Safety loop

Gel permeation columns may be left unattended during stabilization or chromatographic separations. However, if this is done the column should be provided with a safety loop to prevent it from running dry. Two ways of constructing a safety loop are illustrated in Figure 5-9. This device functions as a siphon. When the level of solution from the reservoir reaches the level of the column outlet (dashed line), the siphon is broken and the column flow ceases. 

Figure S-9. Safety loop arrangements that prevent a column from running dry. (A) The safety loop is placed after the column and the end of the outlet tubing is placed above the column. The flow stops when the eluent in the inlet tubing reaches the level of the outlet tubing. (B) The safety loop is placed before the column with the column outlet tubing in any position above the lower loop on the inlet side. The flow stops when the eluent in the inlet tubing reaches the level of the outlet tubing. (Courtesy of I. M. Easterday, Pharmacia Fine Chemicals, Inc.)...

A set of equipment intended to reduce the risk due to a specific hazard (a safety loop). Its purpose is to (1) automatically take an industrial process to a safe state when specified conditions are violated (2) permit a process to move forward in a safe manner when specified conditions allow (permissive functions) (3) take action to mitigate the consequences of an industrial hazard. It includes elements that detect when an incident is imminent, decide to take action, and then carry out the action needed to bring the process to a safe state. Its ability to detect, decide, and act is designated by the safety integrity level (SIL) of the function. See also Safety Integrity Level (SIL). 

FIGURE 10.236 Top view of proper guy-anchor grounding techniques. A properly dressed and installed ground wire prevents surge currents from welding turnbuckles and damaging safety loops. The perimeter ground coimects to the tower base by way of a radial wire. (After [9].)... 

It is quite a common feeling in people s minds that there are special requirements for SIL instruments. Alternatively, some put a lot of stress on selecting SIL for a logic solver as it controls the loop. What is a safety loop To answer these issues, it is important to understand how the safety of a loop can be assessed. In this clause the emphasis will be on calculation of SIL for the safety loop and what this actually means. 

Probability of failure on demand/safety integrity level (PFD/SIL) for a safety loop (typical). 1/P, input O/P, output. For share of SIL requirements see Fig. VIII/l.O-lB. 

From Fig. VIII/1.3-2 it is clear that the architecture constraint is one of the issues that need to be considered while assessing the SIL of a safety loop. The SIL limit of a product is affected by a number of parameters such as architectural constraints, HFT, SFF, PFDavg (fot low demand mode)/PFH (for continuous mode), and maintenance interval. These will be discussed in the following subclauses. 

From Tables VIII/1.3.1-1 and Vlll/1.3.1-2 it is seen that if SFF exceeds 60%, 90%, or 99%, SILl, SIL2, or SIL3 is obtained, respectively, for HFT = 0. Self-declaration for SIL is permitted by lEC 61508 for SILl but requires the third-party certification for SIL2 and higher. So, if there are a number of components in a safety loop, then the SFF for the safety loop can be discovered ... 

Reliability block diagram for safety loop. See text for meaning of abbreviations. 

One may refer to ISA-TR84.00.02-2002—Part 2 (Clause 5.1) for details of the calculation of a safety loop in various combinations. Returning to the original issue of PFDavg of a safety loop one needs to know the test procedure implemented and test intervals so that one can arrive at the maximum SIL for the device or subsystem. Table VIII/1.3.5-1 shows the data from two components one pressure transmitter and one isolator in series. Let us now check the SIL for the subsystem comprising these two components (pressure transmitter and isolator) for checking purposes the test interval as per FMEDA has been ttiken as 1/2/5 years (data). The selection with reasoning is elaborated in the table itself. The data shown in the table are taken from a reputable manufacturer s data in SIL certifications. 

Starting from the safety loop, let us examine how for a particular instrument/device SIL certifications are given and what is the minimum that needs to be seen. During... 

Referring to Fig. VIII/1.4 1 it is seen that the safety loop comprises sensor S1L2, logic solver S1L3, and final element S1L2. (For better understanding of a safety loop Fig. VIII/1.4 2 may be referred to.) Here, focus is on the sensor to check that the certificate conforms to the requirements. As discussed earlier and shown in... 

Conformity of component safety integrity level (SIL) and safety loop (SIF). PLC, programmable logic controller PFD, probability of feilure on demand. 

The brief discussions on SIL, PE systems, and plant emergency systems are now concluded. With these concepts on SIS and SIL discussed in Chapters VII and VIII, respectively, it is time to look at various safety loop components with SIL ratings in SIS applications. These are covered in the next chapter. 

For any safety loop comprising several components, the safety integrity level (SIL) achievement is a joint responsibility of end-user and supplier, as will be clear from Table IX/1.0-1. Why discussing this here These are discussed here to show that equipment manufacturer/system integrator or end-user is not only responsible for the same in isolation. In a safety life cycle, there are several phases involving several activities. So, at various stages there will be involvement of either end-user or sup-plier/manufacturers. The same issue has been elaborated in Clause 1.0.1 refer to Fig. IX/1.0-1 also. 

From the discussions, it transpires that each element of safety loop contributes to PFD for the safety loop. Naturally, PFD from the valve or final elements needs to be considered for SIL. This is very easy simple statement. One needs to look into the issue slightly in depth. From use point of view there can be mainly three pattern of usage of final elements, such as ... 

As final element is the most vulnerable component in a safety loop so, redundancy is another important issue here. Use of redundant valve in series and parallel mode is a related to process concerned but in any case it is normally a costly affair. However, proof testing at regular intervals could be an economic alternative to ensure reliability and availability of final element in the safety loop. This proof testing for final elements is popularly known as partial stroke testing discussed later in this chapter. 

Role of Digital Positioner and Control Valve in Safety Loop... 

Intelligent Transport Systems -The Role of a Safety Loop for Holistic Safety Management... 

The implications of this is therefore that part of the systems safety properties needs to be designed to be monitored and handled during run time. If (when) accidents occur they can then be analyzed, off-line, so that the events that led to the accidents can be understood and similar scenarios prevented in the future. This continuous process of interplay between design-time safety assessments, run-time safety assessments and off-line post analysis is what we term a safety loop. 

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