Trench piping

CONFINED SPACE A spacc which is substantially, although not always entirely, enclosed and where there is a reasonably foreseeable risk of serious injury from hazardous substances or conditions within the space or nearby. The risks may include flammable substances oxygen deficiency or enrichment toxic gases, fume or vapour ingress or presence of liquids free-flowing solids presence of excessive heat. For the purpose of the Confined Spaces Regulations 1997 a confined space means any place, including any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk. 

A "pipe trench" is an installation of process and utility piping on supports within a below grade, open excavated trench. Pipe trenches may provide some protection from damage due to explosion overpressures or explosion fragments or missiles. 

Processing facilities have experienced several serious pipe trench fires. Contributing to the size of the fires were inadequate or plugged drains, lack of isolation valves, pipelines on the ground, or inadequate fire stops along the length of the trenches. Pipe trench fires can result in significant business interruption. 

The presence of a trench/pipe system in Option B, used to remove rain accumulations, increases the risk that contaminated fire water might be inadvertently discharged into the sanitary sewer system. In order to reduce this risk, it is necessary to install a motorized valve in the drainage piping. This valve could be arranged to close automatically in the event of a fire through a signal from the fire alarm panel and manually as needed. It may also be desirable to use a valve which will automatically close in the event of loss of power. Alternatively, this valve could be normally closed and opened when needed. 

Examples of a confined space include manholes, sewers, tunnels, excavations, storage tanks, holds of ships, pits, trenches, ducts, some unventilated areas or rooms within buildings (particularly below ground level), boilers, combustion chambers in furnaces, chambers, vats, silos, pits, trenches, pipes, flues and wells. 

Confined space - means any place, including any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or similar space in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk. 

There are two basics methods for installing plastic pipe in trenches. Pipe with a large diameter is usually joined above ground and then lowered into a narrow trench. Smaller pipe, usually pipe with a diameter up to 8 inches, is often joined in a trench. This requires a much wider trench and it means that installers must be in the trench. When this is the case, appropriate safety procedures must be employed to protect the workers from trench failure and cave-ins. Before we get into deep details, let s discuss the basics of an underground installation. 

When a wide trench is used, with workers in the trench, pipe is often passed from workers above the trench to workers in the trench. Pipe should never be rolled or tossed into a trench. Occasionally, pipe is lowered into this type of workplace with equipment, such as a backhoe. In all cases, the pipe should be handled carefully and laid in the trench gently. 

Enter the applicable letter code for the receiving stream or water body from Section 3.10 of Part I of the form. Also, enter the total annual amount of the chemical released from all discharge points at the facility to each receiving stream or water body. Include process outfalls such as pipes and open trenches, releases from on-site wastewater treatment systems, and the contribution from stormwater runoff, if applicable (see instructions for column C below). Do not include discharges to a POTW or other off-site wastewater treatment facilities in this section. These off-sife transfers must be reported in Pari III, Section 6 of the form. 

Absorption Field A system of properly sized and constructed narrow trenches partially filled with a bed of washed gravel or crushed stone into which perforated or open joint pipe is placed. The discharge from the septic tank is distributed through these pipes into trenches and surrounding soil. While seepage pits normally require less land area to install, they should be used only where absorption fields are not suitable and well-water supplies are not endangered. 

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. 

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. 

In another incident a backhoe ruptured a 3-in. polyethylene natural gas pipeline fortunately the gas did not ignite. The drawings were complex and cluttered, and the contractor overlooked the pipeline. A metal detector was not used. This would have detected the pipe as a metal wire was fixed to it, a good practice. In a third incident a worker was hand-digging a trench, as an electric conduit was believed to be present. It was actually an old transfer line for radioactive waste, and he received a small dose of radioactivity. The planner had misread the drawing. 

Process workers often complain that valves are inaccessible. Emergency valves should always be readily accessible but other valves, if they are operated, say, once a year or less often, can be out of reach. It is reasonable to expect workers to get a ladder or scramble into a pipe trench at this frequency. Designers should remember that if a valve is just within reach of an average person then half of the population cannot reach it. Equipment should be placed such that at least 95% of the population can reach it. Guidance on specific measurements to achieve this objective is available in a number of standard human factors textbooks (see Bibliography). 

Figure4.9 Standard pipe installations for water sluice valve up to 10 inches or 250 millimeter diameter. Notes (1) If a valve extension spindle is required, a galvanized centering support must be provided. (2) Flanged valves with appropriate adapters must be used. (3) Unless otherwise directed, pre-cast concrete units shall be bedded on well-compacted granular material Type A brought up from the base of the trench...

Figure4.10 Standard pipe installations for a double air valve. Note Unless otherwise directed, pre-cast concrete units shall be bedded on well-compacted granular material Type A brought up from the base of the trench.

Discharge pipes can be located in trenches covered by floor plants, and there is no technical reason against laying the pipes directly on the ground, but provision must be made for drainage. 

Soil resistivity surveys are often impractical in built-up areas, but in such areas impressed-current cathodic protection is usually avoided on account of the danger of interaction. Under such conditions adequate protection can be achieved by installing magnesium anodes in the pipe trench should the soil resistivity measurements made when the trench is opened indicate that this is necessary. 

FBE-coated pipe requires careful handling from factory to the pipe trench to avoid mechanical damage. Repairs are undertaken with either trowel or brush-applied, liquid two-pack epoxy resin-based paints or by melt sticks of compressed powder. 

Backfill the soil replaced over the pipe in the trench (general connotation). In cathodic protection, special backfills are packed around the anodes. These backfills are selected to lower circuit resistance of the anode for sacrificial anodes a gypsum/bentonite mixture is used, and for impressed-current anodes, coke breeze. 

Line Wrapping the technique of wrapping a pipeline over the pipeline trench applied only to welded steel pipelines. It is carried out by a linewrapping machine which travels along the pipe, cleaning it, priming it. 

TWo additional observations can be made. First, because a rigid pipe transmits almost all the load of the earth cover to the trench bed, someone will occasionally be heard to say that rigid pipe, such as concrete pipe, does not require side support. This is not true. Second, because of the difference in the ways rigid and flexible conduit distribute the load of their earth cover, flexible piping materials are often said to require less bedding bearing strength, because they impose less of a load... 

In terms of dead loads, the shape of the trench in which the pipe will be buried is also a factor. Generally speaking, a narrow trench with vertical sidewalls will impose less of a load on the pipe than will a wider trench with sloping side walls. It is necessary also to know the modulus of soil reaction (E), which is dependent on the type or classification of the native soil, the backfill material that is contemplated, and the desired consolidation of the backfill material. Soil consolidation is important, because it contributes to the strength of a flexible conduit in a buried pipe system. 

Therefore, before a final wall structure can be selected, it is necessary to conduct a combined strain analysis in both the longitudinal and hoop directions. This analysis will consider thermal contraction strains, the internal pressure, and the pipe s ability to bridge soft spots in the trench s bedding. In order to do this we must know more about the inherent properties of the material we are dealing with that is a structure made up of successive layers of continuous filament-wound fiberglass strands embedded within a plastic matrix. We must know the modulus of the material in the longitudinal direction and the... 

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