A form 2 gasometer is a telescopic, water-sealed gas holder that stores low-pressure fuel gas in two or more nested lifts which rise and fall as gas enters or leaves. Typical municipal holders ran 50,000 to 350,000 m³ capacity at a delivery pressure of 75 to 200 mm water gauge, set by the dead weight of the crown and lift sheets. The telescopic form solved the height problem of single-lift holders, packing more volume into the same footprint. The Oberhausen Gasometer in Germany — now a museum — held 347,000 m³ in this configuration.
Gasometer Form 2 Interactive Calculator
Vary holder diameter and floating lift mass to see the gas delivery pressure, water-gauge head, and cross-section response.
Equation Used
The holder pressure is the dead weight of the floating crown or telescopic lifts divided by the circular holder area. The calculator converts mass to force with standard gravity, then converts pressure to millimetres of water gauge for low-pressure gasometer practice.
- Floating mass is the total dead mass currently supported by the gas.
- Holder cross-section is circular.
- Pressure is reported as both Pa and mm H2O using standard gravity.
- Water density is taken as 1000 kg/m3.
Inside the Gasometer (form 2)
A form 2 gasometer is the multi-lift cousin of the single-lift water-sealed holder. Instead of one bell rising out of a tank, you get a stack of nested cylindrical lifts that telescope upwards as gas fills the holder. Each lift has a U-shaped cup at its bottom edge and a grip on the top of the lift below it. As the upper lift rises, its cup catches water held in the grip of the lower lift, and that annular water seal is what keeps the gas in. No mechanical seals, no packing, no o-rings — just water and weight.
The pressure inside the holder is set by the dead weight of the moving sheet steel divided by its area. That's why the crown sheet weight matters so much. If you lighten the crown by 10% you drop delivery pressure by 10% and your downstream lamp standards or burners run lean. If you let scale and rust build up on the lifts you raise effective weight, push pressure up, and risk popping the cup-and-grip seal when the lifts come together at low fill. The cup must overlap the grip by at least 150 mm of water depth at the worst-case low fill condition — that's the rule of thumb gasworks engineers used, and it's still the right number for a heritage rebuild.
Guide framing handles wind load and keeps the lifts concentric. Spiral-guided holders use helical rails so the whole stack rotates as it rises, which cancels asymmetric wind loading. Frame-guided holders use a vertical lattice tower with rollers. If the rollers seize or the rails corrode you get a lift that hangs up partway, gas pressure spikes, and in the worst historical cases — Stratford in 1948 is the well-known one — the holder vents through a relief or splits a sheet. Tolerance on roller-to-rail clearance is tight: 3 to 6 mm cold, no more, or the lift will chatter and dent the cup as it descends.
Key Components
- Crown (top sheet): The domed or flat steel roof of the uppermost lift. Its dead weight, divided by holder cross-sectional area, sets the gas delivery pressure. A 12 m diameter crown weighing 8,500 kg gives roughly 75 mm water gauge — the classic town-gas figure.
- Lift sheets: Cylindrical steel side walls of each telescopic section, typically 5 to 8 mm plate riveted or welded. Each lift adds its own weight to the column, so a 3-lift holder runs at higher pressure than a single-lift of the same crown.
- Cup and grip seal: The U-channel (cup) at the bottom of an upper lift engages the inverted U (grip) at the top of the lower lift, with water filling both. Cup depth must give at least 150 mm water overlap at minimum fill, with cup-to-grip radial clearance of 25 to 50 mm.
- Outer tank: The fixed water-filled basin the lowest lift sits in. Tank water level must be maintained within ±25 mm of design — too low and the lowest seal breaks, too high and the lift floats unevenly.
- Guide framing: Either a spiral helical rail system or a vertical lattice tower with rollers. Holds the lifts concentric against wind load up to roughly 1.2 kN/m² design pressure, with roller clearance of 3 to 6 mm.
- Inlet and outlet pipework: Gas enters and exits through pipes that pass under the water seal and turn upward inside the holder. The inlet must sit below the lowest operating water level — typically 600 mm submerged — so gas cannot blow back through the seal.
Industries That Rely on the Gasometer (form 2)
Form 2 telescopic gasometers powered urban gas networks for over 150 years and you can still see them as preserved landmarks. They appear wherever someone needed a constant-pressure buffer of low-pressure gas — town gas, biogas, blast furnace gas, coke oven gas. Modern installations are rare but heritage and industrial cases keep the design alive. The water-sealed gas holder principle still beats dry seal alternatives for low maintenance and zero gas-side moving seals.
