NGL Truck Unloading Systems

Safe, metered receipt of NGL from a tanker into a plant depends on three things happening together: controlled management of the pressure difference between the tanker and the receiving tank, measurement of the received quantity in the liquid phase without interruption, and confirmation that the product meets the acceptance specification. The typical receiver is a fractionation unit that separates mixed NGL (Y-grade), or the buffer storage feeding it. Unloading therefore affects not only what goes into storage but also the feed quality of a process unit.

General topics such as suction-side problems and reconciling the received quantity with tank levels are covered on the truck unloading systems page, and the basic pump-versus-compressor comparison on the LPG truck unloading page. Here the focus is on acceptance, pressure and sampling issues specific to NGL.

Acceptance at the plant inlet: what is checked before unloading starts?

With NGL, each tanker may carry product from a different source or a different operating day. Acceptance checks cover shipping documents, tanker identity and the declared composition or vapor pressure. The aim is to catch batches that could fall outside the fractionator inlet specification, such as product with more ethane or heavy ends than expected, before unloading begins. Whether the plant relies on the composition in the shipping documents or on its own sample analysis should be settled in the commercial agreement.

These steps can be tied into the automation system: the unloading pump or compressor does not start until tanker identity and batch data are confirmed and grounding and arm connection permissives are in place. Compared with LPG, this interlock logic adds a product acceptance criterion on top of the usual safety permissives.

Pressure management: tanker, receiving tank and transfer method

What moves the liquid during unloading is the pressure relationship between the tanker and the receiving tank. With NGL the two sides can differ in composition and therefore in pressure: a light batch unloaded into a tank holding heavier product may keep the tanker at higher pressure, while the reverse case can leave the receiving tank at the higher pressure. The transfer method is chosen to work in all of these cases.

·         Pump unloading: A pump moves the liquid while a vapor balance line connects both sides. Because the product at the suction is close to its bubble point, suction piping with low pressure losses and an NPSH margin checked for light mixes are required.

·         Compressor unloading: Vapor from the receiving tank is compressed and pushed into the tanker, displacing the liquid. With NGL that vapor is rich in ethane and propane, so compressor design pressure and safety settings are assessed for a light vapor, and possible condensation at the compressor discharge is taken into account.

·         Flow on pressure difference alone: When tanker pressure is clearly higher, partial transfer is possible, but flow falls as pressures equalize and keeping the meter in the liquid phase gets harder. It is usually treated as an auxiliary condition rather than the main method.

Metering at the end of unloading: avoiding two-phase flow

As the tanker nears empty, the liquid level drops and vortexing and vapor entrainment begin at the outlet. In a pressurized liquefied gas, vapor cannot be vented with a simple air eliminator the way air is removed from atmospheric fuels. Legal metrology documents for measuring systems handling liquefied gases under pressure define a condenser tank as a gas elimination device; separating vapor upstream of the meter and holding back pressure at the meter are considered together.

Operationally, the end of unloading is usually handled with a low-flow final stage, vapor detection through meter diagnostics or density measurement, and an automatic stop linked to it. A sudden drop in the density measured by a Coriolis meter can serve as an indicator of two-phase flow. At plants where metering is used commercially, the treatment of vapor and any liquid heel left in the tanker should be defined in advance.

Sampling to confirm composition

A sample taken from pressurized NGL loses its light components and changes composition if its pressure is released. Samples are therefore collected in pressurized containers, such as piston cylinders, that keep the liquid phase intact. A flow-proportional composite sample taken throughout unloading represents the whole batch better than a single spot sample, particularly when the tanker is stratified or holds a heel from a previous load. How the sample is used commercially is explained on the NGL custody transfer page.

Feeding the fractionation unit

If unloaded product goes straight to a fractionation unit, composition that changes from tanker to tanker can affect column loads and product quality. A buffer tank allows batches to blend and steadies the feed. Passing mass, density and analysis results from the unloading skid to the plant distributed control system (DCS) helps operations see feed changes coming.

Selection criteria

TLY Enerji's contribution

On NGL unloading projects, TLY Enerji can provide engineering support for assessing acceptance and metering requirements, supplying Coriolis meters, density, pressure and temperature instruments, valves and pumps, installing process analyzers, and configuring and commissioning the PLC and DCS side. The fractionation unit and storage facility themselves are outside this scope; the scope of supply is defined in the project specification.

Information to share for an unloading facility

·         Typical and extreme compositions of incoming NGL and what data arrives with the shipping documents

·         Design and operating pressure of the receiving or buffer tank

·         Preferred transfer method or existing pump and compressor infrastructure

·         Number of tankers per day and target unloading time

·         Fractionation unit inlet specification and acceptance criteria

·         Sampling and analysis arrangement: laboratory, online analysis or both

·         Whether the received quantity is the basis for purchase

·         Plant DCS/SCADA interface and hazardous area classification

Related pages

·         NGL loading and unloading skids: Pre-assembled package for unloading equipment and its interfaces.

·         NGL custody transfer metering systems: From received mass and compositional analysis to the commercial quantity.

·         LPG truck unloading systems: Baseline comparison of pump and compressor unloading for propane and butane.

·         Truck unloading systems: General treatment of suction-side problems and quantity reconciliation.

Questions about NGL truck unloading

Should NGL be unloaded with a pump or a compressor?

There is no fixed answer. A pump transfers liquid directly and gives stable conditions for metering, but the NPSH margin at the suction is small. A compressor can empty the tanker more completely, but with NGL the compressed vapor is rich in light components, so pressure and condensation conditions need separate review. Plants that receive a wide range of compositions often consider providing both methods.

Why can't the sample go into an ordinary bottle?

NGL is a liquid only while it is under pressure. In an open container, light components such as ethane and propane flash off immediately and the remaining liquid no longer represents the product. Samples are therefore collected in containers that keep the liquid under pressure, such as piston cylinders, and transported to the laboratory that way. Sample point design and container preparation also affect how representative the sample is.

How does vapor at the end of unloading affect the meter reading?

As the tanker nears empty, vapor can mix with the liquid. Two-phase flow through the meter causes measurement error and may trigger meter diagnostic alarms. To prevent this, a vapor separation element is placed upstream of the meter, back pressure is held downstream, and the end of unloading is handled with a low-flow stage or a density-based automatic stop.

Why does variable composition matter for the fractionation unit?

Fractionation columns are operated for a certain feed composition. If the share of ethane or heavy ends changes markedly from one tanker to the next, column loads and product quality can fluctuate. Blending batches in a buffer tank and passing unloading data to the control system let operations prepare for these changes in advance.