What is an X-mas tree in a fired heater?

"X-mas tree" (or Christmas tree) is a shop and tender name for a cast tube support in a fired heater: a support where the tubes rest. It is a fired heater internal, and it is cast in different grades depending on duty, for example HK40, HP-Nb, 25Cr-20Ni types, 50Cr-50Ni-Nb or CT-15C, not in one grade. It has nothing to do with the wellhead "Christmas tree" valve assemblies used in oil and gas production, and the two are made, specified and bought in completely different ways. If a drawing or tender says "X-mas tree" next to a heater tag, tube supports, hangers or a material such as HK40, it means the heater casting.

Because the name is informal, the drawing defines the part. Send the drawing, not only the name.

What do tube supports, hangers, tube sheets and guides do?

Roof hangers suspend horizontal radiant tubes, or the top of vertical tubes, from the heater roof steel. In top-supported designs they carry the full weight of the coil.
Intermediate and wall supports, including X-mas tree and ladder types, hold horizontal radiant tubes at points along their length so the tubes do not sag between the end tube sheets.
Tube sheets are cast plates with a pattern of holes. In the convection section, intermediate tube sheets carry the tube bank and keep it on pitch while the tubes slide through as they expand.
Guides keep vertical tubes in line at the bottom or top of the radiant section, allowing axial growth while stopping sideways movement.

All of them share one job: carry load at high temperature while letting the tubes move. A support that binds puts bending into the tube. A support that sags lets tubes bow, touch and overheat.

What service conditions do tube supports see?

Temperature. Supports sit in flue gas and are not cooled by the process fluid, so their metal temperature follows the flue gas around them. API 560 therefore sets the support design temperature from the flue gas temperature in contact with the support, with a margin added (for convection supports, plus 55 °C, or 100 °F, in the 5th edition). Purchaser specifications built on API 560 add that no credit may be taken for refractory coatings applied for corrosion protection.
Creep. Dead load from tubes and fluid, held for years at temperature, makes the casting creep. Design stresses are therefore set from creep and rupture data, not room-temperature strength.
Thermal cycling. Start-ups, shutdowns and firing changes heat and cool the support, and friction from tube movement adds load at the tube seats.
Fuel-ash corrosion. Heaters firing residual oil leave ash rich in vanadium and sodium compounds that melt at furnace temperatures and dissolve the protective oxide on 25Cr alloys. Supports and hangers are often attacked even when the tubes, running cooler, are not.
Sulphur and oxidation. Sulphur in the fuel and long exposure to hot oxidising gas consume metal from the surface. API 560 sets a minimum corrosion allowance on exposed surfaces of 1.3 mm (0.05 in) for austenitic and 2.5 mm (0.10 in) for ferritic support materials.

How do cast tube supports fail?

Creep sag and distortion. Arms droop, tube seats drop and the coil follows. The usual cause is metal running hotter than the design temperature, often after a change of fuel or firing rate.
Fuel-ash wastage. Thinning, pitting and scale build-up on the hot faces of supports and hangers in oil-fired heaters. A published study of an Fe-25Cr-20Ni support hanger material found severe hot corrosion under sodium sulphate and vanadium pentoxide deposits.
Embrittlement after long service. 25Cr cast alloys age at temperature. Carbides coarsen, and sigma phase can form, which leaves the metal brittle when cold. An aged support can survive in service and then crack at shutdown or during handling, and aged material is hard to weld repair.
Cracking from casting defects. Shrinkage, gas porosity and sand inclusions at rib junctions and tube seats start cracks under creep and cycling. One refinery reported cast supports cracking from casting voids and changed its approach as a result. Soundness at the highly stressed sections matters as much as the grade.

Which grade should a fired heater tube support be cast in?

Choose by design metal temperature first, then by fuel.

HK40 (25Cr-20Ni) is the standard cast support grade for hot radiant positions. Purchaser specifications based on API 560 commonly allow cast 25Cr-20Ni up to about 1,093 °C (2,000 °F) design temperature where the fuel is clean.
HH (25Cr-12Ni) and HF (20Cr-10Ni) are used at lower temperatures, for example in convection positions. Some purchaser specifications exclude 25Cr-12Ni for supports, so check the project specification.
50Cr-50Ni-Nb to ASTM A560 is the answer to fuel-ash attack. API 560 lists A560 grade 50Cr-50Ni-Cb with a maximum design temperature of 982 °C (1,800 °F). It costs more and is less ductile than HK40, so it pays only where ash attack is the life-limiting mechanism.
HP-Nb (25Cr-35Ni-Nb) is used where a support needs more creep strength than HK40 gives, or where the designer has matched the support grade to an HP coil.

The fuel-ash rule. API 560 says that when the support design temperature exceeds 650 °C (1,200 °F) and the fuel contains more than 100 ppm of vanadium and sodium combined, the support must be protected. The options are a 50 mm (2 in) cover of dense castable refractory, at least 2,080 kg/m³ (130 lb/ft³), for radiant or accessible supports, or a fuel-ash-resistant alloy such as 50Cr-50Ni-Nb. Some purchaser specifications set the limit differently, for example 100 ppm vanadium alone or 150 ppm sodium plus vanadium, so read the project specification.

