Case Study sea water corrosion

Article IndexCase Study of Corrosion of a Vertical Intake Pump in Warm Seawater page 2 page 3 page 4 page 5 All Pages Page 1 of 5

The corrosion-related issues of vertical wet pit pumps handling seawater continue to significantly affect their availability and reliability. For many years, vertical pumps were supplied with cast iron, NiResist and 316 austenitic stainless steel components. The performance of these materials over time is well known and documented.

However, many pumps recently have been supplied with duplex, super duplex and super austenitic stainless steel components for seawater service in both power generation and desalination. With the use of these newer materials, the industry is now faced with new corrosion-related issues, many of which were not anticipated with the newer materials.

This article will discuss a recent corrosion issue on a vertical pump installation manufactured from these newer corrosion resistant alloys in a Middle East desalination plant processing filtered and treated sea water. As a result of the unanticipated corrosion problems, the complete pump was replaced with a revised mechanical design and different materials.

Introduction

Vertical pumps play a major role in today's industrial economy since they are used in many processes to move large volumes of seawater. Seawater reacting with various pump components causes corrosion damage. In these marine environments, the corrosion factors that affect the performance of the pump components are:

Salinity Zone

o Atmosphere o Splash o Tidal o Submerged

Temperature Dissolved Oxygen Biological organisms Design

The damage that the pump components experience depends on the marine elements listed above, the construction materials used for the pump components and a number of pump design elements.

Within the pump, multiple forms of corrosion may occur either independently or simultaneously.

Those types are:

General corrosion Stress corrosion cracking Crevice corrosion Pitting corrosion Corrosion erosion Intergranular corrosion

This article will not explain each of the above corrosion types since this topic has been adequately discussed in previous papers (1). Both the pump supplier and the end user must be able to recognize which of the above forms of corrosion is present in a particular installation, identify the root causes and determine the corrective actions needed to remedy the situation and prevent any future recurrence.

Seawater Corrosion Case Study

The case history is of a moderately sized cooling water pump installed in a reverse osmosis desalination plant in the Middle East. The pump design is shown in Figure 1. The pumps were manufactured from two basic alloys: 2205 duplex stainless steel for the fabricated components and Ferralium alloy for the cast components and the fasteners. Component material composition was confirmed through PMI (Positive Material Identification) conducted by a third party hired by the end user, along with certified mill test reports supplied by the pump manufacturer for specific pump components.

Figure 1

The pump manufacturer implemented many additional component material requirements and

processes as a result of previous corrosion problems with these materials at the same site:

1. The fabricated pump components incorporated a process called "over alloying" to provide enhanced corrosion resistance of the weld filler metal. It was done so the deposited weld material would have better chemistry than the 2205 parent plate material. The filler material used was 2553.

2. The plate used for the fabricated components was ordered to the ASTM specification with the following supplemental material requirements:

Minimum chrome content was 22 percent. Minimum molybdenum content was 3.1 percent. Minimum nickel content was 5.5 percent. Minimum nitrogen content was 0.16 percent.

These requirements were obtained from the material composition of a proprietary 2205 duplex material and were not incorporated into the ASTM specification.

3.  ASTM A240 and A480 table A1.25 "Heat Treat Requirements" were followed.

4. A metallographic examination was performed on a representative sample from each material after heat treatment to ensure proper microstructure and confirm the absence of detrimental phases or any intermetallic compounds.

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Parameter Units Analysis 1 Analysis 2 1)

       Date     6/1/2001Turbidity NTU - -Conductivity Us/cm 52,270 58,000 - 66,800pH - 8.0 7.9 - 8.2Total dissolved solids  TDS

ppm 36,800 43,180 - 45,240

Total suspended solids

ppm - <0.1 - 6

Total hardness as CaCO3 7,400 6,950 - 7177Total alkalinity as CaCO3 120 83 - 116Calcium ppm 400 441 - 551Magnesium ppm - 1410 - 1470Iron ppm 0.07 -Copper ppm 0.05 -

