Oil, gas and chemical companies have tank inventory measurement points that require compliance testing and periodic calibration of level instrumentation to meet industry regulations. Traditionally, these calibrations have required a company to dispatch an employee to the top of the tank roof with a portable level gauge.

Level measurement instruments have greatly improved with respect to calibration and verification in recent years. The latest state-of-the-art technology offers a safer and more efficient way to maintain compliance. Advances in sensor technology now provide device self-checking diagnostics and storage of reference data from previous verification events to address compliance with state and industry test requirements.

Reducing risks

Portable level gauging requires an employee to access the top of the storage tank, open a thief hatch and determine the tank level by means of a plumb bob or hand gauge.

Falls are among the most common causes of serious, work-related injuries and deaths, especially in environments subject to inclement weather, such as snow, ice, rain and/or high winds. Whenever an employee climbs a tank to check a gauge – whether during installation, calibration or instrument inspection – employers must make every effort to reduce risk.

For a cautious employer, this may involve setting up adequate fall-protection systems such as:

  • Constructing guardrails and platforms at the top of the tank
  • Providing safety ropes and harness equipment
  • Erecting scaffolding on the side of the tank

Each operation adds manpower and capital costs and increases the time required to perform the task.

Tank gauging and sampling activities can expose workers to high concentrations of hydrocarbon gases and vapors (HGVs), in some cases at levels immediately dangerous to life and health. Any time a thief hatch is opened, HGVs and oxygen-deficient components are released through the hatch into the employee’s breathing zone.

The potential also exists for life-threatening concentrations of hydrogen sulfide gas to be released where “sour” oil and gas operations are conducted. The vapor space in such storage tanks can have hydrogen sulfide concentrations of more than 100 parts per million (ppm). Hydrogen sulfide concentrations above 10 ppm are considered immediately dangerous to life or health.

The consequences of these occupational injuries can be considerable for the employer in terms of lost production, insurance costs and compensation costs. For the employee, the results can be lost work time and income, medical expenses, and possible long-term health problems or disability.

A better way

Figure 1. A positive reflection is caused when microwaves travel from low to high DC media such as a level reflection from air to oil.

Electronic verification of level and other instruments is widely accepted by regulatory bodies for safety instrumented systems. Today’s advanced sensor technology allows a functional proof test that can be conducted without interrupting the production process. Independent of the process conditions, the proper function of the safety system can be simulated in situ with a functional proof test. TV, one of the most respected authorities in its field, recognizes in-situ electronic verification as a way to prove the safety function of guided wave radar (GWR) level instruments as an alternative to wet testing.

Recent technological advances in guided wave radar level measurement technologies include internal diagnostics similar to those offered in many Coriolis mass flowmeters. Modern guided wave radars offer many features to fulfill the requirements of electronic verification.

With electronic verification, the level instrument and its supporting software can determine instrument accuracy and detect problems. This means no employees must climb a tank unless the verification requires maintenance or replacement of the instrument.

Guided wave radar technology

Guided wave radar, also known as time-domain reflectometry, is a time-of-flight level measurement technology. GWR operates with a microwave launch pulse sent from the transmitter, which follows a wave guide to the surface of the material being measured (see Figure 1). The varying impedance created by the change from the gas phase to the material being measured causes the microwave pulse to reflect and return to the transmitter. The time the pulse takes to reach the material surface and return is divided by two, which provides the distance to the material being measured and allows the level to be calculated.

Even more information can be extracted by reviewing the reflection curve of the return signal because changes in impedance create negative and positive reflections. A negative pulse is created when microwaves travel from a higher to lower dielectric constant (DC) such as the launch signal in which the microwave pulses travel from the process connection to the air at the top of the tank. This negative pulse is commonly referred to as the fiducial pulse.

A positive reflection is caused when microwaves travel from low to high DC media such as a level reflection from air to oil. Positive and negative pulses also occur in the “internal area” (the arrow to the left in Figure 1) due to impedance changes caused by the instrument’s internal, high-frequency module and cable. This reflection curve is a “fingerprint” of the particular instrument and will change when installed in a tank, providing a unique baseline reflection curve for the particular application.

Baseline reference curve

More advanced guided wave radar instruments have the ability to locally store the baseline reference curve (see Figure 2). This reference curve duplicates the current reflection curve recorded for a particular installation at a specific time. The reference curve is saved and access protected by a user password with a time stamp reference to operation time since the instrument was first powered up and put into operation.

guided wave radar

Figure 2. The baseline reference curve (yellow line) matches the current reflection curve at the time of commissioning. A new current reflection curve (red line) is recorded every time the level transmitter is verified. Any significant difference between the two curves indicates problems. The mapping curve (black line) identifies obstacles in the tank.

