A supercritical fluid (SCF) exists:
…at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It can effuse through porous solids like a gas, overcoming the mass transfer limitations that slow liquid transport through such materials. SCF are much superior to gases in their ability to dissolve materials like liquids or solids.
I share this as background to a whitepaper, Ethylene Pipeline Measurement via Coriolis Flow Meter, that Emerson’s Dean Minehart alerted me to. The whitepaper opens:
Ethylene is transported in large quantities by pipeline as a supercritical or dense phase fluid. Typical pipeline pressure is above 1000 psia which allows ethylene to maintain a supercritical state at ambient temperature during shipment and storage.
The traditional way to measure ethylene flows in pipelines is with orifice plates and differential pressure (DP) meters. The drawback of their use is that they:
…require regular maintenance to verify plate dimensions and pressure/differential pressure transmitters are calibrated at some interval. Taking a meter run out of service for maintenance poses potential safety risks (e.g. ethylene volatility) which must be carefully mitigated.
Decomposition is potential issue with supercritical ethylene fluid.
High temperature and pressure ethylene can decompose forming methane and carbon. Decomposition is a chemical reaction that releases heat which can fuel additional decomposition.
Ethylene is a highly flammable fluid and when exposed to air can form an explosive mixture. Auto ignition is possible at high temperatures.
An example is shared of incident presented at the 2010 Global Congress on Process Safety.
The incident occurred when placing the off-line meter run [after maintenance performed] in service. The off-line meter run was inventoried with low pressure nitrogen and 1100 psi ethylene was introduced through a large gate valve. The subsequent quick pressurization resulted in an adiabatic compression of the nitrogen which initiated an ethylene decomposition in the pipe.
A hot spot in the stagnant piping indicated a continuing nonpropagating decomposition taking place. Action was taken to re-establish flow through the pipe with the hot spot to cool the pipe. The hot spot continued for approximately three hours before no additional heat was detected.
Coriolis mass flow meters, such as Micro Motion Elite meters, with advanced on-line diagnostic functions provide a safe maintenance-free alternative to orifice metering in supercritical ethylene pipeline measurement. Smart Meter Verification (SMV) is an on-line, advanced diagnostic function that verifies the Coriolis flow meter, transmitter and interconnecting wiring while a meter is in normal operation.
Read the whitepaper for more on how SMV works to provide high confidence, accurate mass flow measurements and how these Coriolis meters work well in a “pay and check” configuration.
Visit the ELITE Coriolis Mass Flow Meters and Ethylene sections on Emerson.com for more on these and other technologies and solutions to improve ethylene production and transportation safety, reliability and efficiency. You can also connect and interact with other flow measurement experts in the Measurement Instrumentation group in the Emerson Exchange 365 community.