Backup Boiler Sizing: 6 bar/28 TPH, Not 33 bar/32 TPH

The quick answer everyone reaches for first is "buy a copy of the big boiler." That is the right answer only if the backup has to keep the extraction turbo-generator loaded. It does not. The stated duty is process steam, and process steam is taken downstream of two pressure-reducing and desuperheating stations at 3 kg/cm2 / 180 C and 6 kg/cm2 / 350 C. Sizing a packaged boiler for 33 bar, 370 C and 32 t/h to feed a 28 t/h low-pressure header buys pressure part cost, superheater surface, feedwater pump head and code inspection scope that never gets used.

The other quick fix — a plain saturated packaged boiler at 6 kg/cm2 — fails for a different reason, and it fails at the second PRDS station. Work through the duty in order.

Size to the Process Load, Not the Turbine

Start from what leaves the PRDS stations, because that is the only steam the backup has to make.

Twenty-eight tonnes per hour is the number to buy against, not thirty-two. Before you write it on the enquiry, confirm the two loads actually peak together — pull a month of PRDS flow trends. If 23 and 5 never coincide, the boiler MCR drops and so does the fuel bill for the whole life of the machine.

Compare the three candidate specifications on one line each:

Check before moving on: you have a signed-off coincident process flow and the two PRDS outlet conditions in writing.

Fix the Design Pressure from the Extraction Piping

Do not set boiler design pressure from a catalogue class. Take it from the pipe the backup will feed. Find the design pressure of the line running from the steam turbine extraction nozzle to the 6 kg/cm2 process header — that piping design pressure is the design pressure for steam leaving the new boiler. The boiler's master stamping pressure (MAWP) then sits above it, on the order of 5 percent higher, so that the operating band clears the safety valve set pressure.

The mechanism behind that margin: a boiler stamped at exactly the header pressure has its safety valves set at or barely above normal operating pressure. Every load swing, every PRDS control valve step, every burner modulation lag lifts a valve. Lifted valves cut seats, seats leak, and a leaking safety valve on a backup boiler is discovered the week you need it.

1. Read the design pressure off the extraction-to-process line class (P&ID plus line list, not the field gauge).
2. Set boiler outlet design pressure to that value.
3. Set MAWP / stamping pressure roughly 5 percent above it.
4. Check the safety valve set pressure against MAWP and confirm normal operating pressure sits clearly below the set point.
5. Confirm the new boiler's relief capacity discharges the full MCR at accumulation, per the code stamped on the boiler.

Check: operating pressure, design pressure, MAWP and safety valve set pressure written as four distinct numbers in ascending order.

Set Steam Temperature Before You Pick a Boiler Type

This is where the cheap saturated package dies. At roughly 6 kg/cm2 g the saturation temperature is about 164 C; at 3 kg/cm2 g it is about 143 C — read the exact values off your own steam tables at the pressures you settled above. A desuperheating station only sprays feedwater tolower

Throttling will not rescue you. Letting a saturated 15 bar supply down to 6 kg/cm2 is essentially isenthalpic; the temperature barely moves and lands nowhere near 350 C. The heat has to come from a superheater.

So the specification is a packaged boiler with a superheater, generating steam above 350 C at the outlet, at a pressure set by the piping study. State the required outlet temperature and the permissible temperature swing across the 5 t/h to 28 t/h load range in the enquiry — packaged superheaters are often uncontrolled, and their outlet temperature drifts with firing rate.

Check: a temperature line on the datasheet at both minimum and maximum firing, with a stated tolerance.

Confirm Turndown, Feedwater and Fuel Before Ordering

Convert the load profile into a required turndown ratio and put it on the enquiry alongside MCR.

• Heat duty: Q = m x (h_steam - h_feedwater), with m in kg/s and enthalpies in kJ/kg. Vendors quote "from and at 100 C" evaporation; convert with equivalent evaporation = m x (h_steam - h_feedwater) / 2257 so you are comparing like for like.
• Feedwater temperature: the deaerator on the big boiler may be unavailable during its outage. If the backup runs on colder feedwater, duty and fuel both rise. Decide now whether the backup gets its own deaerator.
• Feedwater pump: size for MCR plus continuous blowdown plus PRDS spray water, and for the new MAWP head, not the old boiler's head.
• Fuel: confirm the fuel train, gas pressure or oil supply can hold MCR when the main boiler's fuel handling is isolated for repair.
• Water treatment: a superheater is unforgiving of carryover. Check that the existing chemical regime and blowdown control cover the new drum.

Check: minimum stable firing rate in t/h, and confirmation that it is below the smallest expected process load.

Tie In Without Backfeeding the Extraction Header

The backup joins the process header that the turbine extraction also feeds. When both are live during changeover, the lower-pressure source gets pushed backwards.

1. Land the new boiler's outlet downstream of the PRDS station isolation, on the header the process actually consumes from.
2. Fit a non-return valve plus a positive isolation valve on the backup outlet, so steam cannot track back toward the extraction nozzle.
3. Verify the PRDS pressure controllers do not fight the new boiler pressure controller — one device holds header pressure, the other follows.
4. Warm and drain the tie-in line before opening; a cold spur off a superheated header is a water-hammer source.
5. Trend header pressure through a manual changeover before you rely on an automatic one.

Check: with the backup at pressure and the main boiler on line, header pressure stays flat and no non-return valve chatters.

One boundary worth stating in the scope document: if anyone later proposes feeding the turbo-generator from this backup, the answer is no unless the steam conditions match. Running wet or saturated steam for extended periods into a machine designed for superheated flow damages the blading and voids the turbine warranty. Back-up for process is a different job from back-up for generation.

Verify: Hydro, Pop Test, Then a Full 28 TPH Run

Get it running, then prove it holds. Commissioning acceptance in order:

1. Hydrostatic test at the code multiple of MAWP; witness and record.
2. Safety valve pop and reseat test against the set pressures fixed earlier. Record lift and blowdown.
3. Burner light-off and flame safeguard proving, including all trips: low water, high pressure, flame failure, high stack temperature.
4. Hold minimum firing for at least an hour and log superheater outlet temperature — this is where uncontrolled superheaters over-temperature or under-deliver.
5. Run a full changeover: isolate the main boiler, hold process pressure on the backup alone, then return. Watch for reverse flow at the extraction tie-in.

Stop and call the boiler manufacturer or your Authorised Inspection Agency if any of the following show up: superheater outlet temperature cannot hold 350 C at any point in the load range, a safety valve lifts during normal modulation, or the stamped MAWP does not clear the header design pressure you derived from the piping. Those are pressure-part and code-stamp issues, not commissioning tuning, and field adjustment of set pressures or superheater surface is not yours to make. Bring the piping design pressure documentation to that call — it is the number the vendor will size against.

Frequently Asked Questions