EMERSON ProcessManagement has released theindustry’s first digital DP Levelarchitecture, the Rosemount
3051S Electronic Remote Sensor system.With over ten patents, the 3051S ERSsystem is a breakthrough technology thatreplaces mechanical impulse piping withtwo 3051S pressure sensors that arelinked together electronically.Differential pressure (DP) is calculatedin one of the two sensors and is trans-mitted using a standard two-wire 4-20mA HART signal.
Overview of Traditional DPLevel TechnologyConventional differential pressure trans-mitters are used in level applications bymeasuring the amount of pressureexerted by the liquid level in a vessel. Forexample, a 500 mm column of waterwill exert 500 mmH20 of pressure.However, in many applications, there isadditional vapour pressure above theliquid. Being that the vapour pressure isnot part of the liquid level measurement,the use of impulse piping or capillarywith seals is required to compensate forits presence.
In an impulse piping configuration,
the low side reference leg is either filledwith a column of liquid (wet leg) orwith a suitable dry gas (dry leg). Wetlegs are used when the vapour in thetank will condense into liquid form.Likewise, dry legs are used when thevapour does not condense. While rela-tively simple in concept, impulse pipinginstallations can be difficult to maintain.Evaporation often occurs in wet legs andcondensation can occur in dry legs. Bothconditions will cause measurement errorin the DP transmitter. Additionally,impulse lines can leak, plug, and mayrequire insulation or heat tracing.
Capillary and seal systems eliminatemany of the issues with impulse pipinginstallations. These systems consist ofexternal sensing diaphragms that areconnected to the DP transmitter withoil-filled capillaries. Changes in pressurecause the diaphragm membrane todeflect, and the pressure is transferredthrough the oil-filled capillary to thetransmitter. Oil-filled capillary systemsare carefully welded and manufacturedso that it is a hermetically closed systemfor reliable performance. This type ofconstruction eliminates leak points andplugging that can occur with impulsepiping, and there is no need to fill or
drain the lines. A common capillary seal configura-
tion is the “balanced system”, where theseals and capillary length are identical onboth the high and low side of the DPtransmitter. This type of system is tradi-tionally specified because it supposedlycompensates for all temperature effects.For example, as the outdoor temperaturechanges (whether from night to day orseason to season), the oil volume in thecapillary will expand and contract,causing changes in the internal pressureof the capillary system. These changes inpressure will result in measurementerror, called “seal temperature effect”. Abalanced symmetrical system wasinitially thought to cancel out this error,because the same expansion / contrac-
tion of oil volume will occur on boththe high and low sides of the transmitter.While this may be true, there is anothersource of temperature error that is notaddressed.
Anytime a capillary seal system isinstalled and there is a vertical separationbetween the two seals (such as on allvessels and distillation towers), there is“head pressure” that is exerted on thelow side of the transmitter from theweight of the oil in the capillary. Againas the temperature conditions change,the density of the oil in the capillary willfluctuate and cause a change in the headpressure that is measured by the trans-mitter. This source of error is called“head temperature effect.” Whilebalanced systems can cancel out sealtemperature effect, they do not compen-sate for head temperature effect.Additionally, balanced systems result inextra capillary on the high side of thetransmitter that is often coiled up andadds unnecessary costs. Balanced systemsalso require the use of a pipe stand orother mounting hardware to facilitatethe installation of the transmitter.
Rosemount Tuned-System assembliesoffer a better solution compared tobalanced systems by offering improved
16 www.pacetoday.com.au JULY 2011
MASTERCLASS
A New Guide to Successful Differential Pressure Level MeasurementThe availability of Electronic Remote Sensor technology enables the use of DP Level instrumentation on tall vessels and distillation towers, writes Michael Olivier.
ERS deliversunmatched benefitswhen used on theright applications.
Capillary and seal systemsconsist of external sensingdiaphragms connected to the differential pressuretransmitter with oil-filled capillaries.
performance and easier installations at alower cost. Tuned-System assembliesconsist of a direct mount seal on thehigh-side of the transmitter with a singlelength of capillary and seal on the lowside of the transmitter. The asymmetricaldesign purposely induces seal tempera-ture effects to counteract the headtemperature effects that will always bepresent on any vertical installation. Also,Tuned-System assemblies can be directlymounted to the vessel without the needfor additional transmitter mountinghardware, and installed costs are oftenreduced by 20% by eliminating theunnecessary length of capillary on thehigh side of the transmitter.
Limitations of TraditionalDP Level TechnologyWhile Tuned-System assemblies are aproven and reliable technology, tallvessels and towers have traditionallybeen difficult applications. In particular,these require longer lengths of capillaryto facilitate the installation, resulting inhead temperature effects that are oftentoo great to “tune” out. For example, aninstallation that requires 50 feet (15meters) of capillary will experience asmuch as 15 in.H20 (383 mmH20) ofmeasurement drift for a 50°F (28°C)change in temperature. Additionally,
time-response can be sub-optimal on tallvessels and towers, and installationsbecome increasingly difficult with longerlengths of capillary.
Electronic Remote SensorTechnologyElectronic Remote Sensor technologysolves many of the problems that aretraditionally seen when making a DPmeasurement on tall vessels or towers.Rather than using a single DP trans-mitter with mechanical impulse pipingor capillary, the Rosemount 3051S ERSSystem uses two direct mount gage orabsolute sensors that are connected witha non-proprietary electrical wire. One ofthe two sensors calculates the DP andtransmits it back to the host system /DCS using a standard two 4-20 mAHART signal.
The unique digital architecture of the3051S ERS System enables many bene-fits when used on tall vessels and distilla-tion towers including:• Improved Performance: The 3051SERS System replaces mechanicalcomponents with a digital architecture,resulting in faster time response and amore stable and repeatable measure-ment even in applications with widevarying temperatures.
• Simplified Installations andMaintenance: Cold weather installa-tions often require heat tracing or insu-lation. Impulse lines need to bechecked for leaks, condensation, evap-oration, and plugging. The digitalarchitecture of the 3051S ERS Systemeliminates these practices, resulting incleaner installations with less mainte-nance and upkeep.
• Additional Process Insight andDiagnostics: The 3051S ERS System isa MultiVariable solution that providesadditional process information foroptimised control. In addition to theDP calculation, the 3051S ERSprovides real-time access to the read-
ings from each pressure sensor and ascaled output for tank level or volumemeasurement.
ComplimentaryTechnologiesWhile the 3051S ERS System is greatfor tall vessels and towers, there areapplications where a traditional Tuned-System assembly will continue to be thepreferred technology. The sensors in a3051S ERS System are specified andsized based on the combined static pres-sure and the DP column from the liquidlevel, where as a Tuned-System assemblyis sized just on the DP column. Becauseof this, Tuned-System assemblies willcontinue to be the optimal solution forsmaller, higher pressure vessels, andElectronic Remote Sensor technologywill enable new success on tall vessels,towers, and other similar applications.
[Michael Olivier is Senior MarketingEngineer, Emerson Process Management.]
Emerson Process Management
03 0000 0000
www.emerson.com.au
JULY 2011 www.pacetoday.com.au 17
l Pressure Level Measurement vessels and distillation towers, writes MMiicchhaaeell OOlliivviieerr.
PRESENTED BY
ERS is the digital architecture that eliminates the mechanical limitations ofcapillary solutions.
Additional information on theRosemount 3051S ERS Systemincluding videos, success stories,and product specifications can befound online athttp://www.Rosemount.com/3051SERS
LLEEAARRNN MMOORREE
Various configura-tions are possiblefor DP level meas-urement.
