IES66580B2 - Production of purified water process systems - Google Patents

Abstract

Pipe and vessel components of stainless steel having a ferrite content below 0.5% are used and the internal surfaces are polished to an RA value of at least 0.2 p.m. Components are welded (12) with o.2 second pulse time periods both primary and backing current levels of approximately 52 and 21. This provides a weld heat area which is not susceptible to contamination by development of rouge. A maximum of three tack welds are used (13) before welding. A fluid test (17,20) is carried out. There may be both an hydrostatic and pneumatic test, or one of these carried out, depending on the nature of the project. There is comprehensive weld testing, followed by degreasing (23) using an alkaline solution at a temperature of between 50 and 90°C. After rinsing (24), the internal surfaces are passivated (25) using a 10% nitric acid solution at a temperature of approximately 32°C.

Description

Production of Purified Water Process Systems The invention relates to the production of purified water process systems for industries such as the pharmaceutical, food, and the semiconductor industries.

In recent years, production of stainless steel alloys has improved and it is now possible to purchase alloys which have been treated and have good corrosion-resistance properties. However, significant problems have arisen and continue to arise in purified water process systems despite the improved materials and also improved postfabrication operations such as passivation. These problems have been caused fabrication, and installation, easily damaged or contaminated, often resulting in appearance of rouge (red iron oxide) on the inside surfaces of tanks or at seats and faces of valves. Problems can also arise at welds and at heat zones of welds at which carbon forms carbides with the metallic components to form chromium carbide, which is susceptible to intergranular corrosion. by materials handling, Internal surfaces can be Heretofore, much work has been carried out in developing passivation processes. For example, Japanese Patent Specification No. JP 56119895 describes a process which involves heat treatment at 650°C, following which there is treatment for 500 hours in pure water. In Canadian Patent Specification No. CA 8,322,981, a method is described in which there is initial acid cleaning, following by neutralising treatment and passivation. In Swedish Patent Specification No. SS 9,100,514, a method is described in which hot nitrogen gas is used for drying the internal surfaces at a temperature between 70°C and 90°C.

Soo58C - 2 While these cleaning methods are apparently effective for the desired uses, it has been found that the prior fabrication, cleaning and passivation steps do not provide consistent results in a wide variety of purified water process system projects. The present invention is therefore directed towards providing a comprehensive method for production of purified water process systems, which method is consistent in the results achieved and may be applied universally across a wide range of different types of systems.

According to the invention, there is provided a method of producing a purified water process system, the method comprising the steps of :assembling system components according to a design specification, the components being of stainless steel alloy material having an iron content below 0.8% and a chromium content in the range 15% to 22%; polishing internal surfaces to a roughness of less than 0.3 RA; welding the components together by pulse welding, in which the primary current level is in the range 47A to 57A, and the backing current level is in the range of 17A to 25A for pulse times of approximately 0.2 s each to complete a cycle; carrying out non-destructive weld testing; carrying out fluid testing of the welding system by applying half of design test pressure, followed by incremental pressure build up to a full design test pressure; - 3 degreasing the internal surfaces of the system by circulating an alkaline detergent at an elevated temperature in the range 50°C to 90°C; passivating the internal surfaces by circulation 5 of a nitric acid solution at a temperature in the range 30°C to 36°C; and rinsing the internal surfaces with deionised water.

Preferably, the primary current is approximately 52A and 10 the backing current is approximately 21A.

In one embodiment, pulse welding is performed using welding grade argon gas having a purity of 99.96% or greater.

In one embodiment, joints having a circular cross-section 15 are welded by an orbital welding machine, the head of which is controlled to move in a step-wise manner for a lower portion of the joint and in a continuous manner for an upper part thereof.

Preferably, the internal surfaces of the components are 20 polished to a roughness of 0.2 μχη RA value, and polishing is carried out by an automatic rotating polishing machine gradually polishing in 0.1 gm RA value increments.

In another embodiment, the welds are non destructively tested by 100% boroscopy and at least 10% random radiography tests of visible welds, and 100% radiograph tests of non-visible welds.

Ideally, the degreasing step is carried out at a temperature decreasing from approximately 72° to 64°C over a time period between 2 hours and 3 hours.

