Optimising Level Measurement Performance in Underground Tank Installations

Storage tanks are often buried underground to insulate their contents from very low ambient temperatures, however the problems of condensation, ice formation and low viscosity media often compromise the accuracy of level measurements. Ingemar Serneby, Senior Application Specialist at Emerson Process Management explains how the correct selection and installation of radar devices will minimise these problems to give the best measurement accuracy.

Underground storage tanks are used in many different industries and are very common in the upstream oil and gas facilities, particularly those operating in colder regions. They are installed below ground to insulate their contents from very low ambient temperatures and are often buried up to 1.5m deep. The purpose of these tanks is the same as for their above ground counterparts, functioning as flare knockout tanks, drainage tanks and separators. The tanks are used to store oil, water, natural gas liquids, and mixtures of hydrocarbons and water.
Typically these applications are based around a horizontal bullet tank of 2-3 metres diameter with two nozzles 75-100 mm diameter and approximately 2-3 metres tall. Access to the tanks to measure temperature and/or level is provided by the nozzles or a stilling well with an entrance that sits above ground. One nozzle is typically used for temperature monitoring and another for level measurements, which can be performed by either a continuous or single point measurement, or using a hand-dipping method.
 
Challenges
Condensation is often a problem with underground tanks. Although the storage conditions are usually stable, the contents may enter the tank at a higher temperature and will cool over time to match the storage conditions. As the contents cool, condensation forms on the coldest parts of the tank, which are usually the stilling well or nozzles where the instruments are installed. If the ambient temperatures are low enough, ice will form, which could affect the accuracy and reliability of the level measurements. Low temperatures also increase the viscosity of the contents causing coating and build-up on wetted surfaces as the level rises and falls.
Because access to underground tanks is from the surface using a nozzle or stilling well, only top-down level technologies such as mechanical level gauges or radar based instruments (guided wave or non-contacting) can be used.

Choosing the Right Technology
The traditional approach to measuring level is hand dipping. However, obtaining accurate results can be particularly difficult when the operators are exposed to cold weather, wet or snowy conditions, lightning and high winds. Access may be restricted during these times and also during the hours of darkness. In addition, the need to wear gloves and other protective equipment makes it more difficult for dipping equipment to be used. There is also the possibility of measurements being incorrectly taken or erroneously recorded due to human error.
To provide continuous measurements, some underground tanks have been fitted with a mechanical level gauge. However, these are prone to problems due to sticking caused by coating and the build-up of condensation and ice. The presence of moving parts also means there is a need for regular maintenance or renewal.
Radar technology provides an accurate, versatile and reliable alternative to hand-dipping and mechanical devices for level measurement applications. There are no moving parts to wear or stick and radar devices (particularly guided wave radar) are less sensitive to condensation and the formation of ice. Measurement accuracy is independent of product properties and their integrated probe and transmitter construction means they are virtually maintenance free. However, choosing and installing the correct type of radar is crucial  to achieve the desired measurement accuracy for individual applications.
Guided wave radar (GWR) and non-contacting radar (NCR) are both top down, direct measurement technologies that measure the distance to the surface. Whereas GWR radar uses a probe to ‘guide’ the radar to the surface, non-contacting radar uses an antenna to transmit the signal through the vapour space.
With radar-based technologies, the dielectric constant of the media is a key indicator of the strength of signal that will be reflected back to the gauge. Underground tanks often contain products that have a low dielectric constant, such as hydrocarbons. To improve the maximum measuring range in these materials, technologies are available that provide accurate level measurements even when the returning signal strength is weak.

Both GWR and NCR can be applied to underground applications. As a general guide, GWR is preferred when an oil/water interface measurement is required. GWR is also a better option where condensation is present or the product to be measured has a very low dielectric constant (< 1.9) or a turbulent surface. For many applications, GWR will typically provide greater accuracy than NCR.
NCR is a better option for tanks with a long nozzle where a viscous product can cause heavy coating/build-up on wetted parts such as the probe of a GWR. However, it is important to note that NCR may need air purging to cope with condensation related issues. Cone antennas must be used and the antenna size should closely match the nozzle or stilling well pipe diameter.
When installing either type of radar device in an underground tank with a cathodic protection system (used to reduce corrosion within the tank) the radar needs to be grounded. For locations where surges from lightning may be a problem, the use of a transient terminal block will provide electronic components with a higher degree of protection against transients.

Radar for use in Stilling Wells
Single rigid or flexible GWR probes may be used in stilling wells. For longer nozzles, a stilling well is sometimes required to screen out possible noise induced by the nozzle walls. Care should be taken to ensure that the probe does not touch the side walls of the stilling well. If the probe touches the wall, false reflections will create false level measurements. When used in metal stilling wells, single rigid probes offer a stronger return signal than when used in open applications. This makes them suitable for low dielectric and interface applications.
When using NCR in stilling wells the antenna should match the stilling well pipe size with the gap between the antenna and the pipe wall made as small as possible. Larger gaps may impact performance.

Summary
Radar technology provides an accurate, versatile and reliable means to measure level in underground tanks. Unlike mechanical level gauges, there are no moving parts to wear or stick. Measurement accuracy is independent of product properties and the integral probe-transmitter solution is virtually maintenance free. Radar devices, particularly guided wave radar, are less sensitive to the problems of condensation and ice formation. Non-contacting radar is a good option when viscous product can cause heavy coating/build-up on wetted parts.
Case History
A major oil refiner in Canada needed to automate the level management in its flare knockout tank to prevent overspill situations that could compromise plant safety. The company was using a mechanical float-based system, but this was proving unreliable because of the tendency of the oily water mix to coat the wetted parts. The unreliable instrumentation meant that to prevent an overfill situation and maintain plant safety, trucks were sent to empty the tanks sooner than was necessary - incurring additional operating costs.   
The customer decided to install Emerson’s Rosemount 5402 non-contacting radar on the existing stilling well and the antenna was trimmed to fit. One of the elements of setting up a radar unit for use in stilling wells is the need to compensate for the change in propagation speed of the signal which naturally occurs in pipes. The PC configuration tool - Radar Master automatically initiates this calculation when the user inputs the pipe’s inside diameter, helping to optimise performance. The trending tools built into the software provide redundancy to the DCS trends and allowed the operators to focus more closely on the level readings corresponding to process events.
The accurate and reliable level measurements provided by the Rosemount non-contacting radar increased operator confidence in the level measurement, enabling the tanks to be utilised to their full capacity. This means that fewer trips (by operators) are required to empty the tanks. Overall operation and maintenance costs have been reduced by $10,000 per year.