Antecedentes Nacionales

General Considerations

The MSF process is a materials-intensive process, approximately 25 Kg of heat exchanger tubing is required for each cubic meter of output per day. Evaporator bodies, tube plates, tube support plates, waterboxes, piping and pumps all require considerable amounts of materials. In order to optimise materials selection in terms of cost and performance, it is

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necessary to use a variety of materials, restricting the more expensive alloys to those areas of the plant where they are needed. As the cost of alloy materials, such as stainless steels, have decreased in cost relative to carbon steel and maintenance costs, there has been a tendency for the industry to make greater use of alloy materials to achieve higher reliability and lower overall water costs. Thus, factors influencing materials selection are considered for the main components of MSF units in the following sections.

Heat Exchanger Tubing

A typical MSF unit can be considered in three sections as regards technical requirements for tubing. These are the reject section, brine heater and recovery section. Table 5 [20]

shows that the failure rates for tubing in these three sections of the plant vary significantly.

Table 5. Failure Rates (Including Tube Replacements) for All Alloys in Distillation Plants Tubing

Component Failure Rate (%)

Brine Heater 4.90

Heat Recovery 0.81

Head Reject 2.46

Heat Rejection Section. This section handles natural sea water, and copper-base alloys have often been selected for tubing. The most common cause of failure in these alloys is impingement attack, which can be caused by partial blockage due to debris passing the screens, unsatisfactory flow conditions in the water boxes and unsatisfactory entry conditions to the tube, i.e., any condition that can lead to local turbulence and high velocities (i.e., the normal design velocity is about 2 m/sec.) Table 2 gives data on the resistance of copper base alloys to this type of attack, and it is useful to note that the cost of these alloys is on the same order as their resistance to impingement corrosion.

In unpolluted sea water with good intake screening and flow conditions, the copper base alloys perform well. However, modifications to the sea water composition by pollutants or by over-chlorination can give rise to corrosion problems. These are considered below.

Effect of Sulphides. Sulphides may be present in sea water from the decomposition of organic matter, effluents from nearby sewage works or action of sulphate reducing bacteria (SRB). Normally when sulphides form in the sea water, there is a reduction in oxygen content, and in some cases the oxygen is completely removed. In this case, corrosion is not severe. However, in most cases the sulphides exist in aerated sea water, and this mixture of sulphides and oxygen, or preexposure to sulphides followed by exposure to aerated sea water, is very damaging to most copper base alloys. The sulphides enter the protective films and reduce their resistance so that they are easily damaged by flowing sea water at velocities which they would normally withstand. Table 6 [21] illustrates this effect on four widely used heat exchanger alloys. Sulphide levels as low as 20 ppb can be damaging to copper base alloys.

Although copper base alloys such as cupronickels are not prone to severe pitting or crevice corrosion in static natural sea water, they can suffer this type of attack if sulphides are

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present. Where the heat exchanger has operated in clean sea water and has formed good protective films, these alloys can endure occasional exposure to sulphides. Ferrous sulphate dosing can also reduce the effects of sulphides [22]. However where sulphides occur regularly, the use of titanium can often be justified.

Table 6. Effects of Sulphides and Chlorine on Impingement Attack in Sea Water

Alloy Depth of Attack (mm)

Sea Water 0.1 ppm Sulphides

0.25 ppm Chlorine Aluminium Brass 0.26-0.28 0.56-0.90 0.07-0.20 90/10 Cupronickel 0.04-0.06 0.24-0.44 0.09-0.10 70 /30 Cupronickel 0.07-0.12 0.62-1.03 0.12-0.15 66/30 /2/2 CuNiFeMn 0.01-0.02 0.98 0.05 Velocity in test zone 7.5 m/sec.

Effect of Ammonia. When organic matter decomposes in sea water, ammonia forms as well as sulphides. It may be present from other sources such as effluent from ammonia plants, of which there are several in the Arabian Gulf, and sewage plants. The main effect of ammonia is to cause severe pitting under crevice conditions with heat transfer [23]. Table 7 gives data on the effect of ammonia under crevice conditions. The pitting is often accompanied by deposition of copper, sometimes outside the pit. Addition of iron markedly reduces this type of attack, as shown by data in the table.