In a further embodiment, the passivating step is carried out for in excess of 1.5 hrs at a temperature in the range of 32°C to 33°C.

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which :Fig. 1 is a flow chart illustrating a method of the invention for production of purified water process systems; and Figs. 2 to 5 inclusive are diagrams showing parts of a system which is produced by the method of the invention.

Referring to the drawings, and initially to Fig. 1, there is shown a flow chart of a method 1 of the invention for production of purified water process systems. The method 1 comprises steps 10 to 27 inclusive. Parts of process systems which are produced by the method 1 are shown in Figs. 2 to 5, and to help understand the context of the method 1, these are now described. In Fig. 2, part of a circuit 31 for circulation of purified water is shown. This circuit comprises a flow restrictor 32, which is shown in more detail in Fig. 3. The flow restrictor 32 comprises a throat portion 40 connected by orbital welds 41 to pipes. Various drops 33 are connected in both the circuit 31 and auxiliary cold water sub-circuits 3S and 37. As illustrated in Fig. 4, each drop 33 comprises a Ushaped conduit 45 having orbital welds at each end of two - 5 elbow joints. A valve 46 is connected to the lower portion of the drop 43 and this connects the drop 33 to a further process device such as a vial washer, a sink or an autoclave. As illustrated in Fig. 4, each drop 33 further comprises a thermometer 47 of the wrap thread type. The circuits 31, 36 and 37 further comprise temperature sensors 34 and a heat exchanger 35. A water vessel 50 is shown in Fig. 5 in which there is a bursting disc 51 and a lower outlet 52, which is connected to a tri-clover centrifugal pump 53.

The systems illustrated in Figs. 2 to 5 inclusive are typical of the type of system which may be produced by the method 1 of the invention. It will be appreciated that there is a large number of welded connections.

Accordingly, extensive materials handling and welding operations are required - all providing possible sources of contamination.

Referring again to Fig. 1, the method of the invention is now described. The first step, 10, involves assembly of the various components which are required for the specified system. The material used is a stainless steel having an iron content of less than 0.8% and preferably approximately below 0.5% and a chromium content in the range of 19 to 21%. The chromium content is preferably approximately 18 to 19%. The following is the composition of the important elements in material used. Material Comoos ition . C Si Mn P s Cr o 30 >0.02 Mo 4.0 - 5 <0.70 Fe .0 0.5 <2.0 Ni 24.0 - <0.03 26.0 0.04 <0.015 N - 0.15 19 - 1.0 - 21 Cu 2.0 - 6 An important aspect of the material is its low ferrite content.

In step 11, the internal surfaces of the components are polished to an RA value of 0.2 μπι. This is achieved by use of an automatic rotating polishing machine which gradually polishes the surface from an RA value of 0.9 μπι to 0.2 μπι in 0.1 μπι increments.

In step 12, the current levels and pulse times periods for an orbital welding machine to be used are set. These parameters are extremely important as they set the correct treatment of the weld material and the components to prevent impurities arising. It has been found in particular that a primary current value in the range 4 7A to 57A and preferably 52A and a backing current value in the range 17A to 2A and preferably 21A are particularly suitable when applied in pulses of 0.2 seconds each for a full cycle of 0.4 seconds. This welding method is particularly effective at providing a weld heat zone which does not tend to corrode over time. This is because it prevents excessive heating of the weld to maintain the ferrite content at or below 0.5% in the weld zone.

In step 13, components are tack welded together by mounting on suitable alignment fixtures and no more than three tack welds are made at each joint.

The orbital welding machine is then operated in step 14 according to the settings which have been inputted. The welding gas used is welding grade argon at a purity of at least 99.996%. The orbital welding head moves in stepwise fashion for the lower part of the components, and in continuous fashion for the upper part. This prevents loss of molten steel. As outlined above, the levels of primary and backing current and the pulse times have been found to be particularly suitable for maintaining a low ferrite content. Any non-circular joints are pulse welded with * the same parameters by a linear welding control machine.

When step 14 is complete, a number of sub-assemblies are ready for on-site installation. They are transported by a skid unit and in step 15, the sub-assemblies are installed on-site by making the necessary ferrule joints.

