Utility Failures

Now let us consider utility failure as a cause of overpressure. Failure of the utility supphes (e.g., electric power, cooling water, steam, instrument air or instrument power, or fuel) to refinery plant facihties wiU in many instances result in emergency conditions with potential for overpressuring equipment. Although utility supply systems are designed for reliability by the appropriate selection of multiple generation and distribution systems, spare equipment, backup systems, etc., the possibility of failure still remains. Possible failure mechanisms of each utility must, therefore, be examined and evaluated to determine the associated requirements for overpressure protection. The basic rules for these considerations are as follows ... 

Backup systems which depend upon the action of automatic cut-in devices (e.g., a turbine-driven standby spare for a motor-driven cooling water pump, with PLCI control) would not be considered an acceptable means of preventing a utility failure for normal pressure relief design purposes, even though their installation is fully justified by improved continuity and reliability of plant operations. 

We can consider a range of typical plant emergency situations which may result from utility failures, equipment malfunctions, or plant upsets, and which may result in equipment overpressure along with some guidelines for the evaluation of these emergency conditions and determination of reheving rates. 

Remember that the failure position of a valve refers to its failure mode if there is a utility failure. A valve can mechanically fail in any position it is possible for a fail closed valve to get stuck in the open position. When doing a process hazard analysis it is important to consider all possible failure positions of a valve, and not only the failure position resulting from utility failure. 

OTHER Unplamied Utility failure Does LICV fail closed or... 

Fail-safe procedures Examines the consequences of utility failures, such as loss of steam, electricity, water, air pressure, or inert padding. Describes what to do for each case so that the system fails safely. 

Fail-safe is a concept used to specify the position of process instrumentation in the event of power, air pressure, or other utility failures. For instance, the valve supplying cooling water to a chemical reactor would fail in the open position ( fail open ) in the... 

Occasionally an incident occurs that results in a common mode failure. This is a single event that affects a number of pieces of hardware simultaneously. For example, consider several flow control loops similar to Figure 11-4. A common mode failure is the loss of electrical power or a loss of instrument air. A utility failure of this type can cause all the control loops to fail at the same time. The utility is connected to these systems via OR gates. This increases the failure rate substantially. When working with control systems, one needs to deliberately design the systems to minimize common cause failures. 

The pilot plant must also be carefully designed so that its control and safety systems are, fail-safe, and any unexpected equipment or utility failure brings the unit into a safe and de-eneigized condition. Unexpected or rapid process changes, if they can herald or lead to dangerous conditions (eg, runaway exothermic reaction), should be continuously monitored by appropriate instrumentation and suitable automatic action provided (1,55—67). 

Occasional Several times a year Imprecise communication between production, e.g. tank farm, failure of utilities, failure of a motor, explosive mixture after a failure... 

Building or structure collapse Power or utility failure Extreme air pollution... 

Blocked outlet Utility failure Cooling or reflux failure Inadvertent valve opening Loss of fans... 

OTHER THAN No part of the intention is achieved. Something different happens. Wrong component, startup and shutdown problems, utility failure. 

Unplanned shutdown (OTHER THAN normal operation). OTHER THAN feedwater Utility failure. Not possible Does LICV fail closed or open Same consequences as NO level in drum or MORE level in drum. ... 

Throughout the chapter, several examples will be provided that incorporate pneumatically actuated valves. It is important to note that the same functionality can be obtained with electric motor-actuated valves or electric solenoid-actuated valves. The selection of appropriate valves for a particular furnace and combustion process is a function of cost, performance, availability of clean instrument air, utility failure analyses, and personal preference. 

OTHER THAN NO PART OF THE INTENTION IS ACHIEVED. SOMETHING DIFFERENT HAPPENS. WRONG COMPONENT, STARTUP, SHUTDOWN, UTILITY FAILURE. 

The above discussion pertains to subcooling and superheating effects that tend to persist. Additional considerations apply to handhng subcooled or superheated feeds during ahnormtil operation, such as startup, shutdown, or utility failure. Some of these are ... 

Immediate start-up following utility failure where no equipment maintenance or opening of equipment has been performed. 

Thermal expansion and fire cases are not required to be checked, if the existing equipment is re-used, with the same service and also the same level control setting. Overpressure relief requirements due to each utility failure, fire cases and any other combination scenarios need to be estimated. API 521 (2014) has a comprehensive list of effects for utilities failure. All the PRV manifolds shall be checked to estimate back pressures at the PRVs. PRD overpressure calculations for equipment shall be documented as shown in Table 3.4. Vacuum relief (if the vessel/s is/are not designed to withstand full vacuum) shall also be documented. All the flare scenarios and flare network shall be properly documented. An example of PRV sizing calculations for the system shown in Figure 3.5 is presented in Table 3.4. 

Many mining operations in remote areas have emergency power supplies required for mine evacuation in the event of a power utility failure. The advantage of the PWM AC drive with active front end (AFE) is that the kVA demand is proportional to the real power produced at the hoist motor. Operating at reduced speed reduces the kVA demand proportionally. When considering kVA demand only, it would appear that an emergency generator rated at some fraction of the hoist motor would be able to power the hoist at reduced speed. 

Lack of work on part of operator or not trained properly Procedure modification to face emergency Equipment failure Control system failure Interlock failure without fallback Utility failure or disturbances Environmental effect External factors Human error Multiple failures... 

Equipment failure, for example, mechanical failure, BPCS component failure, software failure, mechanical failure, vessel, piping failure, performance failure, utility failure, etc. 

Bypass, start up override, and manual shutdown operational Well-documented proof test procedure Correct operation of SIS in utility failure, EMI, etc. 

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