LPG Metering Skids

The single most important condition in LPG metering is that the product stays liquid while it passes through the meter. LPG has a high vapor pressure and readily turns to gas when pressure drops slightly; vapor bubbles forming in or just ahead of the meter create two-phase flow in a device calibrated for liquid and corrupt the measurement. That is why an LPG metering skid is more than a meter and a few transmitters. Its core functions are a back-pressure arrangement that holds meter pressure above vapor pressure, a gas elimination element that keeps vapor away from the meter, and density and temperature measurement to convert the result to mass or reference volume.

The general layout of liquid metering skids and the verification approach for custody transfer are covered on other pages. This page looks at how that architecture is adapted for propane, butane and their mixtures.

Why the liquid phase has to be protected

Liquid meters assume single-phase liquid flow. If line pressure approaches saturation because of friction losses downstream of the pump, valve openings or a temperature rise, part of the flow vaporizes. Since vapor occupies far more volume than liquid, a volumetric meter can register liquid that never passed; in a mass meter, two-phase flow degrades stability and the density reading. Legal metrology requirements for liquefied gases under pressure explicitly call for measuring systems designed so the product in the meter remains liquid.

Setting up back-pressure control

The main tool is a pressure-sustaining or control valve downstream of the meter, which throttles the flow to keep meter pressure a safe margin above the product's vapor pressure at the prevailing temperature. In simple systems a fixed-setpoint pressure-sustaining valve may be enough. In more advanced arrangements the batch controller or flow computer watches meter outlet pressure and product temperature and dynamically positions the set-stop control valve, raising back pressure only as much as needed.

Back pressure has a price in the form of pressure drop. Pump discharge pressure has to cover line losses, meter loss, the back-pressure margin and the pressure of the receiving truck or vessel all at once, so the metering skid cannot be designed independently of pump selection.

Gas elimination: not the same as an air eliminator

Air eliminators that vent to atmosphere, as used on fuel skids, cannot be applied the same way to LPG; separated vapor must go back into a pressurized line rather than to air. One gas elimination device defined for pressurized liquefied gas measuring systems is the condenser tank, a pressure vessel ahead of the meter where vapor collects and usually returns to storage via the vapor line. The right arrangement depends on whether the system is tied into a vapor line and whether it serves duties with a high risk of vapor formation, such as unloading.

Mass, volume and density

LPG density changes significantly with both temperature and the propane to butane ratio. Converting line volume from a volumetric meter into mass or volume at reference temperature therefore needs reliable density and temperature values. General volume correction tables for crude oils and refined products do not cover LPG; separate temperature correction procedures exist for LPG and natural gas liquids. Which correction method, which reference conditions and where density comes from should be decided before the skid's calculation unit is configured.

Coriolis mass flow meters measure mass flow, density and temperature simultaneously and calculate volume from mass and density. Where LPG is traded by mass, this can remove the need for a separate densitometer. Turbine and positive displacement meters are also used for LPG; in that case temperature, pressure and often in-line density measurement are essential parts of the conversion chain. Technology choice follows flow range, allowable pressure drop, product variability and the proving method; no single technology is the right answer for every LPG application.

Typical LPG metering skid components

Pressure rating, materials and hazardous area

All piping, flanges, valves and instrument bodies on the skid are selected for a design pressure that accounts for storage vessel design pressure, pump shut-off pressure and thermal expansion scenarios. Liquid LPG trapped between two closed valves can build pressure quickly when it warms, so thermal relief should be considered in such sections. Gasket and seal compatibility with LPG is checked separately.

A metering skid operates in a hazardous area. Even when each device carries its own explosion-protection documentation, that does not cover additional ignition risks from combining them; whoever assembles equipment into a functional unit is responsible for assessing the assembly as a whole. The same logic applies to metrology: a meter's own approval does not by itself cover the complete measuring system with its gas elimination, back-pressure and conversion elements.

TLY Enerji's contribution to LPG metering projects

TLY Enerji provides engineering support for selecting metering technology to suit process conditions, supplying meters and transmitters, installing and commissioning flow meters, and integrating the control system. Its product portfolio includes Coriolis mass flow and density meters, pressure and temperature instruments and control valves. Which skid components fall within the scope of supply is agreed on the basis of project data.

Process data we ask for

·         Product and composition range (propane, butane, mixture; seasonal variation)

·         Minimum and maximum flow, operating pressure and temperature range

·         Duty of the skid: loading, unloading or transfer

·         Required result unit: mass, reference volume or both

·         Whether a vapor return line is connected to the system

·         Pump discharge pressure and allowable pressure drop

·         Whether measurement is for custody transfer, and verification expectations

Related pages

·         LPG custody transfer metering systems: How the commercial unit, corrections and verification are settled.

·         Propane and butane metering systems: How composition differences affect density and meter settings.

·         LPG loading and unloading skids: Integrating the metering skid into the loading and unloading package.

·         NGL metering skids: Metering approach for natural gas liquids with more variable composition.

LPG metering skid FAQ

How much back pressure does an LPG meter need?

There is no single figure. The required back pressure is the product's vapor pressure at the meter temperature, which depends on composition, plus a safety margin. That margin is set by the project specification, the applicable metering standard and the meter manufacturer's guidance. At sites with seasonal blend changes, the product with the highest vapor pressure governs.

With a Coriolis meter, do we still need a densitometer?

Because a Coriolis meter also measures density, a separate densitometer is often unnecessary for mass-based measurement. If results must also be reported as volume at reference temperature, however, you still need to plan how density is brought to reference conditions and how the measured density is verified. The decision depends on the commercial arrangement and proving approach.

Can a fuel metering skid be used for LPG?

Not directly. An LPG skid differs in pressure rating, gas elimination arrangement, back-pressure control, sealing materials and calculation method. Air eliminators venting to atmosphere and volume correction tables defined for atmospheric products are not suitable for LPG. Converting an existing skid is only possible after reassessing all of these points.

Does the meter's accuracy figure describe the skid's accuracy?

No. The accuracy in a manufacturer's datasheet describes meter performance under reference conditions. Total skid uncertainty also includes how well the liquid phase is protected, temperature and density measurement, the conversion calculation and the proving method. Legal metrology documents likewise set separate error limits for the meter and for the complete measuring system.

Why does strainer condition matter so much on an LPG skid?

As a strainer fouls, its pressure drop rises and the liquid reaching the meter arrives at lower pressure. With LPG that drop can bring the product close to its vapor pressure and cause vaporization ahead of the meter. The strainer is therefore sized for low pressure drop upstream of the meter, and differential pressure is monitored to schedule cleaning. A safe way to drain the section for maintenance should also be provided.