Whether corrosion comes from galvanic effects or marine organisms, in flowing seawater or stagnant seawater pumping operations, matching the right materials and costs for the right application is paramount.
Stephen Morrow, Jose Gutierrez and Richard O’Donnell, ITT Goulds
As more production moves offshore and existing fields mature, the number of pumps handling seawater applications is increasing. Vertical turbine pumps offer a flexible design to accommodate any system by multi-staging. The ease of changing the staging on the pumps is also an advantage if it becomes necessary to change the hydraulic characteristics of the system. Stages can be added to the same pump if changes in the system head curve result from an increase in pipe friction due to corrosion. This condition is quite common in seawater systems.
Corrosion in vertical turbine pumps can be minimized, and in many cases, completely prevented by proper material selection. However, high initial cost may prevent the use of those alloys which would eliminate the corrosion problem. But initial low-cost materials are not likely to result in the lowest ultimate cost. In critical service applications, it is wise to select more corrosion-resistant and more expensive materials. This article examines some popular combinations of materials for seawater service, from low-cost, limited service life, to high-cost, long service life.
GALVANIC CORROSION
Since seawater is a highly conductive environment, galvanic considerations normally dictate material combinations. An understanding of the galvanic series of metals is essential to proper material selection. Figure 1 shows the corrosion potential of materials in the flowing sea water.
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Fig. 1. Corrosion potential of materials for use in flowing seawater.
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A few suggestions to combat galvanic corrosion in vertical turbine pumps are:
- Select combinations of metals as close together as possible in the galvanic series with less than 0.25 V potential difference.
- Avoid the combination of a small anode and a large cathode.
- Use insulating washers on bolting and insulating gaskets on mating flanges.
- Use protective coatings on less expensive materials.
- Locate sacrificial zinc or aluminum alloy anodes on the column assembly.
Additionally, marine growths can be a severe problem that is solved by selecting materials with good resistance to marine biofouling, treating the sump with chlorine and using an electrolytic water treatment.
The application and service conditions will also affect the rate of corrosion. A pump that sits idle for long periods of time, such as a vertical fire pump on an offshore installation, will experience a different rate of corrosion than a pump constructed of the same materials operating continuously. Many materials are regarded as corrosion resistant in fast flowing seawater, but under stagnant conditions they are susceptible to pitting and crevice corrosion attack. This reversal in the resistance of metals due to velocity change is due to the depolarization of the hydrogen film which protects the material.
This mechanical depolarization (loss of protective film) is an important aspect of pump design and material selection. Stainless steels and nickel-base materials remain passive at high velocities, but they corrode due to pitting in stagnant water. The copper base alloys are very corrosion-resistant in quiet unpolluted sulfide free water, but as water velocities increase the corrosion barrier can be stripped away. Thus, a proper material combination is highly dependent on the pumping application.
The design of the vertical turbine pump consists of three basic components assembled into a single unit. These are the bowl assembly, column and shaft assembly, and discharge head assembly.
If the service condition is known, the proper mix of materials will determine the ultimate lowest cost by providing good service life.
BOWL ASSEMBLY
The pumping element of a vertical turbine pump is the bowl assembly. It is comprised of the suction bell, one or more intermediate bowls, the bowl shaft, and the impellers. The many components of the bowl assembly create the major concern in material selection. An improper galvanic coupling can create a “dry cell” effect (electron flow between dissimilar metals in contact in a conductive electrolyte solution). The bowl assembly should be mechanically constructed so that field maintenance is easy. Some basic design criteria are:
- Bowls should be flanged, not threaded.
- Bolting material should be cathodic to bowl material, but still as close to it as possible to prevent accelerated corrosion.
- Impellers should be keyed for easy removal.
- Impellers should be of the same enclosed type to allow the use of wear rings, which permit re-establishing the initial running clearances when replaced.
COLUMN ASSEMBLY
The column and shaft assembly is designed to connect the bowl assembly to the discharge head assembly. The column serves two purposes: it carries the weight of the bowl assembly and provides a means of conducting the fluid vertically from the pumping element to the discharge head. Threaded column assemblies are not suitable for seawater applications and should not be used.
