Interlocks and Permissives

A permissive is a special type of interlock that controls a set of conditions that must be satisfied before a piece of equipment can be started. Permissives deal with start-up items, whereas hardwire interlocks deal with shutdown items. A permissive is an interlock controlled by the distributive control system (DCS). This type of interlock will not necessarily shut down the equipment if one or more of its conditions are not met. It will, however, keep the equipment from starting up. 

Temp Indicator Temp Transmitter Temp Recorder Temp. Controller 

In-Line Flow Element with Integral Transmitter. Ex. Mass, Coriolis, Thermal, Int. Orifice 

A simple flow diagram provides a quick snapshot or overview of the operating unit. Flow diagrams include all primary equipment, flows, and numbers. A technician can use this document to trace the primary flow of chemicals through the unit. Secondary or minor flows are not included. Complex control loops and instrumentation are not included. The flow diagram is used for visitor information and new employee training. 

Future Equipment Major Process Minor Process Pneumatic Hydraulic Capillary Tubing Mechanical Link 

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When safety functions are allocated to the safety instrumented system layer, they are known as Safety Instrumented Functions (SIF). The differences and/or relationships between safety functions, safety instrumented functions, safety instrumented systems, interlocks, and permissives are important. Annex H provides a discussion including important historical references to assist in the understanding of the new terms. 

Figure 9-6QQ. Nutter Snap-Grid high-capacity grid packing with good structural integrity (interlocking) and reduced fouling. Used by permission of Nutter Engineering, Harsco Corp., Bull. CSG-2.

Scheme 1, The conceptual progresssion from simple recognition processes—based on donor-acceptor stacks (top) and metal ion chelation (bottom)—to a family of interlocked and intertwined structures and superstructures. Reproduced from ref. 36 (Amabilino and Stoddart, Chem. Rev. 1995, 95, 2725) with permission of the American Chemical Society.

Fig. 6.20 Comparison TEM (left) and AFM (right) of a highly oriented PE film with lamellar structure. The arrow shows the molecular orientation. The TEM sample was shadowed with platinum and the AFM sample was not they were otherwise identical. Both images can be analyzed to give the same values for lamellar thickness and their height above the surrounding film, but the AFM image is clearer. It also shows how the lamellae interlock and branch. (From Jandt [227] reproduced with permission.)...

Implementation of process interlocks within process control systems is perfectly acceptable. Furthermore, it is also permissible (and probably advisable) that responsible operations personnel be authorized to bypass or ignore a process. Safety interlocks must be implemented within the separate safety interlock system. Bypassing or ignoring safety interlocks by operations personnel is simply not permitted. When this is necessary for ac tions such as verifying that the interlock continues to be func tional, such situations must be infrequent and incorporated into the design of the interlock. 

Fig. 20 Vertical model of complementary assembly of peptide mixtures. Hydrophobic interactions are represented by the interlocking of raised sections and holes. The axis is indicated by dots. Reproduced from Takahashi et al. [57] with permission. Copyright Wiley-VCH. Numbers refer to the peptide entries in Fig. 18. Positively charged residues are dark shaded in contrast with the negatively charged residues which are light shaded...

Fig. 7 Schematic view of the different approaches used to model the chiral interactions between DNA double helices, (a) DNA duplexes are viewed as clean cylinders with charged helical patterns of negative phosphates and positive adsorbed counterions. Reproduced with permission from [18], (b) Duplexes interact via steric interlocking of backbone and grooves and through electrostatic repulsion of the phosphate groups. Reproduced with permission from [14]...

Interlocks Some of these are provided for safety and are properly called safety interlocks. However, others are provided to avoid mistakes in processing the batch. When safety is not involved, terms such as permissives and process actions are sometimes used in lieu of interlocks. Some understand the term interlock to have a connection to safety (interlock will be subsequently defined as a protective response initiated on the detection of a process hazard). 

