A gasometer (form 1) is a single-lift, water-sealed gas holder consisting of an inverted steel bell that floats inside an open tank of water, rising and falling as gas enters or leaves. It solves the problem of storing low-pressure fuel gas at a steady delivery pressure without compressors. The weight of the bell pressurises the gas underneath, and the water seal prevents leakage at the bell's lip. Working volumes ran from a few hundred to over 50,000 cubic metres in Victorian-era town gasworks.
Gasometer (Form 1) Interactive Calculator
Vary bell weight, diameter, lift, fill, and seal factor to see gas pressure, storage volume, and water-seal margin.
Equation Used
The bell weight divided by circular plan area gives gauge pressure. The pressure is converted to mm water gauge, then multiplied by the selected seal factor to estimate the required submerged curb depth. Stored gas volume is plan area times lift times fill percentage.
- Bell weight is the effective net downward weight creating gauge pressure.
- Bell plan area is circular.
- Water gauge conversion uses 1 mm w.g. = 9.80665 Pa.
- Seal depth is sized as a factor of pressure head.
How the Gasometer (form 1) Works
A form 1 gasometer is the simplest type — one bell, one tank, one lift. Gas is piped in below the water level through a submerged inlet. As gas accumulates, it displaces water inside the bell and the bell rises bodily out of the tank, guided by external framing. Pressure under the bell equals the bell's submerged weight divided by its plan area, plus atmospheric. Because the bell weight is fixed, the delivery pressure stays roughly constant from empty to full — which is the entire point. Compressor-fed systems vary pressure with demand, but a gasworks burner network was built around a near-constant 75 to 200 mm water gauge, and the gasometer delivers exactly that without any moving pump.
The water seal is the critical detail. The bell's lower edge — called the curb — dips below the tank water line by a depth that must always exceed the gas pressure expressed in equivalent water column. If you only seal 100 mm under the bell and the gas pressure is 150 mm w.g., the gas blows the seal and you get a flame at the lip. Seal depth is typically 1.5 to 2 times the rated working pressure for safety margin. The guide framing — a vertical lattice of rolled-steel columns and circumferential bracing — keeps the bell from tipping or jamming as it rises. Rollers on the bell crown ride against the columns. If the rollers seize or the columns rack out of plumb, the bell binds, you get uneven lift, and the bell can hang on one side leaving a tilted seal that leaks gas at the high edge.
The other failure that ends gasometers is corrosion of the crown plates. Coal gas carries naphthalene, sulphur compounds and moisture, and the underside of the crown sees decades of condensate. Plate thicknesses started at 6 mm and walked themselves down to 2 mm before holes appeared. Heritage restoration jobs almost always involve crown replating or full bell rebuild.
Key Components
- Bell (crown and side sheets): The inverted vessel that holds the gas. Crown plate carries the dead weight that pressurises the gas; side sheets, typically 4 to 8 mm rolled mild steel, transmit weight to the curb. The bell must be heavy enough to hit the design working pressure but light enough that the gas volume can lift it against breeze loads.
- Curb (bell lip): The lower rim of the bell, dipped into the tank water. Seal depth must exceed working pressure expressed as water column by 50 to 100 percent. A 150 mm w.g. holder needs 225 to 300 mm of immersion at lowest bell position.
- Tank: The open-topped water reservoir. Originally brick-lined or riveted iron, later welded steel or concrete. Diameter sets the maximum volume; a 30 m diameter tank with a 10 m bell lift stores roughly 7,000 m— of gas.
- Guide framing: Vertical columns and horizontal lattice girders surrounding the bell. Holds the bell on its vertical axis against wind loads. Column verticality must stay within 1 in 1000 over the full lift height or the rollers bind.
- Crown rollers: Cast-iron or steel rollers mounted at the bell crown that ride against the inside faces of the guide columns. Roughly 8 to 24 rollers depending on bell diameter. Worn flats on rollers cause juddering lift and uneven seal depth.
- Inlet/outlet pipe: Single submerged pipe under the tank floor that both fills and empties the bell. Diameter sized to the works output rate; a 100,000 m³/day works typically used a 600 mm main.
Where the Gasometer (form 1) Is Used
Form 1 gasometers were the standard storage element of every town gasworks from roughly 1820 until the changeover from manufactured gas to natural gas in the 1960s and 70s. They buffered overnight production against daytime peak demand, smoothed pressure across the distribution network, and acted as a visible volumetric gauge — operators could literally see how much gas the town had. A small handful of working or display gasometers survive today, and several have been preserved as listed structures even after the bell was scrapped.
- Heritage gasworks restoration: The Fakenham Gas Museum in Norfolk retains its 1846 single-lift holder as the only complete town gasworks in England, used for static interpretation.
