Permissive interlocks

Operator makes a Sign off of critical steps (double checking) sequential error, Install permissive interlocks to ensure key Omits steps steps are done in proper order Repeats steps Automate process steps Executes in wrong order CCPS G-15 CCPS G-32... 

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. 

An interlock is a device designed to prevent damage to equipment and personnel by stopping or preventing the start of certain equipment functions unless a preset condition has been met. 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 interlock will not necessarily shut down the equipment if one or more of its conditions are not met, but it will keep the equipment from starting up. 

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. 

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.

If the products of combustion can be diluted so that the carbon dioxide content is not greater than 1 per cent it is permissible to discharge them at ground level. This is the principle of the system shown in Figure 19.7, in which fresh air is drawn in to dilute the flue products which are discharged preferably on the same wall as the inlet to balance against wind effects. It is essential to interlock the airflow switch with the burner controls. 

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]...

Figure 10.89 Conceptual production of increasingly interlocked species through the use of multime-tallic helicates. (a) Doubly interlocked [2]catenane from three metal centres, (b) Pentafoil knot from four metal ions, (c) Triply interlocked [2]catenane. (Reproduced with permission from [98]).

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 31 Space-filling representation of the structure in the 110 direction emphasizing the large channels formed by the interlocking of the two bidimensional nets. Reproduced with permission from ref. 101.

Figure 31 A catenate formed by interlocking of rings around a Cu(I) centre. The system is achiral if the ligand is symmetric, but asymmetric substitution of the ligand (R = H R2 = Ph) results in a directionality in the ligand (indicated by an arrow following the shortest path from R] to R2). The resulting catenate exists as two enantiomeric forms. Reproduced with permission from reference 92.

Figure 33 Strategy for the synthesis of a doubly interlocked catenane. Reproduced with permission from reference 96.

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.

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). 

Even when the reactors are inserted, it may be necessary to impose operational restrictions on the system configuration, e.g. it may not be permissible to have all the generators connected when the interconnector is in service. This aspect may be overcome to some extent by introducing a system of electrical or mechanical interlocks. 

---