Why Is 347 Stainless Steel Used for FCC Distributor Trays and Thermowells?

Type 347 stainless steel has a specific role in petroleum-refinery equipment where welded or fabricated components are exposed to elevated temperature and resistance to sensitization-related intergranular corrosion is important.

Fluid catalytic cracking units are one example.
Selected FCC components such as distributor trays, internal supports, thermowells, hanger rods and other reactor-area hardware may use Type 347 / UNS S34700 when required by the process licensor or refinery metallurgy specification.

However, the words “FCC service” do not automatically require Type 347.
Different parts of the same reactor system can experience different temperatures, stresses, sulfur exposure, erosion, process chemistry and mechanical loads.

A distributor tray and a machined thermowell may operate inside the same refinery process unit, but they are not purchased from the same stainless steel product form.

This guide connects:
FCC component → service condition → 347 raw material → ASTM product standard → fabrication → inspection → RFQ.

Quick Answer: Where Does 347 Fit in an FCC Unit?

Subject to the process licensor, refinery metallurgy specification and actual service environment, Type 347 may be evaluated for selected components such as:

• FCC distributor trays;
• gas or catalyst distributor components;
• reactor internal support plates;
• thermowells;
• hanger rods;
• internal brackets;
• support frames;
• screens and fabricated internal structures;
• selected recuperator components;
• thermally exposed reactor hardware;
• selected fabricated parts around high-temperature refinery reactors.

These are potential applications rather than a universal material schedule.

The final grade should be selected according to temperature, process chemistry, sulfur compounds, mechanical stress, erosion, welding condition, shutdown exposure and the applicable project specification.

Why Does Niobium Stabilization Matter in FCC Service?

Type 347 is a chromium-nickel austenitic stainless steel stabilized with niobium.

Conventional austenitic stainless steels can become sensitized after certain combinations of temperature and time when chromium-rich carbides form at grain boundaries.

If the sensitized material is later exposed to a suitable corrosive environment, susceptibility to intergranular corrosion or cracking can increase.

Niobium has a strong affinity for carbon.

The niobium addition in Type 347 helps bind carbon and reduces the tendency for chromium-carbide precipitation under relevant thermal conditions.

This can be particularly useful for refinery components that combine:

• welding;
• repeated elevated-temperature exposure;
• startup and shutdown cycles;
• residual or applied stresses;
• potential exposure to refinery shutdown corrosion mechanisms.

Niobium stabilization solves a specific metallurgical problem.

It does not make Type 347 immune to chloride attack, sulfidation, erosion, carburization, high-temperature oxidation or every sulfur-containing environment.

For a broader grade overview, see the

JISCO JUDE Austenitic Stainless Steel Guide
.

347 Grade Designations and Product Standards

The alloy designation, starting material specification and finished-component design standard should be treated separately.

For example, a distributor tray can be fabricated from ASTM A240 Type 347 plate, while a solid-machined thermowell may begin with a bar product governed by a different material standard.

ASTM A240 Type 347 / UNS S34700 Chemical Composition

The following values are ASTM A240 Type 347 specification limits for applicable flat products.
They should not be described as producer-specific typical chemistry.

Actual heat chemistry should be verified on the Mill Test Certificate and accepted according to the governing product standard and project specification.

Producer “typical” values should not replace the contractual ASTM limits.

Which FCC and Refinery Components Use 347 Stainless Steel?

1. FCC Distributor Trays

Distributor trays help control how gas, vapor or process streams are distributed within refinery equipment.

Selected FCC and reactor distributor components can operate under demanding combinations of:

• elevated temperature;
• thermal cycling;
• sulfur-containing environments;
• erosion;
• welded fabrication;
• mechanical loading.

Where the project metallurgy specification calls for stabilized austenitic stainless steel, Type 347 plate or sheet may be considered for selected tray components.

Important purchasing requirements can include:

• ASTM A240 Type 347 / UNS S34700;
• plate thickness;
• width and length;
• solution-treated condition required by the governing standard;
• flatness;
• cutting tolerance;
• heat-number traceability;
• intergranular-corrosion testing when project-specified.

JISCO JUDE supplies

hot-rolled stainless steel sheet and plate

for industrial fabrication.

2. Catalyst Distribution Components

Catalyst-distribution hardware can include perforated plates, screens, support sections and custom fabricated internals.

Type 347 may be selected where elevated-temperature exposure and stabilization requirements justify the alloy.

