Boiler Blowdown Valves: Continuous and Intermittent Blowdown Selection

Every steam boiler concentrates dissolved solids as water evaporates. If those solids are not removed, they raise the total dissolved solids (TDS), promote foaming and carryover into the steam, and deposit scale that insulates heat-transfer surfaces and can cause tube failure. Blowdown is the controlled removal of boiler water to hold TDS within limits. It comes in two forms - continuous blowdown from the steam-drum surface to control TDS, and intermittent (bottom) blowdown from the mud drum to remove settled sludge - and each needs a different valve arrangement suited to high-pressure, high-temperature, flashing service.

Continuous (Surface) Blowdown Valves

Continuous blowdown draws water from just below the water surface in the steam drum, where TDS concentration is highest, and passes it through a throttling valve to a flash vessel and heat-recovery exchanger. Because it throttles a small flow continuously against high pressure drop with flashing, the control valve needs an erosion-resistant, adjustable trim - typically a needle-type or multi-orifice throttling valve with a hardened seat. The valve is often paired with an automatic TDS-control system that modulates blowdown based on a conductivity probe.

Intermittent (Bottom) Blowdown Valves

Intermittent blowdown briefly opens a large flow from the lowest point of the boiler (the mud drum or bottom header) to blast out settled sludge and scale particles. This service is severe - full boiler pressure, sudden flashing, and abrasive solids - so it uses robust on-off valves, not throttling valves. The classic arrangement is a tandem pair in series: a quick-opening blowdown valve and a slow-opening sealing valve.

The Tandem Valve Arrangement

For bottom blowdown, two valves are installed in series, and the sequence in which they are operated protects the more vulnerable valve from wire-drawing erosion:

1. 1The sealing (slow-opening) valve nearest the boiler is opened first, fully.
2. 2The blowdown (quick-opening) valve downstream is then opened to perform the blowdown.
3. 3To close, the quick-opening blowdown valve is shut first, taking the throttling wear and flashing.
4. 4The sealing valve nearest the boiler is closed last, sealing against boiler pressure with no flow passing - so it never sees erosive throttling and stays bubble-tight.

Blowdown Valve Types Compared

Materials, Pressure Ratings, and IBR

Blowdown valves see the full boiler pressure and temperature, so material and rating must match the boiler design. Common selections include:

• Carbon steel A105 / A216 WCB for low- and medium-pressure boilers.
• Chrome-moly A182 F11 / F22 or A217 WC6 / WC9 for high-pressure, high-temperature boilers.
• Hardfaced (Stellite) seats and discs to resist flashing and particle erosion.
• Pressure class matched to boiler drum pressure - commonly Class 600 to 1500 or higher.
• Graphite packing rated for the service temperature.

In India, boiler-mounted valves fall under the Indian Boiler Regulations (IBR). Blowdown valves on IBR boilers must carry IBR Form III-C certification, be manufactured and tested by an IBR-approved works, and be stamped and documented accordingly. Always confirm the IBR requirement, the boiler design pressure and temperature, and whether continuous or intermittent (or both) service is needed before specifying.

Vajra Industrial Solutions supplies continuous throttling blowdown valves, quick-opening and slow-opening tandem bottom-blowdown valves, and piston blowdown valves in carbon steel and chrome-moly with hardfaced trims, correct pressure classes, IBR Form III-C certification where required, and EN 10204 3.1 material traceability.