DISPLACER devices have long been the traditional method of level measurement in the oil and gas industry. However, they have certain limitations, notably high price, costly maintenance and repair, and inaccurate measurements.
Instrumentation engineers have been working for years on new measuring technologies to overcome these problems while ensuring these alternatives satisfy application conditions such as corrosion, sour gas and explosive atmospheres, and also meet stringent regulatory requirements of the oil and
Instruments based on Time Domain Reflectometry (TDR), also known as Guided Wave Radar (GWR) technology, have recently been proven to overcome the shortcomings of displacer devices.
These instruments operate independent of changes in product density, are unaffected by the upper gas phase and the two side process connections on the vessel.
This measurement principle has been used for many years in the telecommunications industry to detect cable faults and over the last decade, advances in GWR technology have meant that it has been adopted for process level measurement applications.
It is really the development of time of flight (ToF) technology that made TDR possible. The ToF principle operates by radar pulses being emitted from a transmitter towards the liquid surface in a vessel, reflected by the product surface and again detected by a receiver.
From the time of flight of the pulse, together with the known speed of propogation, the distance between the transmitter and the surface is determined and the level is calculated.
In TDR systems, a pulse of microwave energy is emitted from a high frequency (1GHz) generator and guided along the outside of the probe. When this microwave pulse meets a change in dielectric constant (the product surface), an energy reflection is created.
The strength of the energy reflection is proportional to the difference in dielectric constants of the free space and the material being measured. The reflected pulses are transmitted back along the probe to the electronics where a microprocessor analyses and identifies the reflection from the product level.
TDR devices are available with standard process flanges as well as manufacturer-specific flanges to offer a drop-in replacement for displacer systems, eliminating the requirement for costly modifications.
TDR instruments, such as the Endress+Hauser Levelflex, can handle pressures up to 400 bar and temperature ranges up to 400°C and include as option of NACE MR0175 conformity for applications where sour gas must be considered.
In addition, SIL2 conformity is becoming a standard requirement for applications on high demand safety loops, as is the availability of related safety documentation.
On most guided radar systems, 4-20mA/HART is considered as standard. However greater focus is now being placed on fieldbus compatibility with Profibus PA and Foundation Fieldbus widely offered.
Software developed with the experience of ToF techniques has been adapted to the TDR/GWR principle to offer a simple menu-driven configuration of the working parameters of the device.
The performance can therefore be viewed and monitored online or offline and this information can be used as a diagnostics tool locally or directly by the manufacturer.
Displacers are susceptible to build-up, changes in process conditions and mechanical failure. To remove, repair or re-fit displacers is costly and labour-intensive, significantly increasing plant downtime. This often results in overdue replacement, which in turn produces inaccurate level measurement.
A reliable alternative to significantly reduce maintenance costs, downtime and long-term cost of ownership is the TDR technology.
Designed for high precision and reliable operation in high temperatures and pressures, GWR devices are ideal for the rigours of the petrochemical and oil and gas industries, such as in separators and corrosive level applications.