A gas holder (commonly known as a gasometer, sometimes also gas bell, though that term applies to the gas holding envelope alone) is a large container in which 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 gasholders are about 50,000 cubic metres, with 60 metre diameter structures. Gasholders tend to be used nowadays 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.
There are two basic types of gasholder — rigid waterless and telescoping. Rigid waterless gas holders were a very early design which showed no sign of expansion or contraction. There are modern versions of the waterless gas holder, e.g. oil-sealed, grease-sealed and "dry seal" (membrane) types.
Telescoping holders fall into two subcategories. The earlier of the telescoping variety were column guided variations and were built in Victorian times. To guide the telescoping walls, or "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 to provide a seal. The whole tank floats in a circular or annular water reservoir, held up by the roughly constant pressure of a varying volume of gas, the pressure determined by the weight of the structure, and the water providing the seal for the gas within the moving walls. Besides storing the gas, the tank's design serves to establish the pressure of the gas system. With telescoping (multiple lift) tanks, the innermost tank has a ~1 ft wide by 2 ft high lip around the outside of the bottom edge, called a cup, which picks up water as it rises above the reservoir water level. This immediately engages a downward lip on the inner rim of the next outer lift, called a grip, and as this grip sinks into the cup, it preserves the water seal as the inner tank continues to rise until the grip grounds on the cup, whereupon further injection of gas will start to raise that lift as well. Holders were built with as many as four lifts.
Gasometer at Bernau bei Berlin Germany
Origin of the name "gasometer"
The term gasometer was originally coined by William Murdoch, the inventor of gas lighting, in the early 19th century. 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 gas meter (a meter for measuring the amount of gas flowing through a particular pipe). The term "gasometer" is discouraged for use in technical circles, where the term "gasholder" is preferred.
Dry-seal Wiggins type gasholder
A dry-seal gasholder can be designed to have a gross (geometric) volume ranging from 200 to 165,000 m3 (7,100 to 5,826,900 cu ft), whilst having a working pressure range between 15 and 150 millibars (1.5 and 15.0 kPa). 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: the foundation; the main tank; the piston and the sealing membrane. Each of these elements can be divided into various sub-elements and associated accessories.
A concrete and hardcore base designed to withstand the weight of the steel gasholder structure constructed upon it and to withstand dynamic climatic conditions acting upon the gasholder etc.
The main tank is designed to accommodate the design requirements laid down by the customer and climatic conditions There are three main sub-elements to the tank:
- Tank bottom
- The tank bottom forms a gas tight seal against the foundation and is "coned up" to facilitate drainage to the periphery. The bottom is covered with steel plates. The outer annular plates are butt welded against backing strips, whilst the infill plates are lap welded on the top side only. Welded to the bottom infill plates is the:
- Piston support structure
- When the piston is depressurised it rests on a steel framework which is welded to the bottom plates.
- Tank shell
- The shell of the tank is designed to accommodate the imposed loads and the general data supplied by 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% (known as the air space) of the shell has in it various apertures for access and ventilation. Attached to the shell are various accessories:
- Staircase tower
- For external access to the roof of the gasholder and also incorporates access to the inside of the gasholder via the shell access doors. A locked safety gate is usually located at the base of the staircase to prevent any unauthorised access to the gasholder.
- Shell access doors
- Doors located at pertinent points allowing access into the gasholder from the external staircase tower.
- Shell vents
- Allow air to be displaced from the inside of the gasholder as the piston rises.
- Inlet nozzle
- The connection nozzle allowing the stored gas to enter the gasholder from the supply gas main.
- Outlet nozzle
- For the export of the stored gas, this nozzle comes complete with an anti-vacuum grid to protect the sealing membrane during depressurisation. Depending on the operational process the inlet and outlet nozzles maybe a shared connection.
- Shell drains
- Allow 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 manways
- Used for maintenance access into the gas space – only used whilst the gasholder is out of service.
- Earthing bosses
- To ensure that the gasholder is safe during electrical storms etc.
- Volume relief pipes
- Essential 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 to atmosphere at a safe height above the gasholder roof. As the volume relief valves open they actuate a limit switch.
- Volume relief limit switches
- Used to send signals to the control room to confirm the status of the volume relief valves.
- Level weight system
- A 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 actuate limit switches to signal when the gasholder volume has reached pre-defined settings.
- Level weight limit switches
- Used 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 angle
- Welded to the inside of the shell this angular section is where the sealing membrane attaches to the shell.
- Tank roof
- The roof is designed to withstand the local climatic conditions and the possibilities of additional loads, such as snow and dust. The roof of the gasholder is of thrust rafter radial construction and has a covering of single sided lap welded 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 handrailing
- Safety handrailing around the outside of the roof.
- Radial walkway
- For access from the staircase to the centre vent etc.
- Volume relief valve actuators
- Mechanical arms that operate the volume relief valves once the piston fender reaches a certain level.
- Level weight pulley structures
- Steel 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 piston level readings.
- Roof interior lighting nozzles
- For maintenance access to the gasholders interior lights.