Table 7. Crevice Corrosion in Sea Water Containing Ammonia

Alloy Depth of Attack (mm)

No Iron Plus Iron

Ammonia 0 ppm 2 Ammonia 0 ppm 2 Aluminium brass 0.010 0.090 0.010 0.000 90/10 Cupronickel 0.015 0.100 0.015 0.000*

70/30 Cupronickel 0.015 0.075 0.000 0.015 66/30/ 2/2 CuNiFeMn 0.000 0.065 0.005 0.000*

*Incipient pits-too small to measure; average of two tests in each case.

Two month test in Campbell test rig. 0.042 ppm iron added continuously.

Effect of Chlorine. Chlorine, added to control marine growth, can influence corrosion of copper base alloys. As can be seen in Table 8, low levels of residual chlorine are not damaging, indeed in some cases, they reduce corrosion. However, at higher levels, chlorine increases the effect of flow, and impingement attack becomes more likely [24, 25].

Effects of sand. Suspended matter, such as sand, can cause corrosion effects on copper base alloys in sea water. If it forms deposits in tubes, then some deposit attack, which is a

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form of crevice corrosion, can occur beneath the deposit. Under flowing conditions, sand can cause damage to the protective films on copper alloys giving rise to impingement attack. In this context, two factors are important, namely, the sand content and the size of the sand particles [26]. The cupronickels have better resistance to sand than aluminium brass. The 70/30 cupronickel with 2% iron and 2% manganese was developed specifically for use in waters with high sand content, e.g., 4000 ppm, and is often specified for reject section tubing.

Titanium has excellent resistance to sand erosion and is used where very high sand content occurs.

Table 8. Effect of Carbon Steel and Ni Resist Type II on Crevice Corrosion of Type 316 Stainless Steel in Sea Water

Area Ratio Crevice Attack

(SS:C. Steel or Ni Resist) No of Sites

30-day test; multicrevice assembly with 120 crevice sites; flow velocity 0.5 m/sec Heat recovery Section. The data from Table 5 indicate that this section of the plant presents fewer corrosion problems than others. This is to be expected as in the recovery section, the sea water and brine are usually deaerated so that there is little oxygen to support a corrosion reaction. The main corrosion risk is from vapour side corrosion, and as this is more likely in the first few stages, where carbon dioxide is evolved by the decomposition of bicarbonates in the sea water, these are normally vented directly to the vacuum system. Also, the tubing is made from materials with higher resistance to vapour side corrosion, that is to say, 70/30 cupronickel rather than the 90/10 CuNi alloy or aluminium brass.

When vapour side corrosion occurs, it takes the form of fairly uniform thinning of the tube wall which eventually perforates. The attack is often concentrated at the tube plates or tube support junction with the tube and with acidic condensates. This is sometimes reported as a galvanic effect, for example, when the tube support plates are stainless steel, but it can occur when these are of the same material as the tubing or are even of carbon steel.

Brine Heater. The data in Table 5 indicate that corrosion conditions in the brine heater are the most severe in the plant. However, if the causes of these failures are examined [20], it can be seen that most failures are due to mechanical damage during descaling rather than to corrosion. The need, therefore, for tubing in this section is for mechanical strength and for this reason, 70/30 cupronickel is the usual choice, and it is normally used at at 1.2 mm thickness.

Tube Plates

The main requirements of tube plates are mechanical strength and corrosion resistance.

They should be galvanically compatible with the tube material and preferably slightly anodic to it so as to give some cathodic protection. As they are much thicker than the tube, some corrosion can be tolerated provided it does not disturb the tube/tube plate joint which is

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usually made by roller expansion.

Water Boxes

The most popular type of MSF design is the cross-tube type, and this requires a large number of water boxes with interconnecting piping. For large units, the most economic material for water boxes is carbon steel, but this needs protection if severe corrosion is to be avoided. Paint coatings have not proven to be reliable means of protection as they are easily damaged and require increased maintenance and inspection. Thick (3-5 mm) rubber linings can be used for the reject section but are not reliable for the hot recovery sections where they can become detached under vacuum. The most common solution for large water boxes is the use of 90/10 cupronickel clad steel with a 2-3 mm thickness of the alloy material. This has been used in many plants and is an economic and reliable form of construction.

