Power failures

The following possible scenarios shall be considered in analyzing a power failure situation  

Process engineering and design using Visual Basic 

---

Another problem with this approach is common mode failures. A common mode failure is a single event which could lead to the simultaneous failure of several components at the same time. An excellent example of this is a power failure, which could lead to many simultaneous failures. Frequendy, the common mode failure has a higher probabiUty than the failure of the iadividual components, and can drastically decrease the resulting reUabiUty. 

Drive motors should be of the high-starting-torque type and selected for 1.33 times maximum rotational speed. For two- or three-diameter Idlns, the brake horsepower for the several diameters should be calculated separately and summed. Auxiliaiy drives should be provided to maintain shell rotation in the event of power failure. These are usually gasohne or diesel engines. 

Low-loss system. In a low-loss system the winding also resides in a reservoir of liquid helium but a very efficient insulation system enables the magnet to operate for long periods, typically I year or more, between liquid helium refills. An important feature of these systems is that they are relatively immune to short-term electrical power failures, which has enabled complete reliability even in extremely difficult environments. 

Fibrous or particulate filters are not important anymore because membrane filters are relatively compac t and perform veiy well. For filtration by straining, there is an intermediate air velocity at which filtration efficiency is a minimum because different collec tion mechanisms predominate at different ranges of velocity. At low velocities, diffusional and elec trostatic forces on the particle are important, and increased velocity shortens the time for them to operate. At high velocities, inertial forces that increase with air velocity come into play below a certain air velocity, their effect on collection is zero. Surges or brief power failures could change velocity and collection efficiency. 

Valve Failure Positions In the event of instrument air or electrical power failure, valves either Fail Closed (FC), Fail Open (FO), or Fail in the last position (FL). The position of failure must be carefully selected so as to bring the system to, or leave the system in a safe operating state. 

This is provided to prevent reverse rotation of the pump in the event of a power failure or a deliberate shutdown due to backflow of liquid from the rising mains (pipelines). This is located immediately after the last pump stage casing/discharge outlet to prevent the shaft from rotating in the reverse direction. The provision of a non-return valve also ensures that the pump always starts in a shut-off condition, when the power requirement is at a minimum. 

It simulates the motor s cooling-down condition, for at least 30-60 minutes, during a temporary power failure. 

As an energized capacitor retains its charge, even after a disconnection and causes a voltage transient on a reswitching, it is recommended that its circuit be closed only through a contactor or a breaker, with an undervoltage release. So that in the event of a power failure, the circuit will interrupt automatically... 

Power system stability studies can provide some insight into the effects of power failure. The calculations can become tedious if performed manually because of the iterative steps required to obtain satisfactory answers. Therefore, a computer program is used to supply the iterative answers in a short time and with comparable accuracy. 

Step 6. Acceleration calculations should be made to determine the effects of electrical power failure. Overspeeding of the train can be destructive if controls are inadequate to limit the peak speed within reasonable bounds. 

Power failure, affeeting agitator, pumps, instruments. 

Order for restarting equipment after a power failure. 

Required positioning of control valves (i.e. shut, open or as is ) upon power failure. 

Air cooled heat exchangers will continue to operate (but at reduced capacity) due to radiation and natural convection air circulation should a power failure occur. 

Normal Individual and Process Unit Basis for PR Sizing Considerations -The following single contingencies should be considered as the normal basis for evaluating overpressure that can result from electric power failures ... 

Consideration of Plant-wide or Refinery-wide Power Failure - Although not normally used as a basis for sizing pressure relieving facilities, the following general power failures on a plant-wide scale must be considered. 

Instrument Power - Instrument power failures are evaluated on a basis similar to that described for a power failure. Included in the normal considerations for PR sizing should be the failure of power supply to all instruments in and controlled from a single bus bar. Reliability feahires should include secondary selective power supply to control rooms, with emergency generator or battery backup for critical instruments and control computers. Critical controls should be able to continue operation independently of control computers. 

We need to keep in mind the disposal costs in all of the mechanisms for solidification. With the first method, keep in mind that free liquids are typically not allowed in most disposal scenarios. And adding too much adsorbent can substantially add to disposal costs. Make this point clear to your field people. As far as using polymerization catalysts and chemical reagents, keep in mind disposal costs. Ensure that you are cognizant of disposal costs of spent catalyst prior to using this scenario. As far as freezing is concerned, consider the cost to keep the contaminants frozen and what the downsides are. The downsides besides cost include measures in case of power failure and use of freezing equipment after wastes have been disposed. 

---