9.1 Blowdown Control: Surface & Bottom Blowdown

9.1 Blowdown Control: Surface & Bottom Blowdown

Why Boilers Need Blowdown

Feedwater and makeup water always carry some dissolved minerals and suspended solids, even after treatment. As a boiler continuously evaporates pure steam, everything dissolved in the water that entered stays behind and concentrates in the boiler. Left unchecked, rising total dissolved solids (TDS) lead to scale formation on heat-transfer surfaces, foaming, carryover of boiler water into the steam space, and corrosion. Blowdown is the deliberate, controlled removal of boiler water to keep these conditions inside safe limits. Two distinct blowdown practices work together to do this: surface (or continuous) blowdown and bottom (or intermittent) blowdown.

Surface Blowdown

Surface blowdown is drawn from just below the normal water line in the steam drum, where dissolved solids, alkalinity, and floating foam or scum concentrate. Lighter dissolved solids and oily or foaming contaminants tend to collect near the surface rather than settle, so a surface (also called continuous) blowdown line is positioned to skim this concentrated layer off. Its primary purpose is to control TDS and alkalinity in the boiler water; a secondary benefit is removing the floating scum and oil film that promote foaming.

Because TDS control is an ongoing need rather than a periodic one, many plants run surface blowdown continuously through a small continuous-blowdown valve, often tied to an automatic TDS or conductivity controller that modulates flow to hold a target setpoint. Smaller systems may instead crack open a manual surface blowdown valve intermittently through the shift. When TDS climbs too high, steam bubbles do not break cleanly at the water surface; a foam layer builds and can be swept into the steam outlet along with droplets of concentrated boiler water. This carryover contaminates steam lines, can cause water hammer, and deposits solids on downstream equipment such as superheater tubes, control valves, or process equipment.

Bottom Blowdown

Bottom blowdown is taken from the lowest point of the boiler, such as the mud drum or the lowest header, where heavier sludge, precipitated scale particles, and sediment settle out by gravity because they are denser than water and do not stay suspended. Its purpose is to physically remove this settled material before it bakes onto tube or shell surfaces as hard, insulating scale. Unlike surface blowdown, bottom blowdown is periodic and intermittent: short, high-flow blasts lasting only seconds to under a minute, done on a schedule set by the operator based on feedwater hardness, makeup percentage, and load - often once per shift, sometimes more frequently.

The Two-Valve-in-Series Arrangement and Correct Procedure

Bottom blowdown piping uses two valves in series close to the boiler. The valve nearest the boiler shell is a quick-opening (non-throttling) valve. The second valve, farther downstream in the line, is a slow-opening (throttling) valve designed to be safely operated in a partially open position.

The correct procedure is:

1. Confirm the water level in the gauge glass is normal before starting - never blow down with a low or uncertain water level.
2. Open the quick-opening valve (nearest the boiler) fully first.
3. Slowly open the slow-opening (throttling) valve to establish and control the blowdown flow, watching the water level.
4. Blow down for the scheduled duration or until discharge runs clear, per plant procedure.
5. Close the slow-opening (throttling) valve first, slowly.
6. Close the quick-opening valve last, fully.
7. Recheck the water level once flow stops and add feedwater as needed.

The quick-opening valve is opened first and closed last for a specific reason: it must never be used to throttle flow. A valve held partially open under boiler pressure is exposed to high-velocity, flashing water and steam that erodes and scores ("wire-draws") the seat. Because this valve sits closest to the boiler and serves as the primary shutoff, keeping its seat undamaged is essential - a wire-drawn seat leaks even when nominally closed, risking an uncontrolled, unattended discharge. By opening it fully first and closing it fully last, it only ever operates fully open or fully closed, never partially open. The slow-opening valve, positioned farther from the boiler, is built to be throttled safely and is the valve actually used to set the flow rate. This sequence also reduces thermal shock: opening the quick valve fully first lets the piping between the two valves fill and warm at boiler pressure before the operator begins throttling flow through it, rather than exposing a partially open valve seat to a sudden full pressure and temperature differential.

Blowdown Rate, Feedwater Quality, and Efficiency

The right amount of blowdown depends on feedwater and makeup water quality. Harder or higher-TDS makeup water requires a higher blowdown percentage to keep boiler water TDS within design and insurance limits; cleaner makeup water allows less. Excessive blowdown wastes the heat energy already invested in that water (it was heated close to boiler temperature), along with the treated or softened makeup water and the chemicals dosed into it - all of which must be replaced, raising fuel and treatment costs and lowering overall efficiency. Insufficient blowdown allows TDS, alkalinity, and sludge to build past design limits, causing scale on heat-transfer surfaces (scale insulates and raises tube metal temperature, risking overheating and tube failure), higher foaming and carryover risk, and greater corrosion potential from over-concentrated dissolved solids. The best practice is to set the blowdown rate from routine TDS/conductivity testing rather than habit or guesswork, adjusting the continuous blowdown setpoint and bottom blowdown frequency as feedwater and makeup quality change.

Safety Considerations When Blowing Down

Blowdown water leaves the boiler at or near boiler pressure and saturation temperature, flashing to steam and scalding hot water on release - always treat the discharge as capable of causing severe burns. Stand clear of the valve handwheel or lever and never stand in line with the discharge piping. Many plants route blowdown to a blowdown or flash tank, sometimes with heat recovery that preheats incoming makeup water, which safely flashes off steam, cools the remaining water, and brings discharge temperature down to meet facility or local code limits before it reaches a drain or sewer. Never leave a blowdown running unattended, and never blow down when the gauge glass water level is already low or in doubt, since removing more water from a boiler that is already short on water risks uncovering tubes or the crown sheet. After blowdown, confirm both valves are fully closed with no passing or weeping, and that the water level has recovered to normal, before resuming normal operation.

Surface vs. Bottom Blowdown at a Glance