Evaporator Bodies

The most common material of construction for evaporator bodies is carbon steel. In early plants, this was the only material used, but experience has shown that it could suffer severe corrosion, even in deaerated sea water and brine and alloy materials were introduced.

The use of stainless steels such as Type 316 (low carbon or stabilised grades) began with attempts to repair areas of severe attack and proved very successful. In the absence of oxygen, the well-known tendency of these alloys to pit in sea water was inhibited, and provided care was exercised at shutdowns, when oxygen had access to the plant, stainless steel proved reliable.

An alternative to stainless steels for chamber lining is 90/10 cupronickel. This has been used successfully for many years for evaporator shells for small plants [27] and in clad steel plate form can be used to construct large evaporator shells.

Components in the brine spaces such as brine gates, weirs and nozzles, are normally made from stainless steel even in unlined spaces. The galvanic effect is slight and does not usually give rise to any problems. However, where only part lining with stainless steel is used, there is a risk of galvanic attack where the large area of stainless steel meets the carbon steel. This can be controlled by coating the stainless steel in the final lined stage.

Vapour Spaces

The mixture of water vapour, carbon dioxide and air entering the vapour spaces causes condensation on the walls and roof of the vapour spaces. These may cause corrosion as they are slightly acidic (due to the dissolved carbon dioxide), and if sufficient oxygen is present, it may add to the corrosion rate. Corrosion of steel in these spaces has been studied [16], and it was concluded that the main cause was oxygen in-leakage to the vacuum stages. Corrosion in these spaces is normally found in the mid-plant areas, and the highest rates of attack are in those stages immediately before the stage where cascaded gases are vented. Attack is also greatest above the distillate transfer trough where gases from the higher temperature stages are released as the distillate flashes on passing down the plant. Serious cases have been seen in acid dosed plants with excellent deaeration. These findings, together with the absence of carbonates in the corrosion product, which is mostly magnetic iron oxide Fe₃0₄ led to the conclusion that carbon dioxide played only a small part in the attack and that air in-leakage 79

was the basic cause. Careful attention to air tightness in one affected plant led to a 60%

reduction in the corrosion rate.

As stainless steels have very high corrosion resistance to carbonic acid and low-chloride distillate, even when oxygen is present, the use of this material for interstage walls and roof lining can control this type of attack. Stainless steels are often used in these spaces for parts such as tube support plates, distillate troughs, tube bundle stay bars and vent piping.

However attention to air in-leakage is advised where carbon steel is used.

Demisters

Wire mesh demisters are normally fitted to prevent carryover of droplets of brine.

Because the wire is very thin, usually 0.1-0.2 mm diameter, it is necessary to use materials with high resistance to general corrosion. Originally the nickel-copper Alloy 400 was widely used and gave good performance except where sulphides were evolved from the brine. This alloy is still used but most plants now use Type 316 stainless steel which performs well even when sulphides are present.

Venting System

Piping and Ejectors. Vacuum on the evaporator is normally produced by steam ejectors drawing gases from different sections of the plant. The first three stages are normally vented directly to the system, and gases evolved in lower temperature stages are cascaded to two or three points before being drawn off into intermediate ejectors and condensers. Where the last stage is used for deaeration or if a separate deaerator is fitted, the gases are drawn off into the deaerator condenser. Stainless steels Types 304 or 316 are normally used for the vent piping, headers within the piping system and baffles within the tube bundles. At lower temperatures some use has been made of glass-reinforced plastic piping (GRP), but stainless steels are needed for the higher temperatures, and most plants use them throughout.

The ejectors are also made from stainless steel with nozzles in grades such as Alloy 20.

Provided the chloride content of the gases remains low, the stainless steels work well, the only risk being from external stress corrosion cracking at temperatures above about 60°C. To minimise this risk, painting of the stainless steel is advised, particularly under insulation where chlorides can accumulate.

Condensers. Two types of condenser are used in venting systems, namely, shell and tube type, and barometric. As they have different materials requirements, they are considered separately.