After welding, various non-destructive tests are carried 10 out to test integrity. For visible welds, there is 100% boroscopv testing and 10% random radiograph tests. There is 100% testing of non-visible welds. Any new welds which are made as a result of the testing are examined by use of a liquid penetrant test.

When a complete circuit or sub-circuit has been constructed (step 15) a visual check is made in step 16 of all joints. A fluid test which can be either an hydrostatic test 17,18,19 or a pneumatic test 20,21. For particularly critical applications, both tests are carried out to check for any weakness in welded joints.

The hydrostatic test involves pumping water in the circuit to a pressure of 1.5 times design pressure, 50% of the pressure being applied initially followed by 10% increments over one hour time periods. The full pressure is maintained for at least three hours. When completed, the circuit is then vented and drained and is subsequently » dried by passing warm, dry and oil-free air through the circuit.

I* For a pneumatic test, clean, dry air is used at a pressure in the range of 3 - 4 bar, half of the pressure being applied initially and the remainder in 10% increments.

This leaves enough rime for the system to equalise strains. At each increment, the welds are tested by a liquid penetrant test. The system is de-pressurised slowly in step 21 when the test is complete and any necessary repair work is carried out. The duration of the test is one hour. Any faults identified are then repaired in step 22.

After testing integrity of the joints, the internal surfaces of the system are degreased in step 23 by circulating an alkaline detergent to remove any polishing compounds, construction dirt, protective oils or any other contaminant. The temperature is preferably in the range of 50°C to 90°C, and most preferably in the range 64°C to 72°C. For the latter temperature range, there is preferably a time duration of two to three hours, and most preferably 2.5 hours. The alkaline detergent which is used is that marketed under the name METFIN AK10™ and the concentration used is 5 - 100 g/1 of de-ionised water. The contact time is preferably 10 minutes.

After degreasing, the internal surfaces are rinsed in step 24 with de-ionised water at a pH in the range of 5 - 8, preferably approximately 7.

The internal surfaces are then passivated in step 25 by circulation of a warm 10% nitric acid solution at a temperature of 32 - 33°C for a minimum period of 1.5 hours, and preferably 2 hours. It has been found that this results in deposit of an enriched chromium nickel layer which effectively combines with oxygen in the air to perform a dense chemically resistant passive layer of chromium oxide. It is important that all wetted internal surfaces of the system including all pipework and vessels are passivated in this manner.

After passivation, the internal surfaces are again rinsed in step 26 with de-ionised water with a pH of approximately 7. In step 27, there is a final visual inspection and any additional tests deemed appropriate by quality supervisors are carried out.

It has been found that the method of the invention is versatile and comprehensive as it may be used in a wide range of different purified water process system projects to provide the desired quality.

The invention is not limited to the embodiments hereinbefore described, but may be varied in construction and detail.

Claims (5)

1. A method of producing a purified water process system, the method comprising the steps of :assembling system components according to a design specification, the components being of stainless steel alloy material having an iron content below 0.8% and a chromium content in the range 15% to 22%; polishing internal surfaces to a roughness of less than 0.3 RA; welding the components together by pulse welding, in which the primary current level is in the range 47A to 57A, and the backing current level is in the range of 17A to 25A for pulse times of approximately 0.2 s each to complete a cycle; carrying out non-destructive weld testing; carrying out fluid testing of the welding system by applying half of design test pressure, followed by incremental pressure build up to a full design test pressure; degreasing the internal surfaces of the system by circulating an alkaline detergent at an elevated temperature in the range 50°C to 90°C; passivating the internal surfaces by circulation of a nitric acid solution at a temperature in the range 30°C to 36°C; and rinsing the internal surfaces with deionised water.

2. A method as claimed in claim 1, wherein the primary current is approximately 52A and the 5 backing current is approximately 21A.

3. A method as claimed in any preceding claim, wherein pulse welding is preceded by tack welding with three or fewer tack welds while components are supported on an alignment fixture. 10

4. A method substantially as hereinbefore described with reference to the accompanying drawings.

5. A purified water process system whenever produced by a method as claimed in any preceding claim.