- Industrial archaeology: King's Cross Gasholder No. 8, originally built 1850 by the Imperial Gas Light and Coke Company, dismantled and re-erected as a public space in 2018 — guide framing intact, bell removed.
- Biogas storage on farms: Small floating-drum digesters used on dairy farms in Vermont and Bavaria operate on the form 1 principle at 2 to 20 m³ scale, holding methane at 50 to 100 mm w.g.
- Sewage works digester gas storage: Davyhulme Wastewater Treatment Works in Manchester historically used water-sealed holders to store digester gas before piping it to engines on site.
- Brewery and distillery CO₂ recovery (historical): Guinness's St James's Gate brewery in Dublin used small water-sealed bells in the early 20th century to buffer fermentation CO₂ before compressing for cask carbonation.
- Industrial heritage tourism: Vienna's Gasometer A through D in Simmering, built 1896-99 for the Imperial-Royal Vienna Municipal Gasworks, now repurposed as residential and retail but with the original brick guide-frame shells preserved.
The Formula Behind the Gasometer (form 1)
The fundamental sizing relationship for a form 1 gasometer ties bell weight, bell plan area, and delivered gas pressure together. At the low end of typical works practice — say a 50 mm w.g. holder for a small village system — the bell is light and easily disturbed by wind. At the high end, around 300 mm w.g. for industrial supply, the bell gets heavy enough that crown plate thickness becomes a structural rather than corrosion driver. The sweet spot for Victorian town gas was 75 to 150 mm w.g. — enough head to drive distribution across a few kilometres of cast-iron main, light enough that an 8 mm crown lasts decades.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Pgas | Gas gauge pressure under the bell | Pa (or mm water gauge) | inches water gauge (in w.g.) |
| Wbell | Total submerged weight of the bell including crown, sides, rollers, and any added kentledge | N | lbf |
| Abell | Plan (horizontal cross-sectional) area of the bell | m² | ft² |
| hseal | Required seal depth (curb immersion below water surface at lowest bell position) | m | in |
Worked Example: Gasometer (form 1) in a heritage municipal gasworks display holder
You are sizing the bell for a working-display single-lift gasometer at a restored municipal gasworks museum in Carrickfergus Northern Ireland, using a 12 m diameter tank and a target delivery pressure of 100 mm water gauge to feed two demonstration gas-lit lamp standards on the site. You need to find the bell weight, then check seal depth.
Given
- Dbell = 12.0 m
- Ptarget = 100 mm w.g. (— 981 Pa)
- ρwater = 1000 kg/m³
- g = 9.81 m/s²
Solution
Step 1 — compute the bell plan area at the nominal 12 m diameter:
Step 2 — convert target pressure to pascals and solve for required bell weight at the nominal 100 mm w.g. operating point:
That is the dead mass the crown and side sheets together must total. At the low end of the typical heritage-display range — 50 mm w.g., enough to feed a single lamp standard at low flame — the required bell weight halves to roughly 5,650 kg, and you would feel the bell visibly shudder in any breeze above about 4 m/s. At the high end, 200 mm w.g. for a small industrial supply, you would need 22,600 kg of bell, the crown plate would jump from 6 mm to closer to 10 mm, and wind sensitivity drops to almost nothing.
Step 3 — check seal depth at the lowest bell position. Seal must exceed working pressure expressed as water column by at least 50 percent:
Build to 200 mm to give margin against wave action when the bell rises quickly during gas-up. Below 150 mm and you will smell gas at the curb; above 250 mm and you waste tank depth that could otherwise have gone to bell stroke and stored volume.
Result
Bell weight comes out at 11,310 kg for the nominal 100 mm w. g. delivery, with a minimum seal of 150 mm and a recommended build value of 200 mm. In practice that means a bell roughly the mass of a loaded 40-foot shipping container floating gently on a 12 m tank — slow to rise during gas-up, dead steady at full lift. Across the operating range, halving the pressure halves the bell mass and doubles the wind sensitivity; doubling the pressure doubles bell mass and forces a thicker crown. If you measure delivery pressure significantly below the predicted value, check three things in this order: (1) curb plate corrosion creating a partial leak at the seal — symptom is gas bubbling at the tank water surface near one quadrant; (2) accumulated rainwater or naphthalene scale on the crown adding unaccounted weight that is fine, but check the inlet siphon for water blockage which restricts gas flow and depresses the bell; (3) a single jammed crown roller hanging the bell on one column, leaving a tilted seal where the high side reads correct pressure but the low side is venting straight to atmosphere.
