Corrosion is electrochemical current translated into metal loss. The number that matters first is the chloride concentration: 620 mg/L, combined with wet crevices, deposits, oxygen gradients, residual stress, and the maximum metal temperature. The reported fluid is at ambient temperature, but the actual maximum operating and cleaning temperatures still control polymer limits and chloride-assisted localized corrosion.
The existing carbon-steel and cast-iron wet end has reportedly operated for about 15 years, but only with frequent maintenance for severe corrosion. That history proves hydraulic suitability more clearly than material suitability. The replacement must handle 10 m³/h at a reported pressure of 3.2 bar, oily treated water from an LNG effluent-treatment stage, elevated chloride, calcium, magnesium, and sodium, and uncertain solids.
Corrosion, thermal, and mechanical loads
| Quantity or condition | Known value | Selection consequence | Where to obtain the missing limit |
|---|---|---|---|
| Chloride |
620 mg/L; excursions may exceed 1000 ppm
|
Screen stainless steel for pitting and crevice corrosion at joints, deposits, shaft sleeves, and stagnant regions. | Obtain minimum, normal, and maximum laboratory results, including upset and cleaning conditions. |
| pH | 8.8 |
pH alone does not select the alloy or predict deposit behavior. Evaluate it with chloride, oxidants, oil, temperature, and alkalinity. | Read the current laboratory report and operating trend. |
| Temperature | Ambient; no numerical maximum given | Temperature affects corrosion kinetics, stress-corrosion risk, polymer strength, creep, lining adhesion, and thermal expansion. | Read the process design maximum and cleaning or flushing temperature. |
| Hydraulic duty |
10 m³/h and reported 3.2 bar
|
The supplier must confirm that the offered material configuration meets the duty without excessive shaft deflection, impeller stress, or motor load. | Use the pump data sheet and certified performance curve; identify whether 3.2 bar is differential pressure, discharge pressure, or another stated condition. |
| Oil and organics | Oily treated water; composition unknown | Trace organics can swell, soften, craze, or permeate some plastics and elastomers. A generic label such as plastic is not a material specification. | Obtain oil identification, concentration range, cleaning chemicals, and polymer compatibility confirmation. |
| Solids | Believed low, not measured | Particle concentration, hardness, and size determine abrasive wear of the impeller, casing, bearings, and shaft sleeves. Fibres can change clogging risk. | Measure suspended solids, particle-size distribution, and the largest credible particle. |
| Required life | Long service desired; no acceptance period defined | Material cost must be compared against inspection, repair, outage, and replacement frequency. | Set a target operating life and allowable maintenance interval in the purchase specification. |
Symptoms and controlling causes
| Observed or possible symptom | Controlling mechanism | Deciding inspection or test |
|---|---|---|
| General wall loss on carbon steel or cast iron | Broad electrochemical attack in continuously wetted surfaces | Thickness mapping, mass-loss coupons, and comparison of inlet, casing, and discharge regions |
| Deep isolated pits on stainless steel | Chloride-driven passive-film breakdown | Clean deposits from the surface and measure maximum pit depth rather than average wall thickness |
| Attack beneath gaskets, deposits, or sleeves | Crevice chemistry becomes more aggressive than the bulk-water analysis | Open joints and inspect gasket faces, fasteners, shaft sleeves, and stagnant pockets |
| Cracking near stressed stainless components | Combined chloride exposure, tensile stress, and temperature | Record maximum temperature and inspect suspect areas with a suitable nondestructive examination method |
| Rapid nonmetallic impeller damage | Creep, chemical incompatibility, abrasive wear, or excessive shaft deflection | Material-specific compatibility review, dimensional inspection, vibration trend, and solids characterization |
| Recurring seal or bearing work on a vertical unit | Shaft alignment, runout, support, lubrication, or hydraulic loading rather than casing corrosion alone | Alignment, runout, vibration, bearing-clearance, and operating-point checks |
Scaling is not a dependable corrosion allowance. Deposits can create oxygen-starved crevices and conceal pits, so visible scale may increase localized attack even when the exposed surface looks protected. The horizontal carbon-steel or cast-iron pump operating elsewhere is not a direct material trial because its manufacturer, orientation, hydraulics, crevice geometry, and local water conditions differ.
