Slip plates

It was not customary in the company concerned to isolate equipment under repair by slip-plates, only by closed valves. But after the fire, the company introduced the following rules ... 

The valve was operated by compressed air, and the two air hoses, one to open the valve and one to close it, were connected up the wrong way around. The two connectors should have been different in size or design so that this could not occur. In addition, they were not disconnected, and a lockout device on the valve—a mechanical stop—had been removed. It is also bad practice to carry out work on equipment isolated from hot flammable gas under pressure by a single isolation valve. The take-off branch should have been slip-plated, and double block and bleed valves should have been provided so the slip-plate could be inserted safely (Figure 1-1), [16, 17]. 

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. 

Isolations should not be removed until maintenance is complete. It is good practice to issue three work permits—one for inserting slip-plates (or disconnecting pipework), one for the main job, and one for removing slip-plates (or restoring disconnections). 

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 explosion could have been prevented by isolating the reactor by slip-plates or physical disconnection. This incident and the others described show that valves are not good enough. 

A mechanic was affected by fumes while working on a steam drum. One of the steam lines from the drum was used for stripping a process column operating at a gauge pressure of 30 psi (2 bar). A valve on the line to the column was closed, but the line was not slip-plated. When the steam pressure was blown off, vapors from the column came back through the leaking valve into the steam lines (Figure 1-3). 

The company concerned normally used slip-plates to isolate equipment under repair. On this occasion, no slip-plate was fitted because it was only a steam line. However, steam and other service lines in plant areas are easily contaminated by process materials, especially when there is a direct connection to process equipment. In these cases, the equipment under repair should be positively isolated by slip-plating or disconnection before maintenance. 

While a plant was on line, an operator noticed a slip-plate on a tank vent. The slip-plate had been fitted to isolate the tank from the blowdown system while the tank was under maintenance. When the maintenance was complete, the slip-plate was overlooked. Fortunately, the tank, an old one, was stronger than it needed to be for the duty, or it would have burst. 

If a vessel has to be isolated from the vent or blowdown line, do not slip-plate it off, but whenever possible, disconnect it and leave the vessel vented to atmosphere (as shown in Figure I -4). 

If a vent line has to be slip-plated because the line is too rigid to be moved, then the vents should be slip-plated last and de-slip-plated first. If all slip-plates inserted are listed on a register, they are less likely to be overlooked. 

Figure 1-5. A slip-plate left in position for many months had corroded right...

Figure 1-6. A slip-plate bowed by a gauge pressure of 470 psi (32 bar)...

Slip-plates should normally be designed to withstand the same pressure as the piping. However, in some older plants that have not been designed to take full-thickness slip-plates, it may be impossible to insert them. A compromise will be necessary. 

If you are not convinced that all isolation valves should be backed up by slip-plates before maintenance takes place, at least back up valves on lines containing materials that might turn solid and then melt. 

When possible, drain points in a pipeline should be fitted at low points, and slip-plates should be fitted at high points. 

Even though equipment is isolated by slip-plates and the pressure has been blown off through valves or by cracking a joint, pressure may still be trapped elsewhere in the equipment, as the following incidents show ... 

A welder w as constructing a new pipeline in a pipe trench, while 20 m away a slip-plate was being removed from another pipe, which had contained light oil. Although the pipe had been blown with nitrogen, it was realized that a small amount of the oil would probably spill when the joint was broken. But it w as believed that the vapor would not spread to the welders. Unfortunately, the pipe trench was flooded after heavy rain, and the oil spread across the water surface and was ignited by the welder s torch. One of the men working on the slip-plate 20 m away was badly burned and later died. 

Why was a permit issued to remove a slip-plate 20 m away from a welding Job Although vapor should not normally spread this far, the two Jobs were rather close together. 

The foremen who issued the two permits were primarily responsible for operating a unit some distance away. As they were busy with the running plant, they did not visit the pipe trench as often as they might. Had they visited it immediately before allowing the de-slip-plating Job to start, they would have realized that the tw o Jobs were close together. They might have realized that oil would spread across the water in the trench. 

An 80 m3 tank fiberglass-reinforced plastic acid tank was blown apart at the base as the result of overpressure. The vent had been slip-plated so the tank could be entered for inspection. The steel slip-plate was covered with a corrosion-resistant sheet of polytetrafluoroethylene. Afterward, when the slip-plate was removed, the sheet was left behind. This did not matter at the time, as the tank was also vented through an oveiflow line,... 

Flare lines should never be slip-plated with ordinary slip-plates because they may be left in position in error. The sealing plate cannot be left in position when the relief valve is replaced. 

The inlet valve was leaking, and during the two minutes that elapsed after the slip-plate was removed, enough fuel gas for an explosion leaked into the furnace. (Suppose the leak was equivalent to a 1.6-mm [ /(An.] diameter hole, and the gauge pressure of the fuel gas was 0.34 bar [5 psi]. The calculation shows that 80 L [3 ft-] of gas entered the furnace in two minutes. If this bunted in... 

The correct way to light a furnace (hot or cold) that bums gas or burns light oil is to start with a positive isolation, such as a slip-plate, in the fuel line. Other positive isolations are disconnected hoses, lutes filled with water (if the fuel is gas at low pressure), and double block and bleed valves closed valves without a bleed are not sufficient. Then ... 

Before entry is allowed into a vessel or other confined space, the vessel should be isolated from sources of hazardous material by slip-plating or physically disconnecting all pipelines and by isolating all supplies of electricity, preferably by disconnecting the cables. On the whole, these precautions seem to be followed. Accidents as the result of a failure to isolate are less common than those resulting from a failure to remove hazardous materials or from their deliberate reintroduction as described in Sections 11.1 and 11.2. However, the following are typical of the accidents that have occurred. 

The welders should have known that they should never enter a vessel until they or their foreman has checked that the vessel is isolated. by slip-plates or disconnection, and has accepted (and signed) an entry permit. Power supplies should have been disconnected, or if merely locked off, each person entering should have added his or her personal lock and kept the key. It is hard to believe that this was the first time normal good practice had not been followed. The management system was at fault. 

Once a vessel has been connected up to any process or service line, the full permit-to-work and entry procedure should be followed. In the present case, this should have started eight weeks before the incident. And the nitrogen line should have been disconnected or slip-plated where it entered the vessel. 

The incident also shows the importance of placing spades as near as possible to the equipment that is to be isolated, particularly when a vessel is to be entered. Valves should not be left between a slip-plate and the vessel, as liquid can then be trapped between the valve and the slip-plate and enter the vessel if the valve leaks or is opened. 

If the possibility of reverse flow had been foreseen, then a slip-plate could have been inserted in the line leading to the ammonia storage vessel, as described in Section 1.1. 

If a vessel is being prepared for entry, the computer could check that the number of slip-plates (blinds) to be fitted (or pipes disconnected) is the same as the number of connections shown on the drawing. 

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