Gas holder

A gasometer, or gas-holder, is a large container where natural gas or town gas is stored near atmospheric pressure at ambient temperatures. The volume of the container follows the quantity of stored gas, with pressure coming from the weight of a movable cap. Typical volumes for large gasometers are about 50,000 m³, with 60 m diameter structures. Gasometers tend to be used for balancing purposes (making sure gas pipes can be operated within a safe range of pressures) rather than for actually storing gas for later use. Gas more recently was stored in large underground reservoirs such as salt caverns. Nowadays however line-packing is the preferred method.

Throughout the 1960s and 1970s it was thought that gasholders could be replaced with high pressure bullets. However, regulations brought in meant that all new bullets must be built several miles out of towns and cities and the security of storing large amounts of high pressure natural gas above ground made them unpopular with local people and councils. Bullets are gradually being decommissioned. It is also possible to store natural gas in a liquid form and is widely practiced throughout the world.

Gasholders hold a large advantage over other methods of storage. They are the only storage method which keeps the gas at district pressure. Once the District Low Pressure Switch falls and the booster fans come on, the gas in these holders can be at homes, being used in a very short space of time. Gas is stored in these throughout the day when little gas is being used. At about 5pm there is a great demand for gas and the holder will come down, supplying the district.

There are two basic types of gasholders, rigid waterless and telescoping. Rigid waterless gas holders were a very early design which outwardly showed no sign of expansion or contraction. Telescoping holders fall into two subcategories. The earlier of the telescoping variety were column guided variations and were built in Victorian times In order to guide the telescoping lifts they have an external fixed frame, visible at a fixed height at all times. Spiral guided gasholders were built in the UK up until 1983. These have no frame and each lift is guided by the one below, rotating as it goes up as dictated by helical runners. Both telescoping types use the manometric property of water in order to provide a seal.

Gasometers are often a major part of the skylines of low-rise British cities, due to their large distinctive shape and central location. The pollution associated with gasworks and gas storage makes the land difficult to reclaim for other purposes, but some gasometers, notably in Vienna, have been converted into living space and shopping mall.

Most British cities will have several gasholders. London, Birmingham, Manchester, Sheffield, Leeds, Newcastle and Glasgow (which has the largest gasometers in the UK [ cite web | title = National Grid Gasholder Demolition Case Study | url = http://www.nationalgridproperty.com/pdfs/Gasholder.pdf] ) are noted for having many gasholders. Some of these gasometers have become listed buildings. In the past holder stations would have an operator living on site controlling their movement. However with the process control systems now used on these sites, such an operator is obsolete.The tallest gasometer in europe is found in Rome, at ostience next to the Tevere river, the gasometer is 92 meters tall.

The term "gasometer" was originally coined by William Murdoch, the inventor of gas lighting, in the early 1800s. Despite the objections of his associates that his so-called "gazometer" was not a meter but a container, the name was retained and came into general use. The word is also used to describe a meter for measuring the amount of gas flowing through a particular pipe.

Dry Seal "Wiggins" Type Gasholder

A Dry-seal gasholder can be designed to have a gross (geometric) volume ranging from two hundred cubic metres (200 m³) up to one hundred and sixty-five thousand cubic metres (165,000 m³), whilst having a working pressure range between fifteen (15 mbar) and one hundred and fifty millibar (150 mbar). The Dry-seal gasholder is finished with an anti-corrosive treatment to counteract local climatic conditions and also any chemical attack from the stored medium. This anti-corrosive treatment is fully compatible with the sealing membrane and also the environment.

The Dry Seal Gasholder has four major elements:
* 1. The foundation
* 2. The main tank.
* 3. The piston
* 4. The sealing membrane.Each of these elements can be divided into various sub-elements and associated accessories.The foundationA concrete and hardcore base designed to withstand the weight of the steel gasholder structure constructedupon it and to withstand dynamic climatic conditions acting upon the gasholder etc.The main tankThe main tank is designed to accommodate the design requirements laid down by the customer and climaticconditionsThere are three main sub-elements to the tank:Tank bottomThe tank bottom forms a gas tight seal against the foundation and is “coned up” to facilitate drainageto the periphery.The bottom is covered with steel plates. The outer annular plates are butt welded against backingstrips, whilst the infill plates are lap welded on the top side only.Welded to the bottom infill plates is the:Piston support structureWhen the piston is depressurised it rests on a steel framework which is welded to the bottom plates.Tank shellThe shell of the tank is designed to accommodate the imposed loads and the general data suppliedby the client.The shell is of butt-welded design and is gas tight for approximately 40% of its lower vertical height(known as the gas space) at which point the seal angle is located. The remaining upper 60% (knownas the air space) of the shell has in it various apertures for access and ventilation.Attached to the shell are various accessories:

Staircase towerFor external access to the roof of the gasholder and also incorporates access to the inside of thegasholder via the shell access doors.A locked safety gate is usually located at the base of the staircase to prevent any unauthorisedaccess to the gasholder.Shell access doorsDoors located at pertinent points allowing access into the gasholder from the external staircasetower.Shell ventsAllow air to be displaced from the inside of the gasholder as the piston rises.Inlet nozzleThe connection nozzle allowing the stored gas to enter the gasholder from the supply gas main.Outlet nozzleFor the export of the stored gas, this nozzle comes complete with an anti-vacuum grid to protect thesealing membrane during depressurisation.Depending on the operational process the inlet & outlet nozzles maybe a shared connection.Shell drainsAllow condensates within the gasholder gas space to drain away in seal pots.The seal pots are designed to maintain the pressure with the gasholder.Shell manwaysUsed for maintenance access into the gas space – only used whilst the gasholder is out of service.Earthing bossesTo ensure that the gasholder is safe during electrical storms etc.Volume relief pipesEssential fail-safe system to protect the gasholder from over-pressurisation.Once actuated, by the piston fender, the volume relief valves allow the stored gas to escape toatmosphere at a safe height above the gasholder roof.As the volume relief valves open they actuate a limit switch.Volume relief limit switchesUsed to send signals to the control room to confirm the status of the volume relief valves.Level weight systemA mechanical counter balance system to ensure that the pistons moments are kept in equilibrium.The level weights which run up and down tracks located on the gasholder shell also actuated limitswitches to signal when the gasholder volume has reached pre-defined settings.Level weight limit switchesUsed to send signals to the control room to operate import and export valves etc.

