Piping steam from a separate boiler into the autoclave gives cleaner and more controllable heat than a fire under the vessel, and keeps combustion products away from the load. It moves the danger rather than removing it: the supply must be reduced to a pressure the autoclave is rated for, the autoclave’s own relief valve must be able to pass the full flow of the reducing valve if it fails open, and steam that touches instruments or food must come from clean feed water.
Part of Heat Source
Heating an autoclave with steam made in a separate boiler: pressure matching, clean steam, condensate and what a failed valve does.
A Separate Boiler Gives Control And Takes On A Second Pressure Vessel
The carrier recommends external steam for larger or more frequent operations because it gives better temperature control and keeps combustion products away from the vessel. A shut-off valve in the steam line changes the supply at once; a fire cannot be stopped that fast. The arrangement also keeps the flame, the firebox and the dry-wall hazard of Direct Fire Heating out of the autoclave room.
What it adds is a boiler, which is a pressure vessel in its own right and is built and run to the standards in Boiler Construction and Fire-Tube Boiler, with its own Safety Valve and water-level gauge. The autoclave is only safe if the boiler is.
What Happens When The Reducing Valve Fails Open
A steam boiler runs at a higher pressure than an autoclave needs. A pressure-reducing valve in the supply line brings it down to the autoclave’s working pressure. If that valve sticks open, the supply pressure reaches the autoclave. The autoclave relief valve then has to pass the whole stream.
The corpus’s approximate flow formula for a simple valve, from Safety Valve, is Q = 51 × A × P, with Q in lb/h, A the open area in square inches and P the absolute pressure in psi. It shows that flow is proportional to area times absolute pressure. Take an illustrative supply at 60 psi gauge, within the 60-100 psi range the Boiler Construction article gives for a double-riveted mild-steel boiler, and an autoclave relief valve opening at 16.5 psi gauge:
- Supply, absolute: 60 + 14.7 = 74.7 psia.
- Relief, absolute: 16.5 + 14.7 = 31.2 psia.
- For the same open area, the supply passes 74.7 ÷ 31.2 = about 2.4 times as much as a relief valve of that area can vent.
So the relief seat must be roughly 2.4 times the open area of the failed reducing valve, before any margin. This ratio is an order-of-magnitude result from the corpus’s own approximate formula, for flow that stays choked at both valves, and it ignores the real discharge coefficients. The point is the direction: the safe arrangement keeps the supply pressure as close to the vessel’s rating as practical, so the failed-open case is small.
Clean Steam For A Clean Load
Steam that passes into the chamber touches instruments, dressings or food. Feed water for a boiler is often treated: the Boiler Construction article adds soda ash to precipitate hardness, and the boiler is blown down to remove sludge. Droplets of boiler water can carry over into the steam line, and what they carry will land on the load.
There are two ways to deal with this (a recalled design practice):
- Use clean feed water with no additives for a boiler that feeds an autoclave, and let sludge settle and drain often.
- Keep supply steam out of the chamber entirely, running it through a closed coil or a jacket and letting the chamber make its own steam from the water inside. The supply steam then never touches the load.
What You Need
- A boiler with its own relief valve and water-level gauge, built and tested to the boiler articles.
- A pressure-reducing valve and a gauge on the autoclave side of it.
- A check valve on the steam line, so the autoclave cannot draw boiler water or air back along the pipe if its pressure falls on cooling (recalled practice).
- Piping sloped to a low point with a drain for condensate, lagged to save heat.
- An autoclave relief valve sized for the supply, as above.
- An operator trained on both vessels (Mastery Demonstration).
Method: Running On Steam
Step 1: Check the boiler before the autoclave. Water level, relief valve and gauge, in that order.
Step 2: Open the steam valve slowly. Condensate collected in the pipe can slam down the line as a slug of water, which hammers the fittings.
Step 3: Purge the air from the chamber through the vent until a steady steam stream comes out, then close it, as in Medical Instrument Sterilization.
Step 4: Hold the temperature by throttling the steam valve, and watch the gauge and thermometer together.
Step 5: At the end of the hold, close the steam valve first, then let the chamber cool and depressurise.
Common Mistakes
- Connecting the supply with no reducing valve. The autoclave sees boiler pressure.
- Reducing valve and relief valve both set from guesswork. Prove both by test.
- Leaving the steam valve open during cool-down. It keeps feeding the chamber.
- Letting condensate collect in the pipe. It causes water hammer and puts cold water into the chamber.
Safety
Two pressure vessels linked by a pipe fail together
A fault in the boiler, in the reducing valve or in the pipe can drive the autoclave beyond its rating. Never connect an autoclave to a boiler without a reducing valve, a relief valve on the autoclave sized for the failed-open case, and a means of isolating the supply. The boiler is run only by someone qualified to run boilers.
What’s Next
- Learn next: Applications — what the heat is for
- Combine with: Heat Source — a boiler that might already exist in the village