The 50Cr-50Ni grade also has a ceiling. Published tests in real boiler ash showed its corrosion rising sharply at 1,050 °C, and producers quote service limits near 950 °C. Above that, refractory protection or a design change is the usual route.

How are tube supports designed for casting?

Stress basis. Purchaser specifications following API 560 limit dead-load stress to half the stress for 1% creep in 10,000 hours or half the 10,000-hour rupture stress, whichever is lower, and apply a casting factor of 0.8 to all stress values.
Even sections. Heavy junctions next to thin arms solidify last and shrink. Generous fillets and gradual changes of section make sounder castings, and the rib-to-body junctions are where inspection concentrates.
Tube seats. The seat must let the tube slide axially. Seat radius, width and finish come from the drawing, and some designs need the seat machined.
Corrosion allowance. The API 560 allowance (1.3 mm austenitic, 2.5 mm ferritic, on each exposed surface) is added to the section needed for strength.
Marking. Grade and heat number are usually cast in or hard-stamped so each support can be traced to its certificate after installation.
Pinned or bolted assemblies. Large X-mas tree and ladder supports are often cast in pieces and assembled with pins or locking bars, which keeps each casting to a size that can be poured sound and replaced later.

How are cast tube supports inspected?

API 560 sets a baseline and the purchaser adds to it. Editions of API 560 have required:

visual examination of cast radiant supports, hangers and guides, using MSS SP-55 as the reference for surface imperfections, with dimensional checks to an agreed sampling plan;
liquid-penetrant examination to ASTM E165 on austenitic castings (magnetic particle on ferritic), with rib intersections singled out for 100% examination, and penetrant verification of every repair;
radiography of critical sections when the purchaser specifies it, to ASTM E446 or E186, and radiography of major weld repairs. A repair is major when the prepared cavity is deeper than 20% of the section or longer than 250 mm (10 in);
weld repairs made with procedures and welders qualified to ASME Section IX.

On top of that, buyers usually ask for chemistry and tensile certificates per heat, positive material identification (PMI) of each piece, and third-party witness of the inspection.

What should an RFQ for fired heater tube supports include?

The drawing for each item, with revision, and the item name used on the heater (hanger, intermediate support, X-mas tree, tube sheet, guide).
Grade and standard: for example ASTM A351 HK40, A297 HH, or A560 50Cr-50Ni-Cb. Name the standard, not only the grade label.
Design metal temperature, the heater section (radiant, shock, convection), and whether the heater fires gas or oil. For oil, the vanadium and sodium content of the fuel.
The governing specification: API 560 edition, and the purchaser's or EPC contractor's own fired heater specification.
Inspection: visual standard, penetrant extent and acceptance level, radiography zones and severity level, PMI, dimensional checks, and the third-party agency.
Machining of tube seats, bolt or pin holes, and any assembly or trial fit of multi-piece supports.
Marking, certificates and documentation required with dispatch.
Quantity, spares, delivery schedule and any vendor approval the end user requires.

For a replacement with no drawing, a sketch with dimensions, photos of the old support and a PMI reading of its alloy are enough to start. What we need to quote lists the rest.

Does Ni-Chrome cast fired heater tube supports?

Yes. Tube supports are one of our main products. We cast roof and wall hangers, intermediate supports, X-mas tree supports, support ladders, tube sheets, guides and locking bars in HK40, HH, HP-Nb and 50Cr-50Ni-Nb, in sand moulds, with the grade and heat number cast in.

Send a drawing for tube supports →

Frequently asked questions

Is a fired heater X-mas tree the same as a wellhead Christmas tree?

No. A fired heater X-mas tree is a cast tube support inside a heater, the part the tubes rest on. A wellhead Christmas tree is an assembly of valves on top of an oil or gas well. They share a nickname and nothing else.

Why do tube supports run hotter than the tubes?

Tubes are cooled from inside by the process fluid. Supports are not cooled at all, so they settle close to the temperature of the flue gas around them.

When should supports be in 50Cr-50Ni-Nb instead of HK40?

When the heater fires oil with significant vanadium and sodium and the support runs above about 650 °C. API 560 then requires either a refractory cover or a fuel-ash-resistant alloy. On clean gas firing, HK40 is the usual choice.

Can aged tube supports be weld repaired?

Often not reliably. Long service at temperature leaves 25Cr alloys brittle from coarse carbides and sigma phase. A cracked aged support is usually replaced, and any repair needs a metallurgical assessment of the actual part.

Which standard covers fired heater tube supports?

API 560, Fired Heaters for General Refinery Service, covers support design temperature, allowable stresses, materials, the fuel-ash rule and inspection. The castings themselves are bought to ASTM A297, A351 or A560 depending on grade.

Are tube supports sand cast or centrifugally cast?

Sand cast. Supports, hangers, X-mas trees and tube sheets are irregular shapes, so they are poured statically in sand moulds. Centrifugal casting is for round parts such as tubes and sleeves.

Sources

Need tube supports cast? See the castings we make and our alloy grades, or send us a drawing.

Fired heater tube support castings: X-mas trees, hangers and tube sheets