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Improving level measurement in tall vessels

19 January 2016

Pawel Czoczys explains how the latest digital differential pressure level technology is able to solve many of the problems traditionally experienced when using differential pressure measurements to monitor levels in tall vessels or towers. 

To optimise process efficiency and ensure plant safety, it is essential to equip tanks and vessels with reliable and accurate instrumentation. It is also an important factor in inventory management, ensuring that continuous processes are not interrupted due to ingredient shortages.

The traditional approach is to use a single differential pressure (DP) transmitter with mechanical impulse piping for wet/dry legs or fluid filled capillary. Tall towers and vessels require long lengths of impulse piping lines or long lengths of capillary.
 
However, impulse piping lines can be unreliable due to leaking or plugging and can add measurement error due to evaporation in wet legs or condensation in dry legs.  Installations in colder regions may also require impulse piping to be heat traced or insulated which adds to installation costs and maintenance requirements.  

Capillary systems are affected by changes in temperature which causes the fill fluid in the capillary to expand and contract. This effect is magnified in long lengths of capillary which can cause measurement drift concerns. Long lengths of capillary can introduce long response times. The more capillary required, the longer it takes for a change in process conditions to traverse through the capillary to the DP transmitter. 

The maintenance and drifting issues can undermine operator confidence that the process is operating safely. For example, measurement drift caused by plugged impulse lines or a wet leg/dry leg failure could lead operators to believe that a distillation tower is weeping or has liquid/vapour entrainment, which will result in frequent trips into the field to verify measurements and rectify instrument failures. 

DP level technology
Digital DP level technology, such as the Rosemount 3051S Electronic Remote Sensors (ERS) system, can solve many of the traditional problems incurred when performing a DP measurement on tall vessels or towers. Two direct mount gauge or absolute sensors are connected digitally with a non-proprietary electrical wire. One of the sensors calculates the DP and transmits it back to the host system/DCS using a standard 4-20mA HART signal. High and low pressure measurements are fully synchronised to ensure that the differential pressure measurement is accurate. 

Digital DP level technology replaces the long lengths of oil-filled capillary and impulse piping with an electrical wire that is immune to temperature induced effect. This provides more stable and repeatable measurements over a large range of ambient temperatures without fill-fluid density or volume changes affecting the reading. 

Reduced maintenance
The technology also simplifies installation. Because the impulse piping is replaced by electrical wire, which can be fed through floor grates and wound around plant obstacles, installation time is drastically reduced. With the fluid-filled mechanical parts being replaced with an electrical wire, no heat tracing or insulation is required, greatly simplifying installation. 

Because there is no piping there is also no need to check for leaks, condensation or evaporation, which eliminates the need for operator clipboard rounds. 

The technology is being successfully used in the salt unit at one Inovyn chemical facility, where it is helping to optimise the performance of a brine evaporator. The company had been making the measurement using a differential pressure transmitter with impulse piping but salt crystal formation routinely plugged the impulse piping for the DP transmitter, resulting in measurement loss and this required the evaporator to be routinely taken offline to allow maintenance staff to remove the salt build-up and purge the impulse piping. 

The installation of the 3051S ERS system helped optimise control of the brine evaporator. Differential pressure is now computed in one of the two sensors and sent back to the DCS. Since it was installed there has been no loss of brine level measurement due to plugging which has resulted in higher levels of efficiency with the evaporator control, and has helped reduce both operations and maintenance costs and increased availability. 

Multivariable devices extend the benefits of digital DP level technology by providing access to more process variables for optimised control. In addition to the DP calculation, digital DP level technology provides real-time access to the pressure sensor readings and a scaled output for tank level or volume measurement. 

Process insight
Digital DP level technology, with multivariable capability, is particularly beneficial when installed on distillation columns. Foaming, entrainment, where weeping and flooding are prime contributors to dangerous conditions, poor quality and low throughput. The more process information the engineers have about the distillation process, the greater control they will have.  

Emerson’s Rosemount 3051S ERS System provides additional information needed to properly monitor and control the process. In addition to the differential pressure measurement, the two individual sensors can provide real-time access to both high side pressure (PHIGH) and low side pressure (PLOW) measurements. Using the PHIGH and PLOW measurements, along with a temperature measurement, it is possible to find the exact location of the distillate on the vapour-pressure curve. Device dashboard interfaces give a view of everything needed to evaluate the measurement by making all the process information and variables accessible and simple to read. Once the operating engineer knows where the distillate lies on the vapour pressure curve, it is possible to alter the tower parameters to increase throughput and quality. 

Access to the PLOW measurement point as well as the system DP enables the vapour pressure to be actively monitored – a particular beneficial when dealing with gas-blanketing systems such as nitrogen blanketing. Since some fuels require oxygen to combust, an inert gas such as nitrogen is often ‘blanketed’ across the surface to reduce the risk of ignition. Some blanketing gases are also non-breathable substances and sudden loss of vapour pressures could lead to hazardous situation for operating personal.

Summary
Obtaining accurate and reliable level measurements in tall vessels and distillation towers is difficult, particularly in outdoor vessels subjected to varying temperatures. Impulse lines can be unreliable, and long lengths of capillary are hard to work with. Replacing traditional systems with electronic DP transmitters overcomes many of these problems. Because there is no piping to be checked for leaks, condensation or evaporation, operator clipboard rounds are eliminated, reducing costs and enhancing safety for operators and maintenance personnel. 

Digital DP level technology can offer improved measurement and because the impulse piping is replaced by electrical wire, installation is quick and easy. Digital DP level technology does not suffer from problems of drift due to changing temperature and response times are much faster than a traditional system. Digital DP level technology with multivariable capability provides access to more process variables for optimised control and this is useful when installed on distillation columns. The more process information that is available about the distillation process, the greater the control. 

Digital DP level technology can be used is a variety of applications, including distillation columns, chemical storage tanks, pulping vessels, enzyme fermentation, alkylation reaction and beer fermentation, where it can help improve performance, simplify installation and maintenance routines, and reduce costs.
Pawel Czoczys was pressure marketing manager at Emerson Process Management.


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