Sodium ppm - -Potassium ppm - -Chlorides ppm - 20,776 - 24,419Sulphate ppm 2,830 2,000 - 2,908Silica ppm 0.5 -Strontium ppm - -Barium ppm - -Ammoniacal Nitrogen as NH3

ppm - <0.001 - 0.055

Sulphide (H2S) ppm   <0.001 - 0.021Silt density index - - -Oil & grease ppm - -Total organic carbon ppm - -Total oxygen demand COD

ppm - -

Biological oxygen demand  BOD

ppm - -

Total colonies count 1/1 ml - -Coliforms 1/100 ml - -

Table 1

These pumps were installed in the warm waters of the Arabian Gulf with an expected water chemistry as shown in Table 1. They experienced advanced and abnormally severe stages of both crevice and pitting corrosion after only five months of operation. During a planned visual inspection of the pumps, the following observations were reported when one pump was removed from service and disassembled:

Corrosion was found in the tight crevices between the mating surfaces of the submerged flanges outside of the bolt circle.

Corrosion was observed on the mating surfaces of the submerged bolts and nuts. No evidence of significant corrosion was found on any of the plate used in fabrications. The weld material was in excellent condition except for a few areas of pitting that had the

appearance of random, deep pits or gouges. No corrosion was observed on any components above the splash zone.

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The corrosion was a form of localized corrosion usually associated with alloys with good corrosion resistance. Two of the primary forms of local corrosion are pitting and crevice corrosion. Crevice corrosion is a special case of pitting associated with active/passive alloys like austenitic stainless steels.

Mechanical crevices in pumps can be found between components at flanged joints with rabbet aligning fits, between rotor components assembled with loose mechanical fits and under o-rings,

fasteners or marine organisms that often grow on surfaces of a pump component. In these cases, a natural or manmade crevice is formed on a metal's surface. These crevices impede the transport of oxygen to the surface of the stainless steel component. If insufficient oxygen is available to maintain or reform the chromium oxide film, the stainless steel surface will be prone to the form of local corrosion known as crevice corrosion.

Crevice corrosion was observed on the flange faces of the pump components located in the splash zone. Figure 2 shows the Ferralium flange face with severe corrosion damage.

Figure 2

Figure 3 shows crevice corrosion on the head of a Ferralium bolt.

Figure 3

Figure 4 shows crevice corrosion on an axial face of the pump shaft at an undercut area where a split ring is located.

Figure 4

Figure 5 shows both crevice corrosion and pitting on the Ferralium impeller.

Figure 5

The amount of corrosion observed in the relatively short period of time was quite surprising given the construction materials and the additional requirements and processes implemented on this project. Generally speaking, Ferralium is thought to be highly resistant to crevice corrosion. It needs to be stated that no stainless steel is immune to local corrosion attack in circumstances where tight crevice conditions are exposed to warm seawater. In addition, the use of ferric chloride as the specified water treatment process adds to the crevice corrosion potential of the duplex alloy since ferric chloride is used as the etching agent in a number of ASTM metallurgical processes to accelerate the corrosion process.

After lengthy discussions with the end user and consultants, a decision was made to:

Replace the pumps with a different mechanical design Incorporate special features into the pump design to minimize the opportunity for crevice

corrosion Upgrade the material of the pump components Incorporate a cathodic protection system to the pumps

New Pump Arrangement

The original pump was a "pullout" mechanical design that allowed for the pump hydraulic components to be removed without disturbing the pump outer shell assembly and discharge connection. The replacement pumps were manufactured of the "non-pullout" mechanical design. All customer interfaces remained unchanged so that the substitution of the existing pumps with

the new ones could be done without affecting the system. Consequently, no modifications or repairs were required at the end user's plant site.

The new "non-pullout" pump mechanical design offered major advantages over the original design (see Figure 6). The first major advantage was a considerable reduction of the number of crevices and the resulting number of opportunities for crevice corrosion. The original design had 13 main components and 11 flanged joints exposed to seawater, either internal to the pump or on the external outer shell of the pump. Five of these flanged joints were submerged below the maximum water level and were then exposed to the stagnant seawater environment. The non-pullout design used only six main components and five flanged joints that were exposed to seawater either internal to the pump or on the external outer shell of the pump. Only two flanged joints were submerged below the maximum water level. The shaft enclosing tube was also eliminated.

