Process Upset

The wisest fan choice is frequently not the cheapest fan. A small fan operates well on its curve but may not have adequate capacity for maximum flow control, future needs, or process upset conditions. It may be so lightly constmcted that it is operating near its peak speed with no provision for speed increases in the future, if needed. As fan size is increased, efficiency generally improves and wheel speed is lower. These factors decrease operating cost and provide reserve capacity for the future. However, it is also possible to oversize a fan and impair its performance. 

Operability. Hidden costs may result from changes in the way a process operates as a result of a control. For example, enclosure and isolation may diminish the abiUty of workers to observe the process. Upsets and dismptions resulting from this loss of intelligence are expensive and generate resistance to the use of these controls, no matter how effective. 

Normal Operation. The designer of a chemical plant must provide an adequate interface between the process and the operating employees. This is usually accompHshed by providing instmments to sense pressures, temperatures, flows, etc, and automatic or remote-operated valves to control the process and utility streams. Alarms and interlock systems provide warnings of process upsets and automatic shutdown for excessive deviations from the desired ranges of control, respectively. Periodic intermption of operations is necessary to ensure that instmments are properly caUbrated and that emergency devices would operate if needed (see Flow measurement Temperaturemeasurement). 

In order of priority, the factors that govern the selection of industrial dryers are (/) personnel and environmental safety (2) product moisture and quahty attainment (J) material handling capabiHty (4) versatiHty for accommodating process upsets (5) heat- and mass-transfer efficiency and (6) capital, labor, and energy costs. 

The success of EDR ia water demineralization has appareatly resulted from its greater tolerance of particulate and fouling matter compared to reverse osmosis greater forgivingness of process upsets greater tolerance for unskilled operators simplicity ia desiga and constmction of EDR stacks compared to reverse osmosis modules the abiHty to iaspect, clean, or replace one membrane at a time the existence of a comprehensive global sales and... 

Use of Corrosion Probes The major use of corrosion monitoring probes is to measure the corrosion rate in the plant or the field. In addition to corrosion-rate measurements, corrosion probes can be used to detect process upsets that may change the corrosion resistance of the equipment of interest. This is usually equally as important a measurement as corrosion rate since a change in the process conditions can lead to dramatic changes in the corrosion rate. 

The trip valve is provided as a second line of defense in case of overspeed The trip valve is frequently equipped with a trip-actuating solenoid which can be operated by push button, by low od pressure, or by some other process upset. When the speed control functions as described above, the trip will not be actuated by load dump. 

Thrust Bearings Turboexpanders often have process upsets or ice plugging or the hke, which can cause serious thrust-bearing load variations. In applications above 506.6 to 1013.2 kPa (75 -148 psi), the best available thrust bearing usually is insufficient to protect against such high thrust loads. Various indicatious, such as the differential... 

DCS sampling frequency too low or the response time for some analog instruments may be too slow for proper control of transient nature of batch processes and may lead to a process upset. 

In addition to failure as a result of their utihty supply, items of equipment are subject to individual failure through mechanical malfunction. Such items include pumps, fans, compressors, mixers, instruments and control valves. The process upset resulting from such malfunctions (e.g., loss of a reflux pump) may in turn result in emergency conditions and the potential for overpressure. These contingencies should be examined and evaluated. 

Emergency Conditions in Integrated Plants - In integrated plants, a process upset in one unit may have an effect on other units (e.g., loss of flow of a pumparound which is used as a source of heat for reboiling other towers). All possibilities such as these must be carefiiUy considered and the potential for resulting overpressure evaluated. 

Similarly, unit operating staffs can be trained to work together during a process upset using all the skills and resources available. Such training is part of nuclear submarine training ( Submarine , 1992) and... 

Operational Error Design Error Process Upset Mechanical Failure Unknown Natural Hazard Sabolage/Arson... 

At about 12 40 A.M. December 3, 1984, the accident occurred that took 2,500 lives at the Union Carbide (India) Ltd. (UCIL) operated pesticide plant. The first indication of process upset was when the control-room operator (CRO) noted that the temperature in a supposedly refrigerated tank storing methyl isocyanate gas (MIC), had risen to 77° F. The pressure, normally 2 to 25 psi was 55 psi. 

This approach has the advantage of reducing the number of process upsets that must be examined. In a nuclear plant, the hazard has one location - the core a chemical plant, hazards have many locations. In a nuclear plant, the hazard is exposure to radiation and fission products in a... 

Tie actual HAZOP required a 1-week meeting and a 3-day meeting. During the first meeting, the 1 "HAZOPed" each section of the process, identified process upsets with significant... 

Transient - A passing disturbance that in itself is not damaging but the process upset may be damaging. 

To develop a safe design, it is necessary to first design and specify all equipment and systems in accordance with applicable codes and standards. Once the system is designed, a process safety shutdown system is specified to assure that potential hazards that can be detected by measuring process upsets are detected, and that appropriate safety actions (normally an automatic shutdown) are initiated. A hazards analysis is then normally undertaken to identify and mitigate potential hazards that could lead to fire, explosion, pollution, or injury to personnel and that cannot be detected as process upsets. Finally, a system of safety management is implemented to assure the system is operated and maintained in a safe manner by personnel who have received adequate training. 

Safety analysis concepts are discussed in this chapter by first describing a generalized hazard tree for a production facility. From this analysis, decisions can be made regarding devices that could be installed to monitor process upset conditions and to keep them from creating hazards. 

For each piece of equipment (process component), develop an FMEA by assuming in turn that each process upset that could become a potential source occurs. That is, assume a control failure, leak, or odier event leading to a process upset. 

The application of this procedure is best seen by performing an FMEA on a simple two-phase separator. Table 14-3 lists those process upsets that can be sensed before an undesirable event leading to a source of condition occurs. For overpressure, primary protection is provided by a high pressure sensor that shuts in the inlet (PSH). If this device fails, secondary protection is provided by a relief valve (PSV). 

Annunciator panels should be in a central location with alarm annunciators and shutdown annunciators grouped separately. The first alarm and the first shut-down normally sound a horn and are annunciated. This is called first-out indication. Subsequent shutdown or alarm signals received by the panel are either not annunciated or are annunciated in a different manner so that the operator can determine the initiating cause of the process upset. 

Design and maintain a system for process upset detection and shutdown—RP 14C. 

Improving maintenance helps reduce equipment failure. In addition, managing process changes can help avoid process upsets and thus reduce downtime. 

---