19 Propylene Refrigeration Skids for Ethane Carriers: What Repeat-Order Delivery Teaches About Cargo Reliquefaction

Two things are happening at once in the ethylene business, and they are pulling on the same piece of equipment.

On land, a wave of world-scale crackers is coming online. Chinese projects alone account for more than USD 40 billion of steam cracker and derivative capacity under construction, with several million-tonne-per-year units scheduled to start up during the second half of 2026. At sea, the feedstock is getting lighter: US ethane exports have kept climbing, and the fleet that carries ethane across the Pacific has grown from a novelty into an established trade.

Both ends of that chain need the same thing — a reliable propylene refrigeration package. Ashore it sits in the cold section of the cracker. Afloat it sits in the cargo handling room of the carrier, reliquefying boil-off gas so the cargo arrives as cargo and not as fuel.

Over the past two years, Suzhou Lmart Energy Equipment has been awarded three consecutive orders from an international gas handling systems integrator for a combined 19 propylene refrigeration compressor skids destined for ethane carrier newbuildings. Units have been delivered progressively and some remain in production. This article is a technical debrief of that programme — what the duty actually is, what changes when the package goes to sea, and what a repeat order really measures.

Why propylene is the workhorse refrigerant on both sides of the chain

Cracked gas leaving the furnaces is a mixture — hydrogen, methane, ethylene, ethane, propylene, C4 and heavier. Separating polymer-grade ethylene at 99.95 % purity or better is done by cryogenic fractionation: cool the stream in stages and cut it tower by tower. The refrigeration that makes those cuts possible is almost universally a cascade of propylene and ethylene circuits.

• The propylene circuit covers roughly −40 °C to 0 °C. It condenses the C3 splitter and deethaniser overheads and, critically, rejects the heat of the ethylene circuit.
• The ethylene circuit rides on top of the propylene circuit and reaches down towards −100 °C for the demethaniser.

The propylene loop is the foundation of the whole cold section. It usually carries the single largest refrigeration load in the plant, and if it trips, the cold end goes down with it. That is why propylene refrigeration compressor trains are managed as critical machinery, with the same spares philosophy and condition monitoring as the cracked gas compressor.

Propylene — refrigerant designation R1270 — also carries a practical advantage that matters to operators: it is already an inventory item on the site. Top-up, leak handling and disposal ride on chemistry the plant already manages, with no separate refrigerant supply contract, no phase-down schedule and no global warming potential concern of consequence. Its thermodynamics in the −40 °C band are simply good: moderate pressure ratio, high volumetric refrigerating capacity, well-mapped compressor selection curves. This is a route the petrochemical industry has validated over decades, and the marine cargo handling sector adopted it for exactly the same reasons.

Ethane at sea: what the cargo reliquefaction duty actually looks like

Ethane boils at −88.6 °C at atmospheric pressure. A Very Large Ethane Carrier moves it as a refrigerated liquid in cargo tanks, and over a voyage of several weeks heat ingress through the insulation continuously generates boil-off gas.

The commercial logic is unforgiving. Boil-off that is burned, or worse vented, is cargo that was paid for and never delivered. On a long Gulf-to-Asia rotation the cumulative loss is measured in whole percentage points of the cargo value, which is a material share of the voyage economics. Every modern ethane carrier therefore carries a cargo reliquefaction plant: capture the boil-off, compress it, condense it against a refrigerant, and return the liquid to the tank.

The cold source for that condensation is the propylene refrigeration package. Functionally it speaks the same process language as the cracker's propylene loop. The differences are environmental and procedural, and they are where the engineering hours actually go:

• Motion. The package must operate through the vessel's specified list, trim, roll and pitch envelope. Oil separation, oil return, liquid level control and separator sizing all have to be checked against inclined and dynamic conditions, not just the static case.
• Footprint. The cargo machinery room is a fixed volume that was allocated at general arrangement stage. The skid must fit that envelope, land on the designated seatings, and leave defined maintenance access — a pull space for the compressor rotor, tube bundle withdrawal clearance for the exchangers, valve and instrument access for the crew.
• Vibration and structural response. Skid frames are checked for stiffness against hull-induced excitation, and rotating equipment is aligned on a base that will be welded into a moving structure rather than grouted onto a static foundation.
• Certification and survey. Materials, welding, pressure parts and electrical equipment are all built and inspected under classification society rules, with drawing approval before fabrication and surveyor attendance at defined hold points.
• Schedule coupling. Delivery is not tied to a plant turnaround; it is tied to a shipyard's block erection and machinery installation sequence. Being early is inventory; being late is a berth.

Delivery record: three consecutive batches, 19 skids

The purchaser on this programme is an international gas handling systems integrator with a long track record in liquefied gas cargo systems. Client identity, shipyard and hull identification are withheld under our confidentiality practice; the delivery profile is as follows.

Batch	Year awarded	Scope	Qty	Application	Inspection basis
1	2024	Propylene refrigeration compressor skid	6 sets	VLEC series	Classification society survey
2	2025	Propylene refrigeration compressor skid	7 sets	99,000 m³ class VLEC	Classification society survey
3	2025	Propylene refrigeration compressor skid	6 sets	150,000 m³ class ethane carrier	Classification society survey

Three observations from executing it.

The follow-on orders are the real result. A first order proves you passed a vendor audit. The second and third prove the first one did not create problems downstream — no drawing rework loops, no non-conformances discovered at the yard, no documentation gaps found during commissioning. For a series-ship programme, changing refrigeration package suppliers mid-series means new drawing approval, new weld procedure review and a second equipment configuration in the owner's spare parts inventory. Owners and integrators avoid that unless the incumbent forces them to.

