Flow Instruments & Engineering GmbH, a member of the Chart group, has just obtained Measurements Instrument Directive (MID) type approval for its Flowcom 3000 dynamic mass flow meter, designed for the accurate metering of LNG.
The system has no moving parts and is based upon an orifice flow meter in conjunction with accurate density measurement in real-time.
The unique design is already patented and patent-pending in multiple countries and features a thermally insulated pre-cooled meter with a built-in phase separator – ideally suited for, but not limited to, use in LNG fuel dispensers.
Described by the company as the ‘definitive’ answer to metering LNG, the Flow Instruments design uses vacuum technology that minimises heat being added to the system – keeping storage tank pressures to a minimum. The metering chamber has a liquid feed in, return line to storage called a re-circulation line, and a discharge line. This meter chamber piping arrangement minimises the space required in the dispenser and acts as a phase separator.
Most codes, including the International Organisation of Legal Metrology (OIML) guideline have language about phase separating. With adding a temperature probe (not required for density determination – not metrologically relevant), the LNG quality can be measured.
Why submerge a meter?
Without the meter being submerged, the system requires diversion of metered flow to cool the meter. Cooling the meter is absolutely required to accurately meter smaller quantities that are typical when dispensing LNG.
Improper cool-down will result in different levels of two-phase flow. No meter is capable of metering two phase flow and especially ‘Slugs’ that are present at the start of flow through a warm meter and empty hose. A very important benefit of a properly cooled meter is that the minimum allowable transfer quantity can be optimised. Even the smallest start-up errors result in large errors if the quantity transferred is small.
The meter is literally submerged/located inside a larger pipe that has an inlet at the top, meter withdrawal off the bottom and a re-circulation line out of the top. This piping scheme allows flow in and out over the top of the meter – cooling the meter from the outside in.
This method of submerging the meter can only be accomplished with a meter that has no moving parts, like orifice technology.
It is important to note that this unique design does not need to compromise on the most basic metering principle of no diversion of metered flow. The OIML defines metering standards which have been adopted by many countries around the world, including the European Union, by implementing the MID.
The applicable standards for metering the flow of cryogenic liquids, such as LNG, emphasises the requirement that conforming equipment allow no diversion of metered flow.
The OIML International Recommendation, Dynamic measuring devices for cryogenic liquids, paragraph 12.3, Diversion of measured liquid states:
No means shall be provided by which any measured liquid can be diverted from the measuring element of the meter or the discharge line there from.
(OIML R 81: Edition 1998 (E), available at http://www.oiml.org/publications/R/R081-e98.pdf.)