Isolation for maintenance

Some Miscellaneous Incidents Involving Isolation for Maintenance... 

Provision for isolation for maintenance. Where it is necessary to carry out maintenance on a live system, valves must be lockable and may need to be installed in tandem. 

During the Design Review process a key requirement is to evaluate the system design against the requirements for ongoing use and maintenance. For example, if LIMS utilizes analytical equipment interfaces which must be isolated for maintenance, LIMS must be designed to allow for maintenance activities without there being any effect on the operation of the rest of the system and any impact on the use of LIMS. 

The transmission of power around the cell room once it is at the appropriate voltage and current also requires thought. The current is transmitted along bus-bars, normally constructed from copper or aluminium because of their exceptional conductivity, and the dimensions of the bus-bars are carefully calculated because of the considerable investment in the metal. Moreover, the connectors to the cells must be designed to avoid energy loss even at the very high currents and there must be switches which permit individual cells to be isolated for maintenance without disturbing the rest of the cell house. 

The moderator system includes two 100% capacity pumps, two 50% flow capacity heat exchangers cooled by recirculated light water, and a number of control and check valves. Connections are provided for the purification, liquid poison addition, D2O collection, supply, and sampling systems. The series/parallel arrangement of the moderator system lines and valves permits the output from either pump to be cooled by both of the heat exchangers and ensures an acceptable level of moderator cooling when either of the two pumps is isolated for maintenance. Reactor power must be reduced to about 60% if one moderator heat exchanger is isolated. The primary functions of the system are to... 

Pump A had been isolated for maintenance. The maintenance team on day shift had removed a pressure relief valve, and the open flange where the relief valve was normally attached had had a temporary cover fitted which was neither leak-tested nor pressure-tested. A Permit to Work was stiU in force for the job, and... 

Figure 17.9 represents a much more hazardous situation. Vessel A is in use while Vessel B is blocked in and positively isolated for maintenance work—say to change out a catalyst bed. Valve 4 is closed in order to protect workers from a release from Vessel A. If an external fire occurs near Vessel B, it may be overpressured because it is isolated from the relief system. In such situations, Vessel B must be provided with another pressure relief mechanism or the vessel should be open to the atmosphere. 

An isolator, such as at locations C and Ct in Figure 13.29, may sometimes be used to isolate the circuits it is feeding, say, for maintenance or repairs. This isolator is simply a switch and provides no protection to the circuits. For a fault on the outgoing side, the individual outgoing feeders must have their own protection. Fora severe fault elsewhere in the system, there must be a protective feeder closeby, in the upper stream. 

Tsnlatinn Valves - Isolation valves in the individual PR valve outlet lines should be provided to permit safe removal of one PR valve for maintenance during plant operation. 

Isolation Valves for Pressure Relief Systems - Block valves for maintenance isolation purposes are permissible in pressure relieving systems, provided that they are car sealed open and comply with the requirements of CSO valves. The particular locations where such CSO valves are p>ermitted are ... 

The following pages describe accidents that occurred because equipment was not adequately prepared for maintenance. Sometimes the equipment was not isolated from hazardous materials sometimes it was not identified correctly, and so the wrong equipment was opened up sometimes hazardous materials were not removed [1, 2]. ... 

An ethylene compressor was shut down for maintenance and correctly isolated by slip-plates. When repairs were complete, the slip-plates were removed before the machine was tried out. During the tryout, some ethylene leaked through the closed isolation valves into the machine. The ethylene/air mixture was ignited, either by a hot spot in the machine or by copper acetylide on the copper valve gaskets. The compressor was severely damaged. 

A reactor was prepared for maintenance and washed out. No welding needed to be done, and no entry was required, so it was decided not to slip-plate off the reactor but to rely on valve isolations. Some flammable vapor leaked through the closed valves into the reactor and was ignited by a high-speed abrasive wheel, which was being used to cut through one of the pipelines attached to the vessel. The reactor head was blown off and killed two men. It was estimated that 7 kg of hydrocarbon vapor could have caused the explosion. 

The incidents described could occur in many different types of plants and are therefore of widespread interest. Some of them illustrate the hazards involved in activities such as preparing equipment for maintenance and modifying plants. Others illustrate the hazards associated with widely used equipment, such as storage tanks and hoses, and with that universal component of all plants and processes people. Other incidents illustrate the need for techniques, such as hazard and operability studies, and protective devices, such as emergency isolation valves. 

Figure 4.2 shows an example HTA for the task of isolating a level transmitter for maintenance. Redescribing operations into more detailed plans and suboperations should only be undertaken where necessary, otherwise a great deal of time and effort is wasted. Since the description is hierarchical the analyst can either leave the description in general terms or take it to greater levels of detail, as required by the analysis. 

As with the single bus arrangement, each connection is linked to the main bus through a circuit breaker, but the breaker may be bypassed using disconnect switches through a transfer bus and another breaker to the main bus. This permits isolation of the circuit breaker for maintenance without loss of service to the connection. 

With shell boilers, economizers will generally only be fitted to boilers using natural gas as the main fuel and then only on larger units. It would be unlikely that a reasonable economic case could be made for boilers of less than 4000kg/hF and A100°C evaporative capacity. The economizer will incorporate a flue gas bypass with isolating dampers to cover for periods when oil is used and for maintenance. The dampers require electric interlocks to the selected fuel. 

When a boiler may not be shut down for maintenance of the level control chambers isolating valves can be fitted between the water-level control and the steam space. In this instance, the valves must be capable of being locked in the open position and the key retained by a responsible person. When these valves are closed during maintenance periods the boiler must be under manual attendance. Fitting of these valves should only be with the agreement of the insurance company responsible for the boiler. Drains from the water-level controls and level gauges should be collected at a manifold or sealed tundish before running to the blowdown vessel. 

Equipment that needs to be taken out of service for maintenance will normally have an isolation valve on each side. This will include pumps, compressors and control valves. 

Figure 6.4 Experimental set-up for maintenance and monitoring of the isolated perfused lung [139].

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