The 150,000 m³ class raised the bar. Larger cargo containment means proportionally larger boil-off generation, larger reliquefaction capacity and a physically bigger package. Batch three was not a copy-paste of batch two: thermal duty, pipe sizing, structural design of the skid frame and lifting arrangements all went through a fresh review cycle, while the qualified procedures and inspection framework carried over intact.

Survey coverage was continuous, not sampled. Drawing approval, material certification review, weld procedure qualification, in-process inspection and factory acceptance testing were executed with surveyor attendance at each defined hold point — for every skid. Nineteen units means nineteen complete inspection dossiers. There is no volume discount on that.

The onshore duty points behind the marine scope

A fair question from any technical evaluator: is a marine cargo reliquefaction skid the same business as an onshore petrochemical refrigeration plant? Our experience is that the process core is identical and the wrapper is different. Heat balance, refrigerant property data, compressor selection logic, oil system design, capacity control philosophy and anti-surge or capacity slide logic all transfer directly. What changes is the environmental envelope and the certification route.

Across a cumulative refrigeration package record of roughly 130 units, propylene-duty references include:

• 8,330 kW at −32 °C propylene refrigeration system for a propane dehydrogenation unit at a major domestic refining and petrochemical complex.
• 2 × 14,000 kW propylene refrigeration packages for a coastal petrochemical base.
• −40 °C refrigeration package for the ethane storage area of an ethane-to-ethylene project.
• 1,400 kW propylene refrigeration system for a 450,000 t/y polypropylene unit.

The common element across these is the oil-injected screw compressor as the machine at the centre of the package. In propylene duty its behaviour suits the application well: it tolerates wet suction conditions far better than a reciprocating machine, and slide valve capacity control gives smooth part-load operation. That matters enormously for cargo reliquefaction, where boil-off generation swings with seawater temperature, ambient conditions, tank filling level and voyage phase — the plant spends most of its life away from design point.

A point of scope clarity, because procurement teams are right to ask it: Lmart does not manufacture compressors. The screw compressor block is a bought-out item from a specialist manufacturer, selected against the duty. What we manufacture in-house are the pressure vessels — oil separators, economisers, receivers, suction accumulators — and the shell-and-tube heat exchangers, including condensers, evaporators and oil coolers. What we do as an integrator is turn those, plus the driver, the oil system, piping, instrumentation and the control panel, into a tested skid-mounted package with a complete document set. That is the honest boundary of our scope, and stating it plainly is more useful to an evaluator than a vague claim of vertical integration.

Batch consistency is engineered, not promised

Nineteen skids is not one skid built nineteen times. The requirement is that unit 19 is indistinguishable from unit 1 in configuration and in documentation, delivered across a two-year window and against a moving yard schedule. Four things carry that:

1. A deep welding procedure library. Over 600 qualified weld procedure qualification records cover the material and joint combinations typical of low-temperature marine piping and pressure parts. When qualifications already exist, a project does not lose weeks to procedure qualification testing before fabrication can start — and, more importantly, later units are welded to exactly the same qualified parameters as the first.
2. A frozen factory acceptance test protocol. Every skid completes mechanical running, leak and pressure testing, and full alarm and interlock function testing before release, executed to the same written procedure and signed off with the surveyor present. A fixed protocol is what makes the FAT records of unit 1 and unit 19 comparable documents rather than two different narratives.
3. Documentation shipped with the hardware. Material traceability, non-destructive examination reports and inspection records are handed over together with the physical skid. Documentation that arrives weeks after the equipment is a certification problem waiting for the yard's installation team, and chasing it is a job nobody has budgeted for.
4. Parallel bay capacity. An 8,000 m² skid assembly workshop lets several packages be built simultaneously, so cadence follows the yard's erection sequence rather than a serial queue.

One lesson from the programme worth passing on: on repeat batches, schedule risk migrates almost entirely into long-lead procurement. Once the first unit is type-frozen, workshop hours become predictable and stop being the constraint. What decides delivery is whether the compressor blocks, special valves, plate material and instrumentation were locked in early enough. Compressing the shop programme to recover a slipped procurement date rarely works; sequencing the procurement correctly at award almost always does.

A buyer's checklist for propylene refrigeration skid packages

If you are evaluating suppliers for cargo reliquefaction or cracker cold-section refrigeration, these are the questions that separate answers quickly:

• Ask for the duty point, not the model number. Refrigeration capacity is meaningless without evaporating temperature, condensing temperature and cooling medium temperature stated with it. A capacity quoted at −25 °C and one quoted at −40 °C are not comparable numbers.
• Ask what is manufactured versus integrated. A supplier who cannot state the boundary clearly has usually not thought about warranty responsibility either.
• Ask to see one complete inspection dossier from a delivered unit, not a certificate list. The dossier reveals how the quality system actually behaves under survey.
• Ask about the part-load map. For marine reliquefaction, behaviour between 30 % and 100 % capacity matters more than the design-point figure, because that is where the plant will spend the voyage.
• Ask about repeat-order history on series ships. Consecutive batches to the same integrator say more about execution quality than any single flagship reference.
• Ask when long-lead items get locked. The answer tells you whether the supplier has actually delivered a multi-unit programme before.

Closing note

With world-scale crackers starting up through the second half of 2026 and the ethane trade still expanding, propylene refrigeration sits at both ends of the same value chain — in the cold section of the plant and in the cargo machinery room of the ship that feeds it. It is not glamorous equipment. It is, however, equipment whose failure stops a cold end or turns cargo into fuel.

What the 19-skid programme taught us is not a slogan about scale. It is that the inspection record of each individual unit, produced the same way every time, is the only argument that survives a technical audit.

If you are selecting propylene refrigeration packages for an ethylene cracker, a propane dehydrogenation unit or a gas carrier newbuilding, our engineering team can review your duty conditions and return a technical proposal with general arrangement drawings and budget pricing.