Trapped pressure

Trapped pressure is a familiar hazard in maintenance operations and is discussed in Section 1.3.6. Here we discuss accidents that have occurred as a result of process operation. 

Every day, in every plant, equipment that has been under joressure is opened up. This is normally done under a work permit. One man prepares the job, and another opens up the vessel. And it is normally done by slackening bolts so that any pressure present will be detected before it can cause any damage—provided the Joint is broken in the correct way, described in Section 1.5.1. 

Several fatal or serious accidents have occurred when one man has carried out the whole Job—preparation and opening up—and has used a quick-release fastening instead of nuts and bolts. One incident, involving a tank truck, is described in Section 13.5. Here is another  

A suspended catalyst was removed from a process stream in a pressure filter. After filtration was complete, the remaining liquid was blown out of the filter with steam at a gauge pressure of 30 psi (2 bar). The pressure in the filter was blown off through a vent valve, and the fall in pressure was observ ed on a pressure gauge. The operator then opened the filter for cleaning. The filter door was held closed by eight radial bars, which fitted into U-bolts on the filter body. The bars were withdrawn from the U-bolts by turning a lai ge wheel, fixed to the door. The door could then be withdrawn. 

One day an operator started to open the door before blowing off the pressure. As soon as he opened it a little, it blew open and he was crushed between the door and part of the stiucture and was killed instantly. 

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Bolted joints are friendlier than quick-release couplings. The former are usually dismantled by a fitter after issue of a permit to work. One person prepares the equipment and another person opens it up the issue of the permit provides an opportunity to check that the correct precautions have been taken. In addition, if the joints are unbolted correctly, any trapped pressure is immediately apparent and the joint can be remade or the pressure allowed to blow off. In contrast, many accidents have occurred because operators opened up equipment which was under pressure, without independent consideration of the hazards, using quick-release couphngs. There are, however, designs of quick-release couphngs which give the operator a second chance. 

A blowdown valve relieves trapped pressure when the compressor is shut down due to a malfunction or for maintenance. The flowsheet shows an automatic blowdown valve. 

In all three cases the lines were correctly isolated from operating equipment. Work permits specified that goggles should be worn and stated, Beware of trapped pressure. ... 

Other incidents due to trapped pressure and clearing chokes are described in Sections 17.1 and 17.2. 

Before the exchanger left the plant, the free tubes v. ere cleaned with high-pressure water jets. The plugged tubes were opened up by drilling >s-in. holes through the plugs to relieve any trapped pressure. But these holes were not big enough to allow the tubes to be cleaned. 

A permit issued for work to be carried out on an acid line stated that goggles must be worn. Although the line had been drained, there might have been some trapped pressure. The man doing the job did not wear goggles and was splashed in the eye. 

After a predetermined pressure drop, which is referred to as blowdown, the valve closes with a positive action by trapping pressurized air on top of the disk holder. The pressure drop is adjusted by raising or lowering the blowdown ring. Raising the ring increases the pressure drop while lowering it decreases the drop. 

Maintenance errors. One common error is the failure to remove an isolation blind in a vent line when returning a vessel to service. Even a thin sheet of plastic placed over an open nozzle may be sufficient to allow a vessel-damaging vacuum to De produced (BP, Hazards of Trapped Pressure and Vacuum, 2003). 

Lees (Loss Prevention in the Process Industries, 2d ed., Butter-worths, London, 1996), BP (Hazards of Trapped Pressure and Vacuum, 2003), and Kletz (What Went Wrong —Case Histories of Process Plant Disaster, Gulf Publishing Company, 1989) include additional case histories providing valuable lessons about how equipment failures and human errors can combine to inflict vacuum damage. 

When pumps of this type are stopped then air must be admitted to the pump inlet. Should this step in vacuum procedure be overlooked, the oil will seep through the lubricating channels and be forced into the vacuum system by atmospheric pressure, flooding the traps, pressure gauges and all the evacuated space. 

The quartz absorption cell the authors used was 10 cm. long. Because this cell was in direct communication with the purification system, the purified ozone was vaporized directly into it from the sample trap. Pressure measurements were made with a sulfuric acid manometer, by a calibrated expansion method, or with a mercury manometer, depending on the pressure desired. For each preparation of purified condensed ozone, an analysis was made of the purity by conventional chemical methods. For the several preparations analyzed, the purity ranged from 90 to 95% with an average of 92%. 

Trapping pressures. A common observation in fluid inclusion studies is that inelusions reeord maximum trapping pressures equivalent to depths that are mueh shallower than the souree region of the xenoliths (Roedder 1984 Andersen and Neumann... 

We believe that it is likely that a failed caprock will remain close to fracturing conditions (cf. Figs 4 7) in a subsiding basin and that these reservoirs for that reason are unable to build up noticeable new petroleum columns. Our few data points on trap pressures in Halten Vest do suggest this to be the case (Fig. 4). 

For the same strength parameters as previous, the unpressurized small dome is stable, but may be brought close to instability by steady core pressures of the order of 5 MPa. Minor augmentation by uniform rind-trapped pressures of the order of 0.2 MPa is sufficient to promote low-angle failure to as shallow as 40° and remove about 20% of the edifice on one side. Alternatively, if effusive gas discharge is flux, rather than overpressure-controlled, a three-order of magnitude (x5(X)) reduction in permeability in a rind of 5 m will... 

Disposable microprizm and photodiode were placed on a rotary table (10 arc sec resolution). Before depositing metal layers, the glass surface of the microprizm was treated with a chromium mkture, rinsed in bi-distilled water and then dried in a dust-fiee airflow. Au layer (thickness 45 run) was deposited onto the quartz surface of microprizm in special vacuum sets equipped with oil pumps and nitrogen traps (pressure 10 Pa). Before deposition, the mikroprizm surface was ion-bombarded in a vacuum (pressure 1 Pa). In order to increase the chip stability in the aqueous environment, Au film was modified by covalent binding of the thiol contained 12-carbohydrate spacer. 

When opening any joint, pipeline or plant equipment do it carefully remember, there may still be trapped pressure or dangerous materials in a line. 

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