Boiler safety and relief valves: accumulation, blowdown and the two-valve rule

Safety valves and relief valves do different jobs, and the exams test the numbers that go with them: accumulation, blowdown, full lift and the two-valve rule. This guide works those figures in plain language, with a calculation for each. It supports 4B unit B-3, plus the accumulation and full-lift items already in 2A and 3B.

Safety valve versus relief valve

Operators use the two names for two different jobs. A safety valve is for steam: the valve on a power boiler and on a steam heating boiler. It pops open at its set pressure and stays open, discharging at full capacity, until the pressure falls far enough for it to reseat. That reseat drop is the blowdown.

A relief valve protects a liquid system. On a hot-water boiler the vessel is full of water, so the valve that guards it relieves liquid rather than steam. It opens as pressure rises and closes again as pressure falls. Calling the hot-water valve a safety valve, or treating a liquid relief valve as the steam valve on a power boiler, is a common way to miss the question.

Accumulation

Accumulation is the extra pressure a boiler is allowed to reach while its safety valves discharge at full capacity. On an accumulation test the steam outlets are shut and the boiler is fired so the valves have to pass what the boiler can produce. Pressure may rise, but only up to a ceiling.

On a power boiler, Section I sets two ceilings and the lower one governs. While the valves discharge at full capacity, pressure stays within 6% above the highest set pressure, and not above 6% over the MAWP. If the highest valve sits a little above the MAWP, 6% of that set pressure can be higher than 6% of the MAWP, so the MAWP ceiling is the one that counts. The same comparison is on the boiler fittings guide.

Steam heating boilers use a different figure. Section IV limits the rise during full discharge to 35 kPa above the MAWP. The power-boiler 6% does not belong on a steam heating boiler, and the 35 kPa rise does not belong on a power boiler.

Worked example. A power boiler has an MAWP of 3000 kPa and its highest safety valve is set at 3090 kPa. Six percent above the highest set pressure is 3090 x 1.06 = 3275 kPa. Six percent above the MAWP is 3000 x 1.06 = 3180 kPa. The lower ceiling governs, so the highest pressure permitted with all valves discharging is 3180 kPa.

Blowdown

Blowdown is how far below the set pressure a Section I steam safety valve is adjusted to reseat. It is a minimum. The current rule asks for a blowdown of at least 2% of the set pressure or 15 kPa (2 psi), whichever is larger. You may not adjust the reseat drop smaller than that.

Worked example. A Section I steam safety valve is set at 3000 kPa. Two percent of 3000 kPa is 60 kPa, and 60 kPa is larger than the 15 kPa floor, so the smallest blowdown that valve may be adjusted to is 60 kPa. The 15 kPa floor governs only when 2% of the set pressure is under 15 kPa.

Two nearby figures are not the blowdown minimum. On this 3000 kPa valve, 90 kPa is the 3% full-lift figure, and 120 kPa is a 4% figure from older editions. Blowdown rules have changed between editions, so confirm the number against the edition your exam uses.

Full lift

Full lift is the point where a steam safety valve is fully open and can pass its rated capacity. Accumulation is how high the boiler may go with every valve discharging. Full lift is how soon each valve has to be wide open.

Section I requires a steam safety valve to reach full lift at no more than 3% above its set pressure. A valve set at 5000 kPa must reach full lift by 5000 x 1.03 = 5150 kPa. 5300 kPa is 6% above the set pressure, which is an accumulation figure, not the full-lift point. 5500 kPa is 10% above the set pressure, and 5050 kPa is only 1% above it.

The two-valve rule

A power boiler needs more than one safety valve once it passes a size threshold. On a fired boiler the threshold is more than 47 m2 of heating surface. On an electric power boiler it is a power input above 1100 kW. An electric boiler rated at 1500 kW is over that limit, so it needs two or more safety valves. One valve is enough only below the threshold. A high-pressure cutout does not take the place of a safety valve.

When two valves are fitted, the set points may be staggered. One valve is set at the MAWP. The second may be set a little above the MAWP, up to 3%. On an MAWP of 1400 kPa, 3% is 1400 x 1.03 = 1442 kPa, so a second valve set at 1440 kPa is inside that allowance. 1442 kPa is the stagger, not the accumulation limit. With valves set at 1400 kPa and 1440 kPa, the accumulation ceilings are 6% above the highest set pressure (1440 x 1.06 = 1526 kPa) and 6% above the MAWP (1400 x 1.06 = 1484 kPa). The lower one governs, so the boiler stays at or below 1484 kPa.

A superheater takes a share of the relieving capacity, not all of it. On a watertube boiler with an integral superheater, the safety valves on the drum provide at least 75% of the total required relieving capacity, and the superheater valves make up the rest. The superheater valve is arranged to open first so steam keeps flowing through the superheater tubes.

Common exam traps

The 3% feed-pressure rule, which says each feed source has to beat the highest safety valve setting, is a different 3% from full lift and a different rule from accumulation. It is worked in the feedwater guide.

• Mixing the 6% accumulation rule with the 3% feed-pressure rule.
• Mixing the power-boiler 6% with the steam heating boiler rise of 35 kPa. A steam heating boiler at 100 kPa MAWP may reach 135 kPa with the valves discharging, which is 100 + 35. The 6% figure on that boiler, 106 kPa, belongs to power boilers.
• Treating one safety valve as enough when two are required. Past 47 m2 of heating surface on a fired boiler, or past 1100 kW on an electric boiler, one valve does not meet the rule.
• Using only one of the two power-boiler accumulation ceilings. Both 6% limits apply, and the lower one governs.
• Using the 3% full-lift figure, or a 4% figure from an older blowdown rule, as the blowdown minimum.

Keep studying

• Boiler fittings and their code numbers
• Boiler feedwater code rules
• Boiler controls basics
• Low-water cutoffs: what the codes expect
• Refrigeration code safety basics

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Common questions