LPG Tank Farm BOG Recovery: Screw Compressor Packages With Eight Years in Service

1. Why Tank-Farm BOG Became a Line Item Again

For most of the last decade, boil-off gas from pressurised LPG storage was treated as an operating nuisance. Vapour built up, tank pressure reached the setpoint, the relief route opened, and the flare took care of the rest. The volumes looked small on a mass balance, and the capital case for recovery rarely survived a budget review.

Two things changed that arithmetic. The first is feedstock price. Chinese LPG import volumes reached 35.68 million tonnes in 2024, up 9.2 % year on year, and through 2026 the combination of higher Saudi contract prices and elevated freight has pushed landed cost up again. Every tonne that evaporates and burns is a tonne that was paid for at import parity. The second is compliance. Flare load, VOC emissions and carbon intensity are all now reported metrics rather than internal housekeeping, and several of our clients report that vapour recovery has moved from the "nice to have" column of a revamp study into the mandatory scope.

The practical consequence for EPC contractors and design institutes is that BOG recovery packages are appearing in more tank-farm scopes — both on greenfield terminals and, more often, as retrofits into congested existing farms where the compressor has to fit around what is already there. This article walks through the recovery route, a project reference with eight years of operating history behind it, and the selection questions that in our experience actually determine whether the package delivers.

2. Where the Vapour Actually Comes From

Engineers who work mainly with cryogenic, atmospheric storage sometimes assume that pressurised LPG spheres and bullets do not boil off. They do — just through different mechanisms and on a different time signature.

• Ambient heat ingress. No matter how good the coating or insulation, solar gain and ambient temperature drive heat into the liquid. Summer daytime wall temperatures raise vapour pressure directly.
• Transfer and flash disturbance. Ship discharge, truck loading and inter-tank pumping change level and pressure quickly. Flashing during transfer produces short, sharp vapour peaks that dwarf the static rate.
• Diurnal breathing. Day–night temperature cycling makes tank pressure oscillate. The portion above setpoint has to go somewhere.

The important design consequence is that BOG flow is not a steady number. On a propane farm in eastern China, the ratio between a mild winter night and a summer discharge peak can easily be several to one. Any compressor selection that ignores this ends up either unable to hold tank pressure during transfers, or running at a fraction of rated load for most of the year.

3. The Recovery Route in Four Steps

The process itself is not exotic. Vapour is drawn from the tank vapour space, compressed, condensed and returned as liquid:

1. Suction knock-out. Vapour is routed to a suction drum sized to remove entrained droplets under the worst credible upset, not just the normal case.
2. Compression. The package raises pressure from tank vapour-space pressure to condensing pressure, with capacity control tracking the actual vapour make.
3. Condensation. A water-cooled or air-cooled condenser returns the discharge gas to liquid.
4. Return. Condensate is let down or pumped back to storage, closing the loop.

Some operators skip the condensing section and route compressed BOG straight into the fuel-gas header or to a downstream unit as feed. Where a stable consumer exists on the plot, that is the cheaper route. Where it does not — and on merchant tank farms it usually does not — re-condensation back to the tank is the general-purpose answer. Both Zhejiang Satellite phases described below use the recovery route back to storage.

4. Case Reference: Two Phases at Zhejiang Satellite Petrochemical

Zhejiang Satellite Petrochemical is one of China's leading acrylic acid and acrylate producers, and its Jiaxing complex operates one of the larger storage areas in the Yangtze delta. Lmart supplied BOG recovery compressor packages for two consecutive phases, in both cases as equipment supplier to a scheme designed by Wuhuan Engineering.

Phase	Service	Medium	Motor rating	Units	Design institute
Phase 1 — 2017	Acrylic acid unit tank farm	Propylene vapour mixture	450 kW	2	Wuhuan Engineering
Phase 2 — 2018	Propane storage area	Propane / LPG vapour mixture	630 kW	2	Wuhuan Engineering

Both phases were delivered as fully integrated skids. The compressor, drive motor, oil separator, oil cooler, oil circuit, instrumentation and control panel were assembled and string-tested in our workshop, and the completed skid shipped as a unit. On site the scope reduced to foundation placement, nozzle tie-ins and utility connections — site installation measured in days rather than weeks, with hot work inside the hazardous area minimised. That matters more on a live tank farm than any efficiency point on a performance curve.

The propane phase entered service in 2018 and has now been running for more than eight years. Zhejiang Satellite subsequently placed three further orders with Lmart, in 2019 and 2023, for PSA off-gas and tail-gas compressor packages. A comparable reference in the same duty class is the 2021 propane tank-farm BOG project at Huayi Group's Guangxi site — also designed by Wuhuan Engineering, 630 kW, three units delivered.

We are careful about how we describe our role in these projects. The process scheme, the tank-farm hydraulics and the plant integration belong to the design institute and the owner. Lmart's position is that of equipment supplier: we take the specified duty, engineer the package around it, build the pressure equipment, integrate the skid and prove it before it leaves the works.

