In terms of flow path, loading NGL into a truck looks much like loading LPG: liquid from pressurized storage passes through a pump, meter and control valve into the tanker, while the tanker vapor space is tied back to storage through a separate line. The difference lies in the product. Because the composition of natural gas liquids (NGL) changes from batch to batch, compatibility with the tanker design pressure, keeping the pump and meter full of liquid, and how much product may go into the tanker are all set by the expected composition range rather than by a fixed product grade.
The general components and loading profile of a loading point are covered on the truck loading systems page, and the closed loading arrangement for propane and butane on the LPG truck loading page. This page deals only with what NGL changes.
The question to answer before loading: can this tanker carry this mix?
With LPG, the product specification is narrow enough that the pressure match between tanker and product is usually settled in advance. With NGL, vapor pressure climbs as the share of ethane and other light components increases. The tanker pressure vessel must be designed for the vapor pressure of the product at the highest temperature it is expected to reach, so a tanker built for LPG service will not necessarily suit every NGL mix.
In practice this check belongs in the product specification before it becomes a loading system question: the contract sets an upper limit on light ends and a vapor pressure limit for product shipped by road, and composition or vapor pressure data is confirmed for each batch before loading starts. Terminal automation can match that data against the tanker ID and the product class the tanker is cleared for, and use the result as one of the loading permissives.
Closed loading and how the vapor line behaves with NGL
As with LPG, NGL is never loaded open to atmosphere. Vapor compressed in the tanker as liquid enters is routed back to the storage tank through a vapor return line, balancing the pressure on both sides. The point to watch with NGL is that the returning vapor is lighter than the liquid: the vapor phase carries a higher share of ethane and propane. That has two consequences.
· Pressure balance: Starting pressures in the storage tank and the tanker depend on the composition of what each contains. If the tanker holds a heel of a different composition from its previous trip, back pressure at the start of loading can be higher than expected; vapor line size and loading rate are chosen with this in mind.
· Quantity accounting: Returned vapor is part of the product. Where metering is used for sales and a vapor connection exists, legal metrology recommendations call for the returned quantity to be measured and deducted. The detailed calculation is covered on the NGL custody transfer page.
Variable composition and the fill limit
Pressurized tankers are never filled completely; a fill limit leaves room for the liquid to expand as it warms. Even when that limit is stated as a volume percentage, the mass that can go into the tanker depends on the density and thermal expansion of the product. A light NGL mix corresponds to less mass in the same volume and expands more with temperature.
Entering the preset as a fixed mass is therefore not enough for NGL. The batch controller should calculate the allowable quantity from the mass and density reported by the meter, the batch composition and the tanker volume. Coriolis meters measure mass, density and temperature simultaneously, so they feed this calculation directly. The tanker fill limit itself is set by transport regulations and the tanker manufacturer's data; the loading system enforces that limit, it does not change it.
Pump, back pressure and keeping the product liquid
At the suction of a pump drawing from storage, the product sits very close to its bubble point; the net positive suction head (NPSH) margin is small and shrinks further as the composition gets lighter. Pump selection, suction line friction losses and the tank outlet layout are checked against the lightest mix the terminal expects.
Around the meter and control valve, back pressure is held downstream of the meter so the product does not flash. For LPG the back-pressure setting can be fixed for one product; for NGL it must be sufficient for the highest vapor pressure batch at the highest product temperature. In some installations the batch controller manages the setpoint dynamically from measured temperature and batch data.
Design inputs that matter most for NGL loading
Parameter | Why it matters in design | Effect on selection |
Composition range (especially ethane and heavy ends) | Vapor pressure, density and expansion all move with it | Sets pressure rating, back-pressure setpoint and the fill quantity calculation |
Vapor pressure at maximum product temperature | Tanker, piping and relief valves are assessed against it | Determines acceptable tanker types and relief settings |
Tanker fleet and design pressures | Not every tanker suits every mix | May require a tanker-to-product match in the loading permissive |
Loading rate and number of bays | Suction losses and vapor line capacity grow with flow | Drives pump, meter size and vapor line diameter |
Purpose of measurement | Internal dispatch tracking and sales metering have different requirements | Determines whether returned vapor is metered and what verification hardware is needed |
Source of batch composition data | Fill quantity and correction calculations rely on it | Choice between online analysis, lab results or certificate data |
Safety layers
ESD valves, breakaway couplings, grounding and arm position interlocks, gas detection and electrical equipment suited to the hazardous area classification are the basic layers for NGL loading as well. What NGL adds is the need to review thermal relief settings and any line sections that can be blocked in with lighter mixes in mind. The required safety integrity level (SIL) of safety functions follows from a project-specific risk assessment. Pressure rating and ESD equipment of a pre-assembled loading package are covered on the NGL loading and unloading skids page.
Where TLY Enerji fits in
For NGL loading projects, TLY Enerji can support the selection of measurement technology from composition and pressure data, the supply of Coriolis meters, pressure and temperature transmitters, control and safety valves and pumps, PLC configuration of loading control, and field installation, testing and commissioning. Tanker and product specifications remain the responsibility of the owner and the carrier; the scope of supply is defined with the technical specification.
NGL data we need to prepare a quotation
· Expected composition range at the terminal, with examples of the lightest and heaviest batches
· Operating and design pressure of the storage tank and maximum product temperature
· Tanker types to be loaded and their design pressures
· Target loading time or flow rate and number of bays operating at once
· Whether a vapor return line exists and where it ties into storage
· Whether the loaded quantity is used for sales and, if so, which legal metrology framework applies
· Where batch composition data comes from and how often it is updated
· Existing terminal automation and loading permissive signals
Related pages
· LPG truck loading systems: Baseline description of closed loading for propane and butane.
· NGL loading and unloading skids: Pressure rating and safety equipment of the pre-assembled package.
· NGL metering skids: Which measured quantity to use when composition varies.
· NGL custody transfer metering: How returned vapor and component quantities enter the commercial calculation.
· Truck loading systems: General description of the pump, meter, control valve and loading arm chain.
Questions about NGL truck loading
Can an LPG loading arm and meter be used for NGL?
Similar types of equipment can be used; what decides it is the pressure rating, seal and material compatibility, and the setpoints. If an existing LPG line is to be dedicated to NGL, the arm, meter, relief valves and back-pressure setting should be rechecked against the vapor pressure of the lightest mix the terminal expects. Density and correction settings in the measurement software also need updating for NGL.
Why can the allowable load change from batch to batch?
The tanker fill limit leaves room for the liquid to expand as it warms. The mass that fits in a given volume, and how much it expands, depend on the density and composition of the mix. A batch rich in light components allows less mass in the same tanker. That is why the batch controller should calculate the allowable quantity from batch composition and measured density rather than use a fixed value.
Can NGL be loaded without a vapor return line?
In a closed system without vapor return, pressure builds in the tanker and loading slows or stops, while venting vapor to atmosphere is not an acceptable practice for safety and emissions reasons. Arrangements without vapor return are handled only in specific cases where tanker and product conditions allow it, and only after a risk assessment.
How is the back-pressure setting determined for NGL loading?
The setpoint is chosen so that the product in the meter never flashes. That requires looking at the highest vapor pressure batch the terminal expects, the highest product temperature and the pressure losses across the meter and control valve together. Where the composition range is wide, having the batch controller manage the setpoint from temperature and batch data can be considered.