Special Design Features

The redesigned pump used a bolted flange design with blind threaded holes rather that the traditional design with through bolt holes. The non-pullout design also allowed the suction bell and impeller housing to be combined into a single piece, thereby eliminating another bolted flange connection. The flanged joint between the discharge head and the column pipe had been moved above the high water level. Consequently, it was submerged in the stagnant liquid when the pumps were idle.

The line shafting was also revised and simplified, leaving only one pump shaft and one drive shaft. The original pullout pump design used three shaft sections and two shaft coupling assemblies. With the revised design, the shaft coupling was placed above the high water level.

The pump shaft bearing spans and shaft diameter were also revised so that one of the intermediate radial pump bearings could be removed. By doing so, crevices were eliminated at the:

Bearing-to-bearing holder Shaft-to-journal sleeve Coupling sleeve and shaft ends Shaft ends and coupling split rings

Figure 6

As a further improvement, the only remaining intermediate coupling and column radial bearing were placed above the maximum water level and not submerged.

Material Upgrade

The selected material for the fabricated flanges remained duplex 2205 as used on the original pumps; however, the mating faces and area underneath the fasteners were overlaid with Hastelloy C22. The fastener material was also upgraded to Hastelloy C22.

Perhaps the most widely used method (1) to evaluate material resistance to pitting is to determine its PREN (pitting resistance equivalent) where:

PREN = Cr% + 3.3 Mo% + 16 N%

For a stainless steel to have a high resistance to pitting, it should have high chromium, molybdenum and nitrogen contents and sufficient nickel to maintain the austenitic structure. When the PREN exceeds 40, the material is generally considered immune to pitting.

A comparison between the original flange material and the upgraded flange material contacting seawater in the crevice areas provided an indication of the improvement:

Duplex 2205 stainless steel               PREN = 34.5

Hastelloy C22                                                PREN = 66.6

Corrosion engineers have methods for evaluating the relative resistance of stainless steels and nickel alloys to crevice corrosion in salt water. A commonly used method is to determine the Critical Crevice Temperature (CCT) in an accelerated laboratory test. The tests will typically be performed in ferric chloride solution, with pH adjusted to 1, for 24 hours, using a standard crevice assembly. The highest temperature at which the sample shows no crevice corrosion is known as the CCT. A higher CCT correlates well with resistance to crevice corrosion in seawater.

From this perspective, the material changes should dramatically increase resistance to crevice corrosion. The CCT for Hastelloy C22 is somewhere between 50 and 70C, depending on the specific parameters of the ferric chloride test, whereas the CCT for 2205 and Ferralium are 17.5C and 22.5C, respectively.

The original Ferralium 255 impeller was upgraded to Hastelloy C22 in accordance with the ASTM A494 CX2MW specification.

The material of the two pump shaft sections was upgraded from 2205 duplex stainless steel to super duplex stainless steel with a PREN > 40. Other minor rotor parts like the shaft coupling sleeve, journal sleeve, split rings and keys were also upgraded to the super duplex alloy. Rotor fasteners were upgraded from 2205 duplex to Hastelloy C22.

Cathodic Protection System

The cathodic protection system added to the pumps was built around a two phase impressed current system for both the internal and external protection of the pump. Each of the two phases used a different transformer-rectifier for electrical supply, as well as a different reference

electrode. The two phases can work independently, as the conditions of both may vary in different ways-pump in operation or idle, different water levels, etc. Variation of the intensity required for protection was automatically adjusted by the equipment according to feedback from the reference electrodes.

For the reasons discussed above, each pump used its own cathodic protection equipment and the equipment was supplied with visual alarms to indicate malfunction.

The impressed current system for protecting the pump external surfaces consisted of two flat anodes attached to the side walls of the intake structure and one reference electrode. Two flat anodes on the sump walls supplied the electrical intensity, which was conducted through the water to the pump outer surfaces.