The raw-material supplier should not determine catalyst geometry or structural load from material grade alone.

The fabricator should provide:

• component drawing;
• hole pattern;
• thickness;
• flatness;
• edge requirement;
• weld specification;
• service temperature;
• process-licensor material requirement.

3. Reactor Internal Support Plates

Internal support structures can carry screens, catalyst-distribution systems or other reactor internals.

Depending on the engineering design, Type 347 plate may be considered for:

• support plates;
• brackets;
• mounting plates;
• internal frames;
• selected reinforcing components.

Plate thickness should be determined by the structural design rather than copied from another FCC unit.

4. Thermowells

Thermowells protect temperature sensors from process pressure, velocity, vibration and direct contact with the process stream.

Type 347 is an established material option for selected refinery thermowells where the service environment requires a stabilized austenitic stainless steel.

A thermowell purchase must control much more than alloy chemistry.

Important information includes:

• material grade;
• starting bar specification;
• process connection;
• root diameter;
• tip diameter;
• bore diameter;
• insertion length;
• unsupported length;
• process temperature;
• pressure;
• fluid density;
• velocity;
• vibration calculation where required.

The finished thermowell manufacturer’s design responsibility should not be transferred to the raw-material supplier.

5. Hanger Rods and Support Hardware

Refinery equipment may contain hanger rods and thermally exposed support hardware.

Where Type 347 is specified, stainless round bar may be machined into:

• hanger rods;
• support pins;
• fastening components;
• machined mounting hardware;
• selected high-temperature supports.

JISCO JUDE publishes

stainless steel wire rod, round bar and billet

with round-bar production ranges for industrial machining requirements.

6. Screens, Brackets and Internal Frames

FCC reactor systems also use fabricated supports, screens and framework that may experience elevated temperatures and repeated shutdown cycles.

Possible starting products include:

• plate;
• sheet;
• bar;
• precision-cut blanks.

The material grade should match the actual licensor or project metallurgy requirement rather than being selected simply because the component is located inside an FCC unit.

7. Recuperator and Hot Internal Components

Type 347 is also used in selected recuperator and thermal-service equipment components.

Depending on equipment design, these may include:

• support plates;
• tube-support structures;
• shell-related components;
• hot internal brackets;
• thermally exposed fabricated parts.

Where long-term creep strength becomes part of the design basis, the project should determine whether 347H or another elevated-temperature alloy is required.

Component-to-Raw-Material Procurement Matrix

ASTM A240 vs ASTM A479 for FCC Components

ASTM A240/A240M

ASTM A240/A240M covers stainless steel plate, sheet and strip.

It can be relevant to Type 347 raw material used for:

• distributor trays;
• support plates;
• internal brackets;
• screens;
• fabricated reactor components.

ASTM A479/A479M

ASTM A479/A479M covers stainless steel bars and shapes for boiler and pressure-vessel construction.

It may be relevant to selected Type 347 bar products used for machined or pressure-equipment components when required by the project specification.

The buyer should not specify ASTM A240 for a round bar merely because both product forms use Type 347 chemistry.

Procurement rule:
the alloy tells the supplier which stainless steel family is required; the product standard tells the supplier what material product must be manufactured and tested.

Why Does a Thermowell Need More Than a Material Grade?

A Type 347 material certificate does not establish whether a thermowell can safely operate in a given process stream.

A thermowell extends into flowing fluid and is exposed to steady and oscillating fluid forces.

Design review can require evaluation of:

• natural frequency;
• fluid-induced excitation;
• dynamic stress;
• steady-state stress;
• pressure;
• unsupported length;
• geometry;
• material properties at temperature.

The process engineer and thermowell manufacturer should therefore review the complete operating condition.

ASME PTC 19.3 TW and Machined-Bar Thermowells

ASME PTC 19.3 TW is widely referenced for thermowell mechanical design evaluation.

Its scope includes thermowells machined from bar stock, including certain flange-connected, threaded and weld-in configurations.

Thermowells manufactured from pipe are outside the scope of this standard.

This distinction matters for procurement.

A buyer requesting an ASME PTC 19.3 TW calculation should provide the thermowell manufacturer with the required process and geometry data rather than sending only:

“SS347 thermowell required.”

Official reference:

ASME PTC 19.3 TW — Thermowells
.

