Hgs

C*-L*-DR*

* Head must be less than 600 ft/stage (180 m/stage) Materials:

B—Cast iron casing—bronze impeller—cast iron/bronze rings C—All cast iron

DR—Carbon steel casing—cast iron impellers G—All 12% chrome (CA-15 or CA-6NM)

H—Carbon steel case—12% chrome impellers (CA-15 or CA-6NM) J—Cast iron case—bronze impellers—bronze rings L—All cast iron with 12% chrome rings

NM—1- or 2-chrome barrels—12% chrome impeller (CA-15 or CA-6NM) S—All 316 stainless (CF-3M)

* Head must be less than 600 ft/stage (180 m/stage) Materials:

B—Cast iron casing—bronze impeller—cast iron/bronze rings C—All cast iron

DR—Carbon steel casing—cast iron impellers G—All 12% chrome (CA-15 or CA-6NM)

H—Carbon steel case—12% chrome impellers (CA-15 or CA-6NM) J—Cast iron case—bronze impellers—bronze rings L—All cast iron with 12% chrome rings

NM—1- or 2-chrome barrels—12% chrome impeller (CA-15 or CA-6NM) S—All 316 stainless (CF-3M)

represents an industry consensus on the recommended materials choices for the full range of high purity waters.

Saline Water Saline waters have been defined as those that are sufficiently electrically conductive to enable an appropriate pump casing material to galvanically protect the pump internals when the pump is shut down. This corresponds to waters with more than about 1000 ppm chloride. Many pump applications involve saline waters. Among the more common saline water applications are the following:

• Tidal river water The chloride level here can fluctuate significantly with the season and the ingress of salt water from a bay or estuary.

• Groundwater The chloride level and corrosivity can vary over a wide range. Some groundwaters, which are injected by high-pressure pumps into oil formations to enhance output, are very corrosive, due to low pH and very high chloride levels.

• Geothermal water This type may contain high levels of hydrogen sulfide, carbon dioxide, and other gases in addition to chlorides.

• Oilfield brines These are often deaerated, greatly reducing their corrosivity. Less corrosion-resistant pump materials may be used but are susceptible to corrosion during shutdown periods when oxygen cannot be effectively excluded from the water.

• Sea water The chemical composition of seawater is relatively uniform throughout the world. Other factors, including temperature, microbiological activity, and the presence of pollutants can alter the corrosivity of the seawater to pump materials of construction.

For each of the saline waters, a variety of materials has been used for pumps. The choice of materials for a particular application will depend on the water chemistry and other factors including the expected life of the pump, whether it will operate continuously or sit idle for long periods, and user preferences based on previous experiences. Some general considerations will influence material selections.

The materials for saline water pumps must resist erosion corrosion. Ni-Resist and copper base alloys are frequently specified but have velocity limits, above which the protective oxide film is stripped off and accelerated corrosion occurs. Among copper base alloys, nickel aluminum bronze can tolerate the highest velocity. The pump designer needs to be aware of these limitations and use bronze and Ni-Resist only for components when the velocity limits of the materials will not be exceeded.

Stainless steels develop a more tenacious oxide film than bronzes and can tolerate velocities much higher than those seen in pumps without suffering erosion corrosion. However, stainless steels are susceptible to pitting and crevice corrosion in stagnant sea-water. These problems are exacerbated if marine biofouling occurs. Several methods exist for handling this problem. The stainless internals of a pump can be effectively protected by galvanic coupling with Ni-Resist. The combination of a Ni-Resist case and stainless steel internals is widely used because of this favorable galvanic relationship.

To avoid localized corrosion during shutdown in an all-stainless pump, some form of cathodic protection is required. This can be either sacrificial anodes or an impressed current system. It is also possible to construct the pump of stainless grades that are highly alloyed and develop adequate corrosion resistance. This approach requires either 6% molybdenum austenitic grades or 25%, 3% molybdenum duplex grades. These materials are considerably more expensive than standard 300-series austenitic stainlesses and see limited use in critical applications. Higher alloyed duplex grades have become the universal standard for high-pressure injection pumps, especially those used in offshore locations. The high mechanical properties enable the design of lighter, smaller pumps. The weight saving is an important factor in offshore applications.

Many large sea water pumps are constructed of cast iron with bronze internals. Provided the velocities are not too high, this combination of materials has been known to provide approximately 20 years of service in some large vertical pumps. Nickel aluminum bronze is preferred over tin bronze for the impeller because it is stronger, has better resistance to high velocity, and is more easily weld-repaired.

Monel shafting is no longer commonly specified for sea water pumping applications. This material is expensive and will develop pitting in stagnant water. Several grades of stainless steel will provide a combination of strength and corrosion resistance equivalent to Monel at a significantly lower cost. These include Nitronic 50 and several of the higher alloyed duplex grades, such as Ferralium.

When specifying materials for sea water pumps, the designer should also consider whether the water will be chlorinated, and, if so, where the chlorine is to be added. Chlorine is added to cooling waters to kill marine organisms that cause biofouling. The chlorine may be added continuously at low levels or as a shock treatment at periodic intervals. Chlorination at normal levels of up to 2 ppm does not appear to be detrimental to alloys commonly used in saline water pumps. However, an injection should be made far enough upstream of the pump intake so that the dilution occurs ahead of the pump. When an injection is made at or near the pump intake, copper alloys, stainless steels, and Ni-Resist may suffer accelerated corrosion. Recent work has shown that the corrosion rate of stainless steels will begin to increase at chlorine levels of about 5 ppm.

Galvanic considerations will also play a role in the material selection for saline water pumps. In general, the pump internals should be cathodic to the pump case. Coatings should be avoided, especially on the anodic component. Flaws or defects in a coating will expose a small area of base metal. Corrosion will then proceed at a high rate due to the extremely unfavorable area ratio. It is also inadvisable to use carbon or graphite bearings in sea water pumps. These are at the noble end of the galvanic series and are likely to cause a galvanic corrosion of stainless steels or other alloys with which they come in contact.

Table 7 indicates a number of material combinations commonly specified for seawa-ter pumps.

Hydrocarbons Pure hydrocarbons are not corrosive, but they frequently contain small amounts of water or other substances that make them corrosive. The material selection guidelines for a variety of hydrocarbon services have been developed by the American Petroleum Institute and are reproduced in Tables 8 and 9. These tables give general guide-

TABLE 7 Materials for saline water pumps
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