Comparing the current reflection curve to the baseline reference curve with software – such as Endress+Hauser’s DeviceCare – at the time of the initial recording, confirms the difference between the two curves. At the next required verification, a new reflection curve is recorded and compared to the reference curve initially recorded.

Any differences in the relative amplitude in the area of the fiducial pulse or level reflection would indicate either problems, such as material build up on the transmitter, or that the instrument has been removed and re-installed.

Sensor integrity diagnostics

Guided wave radar instruments include enhanced device diagnostics and advanced process diagnostics. Some of the device diagnostics include monitoring the reference pulse, checksum RAM, terminal voltage, cycle time and probe breakage to check the quality of the electrical wiring.

Advanced process diagnostics include the calculation of process media DC (where a change could be related to product quality) and the monitoring of relative echo amplitude (where a change could indicate buildup on the sensor as described above). A modern GWR has more than 80 diagnostic routines permanently running in the background and continuously self-checking and monitoring for correct functionality.

A modern GWR instrument incorporates the latest NAMUR NE107 recommendation for enhanced device diagnostics. Within this recommendation, four categories are defined to classify the diagnostic information (see Table 1).

This enables the operator and service technician to quickly identify the problem, work on the root cause analysis and take the right actions to avoid unplanned plant shutdowns. The NAMUR error message and information can be displayed on the local indicator and transmitted via a digital fieldbus protocol (such as, 4-20mA HART, Foundation Fieldbus or Profibus PA) to the automation system, diagnostic software or an asset management system.

Most modern GWR level instruments incorporate all these functions and approach the solution in different ways.

Table 1. NAMUR NE107 recommended error messages

Advanced functions

Some guided wave radar instruments have other advanced functions. An event logbook is a chronological overview of the event messages generated by an instrument that are saved for access through an “events history logbook.” The event history includes entries for every diagnostic and information event, and each diagnostic event indicates when it occurred and ended.

For example, if power is lost, the event log has a reference to the operation time when power was restored, as well as any new or changed parameter settings. When the reflection from the product surface is lost (echo lost), it shows up in the event log.

The event logbook stores all events occurring between two verification events, typically for up to one year. A minimum of 20, but preferably as many as 100, event messages can be displayed.

A self-device check function determines if the hardware and firmware work correctly and verifies proper installation. Device-check functionality on a GWR evaluates the quality of the launch (fiducial), level and interface signals. In addition, the following preconditions must be met:

  • Basic setup has to be carried out correctly.
  • The probe should be covered by process media, meaning the tank has to be at least partially filled.
  • Initialization of the device check must be made either through the keys on the local display or, preferably, through commissioning software.
  • According to the evaluation results, the GWR qualifies the signal strengths into groups and combines the evaluated signals in the parameter “result device check.”

Commissioning is performed according to the guided wave radar manufacturer’s recommended procedures. Initial electronic verification steps for after the GWR has been commissioned and tank level is confirmed include:

  • Performing a self-device check, which will confirm that the basic setup is correct and analyze signal reflections (fiducial, level)
  • Recording a baseline reference curve, capturing level reflection, fiducial and the internal area reflections, and establishing relative echo amplitude of level reflection
  • Recording a last entry in the device event log
  • Saving the commissioning parameter and reflection curve
  • Verification must also be performed per the manufacturer’s recommendations:
  • Review the device event log. If changes to the configuration or faults have occurred since the last verification, this may indicate a need for further investigation.
  • Review the baseline reference curve and current reflection curve for changes of relative signal amplitudes and changes in the near field between the fiducial and the level reflection. If changes are present, then further investigation may be required.
  • Perform a self-device check.
  • Record a last entry in the device event log.

If everything is the same as when the device was commissioned and all tests are passed, a verification report can be printed.

Today’s advanced guided wave radar level technology allows for electronic verification to ensure compliance in a quick and efficient manner. This technology also offers the highest degree of safety because personnel are not required to engage in risky operations. With electronic verification, the level instrument

Dean Mallon is a national marketing manager for the level products at Endress+Hauser. Previously, he was the regional level product business manager and a rep owner/partner. He remains involved in different committees and currently serves on the American Petroleum Institute 2350 5th edition working group.