Field devices

Commercially available combustible gas detection systems generally use 24 VDC as the power supply for field devices. 24 VDC is inherently safer and corresponds the voltages increasing used by most instrument systems in process areas. A main supply voltage converter can be used to step down or convert from AC to DC power supplies. 

Smart Air Sampling System (SASS) The SASS is an air sampler designed to collect and concentrate biological aerosols into a liquid media for subsequent analysis. The collected liquid sample is then provided to a bio-detection field device to determine whether biological warfare agents are present. The SASS can take a 5-7cc sample in ten minutes, and is battery-operated for up to eight hours. 

Direct digital control systems appeared in the mid-1980s and displaced older analog closed-loop schemes for temperature control. These digital systems improved both accuracy and reliability. The earlier systems were modeled after existing system architectures and did not contain intelligent, standalone field devices. There were numerous interfaces to the various building systems and the major decisions were made at a central computer. 

Some SCADA systems use unique, proprietary protocols for communications between field devices and servers. Often the security of SCADA systems is based solely on the secrecy of these protocols. Unfortunately, obscure protocols provide very little real security. Do not rely on proprietary protocols or factor default configuration settings to protect your system. Additionally, demand that vendors disclose any backdoor or vendor interfaces to your SCADA systems, and expect them to provide systems that are capable of being secured. 

Fig. 4. Principle of function of the field device for online monitoring of inhibitors in sewage systems...

With an automated control system, verifying the sequencing functions may be appropriate during the OQ testing. The first step is verifying the interfaces to the field devices, such as pumps, motors, and valves, and their proper operation. This should include operation of proportional control valves. 

Field Devices Hardware devices typically located in the field at or near the process, necessary for bringing information to the computer or implementing a computer-driven control action. Devices include sensors, analytical instruments, transducers, and valves. 

The author points out statistics that suggest 80 percent of all interlock failures are due to failures of the field device. Those statistics show 45 percent of the interlock failures are measurement device failures and the other 35 percent are caused by valve or valve actuator failures. [27]... 

Where an intrinsically safe circuit is earthed by construction at the place of the field device (e.g. this is frequently the case using thermocouples), an Ex i-isolator with galvanic isolation shall be inserted. The reason is that intrinsically safe circuits shall be earthed at one single point only. 

In practice, these apparatus will be selected according to the type of the sensor or actuator desired. Manufacturers offer a broad range of varying functions. The safety-related data (LZ0, I0, Por CG, L0, see Section 6.9.5) are matched to the customary values of the field devices. Sometimes, variants with different Ex i-parameters are available. 

Figure 6.209 Communication with a HART field device via a handheld terminal or a PC (personal computer).

There are two methods for communicating with a HART field device (Fig. 6.209) ... 

Advantages and disadvantages of HART field devices may be summarized as follows ... 

As a result of current - and voltage - limitations required, and in case of classic Ex i-engineering (see Section 6.9.5), some restrictions are imposed upon the intrinsically safe variant. The number of field devices to be connected to the bus system is limited to approximately five, in particular in case of a current consumption exceeding the designated value of 10 mA. Besides, the length of cables shall be limited to some 100 m due to capacitance and inductance limitations. Away out is demonstrated below (FISCO model, see Section 6.9.6). 

The schematic circuit diagram of an intrinsically safe fieldbus according to IEC 61158-2 (physical layer) is given in Fig. 6.212. The field devices are certified as intrinsically safe apparatus. Four-wire transmitters with an external power supply are explosion protected by an additional type of protection,... 

Deviating from conventional installations (in many cases characterized by several field devices connected to a common terminal box/junction box), the fieldbus cable with its intrinsically safe circuit passes from transmitter to transmitter (Fig. 6.213). Each transmitter is fitted with a branch box (T box) to connect it to the fieldbus (Fig. 6.214). At the physical end of the bus segment, a terminating impedance shall be fitted. 

One single Ex i-isolator only required for several field devices Little space required within the control room... 

Not yet applicable for safety-related signals/functions Limited number of field devices per bus segment (5... 10)... 

Field devices with a higher power demand cannot be fed via the bus (e.g. flow transmitters)... 

Figure 6.216 Barrier as an intrinsically safe power supply of field devices for IEC 61158-2 Heldbus systems. Mixed types of protection, marking Ex II2 (1)G EEx me [ia] IIC T4. Installation in zone 1.

So, there is a need only to verify whether the parameters of each single field device will harmonize with the parameters of the power supply (or not). The capacitances and inductances of the field devices and the cable shall be added as usual. The specifications of the Fieldbus Foundation for safety-related parameters of field devices are summarized in Table 6.46. 

Due to these restrictions the intrinsic safety of a fieldbus will be established in a simple way. However, practice has shown that a rather small amount (4. .. 6) only of field devices depending on cable length can be operated via a single bus following this proceeding. The reason is the limitation of capacitance C and inductance L. In fact, a power supply with high output current provides the current for many field devices (10 mA each). However, the values for Cc and L0 still permissible are accordingly low. In contrast, a... 

Table 6.46 Safety-related parameters of field devices as specified by Fieldbus Foundation...

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