Pump Material Class: Service Data, Not PPM, Decides

The selection request passes from process data to corrosion review, then to mechanical qualification and vendor confirmation. Here, it stops at the first handoff: 80,000 ppm H2S, 150,000 ppm chloride, and -100 °C do not define concentration basis, phase behavior, operating temperatures, or the meaning of “material class.” Follow each value from its source before assigning an alloy.

Where does the material-class decision start?

Start with the process datasheet, not an alloy name. The pump serves a flare knock-out drum in a 1+1 arrangement, but redundancy does not reduce the chemical or low-temperature duty imposed on either pump. The standby unit may experience stagnant liquid, vapor exposure, condensation, or intermittent cold liquid when it starts.

Define whether “class” means a company piping class, a pump specification’s material combination, or individual materials for casing, impeller, shaft, wear components, fasteners, welds, and pressure-retaining parts. Those classifications are not interchangeable.

Input	Reported value	Missing definition	Decision affected
Service	Flare knock-out drum hydrocarbon	Liquid composition, vapor fraction, solids, water phase, and operating modes	Corrosion and erosion mechanisms
H2S	80,000 ppm	Mass, mole, volume, liquid-phase, or vapor-phase basis	Sour-service damage assessment
Chloride	150,000 ppm	Total-stream or aqueous-phase concentration	Pitting, crevice corrosion, and chloride cracking assessment
Temperature	-100 °C	Minimum design, minimum operating, normal operating, or transient value	Impact toughness, fracture resistance, phase state, and corrosion rate
Availability	1+1	Standby condition and changeover sequence	Stagnant exposure and thermal transient cases

The first check is a completed process basis in which every concentration has a basis and phase, every temperature has an operating meaning, and “class” points to a named purchaser or pump classification system.

Does -100 °C describe the metal temperature?

Resolve the temperature before screening alloys. A maximum design temperature, minimum design metal temperature, normal process temperature, startup temperature, and depressurization temperature answer different questions. A value of -100 °C may define a genuine cryogenic exposure, a transient case, or a data-entry error; the process heat-and-material balance and relief or depressurization study decide which case applies.

If the pump or trapped liquid can reach -100 °C, material toughness becomes a primary qualification. Carbon steel is not a single low-temperature performance category: grade, product form, thickness, heat treatment, weld procedure, and impact-test results govern resistance to brittle fracture. Duplex stainless steel also cannot be accepted from its family name alone. Its ferritic-austenitic structure, weld condition, section thickness, and documented toughness at the required metal temperature must be reviewed.

Corrosion and toughness address different failure modes and both remain mandatory. The warmest credible wetted condition may control corrosion, while the coldest credible metal temperature controls brittle-fracture qualification. Supplying only an average temperature can therefore miss both limiting cases.

Temperature case	Use in selection	Required confirmation
Normal operation	Continuous corrosion and phase behavior	Expected liquid and metal temperature range
Warm upset	Potentially faster corrosion and cracking	Maximum duration and composition
Cold startup or depressurization	Thermal shock and minimum metal temperature	Transient calculation and starting sequence
Minimum design condition	Pressure-boundary toughness	Material and weld qualification at thickness

The check is a temperature profile that identifies the minimum metal temperature and the coincident pressure for each casing and wetted component, plus the warm condition used for corrosion screening.

Are the H2S and chloride numbers on usable bases?

ppm is a ratio, not a complete concentration definition. If both reported values are mass fractions, 80,000 ppm equals 8% by mass and 150,000 ppm equals 15% by mass. That arithmetic does not establish whether either constituent resides in the hydrocarbon, vapor, free-water, or deposited phase.

Chloride-driven corrosion generally depends on the chloride activity in an aqueous electrolyte at the metal surface. A total-stream chloride result can conceal a much higher concentration in a small water phase. Conversely, chloride present as dry salt or in a nonconductive phase does not produce the same electrochemical condition until water becomes available. Obtain water content, free-water fraction, aqueous chloride, pH, dissolved gases, oxygen ingress, solids, and sampling temperature.

Do not use the stated comparison of 50 mg/L and approximately 50,000 ppm. For a dilute water-like solution, 50 mg/L is approximately 50 ppm by mass, not 50,000 ppm. In a hydrocarbon mixture, conversion between mg/L and mass-based ppm also requires density and a clear phase basis.

H2S assessment likewise needs partial pressure or phase-specific concentration, total pressure, water presence, pH, temperature, and material strength or hardness controls. A high total-stream concentration alone does not determine the wet sour environment at the metal surface.

The check is a laboratory report and stream table that state units, analytical method, sample phase, pressure, temperature, water content, and density where a volume-to-mass conversion is used.

Which damage mechanism explains each rejection?

Layer one is the physical environment: liquid, vapor, water, deposits, temperature, pressure, velocity, and stagnant periods. Only then map the chemical values to damage mechanisms. Selecting a more highly alloyed stainless material for chloride resistance does not automatically qualify its strength, welds, or low-temperature toughness for sour and cryogenic service.