Wet-end material approaches
| Approach | Corrosion position | Mechanical position | Main qualification work |
|---|---|---|---|
| Replace with carbon steel and cast iron | Known to suffer severe corrosion in this service | Existing configuration has demonstrated long operation despite maintenance | Price as the baseline and quantify corrosion rate, repair frequency, and remaining-wall acceptance limits |
Type 316 stainless-steel wet end |
Better localized-corrosion resistance than lower-alloy stainless steels because of its 2-3% molybdenum content, but chloride crevices remain the controlling risk |
Conventional metallic construction avoids many stiffness and creep questions associated with polymer rotating parts | Supplier review of maximum chloride, temperature, crevice design, fabrication, residual stress, and cleaning chemistry |
| Higher-grade stainless steel | Candidate when chloride excursions, temperature, or crevice severity exceed the accepted range for 316
|
Must still meet shaft, impeller, galling, fabrication, and coupling requirements | Select a named grade only after the water envelope and manufacturer corrosion assessment are complete |
| Nonmetallic construction | Can remove metallic corrosion from selected wetted parts, but oil and trace organics may attack the polymer | Modulus, creep, temperature, impeller tip speed, shaft design, bearing support, and solids wear become primary constraints | Specify the exact resin or composite, reinforcement, allowable temperature, chemical compatibility, and certified duty |
PTFE-lined metal |
The lining isolates the structural metal while intact | Metal carries pressure and structural loads; lining defects, permeation, flange details, and differential expansion control life | Review lining thickness and application method in the manufacturer’s data, holiday testing, repair method, nozzle details, and temperature limits |
| Low-alloy steel | Not a default cure for aqueous chloride corrosion; added strength does not automatically provide a stable passive surface | May satisfy mechanical loads, but that benefit does not resolve wet-end corrosion | Require material-specific corrosion data for the complete water chemistry before considering it an upgrade |
Two numerical screening positions must be reconciled rather than treated as guarantees. One conservative warning places stainless-steel crevice-corrosion concern above 200 mg/L chloride, while another selection position describes type 316 as generally acceptable up to 1000 ppm. These figures refer to different confidence levels and unspecified service details. At the measured 620 mg/L, the decision turns on maximum chloride, temperature, crevice severity, deposits, oxidizing species, stress, and the exact fabricated pump design.
Recommended selection path
Use a type 316 stainless-steel wet end as the first priced upgrade, not as an automatic final selection. Ask the existing pump supplier to quote the current carbon-steel/cast-iron arrangement and the same hydraulic design with stainless-steel wetted components. A like-for-like hydraulic comparison isolates the material premium and retains a configuration with known duty performance.
Request a higher-grade stainless option when the maximum chloride can exceed 1000 ppm, the supplier rejects 316 for the stated temperature or crevice geometry, or inspection access makes localized corrosion unacceptable. Carry a qualified nonmetallic or PTFE-lined option into the comparison when oil chemistry is defined and the manufacturer supplies compatibility and mechanical-duty confirmation. This separates three decisions: resistance of the wetted surface, structural capability of rotating parts, and maintainability of the complete vertical pump.
Service-life cost should include the pump, installation changes, inspection, repair parts, production interruption, and replacement interval. A 15-year calendar age with repeated corrosion work is not equivalent to 15 years of maintenance-free life.
Compatibility and design data package
- Collect time-based water analyses covering normal production, process upsets, shutdowns, flushing, and chemical cleaning. Record chloride, pH, calcium, magnesium, sodium, oil identity and concentration, suspended solids, particle size, and any oxidizing chemicals.
- Define minimum, normal, and maximum fluid temperatures. Ambient is an operating description, not a polymer design limit.
- Confirm the hydraulic basis for
10 m³/hand3.2 bar. Supply suction conditions, liquid level range, vertical setting, operating-point range, starts, and any dry-running exposure to the pump manufacturer. - Document the existing damage by component and location. Separate uniform corrosion, pitting, crevice attack, erosion, shaft wear, seal failures, and bearing problems because one material change will not correct every mechanism.