The shells of shell and tube type condensers are normally made in Type 316L stainless steel to resist the incondensible gases. In some cases, copper alloys such as aluminium bronze are used, but this is not common. For tubing, copper base alloys have a limited life due to vapour side attack in the high concentration of gases drawn from the plant which are mixed with ammonia from the ejector steam when hydrazine is used in the boilers. As the attack is usually fairly uniform, several years of life are possible, and some plants use copper base alloys with regular renewal. However in most cases it is more economic to use a resistant material and thin wall welded titanium tubing 0.7 mm thick is the usual choice.

High alloy stainless steel such as UNS S31254 has been used successfully for tubing in ejector condensers in the Middle East. These were roller expanded into an existing naval

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brass tube plate and were in perfect condition after one year of service [28]. The advantage of using this alloy is the ease of descaling compared to titanium.

For shell and tube units, tube plates and water boxes require similar considerations as for the reject section stages. In general copper base alloys are used with anodes in the water boxes to control galvanic effects when titanium or stainless steel tubes are fitted.

Barometric condensers are essentially vessels into which sea water is sprayed. The large deaerator condenser can be made from GRP as temperatures are low; however, at higher temperatures, metals are necessary. Standard stainless steels such as Type 316L are prone to pitting in the sea water spray. Anodes are sometimes fitted to prevent this, but in a spray situation, they are not reliable. As the higher temperature units are relatively small, it is economic to make them from sea water resistant alloys such as the 6% molybdenum stainless steels.

Sea Water Pumps

Wet pit vertical lift pumps are normally chosen for large MSF units. Pump parts must withstand fast flowing sea water, and the normal choice for those parts exposed to the most severe conditions, such as impellers, is stainless steel. The reason for this is evident from the data in Table 4 where stainless steel Type 316 is seen to passivate in very high velocity sea water. The nickel copper Alloy 400 behaves similarly. Although stainless steels are excellent for flow conditions it is also necessary to consider static conditions, and as can be seen from Table 6, they are then subject to severe pitting. Thus, an all stainless steel pump would have problems at shutdown. Such problems have been reported [29].

One method of overcoming this is to make the static parts of the pumps of a material which gives cathodic protection to stainless steels at shutdowns. The material normally chosen is Ni-Resist, a high alloy cast iron. Table 8 [30] gives data on Ni-Resist and stainless steel galvanic couples showing that this combination can control pitting and crevice corrosion under static conditions.

The usual grade of Ni-Resist used is Type D-2W. This has good sea water corrosion resistance but is subject to stress corrosion cracking in warm sea water and brine. Failures have occurred in service on castings which have not been stress relieved. However, when stress relief heat treated for a few hours at 650-675°C and furnace cooled the alloy has proven resistant to cracking. It is essential, therefore, to apply this treatment before service and after any weld repairs.

In recent years, there has been a trend towards the use of duplex stainless steels in sea water pumps. These are stronger than the standard austenitic alloys such as Type 316 and the grades used have higher resistance to pitting and crevice corrosion. In this case, to protect against pitting at shutdowns, it is advisable to operate the pumps for a short period every few days.

Brine Recycle and Blowdown Pumps

These are in many ways similar to the sea water pumps described above, but are barrel-type or canned pumps. The impeller, shaft and other internals are often made from Type 316 stainless steel or its cast equivalent. The casings are often made of Ni Resist Type D-2W, and these again need to be stress relieved as cracking, particularly of diffusers, has been

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experienced in service. The barrels or cans are usually fabricated from clad steels. The cladding is normally Type 316L stainless steel, but 90/10 cupronickel is also used. In some cases, thick, 3-5 mm, vulcanised rubber is used.

Distillate Pumps

The distillate leaving the plant is slightly acidic but not strongly corrosive until it becomes aerated. However, to maintain its purity, it is usual to use stainless steels for the pumps. Type 316 stainless steel is often used. Although this is not really necessary to handle

The distillate leaving the plant is slightly acidic but not strongly corrosive until it becomes aerated. However, to maintain its purity, it is usual to use stainless steels for the pumps. Type 316 stainless steel is often used. Although this is not really necessary to handle