Figure2.15 Examples of structures in photostructuring glass, (a) long hole, (b) columns and (c) interlocking channels.(With friendly permission of the Technical University of llmenau, Faculty of Mechanical Engineering, Department of Inorganic-Nonmetallic Materials).

Fig. 25. A schematic representation of the interlocking sites model for the binding of anions to transferrins. From Schlabach and Bates (178), with permission.

Figure 2.48. Representations of zeolite structures. Shown are molecular and crystal representations of a polyhedron (A) formed from 24 Si04 tetrahedra. Also shown is the three- dimensional array of LTA, Linde A [Nai2(Ali2Sii204g)]-27H20 formed from interlocking Si04 and AIO4 polyhedra of (pore size, B 4.1 A). Reprinted from Greenwood, N. N. Earnshaw, A. Chemistry of the Elements, 2nd ed.. Copyright 1998, with permission from Elsevier.

It is permissible to switch the flame supervision out of the safety circuitry for a furnace zone when the zone temperature is at or above 1400°F (760°C), per NFPA 86 (sect. 5-9.1). Burners without flame supervision shall be interlocked to prevent their operation unless the furnace is at or above 1400°F (760°C). A MOOT (760°C) bypass controller must be used for this purpose per NFPA 86 (sect. 5-17). The bypass controller and temperature-sensing element must be independent from any other controller or element. Failure of the element must cause the bypass controller to sense a temperature below MOOT (760°C) and therefore shut off the combustion system. Visual indication must be provided to indicate that the bypass controller action is in effect. 

Figure 10.84 (a) and (b) Interlocked lettering from the eighth century Book of Kells, (c) The Borromeo family symbol, (d) The trefoil knot as viewed by Dutch artist M. C. Escher. (Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced by permission). 

Fig. 1. 22 Stacked linear contractile actuator configurations. From left to right. Stacked device wherein alternating layers of elastomer and electrode are stacked together helical device where two complementary helical elastomer strips and electrodes are interlocked folded device where a single strip of elastomer with electrodes on the top and bottom is folded upon itself [258]. Proc SPIE 2007, reprinted with permission...

Fig. 5 Reversible sol-gel transformation of Pd-salicylidene complexes. Sonication favours gelation and induces a conformational change, from bent to interlocked structures. Reproduced with permission from [52]. Copyright 2005 American Chemical Society...

Figure 6.5 From Tillman, K., Thust, A., and Urban, K. (2004) Spherical aberration correction in tandem with exit-plane wave reconstruction Interlocking tools for the atomic scale imaging of lattice defects in GaAs Microsc. Microanal. 10. 185, with permission from Cambridge University Press.

Interlocks are commonly used safety devices. The function of an interlock is to prevent the occurrence of an event in the piesraice of certain conditions. Some interlocks prevent action or motion, others send signals to other devices that prevent the action or motion. They automatically reconfigure or interrupt final control devices if monitored variables deviate significantly from specifications. Typical process variables monitored are flow, pressure, level, and temperature. Typical machine variables monitored are coolant level and temperature, lubricant level and temperature, vibration, speed, etc. Interlocks allow equipment to start and operate only when monitored variables are within designed specifications. Interlocks inhibit unanticipated actuation of equipment and ensure correct startup/shutdown sequences are followed. A permissive interlock will not allow a process or equipment to startup unless certain conditions are met. There ate two types of interlocks—safety and process interlocks. Each serves a different function. 

Fig. 22 Two interlocking CBER systems in the [JW Re, Rh, Re — Rh ]cber+uni for hydroformylation (reprinted with permission from Kliihn and Garland [93]. Copyright (2015)...

Figure 21.30 Geometry of angle (warp) interlock fabric. Source Reprinted with permission from Savage GG, Chapman and Hall, London, 76,1992. (a) No added stuffer yarn, (b) With added warp stuffer yarn (quasi 3-D fabric). Copyright 1992, Springer.

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