Gas pipework

It is additionally recommended in the British Gas publication Guidance Notes on the Installation of Gas Pipework, Boosters and Compressors in Customers Premises (lM/16) that for buildings containing plant over 2 MW total heat input and being supplied with gas at pressures above 1 bar, a remotely operable valve shall be fitted in the gas supply to the building. In the case of large boiler houses, provision for remote operation of the valves shall be provided both inside and outside the building. 

Gas pipework in a user s premises serves the function of transporting the gas from the meter to the point of use in a safe way and without incurring an avoidable pressure loss. For low-pressure installations, the permitted pressure loss is only 1 mbar from the meter to the plant manual isolating valve at maximum flow rate. The pipework must be sized adequately to allow for this. Boosters are sometimes used to overcome pressure losses, but the use of a booster should never be considered a satisfactory substitute for correct design of pipe sizes. Where gas is available at higher pressures it may be permissible to tolerate pressure losses of more than 1 mbar. 

Materials used for gas pipework will depend on application, location, environment and operating pressure. General guidelines are as below. 

Attention is drawn to Regulation 4(2) of the Gas Safety (Installation and Use) Regulations, which prohibits the use of non-metallic materials for gas pipework other than for small portable appliances such as Bunsen burners and lighting torches. 

PRESSURE SYSTEM Defined in the Pressure System Safety Regulations 2000 as a system containing one or more pressure vessels of rigid construction, any associated pipework and protective devices the pipework with its protective devices to which a transportable gas container is, or is intended to be, connected or a pipeline and its protective devices which contains or is liable to contain a relevant fluid, but does not include a transportable gas container. Here relevant fluid is steam any fluid or mixture of fluids which is at a pressure of >0.5 bar above atmospheric pressure, and which fluid or a mixture of fluids is a gas, or a liquid which would have a vapour pressure of >0.5 bar above atmospheric pressure when in equilibrium with its vapour at either tlie actual temperature of the liquid or 17.5°C or a gas dissolved under pressure in a solvent contained in a porous substance at ambient temperamre and which could be released from the solvent with the application of heat. 

In addition, they are usually constructed without isolation valves on the fuel supply lines. As a result the final connection in the pipework cannot be leak-tested. In practice, it is tested as far as possible at the manufacturer s works but often not leak-tested on-site. Reference 32 reviews the fuel leaks that have occurred, including a major explosion at a CCGT plant in England in 1996 due to the explosion of a leak of naphtha from a pipe joint. One man was seriously injured, and a 600-m chamber was lifted off its foundations. The reference also reviews the precautions that should be taken. They include. selecting a site where noise reduction is not required or can be achieved w ithout enclosure. If enclosure is essential, then a high ventilation rate is needed it is often designed to keep the turbine cool and is far too low to disperse gas leaks. Care must be taken to avoid stagnant pockets. 

Water can be trapped behind heat exchanger baffles and then suddenly vaporized by circulation of hot oil. It can also be trapped in dead-ends and U-bends in pipework (see Section 9.1.1). Such U-bends can form when one end of a horizontal pipe is raised by thermal expansion. The trays in a distillation column were damaged during startup when hot gas met water, from previous steaming, dripping down the column [3J. Section 17.12 describes an incident somewhat similar to a foamover. 

Military/government type publication. It lists accidents with fatality scenarios that occurred during operation and maintenance of U.S. interstate gas pipelines from 1950 to 1965. Also listed are individual pipework failures during that time, about 3000 entries, that have been compiled from various sources by the study committee. 

Performance of Pipework in the British Sector of the North Sea Offshore Oil and Natural Gas Failure rates based on 27 actual incidents from UK DOE reports Offshore oil. gas. and process fluid submarine pipelines within the UK Continental Shelf 109. 

The slam-shut valve cuts off the gas supply in the event of predetermined pressure criteria being exceeded. It must be manually reset, having made safe the downstream pipework and the cause of the abnormal pressure removed. 

Gas can feed a fire caused by other sources. The integrity of pipework should withstand an external fire for sufficient time for an external emergency control valve to be operated. 

After testing for soundness it will be necessary to safely introduce gas into the pipework displacing the air or inert gas that is in it. Similarly, if pipework is decommissioned for any reason fuel gas must be displaced by air or inert gas. This is a requirement of the Gas Safety (Installation and Use) Regulations, Regulation 21. Guidance on recommended procedures is given in the British Gas publication Purging Procedures for Non-Domestic Gas Installations (IM/2). 

This is to be used only for buried pipework or in ducts, as it is adversely affected by ultraviolet radiation and its use inside buildings is prohibited by the Gas Safety (Installation and Use) Regulations. For pressures up to 4 bar. 

Pipework should be easily identifiable in accordance with BS 1719. Where the normal pressure in the pipe exceeds 75mbar, consideration should be given to labeling the pipe with the normal operating pressure. In buildings in which there are no other piped flammable gas supplies it is sufficient to paint the pipe yellow ochre or to band it with appropriately colored adhesive tape. In large complex installations, it is desirable to identify pipe contents more precisely, and in those instances, the base color should be supplemented with a secondary code band of yellow and/or its name or chemical symbol. 

It is essential when designing the pipe layout for gas distribution that unavoidable pressure losses are not incurred. For low-pressure gas, the pressure available at the meter inlet will be only 21 mbar, and the allowable pressure loss to the point of use only 1 mbar, although higher pressures may be available in some circumstances. If such a low-pressure loss is not to be exceeded it is essential that the pipework be sized correctly. It is preferable to oversize pipework rather than undersize, particularly as this allows... 

This equation can be used for an orifice plate introduced into pipework as a measuring, throttling or balancing device, and for a jet discharging gas into the injector of a burner at atmospheric or sub-atmospheric pressure. 

There is an alternative to using neat butane vapor, however, which overcomes the need for pipework heating. This is to use a gas-air mixture. A special gas-air mixer is used which ensures that a preset ratio is maintained at all demand rates. The ratio chosen must be well outside the flammable limits. A typical LPG-air-mixing plant is illustrated by Figure 20.7. 

Consumer installations which supply LPG to more than one user (e.g. metered estates, holiday home parks, caravan sites) require the gas supplier (i.e. the site owner/operator, not the LPG supplier) to obtain prior consent from the Office of Gas Supply (Ofgas) and, for the necessary pipework installation, from the Department of Energy, Pipelines Inspectorate. 

Perhaps the most widely known measurement technique is that adopted by the West German Gas Industry and developed by Steinrath for buried pipework. This assigns a value (See Table 2.20) to each parameter measured the summation of these values determines the corrosivity of the soil. The parameters measured are shown in Table 2.20. Although this technique was developed for the pipeline industry it can be used with some success for general soil corrosivity assessment. 

Moist or dry gas use backed carbon, graphite. High pressure work in heavy black iron pipework. High pressure Monel or aluminium iron bronze valves... 

An explosion and fire occurred in the pipework of a vessel in which dilute butadiene was stored under an inert atmosphere, generated by the combustion of fuel gas in a limited air supply. The inert gas, which contained up to 1.8% of oxygen and traces of oxides of nitrogen, reacted in the vapour phase over an extended period to produce concentrations of gummy material containing up to 64% of butadiene peroxide and 4.2% of a butadiene-nitrogen oxide complex. The deposits eventually decomposed explosively. 

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