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 gasholder will operate. The piston is designed to apply an equally distributed weight to ensure that the piston remains level 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 space beneath it. For higher pressure gasholders the infill plates are lap welded on both sides, whereas, 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 structure when 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 is out of service.
- Load cell chain receptacle
- A receptacle for gathering up the load cell chains as the piston rises.
- Piston seal angle
- Welded to the outer top side of the annular plates, this angular section is where the sealing membrane attaches to the piston.
- Level weight rope anchors
- Equally spaced around the periphery of the piston deck are the connections to which the level weight ropes are fixed.
- Piston fender
- The fender is a steel frame structure that is fixed to the piston deck annular plates and acts as a support structure for the abutment plates. Access can be gained to the top of the piston fender from either the shell access doors or roof manways 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 inspection purposes.
- Piston ladders
- Rung ladders complete with safety loops for access to the piston deck from the piston walkway.
- Radar reflector plates
- Used to bounce the radar signal back to the radar instrument for volume indication recording and piston level readings.
- Abutment plates
- Fixed to the outside of the piston fender to form a circumferential surface for the sealing membrane to roll against whilst the piston moves during operation.
- Piston torsion ring
- Around the base of the piston fender is a torsion ring which helps keep the piston shape during pressurisation. Concrete ballast can be added to the torsion ring to increase the weight of the piston and subsequently be a cost-effective way to increase the pressure of the gasholder to the required level.
The seal of the gasholder is designed to operate in the conditions specified by the client and to suit the stored medium. The seal rolls from the shell to the abutment surface of the piston and vice versa providing the piston with a frictionless self-centering facility. During depressurisation the seal (Usually rubber) also provides a gas tight facility that protects the holder from vacuum damage by blocking the gas outlet nozzle. During commissioning of the gasholder the sealing membrane is set into an operating condition. This setting must be carried out every time the gasholder is depressurised, otherwise known as "popping" the seal.
Gasholders hold a large advantage over other methods of storage. They are the only storage method which keeps the gas at district pressure (the pressure required in local gas mains). 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 the holder throughout the day, when little gas is being used. At about 5 pm there is a great demand for gas and the holder will come down, supplying the district.
Gasholders 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 gasholders, notably in Vienna, have been converted into other uses such as living space and a shopping mall and historical archives for the city. Many sites however were never used for the production of 'town gas', therefore the land contamination is relatively low.
Most British cities will have several gasholders. London, Birmingham, Manchester, Sheffield, Leeds, Newcastle, Salisbury and Glasgow (which has the largest gasometers in the UK) are noted for having many gasholders. Some of these gasholders have become listed buildings. The gasholders behind St Pancras station in London were specially dismantled when the new Channel Tunnel Rail Link was being created, with Gasholder No 8 being re-erected on a nearby site as part of a housing development.
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 117 metres (384 ft) tall and is located in Oberhausen.
The gasholders of Provan Gas Works, on the skyline in Glasgow, pipework and the booster house can also be seen.
Gasometer of the MAN type in Stuttgart, Germany
"Gasometer C", a gasholder converted for residential and commercial use in Vienna
Gasometers are comparatively rare in the United States. The most notable of these were erected in St. Louis by the Laclede Gas Light Company in the early 20th century. These Gasometers remained in use until the early first decade of the 21st century when the last one was decommissioned and abandoned in place. The most recently used gasometer in the United States is on the southeast side of Indianapolis but it is to be demolished along with the Citizens Energy Group coke plant. Another pair of holders at the Newtown Holder Station, in Elmhurst, Queens, in New York City, was a popular landmark for traffic reporters until the holders were demolished in 1996.
Gasometers, though once common, have became rare in Australia. Most gasworks within the country were demolished or repurposed and few gasometers remain because of this. A good example of a largely intact gasometer is located at the Launceston Gasworks site in Tasmania. Though the gas bell has been removed, all other components are intact. The remains of two older 1860s gasometers are also visible on site but only the foundations remain.
Other storage systems
Gas more recently was stored in large underground reservoirs such as salt caverns. In modern times 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 liquid form and this is widely practiced throughout the world.
- Gasometer Oberhausen
- Gasometer, Vienna
- Natural gas storage
- Water tower, similar utility storage structures
- Gasværket - a theatre in Copenhagen which was formerly a huge gas holder
- "Gasholders and their tanks" (PDF).
- "archiseek: The Gasworks, Dublin." (PDF).
- "National Grid Gasholder Demolition Case Study".
- "Gasometer Oberhausen".
|Wikimedia Commons has media related to Gasometers.|
|Wikimedia Commons has media related to Gasometer.|
- Use of gasometers in Oil & Gas industry
- Condemned: The great gasometer from BBC News, January 28, 1999
- Gasometer Augsburg in Germany and a list of many Gasometers in Europe
- Gasometer Schlieren, Switzerland
- Gasworks, Dublin, Ireland
- The rise and fall of gasometers Extrageographic magazine
- Gasholders and their tanks