Polythionic Acid SCC During Refinery Shutdowns

Polythionic acid stress-corrosion cracking is an important refinery shutdown damage mechanism for susceptible austenitic stainless steels.

The mechanism can develop when:

• sulfide scale exists on the metal surface;
• the equipment is exposed to oxygen;
• moisture is present;
• the stainless steel is in a susceptible sensitized condition;
• tensile stress is present.

This is one reason stabilized grades such as Type 347 have been used in selected refinery services.

However, material grade alone does not replace proper shutdown procedures.

347 Does Not Eliminate Polythionic Acid SCC Risk

It is incorrect to market Type 347 as completely immune to polythionic acid stress-corrosion cracking.

The actual risk depends on:

• material condition;
• thermal history;
• welding;
• stress;
• sulfide-scale formation;
• shutdown moisture;
• oxygen exposure;
• cleaning and neutralization procedures.

Refinery shutdown procedures and the project corrosion-management plan should therefore remain part of the material strategy.

For a broader refinery discussion, see:

347 Stainless Steel in Refineries: Applications & Buying Guide
.

When May ASTM A262 Testing Be Requested?

ASTM A262 contains practices for detecting susceptibility to intergranular attack in austenitic stainless steels.

Depending on the refinery specification, a purchaser may require an applicable ASTM A262 practice for selected 347 material.

This test should not automatically be added to every commercial Type 347 order.

The RFQ should define:

• required ASTM A262 practice;
• specimen condition;
• acceptance criteria;
• sampling requirement;
• reporting documentation.

Official reference:

ASTM A262 — Intergranular Attack Testing
.

347 vs 347H

Type 347 and 347H use the same niobium-stabilization principle but have different carbon requirements.

Standard Type 347 is appropriate for many stabilized corrosion and elevated-temperature applications.

347H uses a controlled higher-carbon range and may be selected when elevated-temperature mechanical properties such as creep strength become an important part of the design.

The decision should follow the component calculation and project specification.

“High temperature” by itself is not sufficient justification to substitute 347H for 347.

347 vs 321 for FCC Internals

321 and 347 are both stabilized austenitic stainless steels.

• 321: titanium stabilized.
• 347: niobium stabilized.

Both can be used in elevated-temperature refinery service.

The final choice can depend on:

• process-licensor specification;
• fabrication route;
• welding practice;
• product form;
• temperature;
• refinery operating history;
• material availability.

The article should therefore avoid claiming that Type 347 is universally superior to Type 321.

347 vs 316L for Refinery Internals

Type 347 and Type 316L solve different materials problems.

347 primarily uses niobium stabilization to address sensitization-related concerns.

316L uses low carbon together with molybdenum, which provides greater resistance to pitting and crevice corrosion than conventional Cr-Ni grades in many chloride-containing environments.

Where chloride-related localized corrosion controls material selection, 316L or a higher-alloy material may provide a better corrosion margin.

Where elevated-temperature stabilization is central to the metallurgy specification, Type 347 may become more relevant.

Cutting, Machining and Welded Fabrication

FCC internals can require a combination of:

• plate cutting;
• hole cutting;
• precision blanking;
• bending;
• machining;
• welding;
• surface cleaning.

JISCO JUDE provides

stainless steel cutting, slitting and cut-to-length services

for drawing-based material preparation.

For machined thermowells, hanger rods and other cylindrical components, buyers can review:

Stainless Steel Wire Rod & Round Bar
.

The final fabricator remains responsible for pressure design, structural calculations, welding qualification, machining tolerances and finished-component acceptance.

What Should a 347 FCC Internals RFQ Include?

1. Process unit: FCC, hydroprocessing, reactor system or other refinery unit.
2. Finished component: distributor tray, support, thermowell, hanger rod, bracket or screen.
3. Exact grade: Type 347 / UNS S34700 or 347H where specified.
4. Material standard: ASTM A240, ASTM A479, EN, JIS or project specification.
5. Product form: plate, sheet, bar or machined blank.
6. Dimensions: thickness, width, length or bar diameter.
7. Drawing: provide the latest approved fabrication drawing.
8. Operating temperature: normal value.
9. Design temperature: maximum specified value.
10. Operating pressure: where applicable.
11. Process medium: actual refinery stream.
12. Sulfur compounds: where relevant.
13. Chlorides: where relevant.
14. Erosion exposure: especially for catalyst-containing streams.
15. Welding: state applicable WPS or project requirement.
16. Heat treatment: where specified.
17. ASTM A262: state the required practice when applicable.
18. PMI: required or not required.
19. NDE: PT, UT or other project requirement.
20. Surface: required delivery condition.
21. Cutting tolerance: where blanks are requested.
22. MTC: state required documentation.
23. Quantity: prototype, turnaround or production requirement.
24. Destination: country, port and requested delivery date.

