05 December 2005 00:00 [Source: ICB]
The application of vortex metering technology has provided more reliable steam flow measurement and significantly reduced plant downtime for Shell Chemicals at its ethylene plant in Moerdijk. Mark Whitfield reports
In 2002, Shell Chemicals undertook a three-month trial of vortex metering technology from Emerson Process Management on steam lines at its Moerdijk cracker in the Netherlands. Following the success of the testing programme, the sensors are planned as replacements on all 20 furnaces at the site. The first eight units changed have now been in operation for more than two years.
The naphtha cracker mixes accurate proportions of light distillate feedstock (LDF) – such as naphtha or ethane – and steam, and heats these to a high temperature in a furnace: this cracks the LDF into basic feedstocks such as ethylene for use in downstream processes, but also creates some carbon and coke deposits that are re-circulated with the process water.
Neil Jackson, Rosemount Flow business director at Emerson Process Management, explains: ‘In a naphtha cracker the steam condensate is recycled, and particulates and hydrocarbons in the recycled steam flows can easily causing “plugging” of the relatively small-bore impulse lines associated with some other flow measurement methods. This plugging or clogging can result in a complete loss of flow measurement signals, which causes the cracking furnace to be shut down.’ A monthly schedule of cleaning each flow meter had been established to overcome this problem.
The Rosemount 8800CD dual vortex meters use a stainless steel ‘shedder bar’ across the flow of steam. Emerson likens this to a flagpole in the wind – wind makes a flag flap because of vortex shedding through the von Karman effect.
In the meters, the eddies or vortices downstream of the obstruction shed from side to side and subject the bar to twisting forces. The top of the shedder bar has a mounting rod that twists and a piezoelectric element in the rod transmits the distortions as an electrical signal. As the frequency of the vortex shedding is proportional to flow, this enables accurate flow measurement.
The profile of the shedder bar, a T-shape in section, provides a wide linear and ?accurate operating range. ‘Particularly in naphtha crackers, vortex flow measurement technology is the preferred method compared with other measurement techniques because of the high accuracy, typically of ?+/-1.35% of rate for steam measurements; high rangeability – which can exceed 30:1 – and the ability to accurately measure gas and liquid flows in the same meter when needed,’ says Jackson.
‘The simple all-welded construction, with no crevices to clog, allows the Rosemount vortex to ignore all problems associated with the clogging particles. Emerson has applied these sensors to such catalytic cracker applications since 1994, and units have been in service continuously over this period without any maintenance caused by a need to clean out the meters,’ he adds.
The Moerdijk cracker requires the mixing of accurate proportions of steam and the LDF, so the flows of both have to be measured accurately. The dual vortex meters provide two flow measurements on the main steam lines.
The first is the steam flow measurement signal, which is fed into the process control system and ensures that the mixture is correct. The second is the secure safety instrumental system (SIS) signal: as part of the safety shutdown system, it cannot be linked to the process controllers, because a fault on the latter could invalidate the flow reading used in the shutdown system and jeopardise the plant safety. The two must be separate, but measure the same flow.
Until recently, the requirements of SIS safety systems dictated that system components had to communicate using analogue technology, at 4-20mA. This output from the sensors is used by the meters and feeds into the process control system.
The new meters are ‘smart’, HART compatible and can be accessed by smart communicators. HART is a communications protocol that has become widely recognised as an industry standard for digitally enhanced 4-20mA smart instrument communication.
The signals are based on the failure modes, effects and diagnostics analysis (FMEDA) assessment of the vortex sensor technology. The independent FMEDA assessment is carried out by Exida, a specialist in this area.
Since installing the vortex flow meters on eight of the steam lines two years ago, none of the units has had to be removed for any cleaning. The savings in downtime and the resulting increase in plant efficiency are obvious. There had previously been a monthly schedule of cleaning the 20 dual flow meter systems on each line. This involved taking the line out of service, unbolting the two DP flow meters and removing associated impulse lines, cleaning and reassembly.
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