Propane, butane and mixed natural gas liquids (NGL) can share pumps, piping and loading infrastructure, but doing so safely and without commercial disputes depends on four conditions. The pressure basis must follow the most volatile product; product changes must follow a defined sequence with the interface managed; meter and control settings must switch automatically with the product; and interlocks must stop the wrong product from reaching the wrong tank or tanker. Miss any one of these and the result is either off-spec product or a safety risk.
How vapor pressure and density differences between propane and butane affect measurement is covered on the propane and butane metering page, and the choice of measured quantity for variable NGL on the NGL metering skids page. This page is about operating several products on one transfer system.
Which products can share a line?
All of these hydrocarbons are gases at atmospheric pressure and stay liquid only under pressure; what separates them is how much pressure they need at a given temperature. Mixed NGL containing ethane is more volatile than commercial propane, and propane more volatile than butane. The design pressure, relief settings and tanker connections of a shared line must therefore cover the most volatile product. Once pressure compatibility is settled, the next question is specification sensitivity: a small amount of ethane-bearing mix or butane in a high-purity propane batch can push its vapor pressure or residue properties off-spec. Typical layouts are:
· Dedicated line per product: Lowest contamination risk, highest investment and space demand; preferred for specification-sensitive products.
· Shared line, sequenced operation: Products move in a planned order; the interface is measured and routed to a suitable tank.
· Shared header, dedicated loading points: The pump header is common while the final section and loading arm are product-specific, keeping line hold-up small.
· Separate line for mixed NGL: Ethane-bearing blends are kept apart from lower-rated propane and butane infrastructure.
Product changes and the interface
When the product changes in a pressurized, liquid-full line, the previous product remains in the pipe and a mixed zone forms where it meets the new one. Because the two products differ in density, an in-line density measurement can track this interface. Everything that passes before density settles inside the new product's expected window is treated as interface and routed to a mixed-product tank or to the tank of the lower-value product.
1. Plan the product sequence in the direction where mixing does least harm, for example light to heavy or narrow-spec to wide-spec.
2. Know the liquid hold-up of the line and calculate the displaced quantity for each change in advance.
3. Log density, and temperature where needed, throughout the change; the start and end of the interface are taken from this record.
4. Route the interface to the tank whose specification is least affected, with the control system confirming diverter valve positions.
5. For truck loading, sweep the volume up to the loading point with the new product so the first batch after a change does not carry the previous product.
Draining a liquid-full system of pressurized product adds flashing, chilling and relief load. Product changes are therefore usually done by displacement without emptying the line, with full draining and nitrogen purging kept for maintenance or long shutdowns.
Pump selection: low viscosity and a narrow NPSH margin
Propane, butane and NGL are stored at their own vapor pressure, in other words at the boiling point. Since net positive suction head available (NPSHA) is the absolute head above the liquid's vapor pressure, the margin in these services comes mainly from tank level minus suction line losses and is often small. If suction pressure falls below vapor pressure, cavitation begins: vapor bubbles erode the impeller, cut pump performance and can send two-phase flow to the meter.
· Margin changes with the product: With the same pump and tank arrangement, each product gives a different NPSH margin, so selection is checked against the least favorable product at the highest temperature.
· Suction design: A short, low-resistance suction line, flooded suction from the tank bottom and, where useful, a vapor return connection on the suction side protect the margin.
· Low viscosity: Thin liquids increase internal slip and weaken lubrication, which shapes the choice of pump type, seals and bearings.
· Sealing: Mechanical seal selection for a liquid at its boiling point, and detection of any leakage, are assessed separately.
· Minimum flow and dry running: Running against a closed valve can heat and vaporize the product, so a minimum flow line or protective logic is considered.
Product recipes and control settings
Adjusting settings by hand at every product change invites errors. Instead, a recipe is defined for each product in the flow computer, batch controller or control system, and all related settings load together when the product is selected.
Setting | Why it varies by product | How it is handled |
Back-pressure set point | The pressure needed to keep the meter in liquid phase depends on vapor pressure | Product- and temperature-dependent setting, highest for the most volatile product |
Meter factor | Meter response can shift with fluid properties | Separate proving and a separate factor per product |
Density check window | Expected density is a practical check on product identity | Alarm or transfer stop on out-of-window readings |
Flow and pump limits | NPSH margin and line velocity differ by product | Product-specific maximum flow, ramp and pump speed limits |
Fill limit | The vapor space to leave in a tanker depends on product density and expansion | Maximum fill quantity calculated per product and temperature |
Alarm and trip values | Normal operating pressure differs between products | Product-specific high-pressure alarms; safety trips fixed to the design basis |
Preventing wrong-product transfers
In a multi-product transfer system, the most expensive mistake is sending a product to the wrong tank or to a tanker not suited to it. That risk is handled in layers: orders and tanker assignments are made per product; the control system compares the selected product with header and tank valve positions; density measured at the start of transfer is checked against the expected window; and lines and connections are labeled by product. Before loading, the product is also checked against the tanker's design pressure and permitted product list.
How TLY Enerji helps
For multi-product transfer systems, TLY Enerji provides engineering support in selecting measurement and control technology based on the product slate and operating scenarios. Depending on scope, this can include supply of pumps, valves, Coriolis meters and density measurement, PLC and SCADA automation for recipe management and interlocks, system integration, installation, testing and commissioning. The scope of supply is agreed through the technical specification.
Information to prepare for a multi-product transfer study
· Product list with specification or typical analysis for each
· Flow diagram showing which tanks feed which tanks or loading points
· Expected frequency of product changes and preferred sequence
· Tank or product that can receive interface material
· Existing pump and suction arrangement and tank levels
· Target flow per product and maximum operating temperature
· Existing control system, batch controller or terminal automation infrastructure
Related pages
· Propane and butane metering systems: How property differences between propane and butane affect measurement.
· NGL metering skids: Choosing mass, volume or component measurement for variable composition.
· NGL custody transfer metering: Commercial quantity and composition accounting after a product change.
· NGL truck loading systems: Where NGL truck loading departs from LPG practice.
· Multi-bay and multi-arm loading terminals: Planning the bay, arm and product matrix at multi-product terminals.
Questions about multi-product NGL transfer
Can propane and butane go through the same meter?
Yes, provided the meter is proved separately for each product and the matching meter factor, back-pressure setting and density window load automatically when the product is selected. In custody service, the effect of the previous product left in the line on the first batch after a change, both on measurement and on delivered quality, should also be assessed.
Where does the interface material go?
Usually wherever the specification suffers least: a mixed NGL tank, the tank of the product with the wider specification, or a tank set aside for reprocessing. The right choice depends on interface volume and the plant's tank layout. Confirming the diversion through valve position checks in the control system keeps the mix from ending up in a product tank by mistake.
Why is ethane-bearing mixed NGL kept off the propane line?
Ethane raises vapor pressure markedly. The pressure rating, relief settings or connected tankers of a line designed around propane may not cover it. In addition, ethane-bearing product entering a propane tank or line can push propane vapor pressure off-spec. For both reasons, blends containing ethane are usually moved on separate infrastructure.
Why might a pump cavitate after a product change?
The usable margin at pump suction is the pressure above the liquid's vapor pressure. Switching to a more volatile product, or a rise in temperature, can shrink that margin. If the pump and suction line were only checked for the heavier product, vaporization at the suction can start once the lighter product is running. Pump selection should therefore be verified for the least favorable product at the highest operating temperature.