When to Use a Gasometer (form 1) and When Not To
Form 1 (single-lift) gasometers are the simplest gas holder design. They compete against telescopic (multi-lift) holders, which stack two or more bells to multiply capacity per unit tank area, and against modern dry-seal piston holders like the Wiggins type, which eliminate the water tank entirely. The choice comes down to volume per footprint, capital cost, and pressure regulation quality.
| Property | Form 1 (single-lift) gasometer | Telescopic (multi-lift) gasometer | Wiggins dry-seal holder |
|---|---|---|---|
| Storage volume per tank footprint | 1× (baseline) | 2 to 6× depending on number of lifts | 3 to 8× — limited only by piston travel |
| Working pressure range | 50 to 300 mm w.g. | 50 to 300 mm w.g. (same per lift) | 100 to 500 mm w.g. |
| Pressure constancy across stroke | ±2% (very steady — fixed bell weight) | Stepwise — pressure jumps as each lift engages | ±5% (membrane and piston friction) |
| Capital cost (per m³ stored) | Highest of the three | Moderate | Lowest at large scale |
| Typical service life | 80 to 150 years (heritage examples still standing) | 60 to 100 years | 40 to 60 years (membrane replacement at 20 to 30) |
| Wind sensitivity at part-lift | High at low fill — bell wobbles in guides | Moderate — multiple bells share load | Low — piston is mechanically constrained |
| Maintenance demand | High — water tank, curb corrosion, rollers | Highest — every cup-and-grip seal multiplies | Moderate — periodic membrane and piston seal checks |
| Suitability for biogas / small-scale use | Excellent — 2 to 50 m³ farm digesters use this exact form | Overkill at small scale | Capital cost rules it out below ~5,000 m³ |
Frequently Asked Questions About Gasometer (form 1)
The most common cause is one or two crown rollers seized on their pins, which forces the bell to slide rather than roll up the affected guide column. Friction is uneven around the perimeter, so the side with free rollers lifts first and the bell tilts. Check by lifting the empty bell manually with a chain hoist 50 mm at a time and listening for groan or scrape on a particular column.
The second cause is guide column verticality drift — heritage frames settle over decades and a column 30 mm out of plumb over a 10 m lift will cause progressive jamming as the bell rises. Survey the columns with a theodolite before blaming the bell.
Run the footprint number first. A form 1 holder for 5,000 m³ at 8 m lift needs a tank roughly 28 m diameter. A two-lift telescopic at the same volume needs about 20 m diameter — a 50% reduction in plan area, which matters if you are squeezing it onto a working farm.
Then weigh the maintenance penalty. Telescopic holders have cup-and-grip seals between the lifts that need annual inspection and water level management on each cup. For a small biogas operator without a dedicated gas engineer on staff, the form 1 wins on simplicity even at the cost of footprint.
Two things commonly cause an over-reading. First, accumulated naphthalene and tar scale on the crown underside adds real mass that you didn't include in the bell weight. On a 50-year-old town gas holder this can be 200 to 500 kg of crud per square metre of crown — enough to push pressure noticeably above design.
Second, your pressure tap may be reading dynamic rather than static pressure. If the tap sits in the inlet main close to a bend or a partially closed valve, you pick up velocity head. Move the tap to the bell crown bleed point — dead air, true static — and re-read.
Yes in principle, no in practice without a structural review. The crown plate was sized to carry its own weight plus a dead-load allowance, not arbitrary added mass. Stacking concrete blocks on a 6 mm crown that is already 60% corroded to 4 mm will buckle the central plate and dish it inward.
The other limit is seal depth. If you raise pressure from 100 mm w.g. to 150 mm w.g. you must also deepen the curb immersion by at least 75 mm, which means raising the tank water level — and that may flood the gas inlet pipe nozzle if it wasn't designed for it. Always recheck the seal geometry before adding weight.
Around 1.5 m diameter is the floor for a useful working holder. Below that, the surface-area-to-volume ratio of the bell gets so high that wind catches it sideways and tips it in the guides, and the proportional seal depth eats most of the tank height. Small biogas digesters at 1 to 3 m³ get away with it because they are sheltered inside a digester pit, but anything exposed wants to be at least 2 m across.
At the upper end there is no practical maximum — Oberhausen's holder reached 117.5 m diameter and 67 m tall. Bell stiffness and crown plate thickness scale with diameter, so cost per m³ stored actually drops as you go bigger.
Wind pushes the bell sideways within the guide framing slop, which momentarily reduces seal depth on the windward side. If your design seal margin is only 25% above working pressure, a 50 mm bell shift in a tank with 200 mm seal depth drops the windward immersion to 150 mm, which is exactly the working pressure — and you breach.
Fix it by increasing the seal margin to 75 to 100% above working pressure, or by tightening the guide roller clearances. Roller-to-column gap should be 3 to 5 mm; if it has worn out to 15 mm or more, the bell physically translates that distance under wind load.
References & Further Reading
- Wikipedia contributors. Gas holder. Wikipedia
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