An LPG loading and unloading skid is a pre-assembled package that brings together, on one steel base frame, the pump or compressor that moves product into or out of a road tanker, the liquid and vapor lines, emergency shutoff and relief valves, a metering run where required, and a local control panel. What makes such a package useful is less its equipment list than a clearly defined boundary: how it ties into the storage vessel, the tanker, power and control signals, relief and vent routing, and the foundation. If those interfaces are left open until construction, most of the benefit of pre-assembly is lost in the field.
The general logic of the loading and unloading chain is covered on our Truck Loading Systems and Truck Unloading Systems pages. This page looks at what changes when the product is LPG, a liquid held under pressure, and how that shapes package scope and site interfaces.
Why loading and unloading are often combined for LPG
At many LPG storage sites the same truck bay handles both receipts and dispatches. Instead of building two separate systems, a single package with a valve manifold that reverses flow direction can serve both duties. LPG lends itself to this because transfer is closed in both directions: the liquid line carries product while the vapor line balances pressure between tanker and storage vessel. For loading, a pump draws from storage and pushes into the tanker. For unloading, either a pump is used or a compressor draws vapor from the storage vessel and pushes it into the tanker vapor space, displacing the liquid. The pump-versus-compressor choice is discussed on the LPG Truck Unloading Systems page.
Package scope: base equipment and project options
Equipment group | Role in LPG service | Scope |
Transfer pump | Moves liquid from storage to tanker; because product in the vessel is near its boiling point, suction layout is decisive | Base scope where loading is required |
Compressor | Transfers vapor between vessels to push liquid out; can recover part of the vapor left in the tanker at the end of unloading | Option, depending on unloading method |
Liquid and vapor manifold | Switches flow direction between loading and unloading while keeping the two phases separate | Base scope |
Strainer and vapor eliminator | Protects pump and meter from debris and limits vapor reaching the meter | Base scope |
Meter and back-pressure valve | Measures quantity and holds line pressure so the product stays liquid in the meter | Base scope if metering is needed; custody transfer terms defined separately |
Emergency shutoff valves | Remotely close liquid and vapor lines close to the tanker connection | Base scope |
Thermal relief valves | Prevent pressure build-up in liquid trapped between two closed valves | Base scope |
Hose or loading arm | Makes the tanker connection; selected together with a breakaway coupling | Within skid scope or separate, per project |
Local control panel | Starts pumps and valves, collects permissives, processes ESD inputs | Base scope; link to plant control system per project |
Ground verification | Blocks transfer until tanker grounding is confirmed | Base scope |
Site interfaces to settle at the skid boundary
In a pre-assembled LPG package, most problems arise at the edge of the frame rather than inside it. For each interface below, the split of responsibility should be written down.
Process connections to storage
The pump suction line from the vessel liquid outlet to the skid should be short, straight and free of high points where vapor can collect. Since LPG sits at saturation pressure in storage, every bit of suction-side pressure loss raises the risk of the product flashing at the pump inlet. Skid location is therefore chosen on available net positive suction head (NPSH), not just on free space. Where the vapor return line enters the vessel vapor space, and where the pump minimum-flow bypass returns to the vessel, are part of this interface too.
Tanker-side connection
The type, size and position of liquid and vapor connections on the tanker vary by country and tanker builder. Hose or arm ends must match them, and approach direction, connection height and hose working envelope should be checked together. If the terminal is expected to trigger closure of the tanker's internal valves remotely, the source of that signal and who is responsible for it need to be defined.
Relief, venting and hose depressurizing
Relief valve outlets, and the product left in the hose or arm after transfer, need a destination: the vapor return system, the plant flare or vent system, or a path back to storage. The choice depends on existing site infrastructure and directly affects skid design.
Power, control and safety signals
The hazardous area classification, supplied by the owner's study, drives the choice of motors, transmitters and panels on the skid. Power supply, signals to and from the plant emergency shutdown (ESD) system, links with gas and fire detection, and data passed to the plant control system should each be listed in an interface schedule. The interlock logic itself is covered on the page on ESD and safety interlock systems for LPG terminals.
