Easy proof testing of guided wave radar sensors

16 September 2014

The latest overfill protection standard API/ANSI 2350 states that device verification of most overfill prevention sensors should be carried out semi-annually. Carl-Johan Roos, functional safety officer at Emerson Process Management, explains how this can be achieved without having to raise the level in the tank above its maximum working level.

Having access to accurate tank level data is important for process control and inventory management, particularly in tank storage applications within the oil and gas, chemical, petrochemical and pharmaceutical industries.

Overflow presents a risk to the environment and damage to the plant. It is also potentially harmful to workers and people living nearby. Several high profile and serious accidents involving storage tanks have highlighted the need to reduce the risk of an overfill incident occurring. 

In response, the petroleum industry created the API/ANSI Standard 2350 Edition 4: ‘Overfill Protection for Storage Tanks in Petroleum Facilities’. This standard, released in 2012, is a description of the minimum requirements for atmospheric bulk liquid storage tanks to comply with modern best practices. The main purpose of the standard is to prevent overfills and improve safety. Another common, yet important result of applying this standard, is increased operational efficiency and higher tank utilisation.

The standard applies specifically to large, non-pressurised, above-ground petroleum storage tanks and specifically to overfill prevention. Because API 2350 is the latest overfill prevention standard, representing today’s best industry practices, many companies are adopting it as the new benchmark. Since the standard is relatively generic, users have successfully applied it to normal process level applications.

API 2350 does not compete with other, more generic, safety standards, but is intended to complement them. Using Safety Instrumented Systems (SIS) designed in accordance with IEC 61511 is one example of how to fulfil some of the requirements in API 2350.

Regular testing 
API 2350 places high importance on regular proof testing. The guidelines state that all overspill protection systems (OPS) that are required to terminate receipt must be tested annually, while the High-High sensor alarm must be tested semi-annually. Additionally, continuous level sensors should be tested once a year and point level sensors, semi-annually. 

In many cases, high level alarm testing in process applications requires the fluid level in the vessel to be raised to the high level alarm limit. The fluid has to be moved in and out of the tank under test –increasing the risk of spills. The process can take up to half a day to complete, which can interrupt normal activities. It would also require supervision, with operators monitoring the tank level. This can pose health and safety risks due to the possibility of exposure to the tank contents.

While this may have been an acceptable practice in the past, the latest version of API 2350 does not recommend that the tank level be raised above the maximum working level. So what other options are there to test the high level sensor alarm? The regulations do permit simulating overfill conditions to activate the detector and generate an alarm signal. 

Emerson’s vibrating fork liquid level switch, for example, features a magnetic test-point located on the side of the housing. This is activated by holding a magnet to the test-point which causes the output state to change – simulating the alarm condition and enabling a functional test of the switch and the system connected to it.

However, the ability to simulate an alarm condition has not been available for all level technologies. For example, Guided Wave Radar (GWR) sensors, which are widely used for level applications for inventory management and to generate high level alarms, do not feature overfill simulation technology. However, recognising the benefits that this feature would provide has led to the introduction of an automated high level alarm testing function for Emerson’s GWR transmitters. 

Emerson Process Management’s verification reflector function is now available with the 5300 series GWR transmitters. This function is designed for applications requiring periodic transmitter integrity tests to ensure that the level measurement device is functioning correctly and prevents overfilling. The verification reflector functionality enables automated transmitter integrity tests without stopping the process or manually raising product level in the vessel. In addition to meeting the recommendations of API 2350, it reduces the risk of accidental spills and the high level alarm testing process can be completed more quickly. It also tests the loop from the device to the DCS as well as the device itself.

Compared with traditional diagnostics, which only monitor the transmitter electronics, the verification reflector can also be used to diagnose problems with the upper parts of the probe inside the tank, such as product build-up, corrosion monitoring and other process-related conditions.

How it works
In a GWR installation, the GWR is mounted on top of the tank or chamber with a probe extending the full depth of the vessel. A low energy pulse of microwaves is sent down the probe and when it reaches the media surface, a reflection is sent back to the transmitter which measures the time taken for the pulse to reach the media surface and be reflected back and an on-board microprocessor accurately calculates the distance to the media surface using ‘time-of-flight’ principles.

The verification reflector function uses an adjustable reference reflector fitted to the probe of the guided wave radar at a desired height to generate a unique echo signature. The device constantly tracks the reflector echo to determine if the level is above or below the alarm limit.

A ‘test’ function built into the device software verifies that the GWR has been correctly configured and is correctly tracking the reflector echo. It also confirms that the alarm loop is working with a high level alarm being displayed in the control room. This ‘test’ function can be accessed remotely using Emerson’s AMS Suite or Rosemount Radar Master software packages, as well as locally using a hand-held Field Communicator.
The new verification reflector for 5300 GWR has been designed with applications requiring periodic transmitter integrity tests, to ensure that the level measurement device functions correctly and that overfilling will not occur. It allows for remote proof-testing without the need for operators to climb the tank, and it is able to diagnose any problems with the upper parts of the probe inside the tank, such as product build-up, corrosion monitoring and other process related conditions. It also enables automated device integrity verification without stopping the process or manually raising product level in the vessel – saving time, enhancing safety and reducing the risk of spills.

Benefits of GWR level transmitters
GWR level transmitters are suited to use in storage and buffer tanks containing oil, condensate, water, or chemicals and can also measure both level and interface level in separator applications. They are often the technology of choice for use in waste tanks and underground tanks such as sump pits, and can be used in both chamber and pipe installations.

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