While all vertical pumps consist of the same basic components, there are two distinct variations in the column design. These are open lineshaft construction and enclosed lineshaft construction. With open lineshaft construction, the bearings are lubricated by passing the pumped fluid around and through the bearings.
The enclosed lineshaft construction involves enclosing the lineshaft with a series of threaded connecting tubes. These tubes separate the pumped fluid from the shaft and also house the shaft support bearings. Lubrication, usually oil, is introduced at the top end of the tubing and drips down through the bearings. A drain port in the lower end of the tubing allows the oil lubricant to flow out. The tube and shaft assembly is supported in the column pipe at intervals by spiders to hold it in the center.
The enclosed lineshaft is commonly used for deep well units. This construction is not recommended on seawater service, as it increases initial cost without increasing reliability. It also compounds disassembly problems should maintenance become necessary. For these reasons, open lineshaft construction, due to its simplicity of design, is recommended for seawater service.
With open lineshaft construction, selection of the bearing material depends on pump setting, and the silt and sand content of the pumped fluid.
DISCHARGE HEADS
The discharge head serves several functions. It turns the flow from vertical to horizontal and provides a seal area where the shaft passes through to the atmosphere. It also aligns and supports the driver, and supports the column and bowl assembly. A typical “L” Discharge Head is the most commonly used, especially in seawater applications.
On 12-in. and larger heads, a combination of carbon steel and alloys can be furnished as follows:
- 316SS RF discharge flanges
- The motor base should be made of coated carbon steel, except for the fittings which will be of 316SS material.
- The 316SS top column flange is bolted to the head base plate or to the discharge head extension pipe. Since these components have registered fits, the discharge head extension pipe and column flange should be furnished in 316SS material. When the column is bolted directly to the head base plate, an alternative to a 316SS plate, is a stainless steel weld overlay applied to the wetted exposed surface of a carbon steel coated base plate.
- Stuffing boxes, the discharge head stuffing box plate and the mechanical seal housings should be 316SS material.
In the Al-Bronze alloy construction, the discharge head should be fabricated from Ni-Al-Bronze.
SUMMARY
The proper selection of vertical turbine pumps depends on many factors, including understanding of the service and operating conditions, depth or length of pump, abrasive and corrosive qualities of the seawater, materials availability; and a complete analysis of the seawater that is to be pumped, including biocide treatments and macro-fouling practices.
The following questions will help determine which type of pump construction best meets your needs:
- How much money do you initially want to invest?
- How long will the pumps be in service?
- Will the pumps operate continuously or intermittently?
- Is the seawater being chemically treated?
- Are you willing to invest in more expensive materials initially to reduce the maintenance expenses associated with replacing pumps and parts?
Answers to these questions will tell you whether you need the protection afforded by the more expensive materials, or coatings on less expensive materials. 
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THE AUTHORS
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Stephen Morrow is the Chief Metallurgist and Principal Materials Advisor for the ITT Industrial Process Group providing metallurgical and other materials related technology support for the Fluid Technology Business interests of ITT globally. He has over 30 years broad based manufacturing experience in Metallurgical and Corrosion Engineering with a Bachelor of Science in Metallurgical Engineering from Lafayette College. Steve has been a member of ASM for more than 25 years and NACE for last 20 years and plays an active role in providing technical support to ITT customers and product specialists.
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Jose Gutierrez is Engineering and Marketing Manager for the Vertical Products Operation of ITT and responsible for R&D, field service, application engineering, testing and the Oil & Gas market cells. Jose has a BS Mechanical Engineering, MS Hydraulic Engineering and MBA/International Business He has 25 years of experience in the pump industry on all types of centrifugal pumps. Jose has been published in ASME and Hydraulic publications and recently received his first patent for the ITT “O” Head design.
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Richard O’Donnell is the Technical Service Manager, responsible for R & D and Field Service, at ITT Vertical Products Operation. He has been involved in various engineering and technical functions throughout his 34 year career. He graduated from Wentworth Institute with a degree in Metals Engineering and also has a BS in Mechanical Engineering from Northrop University. During his career he has served on various ASME, Hydraulic Institute Standards and API - 610 committees
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