High-demand-mode examples

The batch process is tripped once every 3-4 batches, and 100 batches are produced annually. This means that the SIF operates approximately 25 to 35 times per year, which is high-demand mode operation per the rule of 1 demand per year. Looking at this on the basis of the hazard rate equation  

HR = DR PFDavg = 35 demand/year 0.0025/demand = 0.085 events/year 

As shown in the previous examples, the situations that are considered to be high demand or continuous demand are actually BPCS functions that are required to be extremely reliable because there is no back-up protection system. An SIS operating as a protection system (i.e. low demand) is used to move a process to a safe state upon detection of an abnormal condition. If a process condition always occurs at the end of every batch, then it is not unexpected. Many practitioners believe that high-demand mode safety functions should not exist in the process industry, and where they are identified, they should be re-engineered to convert them to low-demand mode. 

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The team assesses the number of demands placed on each SIF to ensure they are consistent with what was originally defined. For example, if the original premise was that the SIF would have about one demand for every ten years, but, in reality, the SIF is exposed to one demand a year, the team modifies the PHA assumptions based on this new operating experience. This may result in a higher SIL requirement or in the determination that the SIF is operating in high-demand mode rather than low-demand mode. 

Gas Pump Control Shutoff Gas stations employ fuel pumps that are designed to stop flow of gas when the car s gas tank is full. When the gas pump control fails to function, the car s gas tank is overfilled. The fuel pump shutdown represents an example of a protective system operating in high-demand mode, as there are several demands on the fuel pump shutoff a day. 

The high-temperature demand mode ink-jet process used in printing UV-curing polymer microlenses can be used to create highly controlled spacers in flat panel displays. Figure 11-26 shows an example of printed spacer bumps that would meet the physical and thermal (in excess of 200°C) durability requirements for flat panel displays. Bumps as small as 25 pm diameter and 10 pm high can be created, and bumps this size or larger would span the requirements for most spacers in displays. 

Besides these high-throughput applications, droplet generators can also be used in the drop-on-demand mode. They can be used to deposit lubricants where needed during manufacture, assembling or operating. For example special oils can serve as lubricants in small spindle motors without ball bearings. Therefore, the lubricant has to be provided with precise control of quantity and position. 

ANSi/iSA-84.00.01-2004-1 includes high demand/continuous mode of operation, as well as demand mode. ANSI/ISA-84.01-1996 only addressed demand mode because the vast majority of SIS operate in a demand mode. ISA-TR84.00.04-1, Annex I, discusses the differences between demand mode and high demand/continuous mode. It also provides an example to illustrate the importance of properly defining any high-demand/continuous-mode SIS. 

The rationale behind the definitions of iow demand mode and high demand or continuous mode in lEC 61508 is based on the failure behaviour of a safety-related system due to random hardware faults. Underlying much of the reasoning is the distinction between safety-functions that only operate on demand and those that operate continuously . A safety function that operates on demand has no influence until a demand arises, at which time the safety function acts to transfer the associated equipment into a safe state. A simple example of such a safety function is a high level trip on a liquid storage tank. The level of liquid in the tank is controlled in normal operation by a separate control system, but is monitored by the safety-related system. If a fault develops in the level control system that causes the level to exceed a pre-determined value, then the safety-related system closes the feed valve. With such a safety function, a hazardous event (in this case, overspill) will only occur if the safety function is in a failed state at the time a demand (resulting from a failure of the associated equipment or equipment control system) occurs. A failure of the safety function will not, of itself, lead to a hazardous event. This model is illustrated in Figure 4. 

Low demand mode Where safety function is only performed on demand to transfer equipment under control (EUC) into a specified safe state and the frequency of demand is less than one per year. Fig. VII/1.4-1 is an example of demand mode of operation. This is because normally the level in the tank will be maintained by sensors Sn and control valve Cy, but in case of a high level to avoid a hazardous situation, separate protection sensors will come into play and close the shutdown valve Sy. So safety comes on demand. 

An interesting example of t 3 coordination of a boratabenzene to a transition metal has been observed for a Zr(IV)-boratanaphthalene complex (Scheme 20).35 Thus, treatment of the illustrated 1-boratanaphthalene with Cp+ZrCl3 furnishes an adduct in which the metal-carbon distance for the 3, 4, and 5 positions of the heterocycle (2.53-2.56 A) is significantly shorter than for the 2 and 6 positions (2.77-2.81 A). It is suggested that this distorted bonding mode is the consequence of the high electron demand of Zr(IV), which prefers coordination to the most electron-rich carbon atoms. ... 

In general, most converters are tested on the bench with the electronic load set to constant current (CC mode). True, that s not benign, nor as malignant as it gets. But the implied expectation is that converters should at least work in CC mode. They should, in particular, have no startup issues with this type of load profile. But even that may not be the end of the story Some loads can also vary with time. For example, an incandescent bulb has a resistive profile, but its cold resistance is much lower than its hot resistance. That s why most bulbs fail towards the end of their natural lifetime just when you throw the wall switch to its ON position. And if the converter is powering a system board characterized by sudden variations in its instantaneous supply current demand, that can cause severe problems to the converter, too. The best known example of this is an AC-DC power supply inside a computer. The 12V rail goes to the hard disk, which can suddenly demand very high currents as it spins up, and then lapse back equally suddenly into a lower current mode. 

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