Observed condition or symptom	Mechanism to investigate	Discriminating check
Localized pits on wetted surfaces	Chloride pitting or deposit attack	Aqueous chloride, deposits, temperature, oxygen, and surface condition
Attack at gaskets, wear rings, or deposits	Crevice corrosion	Crevice geometry, stagnant chemistry, and shutdown condition
Cracking in stressed stainless components	Chloride-assisted or sour-environment cracking	Temperature, stress, hardness, microstructure, and crack morphology
Cracking in carbon or low-alloy steel	Wet H2S-related cracking or brittle fracture	Water phase, H2S condition, hardness, impact records, and fracture appearance
Damage after cold startup	Low-temperature embrittlement or thermal transient	Minimum metal temperature, pressure, thickness, and impact qualification
Standby pump degrades faster	Stagnation, condensation, or concentrated deposits	Standby temperature, drain condition, preservation, and changeover history

The check is a damage-mechanism register linking every credible operating mode to affected pump components and a measurable acceptance requirement.

How should candidate alloys be screened?

Use vendor capability to define the candidate set, then send that set through corrosion and mechanical review. Carbon steel, cast 2205 duplex stainless steel, and 2507 super duplex stainless steel were identified as possible families. A further designation, Ferilium 255, was mentioned; verify its exact trade name, specification, availability, casting route, and properties before placing it on a datasheet.

The chloride figures previously used as informal screening points were carbon steel below 20,000 ppm, cast 2205 up to about 70,000 ppm, and 2507 above 70,000 ppm. Service experience with 2205 at 150,000 ppm was also reported. These values are not universal material limits. They omit the concentration basis, water chemistry, temperature, oxygen, fabrication condition, and required life. At the reported 150,000 ppm, a metallurgist must evaluate the actual aqueous environment before accepting 2205 or any alternative.

Candidate	Reason to retain	Reason it can still fail screening
Carbon steel	May be available in grades with controlled low-temperature toughness	Wet sour damage, corrosion allowance, or chloride-bearing water may govern
2205 duplex	Candidate for chloride resistance in some services	150,000 ppm exceeds the cited informal 70,000 ppm screening point; toughness and weld properties also require qualification
2507 super duplex	Common candidate when chloride exposure rejects 2205	Family name alone does not establish cryogenic toughness, sour compatibility, casting quality, or weld acceptance
Ferilium 255 designation	Raised as a sour-service candidate	Exact designation, governing recommendation, supply route, properties, and lead time require direct confirmation

The check is a signed comparison from corrosion and materials engineering that records why each candidate passes or fails every damage mechanism, rather than selecting from chloride ppm alone.

What must the pump vendor qualify component by component?

A pump material class is a collection of parts, not merely a casing alloy. Request a component schedule covering casing, cover, impeller, shaft, sleeves, wear rings, fasteners, seal chamber, weld repairs, gaskets, and mechanical-seal wetted parts. Record the base material specification, product form, heat treatment, maximum hardness, corrosion allowance where applicable, and required low-temperature tests.

For cast duplex or super-duplex parts, obtain the proposed casting specification, heat treatment, microstructure controls, weld-repair procedure, and corrosion and mechanical acceptance records required by the project. For carbon steel, obtain the exact low-temperature grade, thickness-qualified impact properties, welding procedure, heat treatment, and hardness controls. Review dissimilar-metal couples and crevices rather than approving components independently.

Ask the vendor to identify any exception where a small wetted item uses a less resistant material than the main casing. Mechanical-seal faces and secondary seals also need compatibility with the hydrocarbon mixture, H2S, chloride-bearing water, and minimum and maximum temperatures.

The check is a line-by-line material schedule with no generic entries such as “duplex,” “stainless,” or “carbon steel” where an exact specification, grade, and qualification condition are required.

How is the selected class verified end to end?

1. Freeze the normal, startup, shutdown, standby, upset, and depressurization cases, including the coincident pressure, temperature, and phase composition.
2. Confirm the concentration basis for 80,000 ppm H2S and 150,000 ppm chloride; obtain aqueous-phase chemistry and H2S partial-pressure inputs where applicable.
3. Define the governing material-class system and list every pressure-retaining and wetted component within it.
4. Have corrosion and materials engineering screen carbon steel, 2205, 2507, and any proprietary candidate against corrosion, cracking, hardness, welding, and low-temperature toughness requirements.
5. Require the pump vendor to return exact material specifications, product forms, heat treatments, weld procedures, impact-test basis, hardness limits, and exceptions.
6. Compare the vendor return with the process cases and component schedule. Reject substitutions that change metallurgy, heat treatment, toughness, hardness, or corrosion resistance without renewed review.
7. At receipt, reconcile material certificates and test records to component identification. During commissioning, verify standby drainage or preservation, changeover operation, leak tightness, vibration, and stable hydraulic operation under the approved process envelope.

The end-to-end check passes only when the process basis, materials review, vendor schedule, certificates, installed component identification, and commissioning records trace to the same approved class.

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