Boiler feedwater code rules, explained

Feedwater questions show up across the 4th Class papers and again at 3rd Class. This guide explains, in plain language, what ASME Section I and ASME B31.1 expect from a boiler's feed system and why. It lines up with 4B unit B-2, 4A unit A-12 and the piping section of 3A.

Two ways to feed the boiler

The basic idea in Section I is simple: a boiler that loses its feed can run itself dry, so a larger boiler needs a backup. Once a boiler has more than 47 m2 of water-heating surface, the base rule calls for at least two means of feeding water, so one failed pump does not starve the boiler.

There are exceptions. Some boilers fired by gas, oil or fuel burned in suspension may use a single feed source when the heat input can be shut off before the water level gets too low. A solid fuel bed is different. It keeps releasing heat after a trip, so the second feed source has to be independent of the first, for example a steam turbine drive backing up an electric motor drive.

• A low-water cutoff protects the boiler but does not feed it, so it never counts as a means of feed.
• Independent means the two sources cannot be stopped by the same single failure, such as one power outage.

Feed pressure has to beat the safety valve

Each source of feed has to be able to push water into the boiler at a pressure a little above the highest safety valve setting: 3% above it. The reason is the upset case. When pressure climbs to the safety valve setting and into accumulation, that is exactly when the boiler most needs water, so a pump sized for normal operating pressure is not enough.

Worked example: the highest safety valve on a boiler is set at 1500 kPa. The feed source must deliver at no less than 1500 x 1.03 = 1545 kPa. Matching 1500 kPa leaves no margin while the valve is lifting, and a 6% figure belongs to the accumulation rule, not the feed rule.

Design pressure of the feed piping

Boiler external piping is shared between two codes. Section I handles the administrative side, such as marking, data forms and inspection, while B31.1 sets the technical rules for design, materials and fabrication.

For feed piping from the boiler back to the required check valve, B31.1 adds a margin to the boiler MAWP: 25% of the MAWP or 1550 kPa, whichever is smaller. Between the check valve and the feedwater regulating valve, the design pressure has to be at least the pump pressure needed to feed the boiler, which can sit well above the MAWP when the regulating valve throttles.

• MAWP 1000 kPa: 25% is 250 kPa, smaller than 1550 kPa, so the design pressure is 1250 kPa.
• MAWP 8000 kPa: 25% is 2000 kPa, so the 1550 kPa margin governs and the design pressure is 9550 kPa.

Check valve and stop valve order

The feed line next to the boiler carries a stop valve and a check valve. The stop valve sits closest to the boiler, with the check valve upstream of it. That order lets you isolate and service the check valve without draining the boiler, and the check valve stops boiler water from backing out into the feed line if the pump stops.

Exam questions also mix this up with the steam side. When boilers with manholes share a common steam header, each needs two stop valves on its steam outlet with a free-blow drain between them, and B31.1 prefers a stop-check valve nearest the boiler. That double block and bleed is about protecting someone working inside the boiler, which is a different purpose from the feed line valves.

Common exam traps

• Using normal operating pressure instead of the highest safety valve setting for feed pressure.
• Using 6% (the accumulation figure) instead of 3% for the feed source.
• Adding 1550 kPa to the MAWP when 25% of the MAWP is the smaller margin.
• Rating the pipe between the check valve and the regulating valve at boiler MAWP only.
• Counting a low-water cutoff as a means of feed.

Keep studying

• Boiler safety and relief valves
• Boiler fittings and their code numbers
• Low-water cutoffs: what the codes expect
• How to pass 4B

Try some sample questions

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Common questions