- Heritage and museums: The Oberhausen Gasometer in Germany — 117 m tall, 347,000 m³ — preserved as an exhibition hall after decommissioning in 1988.
- Municipal gasworks restoration: Fakenham Gasworks Museum in Norfolk, the only complete surviving town gasworks in England, with its working multi-lift holder.
- Steelworks: Blast furnace gas storage at integrated steel plants such as Tata Steel Port Talbot historically used multi-lift water-sealed holders to buffer BF gas before it fed boilers and stoves.
- Coke plants: Coke oven gas buffering at coking plants such as the former Hamborn works, where COG had to be stored at constant pressure to feed downstream chemical recovery.
- Sewage treatment biogas: Older municipal sewage plants like Mogden in west London used water-sealed lift holders for digester biogas before flexible membrane holders took over from the 1990s.
- Urban landmark preservation: Vienna Gasometer complex — four 70 m diameter brick-clad holders converted to apartments and offices, with the original telescopic lifts removed but the outer envelope preserved.
The Formula Behind the Gasometer (form 2)
What you need to compute first on any form 2 holder is the delivery pressure as a function of fill state, because pressure is what your downstream burners actually see. The pressure is set by the weight of whatever sheet metal is currently floating on the gas. At the low end of typical operation — only the crown lifted, lower lifts still nested — you get the lightest column and the lowest pressure. At the high end — all lifts extended — you get the maximum column weight and maximum pressure. The sweet spot is wherever your downstream appliances were designed for, and a well-sized holder picks lift weights so pressure stays inside ±10% across the full stroke.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| pgas | Gas delivery pressure inside the holder, expressed as head of water | Pa (or mm H₂O) | in H₂O |
| Σ Wlift,i | Sum of dead weights of all currently-floating lifts including crown, sheets, and any ballast | N | lbf |
| Aholder | Cross-sectional area of the holder at the lift cross-section | m² | ft² |
| n | Number of lifts currently extended (1 to total lift count) | dimensionless | dimensionless |
Worked Example: Gasometer (form 2) in a 2-lift heritage demonstration gasometer
You are sizing a 2-lift form 2 telescopic holder for the Hornsey gasworks heritage display in north London, feeding a small ring of demonstration street lamps. Holder diameter is 8.0 m, the crown plus upper-lift sheet assembly weighs 4,800 kg, and the lower-lift sheet weighs 3,200 kg. You need to know what gas pressure the downstream burners will see at empty (only crown floating), nominal (one lift up), and full (both lifts extended).
Given
- Dholder = 8.0 m
- Wcrown+upper = 4,800 kg
- Wlower = 3,200 kg
- g = 9.81 m/s²
- ρwater = 1,000 kg/m³
Solution
Step 1 — compute holder cross-sectional area:
Step 2 — at the low end of the operating range, only the crown and upper lift float (the lower lift is still resting in the tank). The floating weight is just the upper assembly:
That is plenty for open-flame gas lamps, which are happy anywhere between 60 and 150 mm water gauge. At this fill the holder is barely above the tank rim — visually you'd see only the upper lift's crown poking out.
Step 3 — at the nominal mid-stroke, the upper lift has lifted out and pulled the lower lift partway up. Both lift weights now hang on the gas:
This is the sweet spot for the burners and corresponds to the holder being roughly half-full visually, with both lift seams visible above the tank.
Step 4 — at full extension both lifts are floating, but the floating weight is unchanged from step 3 because no new mass joined the column. So:
Pressure is flat across the full stroke once both lifts have engaged — that is the whole point of the telescopic design. The only pressure step is the transition from one-lift floating (95 mm) to two-lift floating (159 mm), which happens over about 50 mm of holder travel as the cup picks up the lower lift.
Result
Nominal delivery pressure with both lifts floating is 159 mm H₂O, well inside the 60-200 mm water gauge band that town-gas appliances were designed for. At empty (one lift) you see 95 mm — burners run a little lean but still light cleanly; once the second lift engages you jump to 159 mm and pressure stays flat across the rest of the stroke, which is exactly what the telescopic form is meant to do. If your measured pressure differs from this, the usual culprits are: (1) cup-and-grip seal leaking because water level in the tank dropped below the 600 mm minimum and the seal broke, dumping gas and dropping pressure to near zero, (2) a lift hung up on a corroded guide roller so the second lift never engaged and you stay stuck at 95 mm, or (3) ice or debris in the cup adding parasitic weight and pushing pressure 15-20% above prediction.