Contents scale

On the gasholder shell is a painted scale displaying the volume of gas stored within the gasholder.An arrow painted on an adjacent level weight indicates the current status.Also painted on the scale is the location of the piston in relation to the shell access doors.Seal angleWelded to the inside of the shell this angular section is where the sealing membrane attaches to theshell.Tank roof

The roof is designed to withstand the local climatic conditions and the possibilities of additional loadssuch as snow and dust.The roof of the gasholder is of thrust rafter radial construction and has a covering of single sided lapwelded steel plates.The roof has various accessories attached including:Centre vent

Allows air to enter and exit the gasholder as the storage volume changes.Roof vents

Small nozzle around the periphery used for the installation of the seal.Roof manways

Allows access down to the piston fender when the gasholder is full.Circumferential handrailingSafety handrailing around the outside of the roof.Radial walkway

For access from the staircase to the centre vent etc.Volume relief valve actuatorsMechanical arms that operate the volume relief valves once the piston fender reaches a certain level.Level weight pulley structuresSteel structures mounting the level weight rope pulleys and rope separators.Load cell nozzles

For maintenance access to the load cell instrumentation used for volume recording purposes.Radar nozzles

For maintenance access to the radar instrumentation used for volume recording purposes and pistonlevel readings.Roof interior lighting nozzlesFor maintenance access to the gasholders interior lights.

The Piston

The gasholder piston moves up and down the inside of the shell as gas enters and exits the gasholder.The weight of the piston (less the weight of the level weights) produces the pressure at which the gasholderwill operate. The piston is designed to apply an equally distributed weight to ensure that the piston remainslevel at all times.The piston made up of the following sub-elements:

Piston deck

The outer annular area is formed from butt welded steel plates resting on steel section rest blocks.Lap welded steel infill plates form a dome profile to withstand the gas pressure in the gas spacebeneath it. For higher pressure gasholders the infill plates are lap welded on both sides, where as,low pressure gasholders are only welded on the top side.The fully welded piston deck forms a gas tight surface, which rests on the piston support structurewhen the gasholder is depressurised.The following ancillary items can be found on the piston deck:

Piston manway

Used for maintenance access below the piston into the gas space – only used whilst the gasholder isout of service.

Load cell chain receptacle

A receptacle for gathering up the load cell chains as the piston rises.Piston seal angleWelded to the outer top side of the annular plates, this angular section is where the sealingmembrane attaches to the piston.Level weight rope anchorsEqually spaced around the periphery of the piston deck are the connections to which the level weightropes are fixed.

Piston fender

The fender is a steel frame structure that is fixed to the piston deck annular plates and acts as asupport structure for the abutment plates.Access can be gained to the top of the piston fender from either the shell access doors or roofmanways depending on the gasholder volume.Attached to the piston fender are the following items:

Piston walkway

A platform around the top of the piston fender equipped with safety handrailing - used for inspectionpurposes.

Piston ladders

Rung ladders complete with safety loops for access to the piston deck from the piston walkway.Radar reflector platesUsed to bounce the radar signal back to the radar instrument for volume indication recording andpiston level readings.

Abutment plates

Fixed to the outside of the piston fender to form a circumferential surface for the sealing membrane toroll against whilst the piston moves during operation.Piston torsion ringAround the base of the piston fender is a torsion ring which helps keep the piston shape duringpressurisation.Concrete ballast can be added to the torsion ring to increase the weight of the piston andsubsequently be a cost effective way to increase the pressure of the gasholder to the required level.The sealing membraneThe seal of the gasholder is designed to operate in the conditions specified by the client and to suit the storedmedium.The seal rolls from the shell to the abutment surface of the piston and vice versa providing the piston with africtionless self-centring facility.During depressurisation the seal also provides a gas tight facility that protects the holder from vacuumdamage by blocking the gas outlet nozzle.During commissioning of the gasholder the sealing membrane is set into an operating condition. This settingmust be carried out every time the gasholder is depressurised, otherwise known as "popping" the seal.

See also

*Gasometer, Vienna

External links

* [http://news.bbc.co.uk/1/hi/uk/264609.stm Condemned: The great gasometer] from BBC News, January 28, 1999
* [http://www.gaswerk-augsburg.de/europa.htm Gasometer Augsburg in Germany and a list of many Gasometers in Europe]
* [http://www.gasometer.de/eng/index.asp Gasometer Oberhausen, Germany]
* [http://www.gasometer.org/en/ Gasometer Vienna, Austria]
* [http://www.gasometer.ch Gasometer Schlieren, Switzerland]
* [http://www.extrageographic.org/magazine/features/2008/080810_they_hold_gas.html The rise and fall of gasometers] Extrageographic magazine
* [http://www.gasholders.co.uk Modern Low Pressure Dry Seal Type Gasholders]

References