5. Why an Oil-Injected Screw Package for BOG Duty

Reciprocating and screw machines both appear on BOG services, and there is no universally correct answer. For the Zhejiang Satellite phases the selection landed on oil-injected screw compressor packages for four reasons that hold across most pressurised LPG tank farms.

Stepless capacity control across a wide turndown. A slide valve gives continuous modulation over roughly 10–100 % of rated capacity. Clearance-pocket control on a reciprocating machine is stepped, which on a swinging vapour make means either frequent start–stop cycling or recycle flow that burns power for no product.

Tolerance of occasional liquid carry-over. A suction drum is mandatory and no compressor selection substitutes for it. But upsets happen, and a rotary positive-displacement machine with oil injection handles small intermittent slugs far better than a reciprocating machine, where liquid at the suction damages valve plates at best and connecting rods at worst.

Low vibration, which is what makes skid mounting practical. With no reciprocating inertia forces to react, the package sits on a structural steel base frame rather than a large mass-concrete block. This is the enabling condition for workshop string testing and fast site commissioning — and on a retrofit into an existing farm, it is often the difference between a feasible layout and an impossible one.

Long intervals between major overhauls. Valves and piston rings are consumable items on a reciprocating machine and set the maintenance rhythm. Screw packages in clean, correctly filtered service run overhaul intervals measured in tens of thousands of hours, which suits a tank farm expected to stay available all year.

Where screw packages are not the answer: very high discharge pressure duties and very small vapour flows. If the process demands discharge well above the range a single oil-injected screw stage comfortably covers, or if the flow is so low that the machine would spend its life at minimum slide-valve position, a reciprocating selection deserves a serious look. The right method is to run the numbers for the specific duty rather than to pick a machine type first and justify it afterwards.

6. Five Decisions That Decide the Project

Having worked through a number of these packages with design institutes and owners, the same questions come back. Getting them settled before the enquiry closes is worth more than any amount of datasheet refinement afterwards.

1. What vapour flow basis are we selecting against? Size only for static evaporation and the package cannot hold tank pressure during ship discharge. Size for the absolute peak and the machine idles at part load for most of its life. The workable pattern is usually a main package covering the normal make, with turndown capability and, where availability demands it, a spare unit absorbing the peaks. That decision belongs jointly to the design institute and the package supplier and should be taken early.

2. What is the real composition range? Tank-farm vapour is never pure propane. It is a mixture of propane, butanes and olefins that shifts with the cargo. Thermodynamic calculation, oil selection, separator sizing and condensing pressure all have to be checked against the actual composition envelope, not a nominal single-component case. On the Zhejiang Satellite packages the verification was run across the composition range the owner provided.

3. Where exactly does the skid stop? Suction drum, condenser, return pump — each can sit on the skid or loose in the field. The more that is integrated, the less site work and the shorter the commissioning window, but transport envelope and lifting access set a hard ceiling. Confirm shipping dimensions with the logistics contractor before freezing the battery limit, not after.

4. Hazardous-area classification and interlock philosophy. Tank farms are classified areas. Motor, instrument and panel certification requirements, plus the interface and interlock scheme with the plant DCS, should be frozen early. Late changes here generate rework out of proportion to their apparent size.

5. Which pressure code, and who certifies? The vessels in the package — suction drum, oil separator, and often the condenser — are pressure equipment in their own right. Whether they are built to ASME BPVC Section VIII Division 1, EN 13445 with PED conformity assessment, or the local code, that decision drives inspection, documentation and lead time. It is worth stating in the enquiry rather than leaving to a technical clarification round.

7. What Lmart Builds, and What We Integrate

We think it helps buyers to be explicit about scope, so we are direct about ours.

Manufactured in-house: pressure vessels to ASME BPVC Section VIII Division 1 under our U-Stamp and to PED 2014/68/EU, shell-and-tube heat exchangers to TEMA (condensers, oil coolers, evaporators), and the structural skid itself — base frame, piping, and workshop assembly.

Integrated, not manufactured: the compressor element itself. Oil-injected screw airends are sourced from established compressor OEMs and selected for the specified duty. Lmart's engineering contribution is the package around it: thermodynamic verification against the actual gas composition, oil-circuit and separation design, cooler sizing, skid layout, instrumentation and control integration, and the string test that proves the assembly before shipment. Drive motors, instrumentation and control hardware are likewise selected and integrated to the project specification.

That division of labour is normal in the packaging business, and stating it plainly avoids the awkward conversation later. What the owner buys from us is a proven, tested, documented package — and the pressure equipment inside it built under our own certifications.

8. Closing

BOG recovery on pressurised LPG and propylene storage is mature technology with a calculable return. The engineering risk sits less in the compression step than in the framing: what flow basis, what composition envelope, what skid boundary, what hazardous-area and code requirements. Settle those, and a package like the ones running at Jiaxing since 2017 and 2018 becomes a straightforward, bankable revamp rather than a research project.

If you are evaluating vapour recovery for an LPG, propane or propylene tank farm, our engineering team can provide comparable project references and a technical proposal with general arrangement drawings and budget pricing within 48 hours.