Gasometer (form 2) vs Alternatives
Form 2 telescopic holders compete with single-lift water-sealed holders and modern dry-seal (Wiggins or piston-type) holders. The choice comes down to footprint, capacity, and how much you want to spend on water management. Here's how they stack up on the dimensions that matter for a working installation.
| Property | Form 2 telescopic gasometer | Single-lift water-sealed holder | Dry-seal (Wiggins) holder |
|---|---|---|---|
| Capacity per unit ground area | High — 2 to 6× single lift | Low | Medium-high |
| Delivery pressure stability across stroke | Excellent once all lifts engaged (±5%) | Excellent (single weight) | Excellent (controlled by piston ballast) |
| Capital cost (per m³ stored) | Medium | High (large footprint) | Highest (precision seals) |
| Maintenance interval (full inspection) | 10-15 years for lift sheets, annual for water level | 10-15 years | 2-5 years for seal grease and piston bearings |
| Service life | 80-120 years (Oberhausen 1929 still standing) | 80-120 years | 40-60 years |
| Water consumption / freeze risk | High — full tank plus seal water, freeze-prone | High | None |
| Operating pressure range | 50-300 mm H₂O | 50-200 mm H₂O | 100-500 mm H₂O |
| Best application fit | Large municipal town gas, BF gas buffering | Small works, demonstrations | Modern biogas, cold climates |
Frequently Asked Questions About Gasometer (form 2)
That step is normal and unavoidable in form 2 design — it happens at the moment the upper lift's cup picks up the lower lift's grip. Before pickup, only the upper assembly hangs on the gas. After pickup, both lifts hang. The floating weight roughly doubles in the space of 30-80 mm of vertical travel, and pressure jumps in proportion.
If the jump is bigger than your calculation predicts, the most likely cause is parasitic mass — water trapped on top of the lower lift's crown sheet from a leaking cup above, or ice in winter. Drain holes in the lower crown should clear standing water within minutes; if they don't, they're plugged.
The trade is footprint versus height versus pressure regulation complexity. More lifts means smaller diameter for the same capacity, which saves land but adds height — and every extra lift adds a cup-and-grip seal that has to be maintained. Wind load on a tall narrow holder also climbs with the square of height, so guide framing gets expensive.
The historical rule was to keep the height-to-diameter ratio below about 1.5 for spiral-guided holders and below 1.0 for frame-guided. If your site is wind-exposed or earthquake-prone, fewer lifts and a wider footprint wins. For a museum demonstration build, 2 lifts is almost always the right answer — easy to maintain and visually obvious to visitors how the mechanism works.
Tilt during travel is a guide-framing problem, not a seal problem. In a frame-guided holder it usually means one or two of the vertical rollers have seized or fallen out of their track, so that side of the lift hangs up while the other side rises freely. In a spiral-guided holder it means the helical rails are no longer concentric — often from foundation settlement on one side of the tank.
Diagnostic check: measure crown elevation at four points 90° apart as the holder fills. More than 25 mm difference across the diameter and you have a guide problem. Left uncorrected, the cup-and-grip seal will scrape on the high side and eventually score the cup wall, which then leaks gas.
The inlet dip pipe must be submerged deeper than the maximum gas pressure expressed as water head, plus a safety margin of at least 50%. So for a holder delivering 200 mm water gauge, submerge the inlet pipe at least 300 mm below the lowest tank water level. Less than that and gas blows back through the inlet seal at peak fill, which sounds like a deep gurgle and bleeds gas to atmosphere.
Most heritage holders used 600 mm submergence as a blanket spec because it covered any reasonable lift configuration without recalculation.
Slow pressure drift with no draw-off is almost always temperature-driven. Gas in the holder cools at night, contracts, and the lifts settle slightly. Because the holder pressure is set by weight not by gas volume, the pressure itself shouldn't change — but if the seal water also cools, the water column on the cup-and-grip rises slightly (denser water) and adds resistance, which the gauge reads as a pressure drop.
Second possibility is a slow seal leak. A 1-2 mm gap in the cup overlap will lose gas at a rate you can't hear but a U-tube manometer will see over a couple of hours. Soap-test the cup edges with the holder near full to confirm.
You can, with two specific changes. Biogas is roughly the density of town gas so the pressure equation is unchanged, but biogas contains H₂S which attacks both the steel lift sheets and the seal water. You need either a corrosion allowance of 3-5 mm on lift plate thickness and a sacrificial coating, or upstream H₂S scrubbing to below 100 ppm.
The seal water also goes acidic — pH drops to 4-5 within months of biogas service — so it must be circulated through a neutralisation loop or replaced quarterly. Most modern biogas plants gave up on water-sealed holders for exactly this reason and went to flexible double-membrane holders, which is why you rarely see new builds today.
References & Further Reading
- Wikipedia contributors. Gas holder. Wikipedia
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