Safeguards normal operations

Maintenance embraces regular inspection, periodic examination by competent persons , and repairs. All maintenance on equipment used with chemicals should be properly planned and recorded. Maintenance operations, particularly those which are non-routine, require a sound system of work with strict administrative procedures, e.g. permit-to-work, to avoid risks arising from modifications to established safe practices. A permit-to-work should be used wherever the method by which a job is done is likely to be critical to the safety of the workers involved, nearby workers, or the public it is required whenever the safeguards provided in normal operations are no longer available. Examples include ... 

The HAZOP analysis technique uses a systematic process to (1) identify possible deviations from normal operations and (2) ensure that safeguards are in place to help prevent accidents. The HAZOP uses special adjectives (such as speed, flow, pressure, etc. see table 5.5) combined with process conditions (such as more, less, no, etc. see table 5.6) to systematically consider all credible deviations from normal conditions. The adjectives, called guide words, are a unique feature of HAZOP analysis. 

The essential feature of safeguards is that they are dedicated to safety and play no other role in plant operations. Therefore, normal operating procedures are not safeguards—even though they can help minimize the chance of an event occurring. 

Most process systems are protected by layers of safeguards as shown in Figure 15.38. The lowest layer (which is shaded) consists of normal operational controls, which are not safeguards per se, even though they control the great majority of deviations. The upper four layers, which are implemented in ascending order once the Safe Limit Value has been breached, represent increasing levels... 

Although normal operational responses such as these will handle the vast majority of hazardous situations it could be argued that they are not true safeguards because they are not dedicated to safety. Hence the following are not trae safeguards ... 

A normal operating response usually takes place before a safe limit has been breached. If the safe high level for T-101 is designated as 95% then normal operations will be conducted as the level rises toward the 95% value. Once the level goes above that point, the systems is, by definition, in an imsafe condition and the safeguards proper take over. 

Each IPL must be specific, independent, dependable, and auditable. No distinction is made between safeguards that are purely for safety and those that address normal operations. For example, an operating procedure for a routine task and a pressure safety rehef valve both count as IPLs. 

The automated system being presently developed allows solution of problems of plant automatic control, monitoring and diagnosis in normal operation and reliable actuation of safeguards, as well as to diagnose the plant actual state if an accident occurs... 

Node-oriented PTU In computer architecture nodes normally operate independently for their associated application. In a node-oriented FTU, there is replication of the node(s). In case of failure of the primary node, the designated replicas of the primary nodes will take over with a necessary mechanism to allow just one replicated unit to take over to avoid collisions or contention [9]. There are a number of hoiler controls in a power plant, namely, a closed loop control such as a superheater temperature control, furnace safeguard and supervisory system, high pressure bypass, etc., each with different apparently independent applications. In node-based architecture, there may be FTUs for each of these control systems acting as one node. In a particular node, when one of the main units fails any one of the standby controllers takes over irrespective of the application. Here the major concern is the size and capacity, and also the changeover mechanism. 

The best method should, ideally, be chosen by the designer as early in the life of the machine as possible. It is often found that safeguards which are bolted on instead of built in are not only less effective in reducing risk, but are also more likely to inhibit the normal operation of the machine. In addition, they may in themselves create hazards and are likely to be difficult and hence expensive to maintain. 

It is important to distinguish between hazards associated with normal operating conditions, where injury might result from human errors and the abuse of safe systems of work, and contingent hazards which follow directly from the failures of machine controls, components or safety devices. Safeguards must be selected to cope with both hazardous conditions. 

Normal operation shall be safeguarded by the operating systems. Moreover, the power plant shall be based on... 

Some process operations use operating relief valves as part of their normal operation, often in batch processes. For example, a chemical reaction may generate increasingly high temperatures and pressures. When the internal pressure rises above a certain point, the relief valve opens and the system pressure is reduced. In such situations, the relief valve is not a true safeguard because it is probably not as reliable as a true PSRV and it may not have been designed for the worst case scenario. 

The danger with such a system is that someone may accidentally leave the bypass line open, thereby bypassing the safety safeguard created by the SDV. The normal operating procedure in such a case would be to either car seal or lock closed the bypass valve. Nevertheless, the possibility remains that it could be left open after restart, particularly if there have been a series of spurious trips and the operator has decided to leave the valve open until the situation is finally stated out. He may then forget to close or seal the valve, especially if he has had many other tasks to carry out at the same time. 

Selection of safeguards to allow for safe access to the danger zone during normal operation, e.g. interlocking guard, two-hand control device... 

PERMIT-TO-WORK A document needed when the safeguards provided in normal production are unavailable and the manner in which a job is done is critical to safety. Identifies conditions required for safe operation. 

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