Sizing an acetylene plant against real demand not peak demand

The number that sizes an acetylene plant is not the one people usually quote. It is not the busiest week last year, and it is not what the works might need one day.

It is steady daily output across the hours you genuinely run.

That distinction is the whole guide. Almost every badly sized plant we are asked to look at was sized against a peak that occurs a few times a year, and the owner is then carrying a machine, a licence and an operator for a demand that arrives on a handful of days.

1. Cylinders filled in a normal day — from your own records, not an estimate
2. × gas per cylinder — giving total cubic metres per day
3. ÷ hours actually run — the shift you work, not the hours the gate is open
4. + a margin for filling and purge losses

That gives cubic metres per hour, which is the unit a generator is rated in. Sizing exactly to the calculated figure leaves nothing for the day something takes longer than it should.

Step three is where the arithmetic usually goes wrong. A plant that is nominally available twelve hours a day is not generating for twelve hours: carbide changeovers, hopper refills, slurry discharge and the settling time cylinders need after filling all come out of that window. Use the hours the generator is actually making gas, which is a number your logbook knows and your memory does not.

IS 8471 classifies generators by capacity, and the boundaries are worth knowing before you talk to anyone, because they change which rules apply to your installation rather than merely which machine you buy.

Portable generators are capped — limited output, and a limited carbide holding capacity. Above that a generator is stationary, and stationary machines divide again into small and large at 5 m³/h. Every plant in the KIGL-DA range is a large stationary generator by that definition, which is the category most industrial users land in as soon as they are filling seriously.

The standard also rules a whole design out. Displacement-type generators, where acetylene is made by intermittent contact between water and carbide suspended in a basket, are stated not to be considered satisfactory. If one is offered to you at an attractive price, that is the clause to ask about.

A plant sized to the peak spends most of the year idle, while carrying:

• the capital
• the maintenance regime
• the licence and inspection burden
• the operator time
• the floor space

None of that scales down when the plant is doing nothing. Size to steady demand and cover peaks from cylinder stock, which is exactly what stock is for.

There is a second cost that is easier to miss. A generator run far below its rated output cycles more and sits idle longer between cycles, and an acetylene plant does not enjoy sitting idle: slurry settles, the agitator has to be timer-driven to stop it seizing, and a machine that is rarely at working temperature gets its condensate and drying stages neglected because nothing appears to be wrong. An oversized plant is not simply an expensive plant; it is a harder plant to keep in good order.

A generator is not the whole plant, and the rest of the chain has to match it:

A generator larger than the filling bay behind it makes gas nobody can package. It is a common and frustrating way to spend capital, and it is invisible until commissioning.

The one most often forgotten is the last. Cylinders are not a consumable but a circulating asset: every cylinder is either being filled, in stock, with a customer, or coming back. Raise output without raising the fleet and the plant simply waits for empties.

Material graded 4 to 50 mm reacts predictably. Outside that band:

• Fines react too fast, spiking pressure and temperature
• Oversized lumps react incompletely, so carbide leaves in the slurry and yield falls

Poor grading makes a correctly sized plant behave like an undersized one, and since carbide dominates production cost, it shows up in the cost per cubic metre as well as in the output.

The tell is in the slurry. Unreacted carbide leaving with the discharge is carbide you bought, paid to handle, and never turned into gas. If consumption per cylinder is creeping up while output holds, look at the grading and at the agitator before you look at the generator.

The generator gets pushed toward the top of its band, which means pressure approaching 0.92 kg/cm²g and temperature approaching 70 °C. Neither is a place to operate continuously.

Both of those are KIGL's own operating limits, and both sit below what the standard permits. IS 8471 caps gas at the generator outlet at 75 °C, and any external part of the machine at 80 °C at 130 % of rated capacity. Running against our limit therefore still leaves margin against the standard's, which is the point of setting it there. It is not margin to be spent.

The answer to being short of gas is never to raise the setpoint. The full operating envelope is on acetylene plant operation and maintenance.

KIGL builds three standard machines — KIGL-DA-25, KIGL-DA-50 and KIGL-DA-100 — and the choice follows the numbers rather than a preference.

Tell us cylinders per day, hours run, cylinder size, and how much the demand varies, and we will tell you which one fits. We will also tell you if the answer is that a plant is not warranted yet. Below a certain steady demand, buying cylinders remains cheaper, and that is a conversation worth having before the capital is committed rather than after.