- Define the required operating life, inspection interval, allowable metal loss, acceptable leakage risk, and spare-parts strategy.
- Issue the same data package to bidders and require a component-by-component material schedule for casing, impeller, shaft, sleeves, fasteners, bearings, gaskets, seals, lining, and reinforcement.
The commercial name of a plastic or stainless family is insufficient. Record the exact offered grade, resin, reinforcement, elastomer, and lining system in the purchase data. Require the manufacturer to identify excluded chemicals and operating conditions.
Vertical-pump mechanical qualification
A nonmetallic casing can be chemically suitable while a nonmetallic impeller or shaft fails mechanically. The impeller carries centrifugal and hydraulic loads; the shaft carries torque and bending load. Lower stiffness, temperature-dependent strength, creep, molding stresses, and differential thermal expansion can affect clearances and balance. Many workable designs combine corrosion-resistant wetted surfaces with metallic or composite structural members, but the supplier must define the actual construction.
For each nonmetallic or lined proposal, request the allowable operating-temperature range, pressure rating at that temperature, impeller speed limit, shaft construction, maximum permitted solids, dry-running restriction, bearing arrangement, and vibration acceptance criteria. For a lined pump, inspect transitions at flanges, nozzles, fastener penetrations, and other discontinuities. Damage at one discontinuity can expose the structural metal and concentrate corrosion beneath the lining.
The reported ambient temperature reduces concern compared with hot service, but it does not settle the question. Solar heating, stagnant conditions, cleaning fluids, or process excursions may set the maximum temperature. Read that value from the process design basis and operating history.
Procurement, commissioning, and verification
- Obtain comparable quotations for the existing construction, a type
316wet end, and any qualified higher-alloy, nonmetallic, orPTFE-lined alternatives. - Review each deviation against the chemistry envelope and mechanical data package. Reject generic statements such as suitable for treated water when oil identity, chloride maximum, solids, and temperature limits are absent.
- Record baseline casing thicknesses, shaft runout, impeller clearances, vibration, motor current, flow, and pressure during commissioning. These readings distinguish corrosion from alignment, wear, or off-design operation later.
- Inspect accessible crevices, gasket faces, shaft sleeves, and deposit-covered areas after the first planned operating interval. Use maximum pit depth and localized damage location, not average appearance, to judge stainless performance.
- For lined construction, perform the manufacturer-specified lining-integrity test before service and after maintenance. Repair or replace damaged lining before exposing the underlying metal.
- Trend chloride and temperature with inspection results. Reopen material selection when a chemistry excursion exceeds the supplier’s accepted envelope or when the measured attack cannot meet the required life.
Successful verification means the pump reaches the specified duty without abnormal vibration or motor load, rotating clearances remain stable, and measured corrosion or wear projects beyond the required inspection interval. Compare results at repeatable operating points because changing flow can alter vibration, erosion, and hydraulic loading.
Frequently asked questions
Can I use 316 stainless steel at 620 mg/L chloride?
316 is the first metallic upgrade to evaluate, but 620 mg/L lies above the cited 200 mg/L conservative crevice warning and below the cited 1000 ppm general selection position. Approval must use maximum chloride, maximum temperature, crevice geometry, deposits, cleaning chemistry, and the pump manufacturer’s material assessment.
Does scale protect a stainless-steel pump from chloride pitting?
No reliable credit should be assigned to scale. Deposits can form crevices, concentrate aggressive chemistry at the metal surface, and hide localized attack until pit-depth inspection.
When should I stop material selection and contact official support?
Stop when maximum chloride or temperature is undefined, the oil cannot be identified, solids remain unmeasured, or the bidder cannot provide an exact wet-end material schedule and compatibility limits. Escalate to the pump manufacturer’s official engineering support and the proposed material supplier with the water analysis, duty point, damage record, and required service life. Request written confirmation for the complete pump construction rather than a generic material recommendation.