Boiler fittings and the numbers the codes use
Fittings questions are mostly number questions: how high, how big, how many. This guide groups the numbers that come up most for the gauge glass, the pressure gauge and the blowoff, in our own words. It supports 4B unit B-3, 4A unit A-12 and the regulation unit of 5th Class.
Gauge glass
The gauge glass shows the operator the real water level, so the codes care about where its lowest visible point sits. On a typical watertube power boiler under current Section I, the lowest visible level sits from 50 mm to 75 mm above the lowest permissible water level set by the manufacturer. On a low-pressure heating boiler under Section IV, it sits at least 25 mm above the lowest safe level.
Higher pressure means more redundancy. Above 3 MPa, Section I calls for two gauge glasses, or one glass plus two independent remote level indicators. The pipe connections between the boiler and the water column are at least NPS 1 (DN 25), so they are less likely to plug with sludge.
Pressure gauge
The pressure gauge dial is sized against the safety valve setting, not the MAWP. Section I wants the dial graduated to about double the safety valve setting and never less than 1.5 times it. With a valve set at 1200 kPa, a 0 to 2400 kPa dial fits, while 0 to 1500 kPa falls below the 1.5 times floor.
A steam gauge is normally connected through a siphon or similar water seal so live steam does not reach the gauge tube. On a steam heating boiler, if the gauge line can be shut off at all, it is by a cock at the gauge with a tee or lever handle that lies parallel to the pipe when open, so anyone can see at a glance that the gauge is in service.
Blowoff
Once a power boiler's MAWP is above 690 kPa, B31.1 wants two valves on each bottom blowoff line: either two slow-opening valves, or a quick-opening valve next to the boiler followed by a slow-opening valve. A slow-opening valve is defined by turns, at least five full turns from shut to open, which forces a gradual opening and limits shock.
Material limits are narrow. Where the blowoff design pressure is 1725 kPa or less, a cast iron blowoff valve is limited to DN 65 (NPS 2 1/2). On low-pressure steam boilers, Section IV sizes the bottom blowoff from the required safety valve capacity, with DN 50 (NPS 2) as the top step for the largest capacities.
Where blowoff goes to a sewer, CSA B51 adds a Canadian rule: a boiler above 103 kPa needs a registered blowoff vessel or another registered cooling device in between, bringing the water down to 65 °C or lower.
Section I and Section IV side by side
A quick comparison of the numbers people mix up most. Figures are summarized in our own words and can vary by edition and boiler type.
| Item | Section I (power boilers) | Section IV (heating boilers) |
|---|---|---|
| Pressure and temperature scope | Above the heating boiler limits | Steam up to 103 kPa; hot water up to 1100 kPa and 120 °C |
| Lowest visible gauge glass level | 50 mm to 75 mm above the lowest permissible level (typical watertube) | At least 25 mm above the lowest safe level |
| Pressure rise with safety valves discharging | 6% above the highest set pressure, and not above 6% over MAWP | 35 kPa above MAWP on steam heating boilers |
Keep studying
- Boiler safety and relief valves
- Boiler feedwater code rules
- Low-water cutoffs: what the codes expect
- Boiler controls basics
Try some sample questions
These come from the live SteamTicket question bank. Pick an answer, then open the explanation.
Sample 1. On a typical watertube power boiler, where should the lowest visible water level in the gauge glass sit relative to the lowest permissible water level set by the manufacturer?
- A. At least 100 mm above it
- B. Exactly level with it
- C. Between 50 mm and 75 mm above it
- D. At least 25 mm above it, with no upper limit
Show the answer
Sample 2. A power boiler's safety valve is set at 1000 kPa. Which pressure gauge range best meets Section I?
- A. 0 to 2000 kPa
- B. 0 to 1200 kPa
- C. 0 to 5000 kPa
- D. 0 to 1000 kPa
Show the answer
Sample 3. Under B31.1, what makes a blowoff valve a slow-opening valve?
- A. It carries a pressure class of at least Class 300 at the blowoff temperature
- B. Its operator must be turned at least five full turns from shut to open
- C. It has a spring return that closes it slowly when released
- D. It cannot travel from shut to fully open in less than 10 seconds
Show the answer
Sample 4. On a low-pressure steam heating boiler, where must the lowest visible part of the gauge glass be?
- A. At the top of the tubes
- B. Anywhere the installer can see it
- C. At least 150 mm above the crown sheet
- D. At least 25 mm higher than the lowest safe water level
- E. Exactly at the lowest permissible water level marked on the boiler
Show the answer
Common questions
Is the pressure gauge range based on MAWP?
What makes a blowoff valve slow-opening?
Why do the gauge glass numbers differ between my textbook and the app?
Practise these code questions in SteamTicket
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