Instrumentation

What is Proof test in instruments?

What is proof testing?

Proof-testing is to identify random hardware failures to confirm the proper functioning of the commissioned equipment already in service.

Proof testing is performed periodically as part of the process of commissioning or managing change to detect systematic (human) mistake.

Proof testing reduces the probablity of possible failures in a safety system. Proof testing is generic and applies to all types of machinery.

Proof-testing is an essential component of IEC 61511 with many demands that are presented throughout the life cycle of safety.

Proof Requirement:

According to IEC 61511, proof test requires these safety requirements specification (SRS)

  • It is suggested that the proof-testing interval specifications be specified.
  • Any requirements on overrides/inhibits/bypasses shall be documented
  • The test can be performed either at the end of the test or by one element at a time.
  • The test method shall include overrides / inhibits / bypasses and how they are cleared and informed to carriers
  • Procedures to avoid deviations during execution should be realistic and take into account both process and private safety issues.
  • The documentation shall also include directions for the maintenance of process safety during the test and the detection of failure behaviour.
  • Proof testing is carried out by skilled staff who are correctly trained and perform the procedure in accordance with the guidelines without any deviations.

Proof testing methods for level measurement:

Live Simulation of Alarm Condition:

An intuitive proof-testing technique is to increase and decrease the real level of the product to confirm that the output signal of the level sensor will work as anticipated.

Bucket Testing:

Another traditional technique of proof-testing is to remove and expose the point level sensor to the alarm condition. This is often done in practice by inserting the tool into a product-filled bucket.

This method requires a visit to the tank and access to the level sensor while the tank is temporarily taken out of operation

The procedure may be a direct safety concern for the test personnel as it exposes both the tank to the atmosphere and the contents of the bucket may be dangerous

Test Chambers:

Live simulation is to mount the level sensor inside a chamber that can be insulated from the tank mechanically. The chamber can be filled and drained with product by using internal links, thus simulating an alarm situation.

Test Levers

The most common concept is the use of test levers to simulate the alarm situation mechanically. While the levers may be loaded from the spring and initially intended to be unsafe, empirical evidence has shown that this is not often the case.

This method is used to eliminate the dismantling and level sensor isolation issues,





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