Example 347 FCC Distributor-Tray Material RFQ

Example Type 347 Thermowell-Bar RFQ

Incoming Inspection

Depending on the FCC project, incoming inspection may include:

• grade / UNS verification;
• heat number;
• Mill Test Certificate;
• chemical composition;
• niobium requirement;
• mechanical properties;
• plate thickness;
• bar diameter;
• surface condition;
• flatness or straightness;
• cut dimensions;
• PMI where specified;
• ASTM A262 results where required;
• full material traceability.

The inspection plan should follow the project specification rather than applying every possible test to every Type 347 order.

How JISCO JUDE Supports FCC Equipment Manufacturers

JISCO JUDE supplies stainless steel flat products, round bar and processing services for refinery, petrochemical and high-temperature equipment manufacturers.

Relevant resources include:

• Type 347 Austenitic Stainless Steel
• Hot-Rolled Stainless Steel Sheet & Plate
• Cold-Rolled Stainless Steel Sheet & Coil
• Stainless Steel Round Bar & Billet
• Cutting, Slitting & Cut-to-Length Services

For broader refinery material-selection information, buyers can also review:

JUDE Steel 347 Stainless Steel in Refineries
.

Specific Type 347 product form, dimensions, surface, processing tolerance, mill route, certification and project inspection requirements should be confirmed for each inquiry.

Frequently Asked Questions

Why is Type 347 used in some FCC reactor internals?

Type 347 is niobium stabilized and is used in selected refinery components where elevated-temperature exposure, welding and resistance to sensitization-related intergranular corrosion are important. The refinery or process-licensor metallurgy specification should determine whether 347 is required.

Can FCC distributor trays be made from ASTM A240 Type 347?

Yes, ASTM A240 Type 347 sheet or plate can be used as starting material for selected distributor trays where the equipment design specifies this alloy. The finished tray must still satisfy structural, fabrication, welding and process requirements.

What product form is normally used for a Type 347 thermowell?

Many process thermowells are solid-machined from round bar. The exact starting-material specification should be stated by the thermowell manufacturer or project specification.

Does ASME PTC 19.3 TW cover thermowells manufactured from pipe?

No. The current ASME PTC 19.3 TW scope applies to specified thermowells machined from bar stock. Thermowells manufactured from pipe are outside its scope.

Is Type 347 immune to polythionic acid stress-corrosion cracking?

No material should be described as universally immune. Type 347 stabilization reduces susceptibility associated with sensitization, but actual PASCC risk also depends on thermal history, stress, sulfide scale, oxygen, moisture and shutdown procedures.

Should I specify ASTM A262 for every 347 refinery component?

No. ASTM A262 testing should be required when specified by the refinery, process licensor, purchaser or project metallurgy specification. The required practice and acceptance criteria should be stated in the RFQ.

Should an FCC component use 347 or 347H?

347H should be considered where the high-temperature mechanical design specifically requires its controlled higher-carbon range and associated elevated-temperature properties. High operating temperature alone does not automatically justify substitution.

Is 347 better than 321 for FCC service?

Not universally. Both are stabilized austenitic stainless steels. Type 347 uses niobium and Type 321 uses titanium. The final selection depends on the licensor specification, product form, temperature, fabrication and refinery operating experience.

Is 347 better than 316L for refinery internals?

They solve different problems. 347 is primarily selected for stabilization in appropriate thermal environments, while 316L provides additional localized-corrosion resistance in many chloride-containing services because of its molybdenum content.

What should I send JISCO JUDE for a Type 347 FCC material quotation?

Send the equipment or component drawing, exact grade and material standard, product form, dimensions, operating and design temperature, process medium, welding and inspection requirements, quantity and delivery destination.

Request Type 347 Material for FCC and Refinery Internals

Technical note:
This article is intended for preliminary raw-material selection and procurement guidance. Final FCC or refinery component material selection must follow the process-licensor specification, process chemistry, design temperature and pressure, erosion and corrosion mechanisms, structural loading, fabrication method, welding procedure, shutdown strategy and applicable engineering codes.