Solvent Metering Skids

Three solvent properties drive meter selection. Most hydrocarbon solvents have very low electrical conductivity, viscosity is typically low, and all solvents are volatile and flammable. Low conductivity largely rules out magnetic flow meters; low viscosity makes internal slippage the key question for positive displacement meters; volatility and flammability mean that all electrical items on the skid must suit an explosive atmosphere and that the liquid must not flash while being measured. Coriolis mass flow meters and positive displacement meters are therefore the options most often evaluated, with the final choice set by the quantity basis, flow range and product list.

The general skid layout is described on the custody transfer truck loading skids page; this page covers how solvent properties change technology choice and skid arrangement.

Why are magnetic flow meters unsuitable for most solvents?

A magnetic flow meter measures the voltage induced when a conductive liquid moves through a magnetic field, so the liquid must exceed a minimum conductivity. That threshold is a manufacturer value tied to model and size, not a universal constant; one product family, for example, specifies 1, 3 or 5 µS/cm depending on meter size. Paraffinic, aliphatic and aromatic hydrocarbon solvents fall far below that level and are outside the normal range of magnetic meters.

Alcohols and ketones are less clear-cut. Electrostatically they count as high-conductivity liquids, yet that scale sits well below what magnetic meters require, so being conductive from a static point of view does not make a solvent measurable with a magnetic meter. The decision rests on comparing the product's measured conductivity with the threshold on the chosen meter's data sheet; the general conductivity split is covered on the chemical metering skids page.

Coriolis: mass, density and product verification in one instrument

A Coriolis mass flow meter is independent of conductivity and measures mass flow, density and temperature simultaneously, calculating volume from mass and density. Where solvents are sold by mass, it gives the invoicing unit directly; where they are sold by volume, the measured density can convert actual volume to volume at base conditions.

Density also serves as a product check: since each solvent has a known density at a given temperature, comparing the in-line value with the expected one can flag a wrong product or contamination early.

At low flow, Coriolis accuracy is governed by zero stability. During the slow-start step that limits static charge in truck loading, the meter runs at the bottom of its range, so sizing should be checked against that start-up flow as well as full flow.

Positive displacement meters: slippage at low viscosity

A positive displacement meter counts volume directly by dividing the liquid into chambers of known volume and can run with mechanical or pulse output without external power. Small clearances remain between the moving parts and the housing, however. A low-viscosity solvent slips through them more easily, and this unmeasured flow increases error, especially at low rates. A positive displacement meter for solvent service should therefore be checked against the manufacturer's low-viscosity flow range and accuracy data, with the meter factor established on the actual product.

Bearings and seals of the moving parts are wetted by the product. Ketones and aromatics can swell some elastomers, so gasket and O-ring materials are confirmed for each solvent from manufacturer compatibility data, and a meter serving several solvents is specified for the most demanding one.

Comparison of technologies for solvent metering (qualitative)

Only liquid should pass through the meter

A volatile solvent can carry gas when pressure drops at the meter inlet or when a drained line is refilled, and vapor or air passing through the meter distorts the registered quantity. A gas elimination device, one of the defined elements of a dynamic liquid measuring system, is installed upstream of the meter. Sufficient back pressure downstream, usually held by the control valve or a back pressure valve at the skid outlet, also helps keep the solvent liquid at the point of measurement.

Static electricity and explosive atmospheres on the skid

Pumps, strainers and fine filters, the main charge generators in a solvent line, usually sit on the metering skid or just upstream of it, so the skid layout must account for the distance and pipe volume between the filter and the fill point. Electrical continuity is provided across meter flanges, valve bodies and spools, and the skid frame is bonded to the facility grounding system.

Meter electronics, transmitters, the batch controller and junction boxes are selected with a protection method suited to the zone set by the hazardous area classification. Coriolis explosion protection approvals can vary with model and electronics configuration: approvals stated for remote electronics may be more restrictive for electronics mounted on the sensor.

Custody transfer or process measurement?

If the meter reading is the basis for invoicing, the system may fall under legal metrology rules for dynamic liquid measuring systems, depending on national legislation and the product. Under those rules the accuracy class and maximum permissible error apply to the complete measuring system, with a tighter limit for the meter itself. For in-plant transfer or batch dosing, the process specification governs. Either way, the contract should state whether mass, actual volume or volume at base conditions is used.

Selection criteria

How TLY Enerji contributes

At solvent metering points, TLY Enerji can support technology selection between Coriolis, positive displacement and other flow meters based on process data, supply of meters, gas eliminators, valves and transmitters, flow meter installation and commissioning, and transfer of measurement data to a PLC-based control system. Whether the skid frame, piping and legal metrology approval steps are in scope is agreed against the project specification.

Information needed to select a solvent meter

·         List of solvents to be measured, with conductivity, density and viscosity data for each

·         Operating temperature and pressure range, and the product vapor pressure

·         Minimum, normal and maximum flow, plus the flow of the slow-start step

·         Invoicing basis: mass, actual volume or volume at base conditions

·         Whether the measurement falls under legal metrology

·         Skid location and hazardous area classification

·         Batch controller, PLC or terminal software the meter will connect to

Related pages

·         Solvent batch transfer systems: Using the selected meter to transfer set quantities of solvent.

·         Chemical metering skids: General framework for the conductivity split and wetted material selection.

·         Custody transfer truck loading skids: Metering skid components and custody transfer requirements.

·         Solvent truck loading systems: Why loading starts slowly and why relaxation volume follows the filter.

Frequently asked questions about solvent metering

Can ethanol or acetone be measured with a magnetic flow meter?

Although these products count as conductive in electrostatic terms, the minimum conductivity of a magnetic meter is defined on a different scale. Measure the product's actual conductivity and compare it with the threshold on the chosen meter's data sheet. If the threshold is not met, or the product varies, a Coriolis meter is the safer choice. If hydrocarbon solvents share the line, a magnetic meter is ruled out anyway.

Can the density reading of a Coriolis meter catch the wrong product?

Where solvent densities differ clearly, comparing measured density with the expected value can give early warning of a wrong product or contamination. For products with similar densities the method may not discriminate. It does not replace the product acceptance procedure; treat it as an additional safeguard configured as an alarm in the control system.

Why are positive displacement meters sometimes avoided for solvents?

Low-viscosity liquids slip more easily through the clearances between the moving parts and the housing, and that quantity goes unmeasured, most noticeably at low flow. Moving parts and seals may also be incompatible with some solvents. Confirm the manufacturer's low-viscosity data and material compatibility before choosing, and establish the meter factor with the actual product.

Can several solvents share one meter?

Technically yes, but at every changeover the product left in the meter and piping mixes with the next one, so dedicated meters are preferred for high-purity solvents. If a meter is shared, the skid is designed to be drainable, the changeover sequence is defined, and for positive displacement meters it is taken into account that the meter factor may differ from product to product.