ⁰ C. For this Reason is, according to a development of the method, the existing in the secondary space of the steam generator cleaning solution heated to a temperature in the aforementioned range. The dissolved deposits are removed by draining the cleaning solution from the secondary chamber of the heat exchanger. The unresolved deposits, which have accumulated predominantly at the bottom of the heat exchanger, are removed by mechanical cleaning, for example by rinsing, from the heat exchanger.

According to a further embodiment, the heat exchanger is the steam generator of a nuclear installation. For steam generators in nuclear facilities, the deposits consist mainly of magnetite. Particularly advantageous can with the inventive method of

Steam generators are freed as part of a so-called maintenance cleaning (maintenance cleaning) of magnetite-containing coverings.

The inventive method for cleaning a heat exchanger will be explained in more detail below by way of example with reference to a steam generator of a nuclear installation. The drawing in: Fig. 1 shows a highly schematic steam generator in a longitudinal section and in

Fig. 2 is a detail view of such a steam generator.

The heated in the reactor core of a pressurized water reactor primary coolant flows through the primary chamber 5 of the indicated in Fig. 1 steam generator 2. In the lower part of the steam generator 2 is a plurality of U-shaped bent tubes 4, which are also referred to as a tube bundle. For green the sake of clarity, only two U-tubes 4 are shown. The primary coolant entering the primary chamber 5 flows through the U-tubes 4 while giving off part of its heat to a secondary coolant present in the secondary chamber 6. The steam generator 2 in the lower region of the secondary chamber 6 supplied, now heated or evaporated secondary refrigerant is removed from this in the upper area, and used to operate a generator. During operation of the steam generator 2, deposits 12 are formed in the secondary chamber 6. These form in the region of the holders 8, but especially on the outer or shell sides of the U-tubes 4 themselves.

1 shows a section of the steam generator 2 known from FIG. 1 in the region of the bent U-tubes 4. By way of example, a U-tube 4 through which primary coolant flows is shown, which is held by a holder 8 and a bottom plate 10 penetrates into the primary region 5 opens. At the transitions between the holder 8 and the U-tube 4, and at the transitions between the bottom plate 10 and the U-tube 4, as well as on the shell side of the U-tubes 4 are themselves

Deposits 12 available. In this case, the quantitatively predominant part of the deposits 12 is on the surface of the U-tubes 4 itself.

The sequence of a two-stage purification of the steam generator 2 will be explained below, wherein by way of example the deposits are to contain for the most part iron oxide (magnetite) and to a lesser extent copper:

After switching off the reactor on the primary side of the steam generator 2, the secondary coolant is first discharged from the steam generator 2. Subsequently, the secondary space 6 subjected to a vacuum or evacuated. In this case, the negative pressure is chosen to be at least so great that at the given temperature the negative pressure is sufficient to evaporate the secondary coolant, usually water. Alternatively, the secondary space 6 of the steam generator 2 is dried by introducing hot air. The impurities 12 dry very rapidly under the conditions described, with their surface forming cracks. As already mentioned, due to the loss of volume occurring during drying, the deposits partly burst from their base. The chipped

Deposits accumulate in the region of the lower tube plate 10 of the steam generator 2. The secondary chamber 6 of the steam generator 2 is preferably kept under vacuum, while in this the cleaning solution is introduced. In this case, the secondary chamber 6 of the steam generator 2 is preferably filled to the upper edge of the tube bundle with cleaning solution.

The cleaning solution used to dissolve the magnetite coatings contains a complex-forming acid, for example ethylenediaminetetraacetic acid (EDTA), an alkalizing agent, for example ammonia, morpholine or a mixture of the substances mentioned, and a reducing agent, for example hydrazine. To remove the magnetite-containing deposits, other well-known cleaning solutions may also be used.

To improve the cleaning effect, the cleaning solution is heated to a temperature in the range of 40 ⁰ C to 160 ⁰ C. This is preferably done by introducing hot steam into the steam generator. Alternatively, the cleaning solution is heated by means of the main coolant pump via the primary circuit of the nuclear facility. If the cleaning solution heated to the extent that it boils, this leads to a thorough mixing of the cleaning solution. Alternatively, inert gas is pressed into the steam generator for thorough mixing of the cleaning solution. Spent and unconsumed cleaning solution are mixed, in particular unused cleaning solution in places where still deposits 12 are present, so that they can be resolved. The deposits 12 are additionally removed mechanically by the boiling cleaning solution from the surfaces of the steam generator.

The magnetite deposits dissolved by the cleaning solution are removed by draining the cleaning solution from the secondary chamber 6. The remaining, not solved by the cleaning solution magnetite deposits, which are piled on the tube sheet 10 are removed mechanically, for example by rinsing the tube sheet 10, from the secondary chamber 6.

Before the copper-containing deposits 12 are subsequently removed from the steam generator 2, it is dried again. This further drying step again leads to a physical / mechanical destabilization of the deposits 12 remaining after the first cleaning step.

The copper-containing deposits 12 are dissolved by forming water-soluble complexes of the copper compounds. As a complexing agent, for example, ethylenediamine (EDA), ethylenediaminetetraacetic acid (EDTA) in amoniakalischer solution under oxidizing conditions is suitable. Oxidizing conditions are achieved, for example, by metering in hydrogen peroxide and / or blowing in air or oxygen. After the dissolution of the copper-containing storage ments 12, the cleaning solution is discharged from the steam generator 2.

Claims

claims

1. A process for purifying the secondary space (6) through which a secondary coolant flows during operation of a heat exchanger (2) of a nuclear installation, of deposits (12) formed during operation and on the surfaces of the secondary space (6), characterized by the following the steps:

Drying of the deposits (12) in the case of the secondary coolant (6) which is predominantly emptied by the secondary coolant,

- Introduce a cleaning solution in the secondary chamber (6).

2. The method according to claim 1, characterized in that the secondary space (6) for drying the deposits (12) is subjected to negative pressure.

3. The method according to claim 1 or 2, characterized in that the cleaning solution is introduced into the pressurized with negative pressure secondary chamber (6).

4. The method according to any one of the preceding claims, characterized in that the cleaning solution is heated to a temperature between 40 ⁰ C and 160 ⁰ C.

5. The method according to claim 4, characterized in that the cleaning solution is heated by hot steam is introduced into the secondary chamber (6).

6. The method according to any one of the preceding claims, characterized in that the cleaning solution in the secondary chamber (6) is brought to boiling.

7. The method according to any one of the preceding claims, characterized in that in the secondary chamber (6) existing deposits (12) are at least partially removed by rinsing, from this

8. The method according to any one of the preceding claims, characterized in that the secondary space (6) of the heat exchanger (2) of deposits (12) is cleaned, which are predominantly magnetite.

9. The method according to any one of the preceding claims, characterized in that is cleaned as a heat exchanger of the steam generator (2) of a nuclear facility.