Safety in field instruments and devices

We have reached the stage in the safety life cycle where we need to be sure of getting things right in the design of the field instrumentation to be used in any plaimed SIS. 

The material in this book is based on the process instrumentation point of view. The principles laid down for the process control industry appear to extend pretty well into any automated plant or machinery system. The difference in machinery systems is one of speed and perhaps more specialized sensing. Issues of diagnostics, redundancy and installation are all common to many industries. 

Some important points to keep in mind when dealing with sensors and actuators are summarized. The next paragraphs will consider these and other issues. 

Consider the 1st point in the above list A typical reliability table will illustrate why the field devices are the major contributors to the possible failiues of most safety functions. 

Typical reliability data shows how field devices dominate SISfailure rates 

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Chapter 7 Safety in field instruments and devices. This chapter examines the... 

While benchtop instruments can use pressurized tanks as the hydrogen source, carrying pressurized hydrogen is very inconvenient in field apphcations, and many believe tanks in the field present a major safety hazard. Alternatively, small hydrogen alloy storage devices that contain reasonable amounts of hydrogen (in the form of a metal hydride) are available, and these can be used safely in the field. 

The frequency of radiation used in NMR depends on the field strength of the magnet used in the instrument. Frequencies range from 60 to 600 MHz for commercial instruments. Modern NMR equipment uses RF pulses in Fourier transform systems. There are no known safety concerns associated with these frequencies of radiation. However NMR magnetic fields can have adverse effects on pacemakers and other medical devices (see below under magnetic fields). 

These three factors served to encourage die development of the field and at the same time served to define the field itself. The increased prevalence of technology and medical instrumentation in hospitals meant that hospital organizations had to develop ways to take care of these devices. With this rapid proliferaticm of what were primarily electrical devices in the vicinity of the patient, some assurance of the electrical safety of the patient needed to be provided. Finally, the skills, training, and education of engineers and technologists who were to become involved in these activities needed to be vetted. 

One very sueeessful measure we took was to run part of the software system test suite in the same environment as its expected final use. Hence, it proved to be an intelligent idea since we discovered many defects we would not have discovered until the instrument would have been in the field. This was because another factor to consider is the environment where the device is used, e.g. if the user works in an extremely noisy environment, then the effect of risk mitigations based on audible signals and alarms may be altered or invalidated. The same applies to visual alarms when the user is operating with other devices at the same time and not paying attention to the computer screen. Note also, as eommented before, that users may interfere with the expected use. Indeed, the noisiest alarm that mitigates a safety eritieal situation may become useless if the user turns off the computer... 

For specific hardware or software items a safety manual is called for. Thus, instrumentation, PLCs, and field devices will each need to he marketed with a safety manual. Re-useable items of code and software packages will also require a safety manual. Contents should include, for hardware (software is dealt with in the next chapter) ... 

It gives application specific guidance on the use of standard products for the use in safety instrumented systems using the proven-in-use justification. The guidance allows the use of field devices to be selected based on proven in use for application up to SIL 3 and for standard... 

An effective way to ensure an accurate record of the installation checkout is to make up a composite package of data sheets and records for each field device. For example a copy of the sensor instrument specification can be clipped to the device checkout form for each safety function as shown in Figure 11.5. 

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