The impressed current system to protect the pump internal surfaces was intended to protect the inner surface of the column pipe, casing, suction bell, shaft and impeller. This equipment consisted of ten anodes fixed to 1.5 in NPT connections at selected locations along fabricated sections of the pump (see Figure 6), one reference electrode and one brush ring fixed to the pump shaft between the stuffing box and the thrust bearing housing. Ten anodes supplied the electrical intensity. The active length of the anodes fell inside the pump so that the intensity could be distributed on the inner surface of the pump and shaft. The brush collector ring was used to ensure that the circuit was closed along the shaft and to prevent any stray currents.

Conclusion

Corrosion problems associated with centrifugal pumps in warm seawater can vary widely and be complex. This article discussed a situation where a revised mechanical design, upgraded materials and a corrosion protection system were all incorporated into the solution to a complex corrosion problem.

The solution to corrosion problems can often be complicated and expensive for both the pump supplier and user. When specifying a pump for a seawater environment, both the end user and the supplier should be familiar with and share their previous experience with pumps in the same liquid environment.

Finding the Root Cause of Failure

Written by Dr. Lev Nelik, P.E., APICS

Pumps and Systems, April 2009

When a critical raw water intake vertical turbine pump at a local water treatment plant failed, there was no time to waste. However, before our repair shop pulled the pump, my repair manager Jerry wanted to find out as much about its history as possible. Speaking with operators, we learned that the pump experienced vibrations since its last repair, and these vibrations were

gradually increasing with time.

The plant contracted divers to examine the sump near the pump bell area, and they discovered that the lower bushing was almost gone. Since the pump intake comes from the river, it was assumed that sand, which is normally present in such applications, entered a lower bushing, wore it out and caused eventual failure.

Therefore, the plant attributed the failure to the "nature of the beast," and wanted to have the bushing replaced, with little interest in the analysis.

When the pump arrived at the shop, it quickly became obvious that there was more damage to the internals than simply a failure of the lower bushing worn away by sand. Both stage bushings were also worn and damaged badly, as a result of the shaft oscillating after the failure of the lower bushing. At first, it was attributed to impellers contacting the casing rings, as evident by the severe damage to both impeller ring and bowl wear rings. Fortunately, there was little damage to both impellers and bowls. The shaft, however, was severely bent, running out almost 1/8-in, and needed to be either straightened or replaced.

The perception of the root cause of failure started to change after a close examination of the suction bell housing revealed a crack (see Figure 2).

Figure 2. Ultimate root of failure: a crack at the bell housing from a thinned wall during the previous repair assembly

Measuring the thickness of the housing wall that held the lower bushing clearly revealed what had happened. After the past repair, the wall was machined out to accept a new bushing. The new

bushing was made with a larger OD to "chase" the reduced ID of the bore to which it was seated. That made the bushing wall much thicker, while the housing wall got unacceptably thin (like a side of a tooth after each cavity repair by the dentist). When over-pressed to the housing, the bushing cracked at assembly, and the pump was doomed.

To correct the problem, the suction bell hosing had to be restored to its original wall thickness (see Figure 3). A new bronze bushing was machined with the correct wall thickness, as well as a correct press fit.

Figure 3. Bell housing wall repaired, and a new bushing with proper wall thickness and correct dimensions installed

When the repaired pump was delivered to the customer and installed, vibration readings were taken again and showed low values. A spectral analysis indicated no significant peaks across a wide range of frequencies. The pump is now being monitored on a regular basis, with vibration trends plotted and reviewed by our technicians and the plant operating personnel. All of its sister pumps are being monitored, with trends plotted on SCOPETM (Specific Comprehensive Operation of Pumping Equipment).

Had the root cause not been identified, more pumps could have failed catastrophically and unpredictably. Understanding the root cause is important to learn lessons and prevent similar issues from happening in the future.

Dr. Nelik (aka "Dr. Pump") is president of Pumping Machinery, LLC, an Atlanta-based firm specializing in pump consulting, training, equipment troubleshooting and pump repairs. Dr. Nelik has 30 years experience in pumps and pumping equipment. He has published more than 50 documents on pump operations, the engineering aspects of centrifugal and positive displacement pumps, and maintenance methods to improve reliability, increase energy savings and optimize

pump-to-system operations.