Pressure rating, materials and low-temperature cases
Skid design pressure starts from the vapor pressure of the product at the highest expected operating temperature, plus the additional head a pump or compressor can develop. Propane-rich product has a higher vapor pressure than butane-rich product at the same temperature, so the product slate directly affects pressure class. Rapid depressurization can also chill LPG as it vaporizes, taking some line sections below ambient; the minimum design temperature of materials should account for that case. Gaskets and hoses must be compatible with LPG.
How metering and safety fit into the package
Once a meter run is added, keeping the product liquid in the meter becomes the central issue; back-pressure control and a vapor eliminator serve that purpose, as detailed on the LPG Metering Skids page. Where the quantity is used for invoicing, the measuring system is assessed as a whole, and one meter's approval does not cover the complete system including returned vapor. On the safety side, emergency shutoff valves, breakaway couplings, grounding and position permissives are built into the package, while the required reliability of each function is derived from risk assessment.
How TLY Enerji supports LPG transfer projects
TLY Enerji provides design, procurement and engineering services for LPG truck loading and unloading stations. Depending on project scope, this can include selection and supply of LPG pumps, flow meters, pressure regulators, relief valves and control systems, along with engineering support for integration, installation, testing and commissioning. The skid's scope of supply and the split of responsibility at site interfaces are agreed for each project against the technical specification.
Information needed to define the package
· Operating modes: loading only, unloading only or both, with daily tanker counts and the split between modes
· Products handled: commercial propane, butane or mixtures, and whether the product changes seasonally
· Storage vessel design pressure, liquid and vapor nozzle locations, distance and elevation difference to the skid
· Target transfer rate and acceptable tanker turnaround time
· Whether unloading will use a pump or compressor, or whether that is left to the engineering study
· Tanker connection types and preference for hoses or loading arms
· Available flare, vent or vapor return infrastructure for relief and hose depressurizing
· Hazardous area classification and the signals to exchange with the plant ESD and control systems
Related LPG applications
· LPG truck loading systems: Closed loading, fill limits and how the loaded quantity is determined.
· LPG truck unloading systems: Pump versus compressor unloading compared.
· ESD and safety interlocks for LPG terminals: Logic behind the shutoff and permissive functions built into the skid.
· LPG metering skids: Keeping LPG liquid in the meter and meter run design.
· LPG vapor return systems: How the vapor line affects pressure balance and transfer time.
Questions about LPG loading and unloading skids
Should loading and unloading share one skid or use two?
If the same bay handles receipts and dispatches and both modes run at similar rates, a single package with a reversing manifold can save space and cost. If rates, tanker types or operating hours differ widely, or both operations must run simultaneously, separate skids keep operation simpler. The choice should weigh the daily operating profile against redundancy expectations.
How is the skid pressure rating determined?
The starting point is the vapor pressure of the most volatile product at the highest expected temperature. Pump or compressor head and pressure surges in the line are added to that. Storage vessel and tanker design pressures also set limits for the connected system. The final value comes from engineering calculation against the project specification and applicable pressure equipment regulations.
Why does an LPG skid need thermal relief valves?
After a transfer ends or an emergency shutdown closes the valves, liquid LPG can be trapped between two closed valves. As sun or ambient heat warms it, the liquid tries to expand and pressure in the closed volume rises quickly. A thermal relief valve releases that pressure to a safe destination and protects piping and equipment. Where its outlet goes depends on the site's relief infrastructure.
Can the meter on the skid be used for custody transfer?
It can, but that decision affects the whole measuring system. Keeping the product liquid in the meter, vapor elimination, temperature and density measurement, proving connections and the treatment of returned vapor in the quantity calculation all have to be designed together. The legal metrology requirements that apply to invoiced quantities depend on national regulations and the contract.