Main Steam Stop, Feed Check and Blowdown Valves
Three valves sit where the boiler meets the rest of the plant. The main steam stop valve lets steam out, the feed check valve lets water in but never back, and the blowdown valves let sludge and dissolved solids out. Each has an operating rule that, broken, damages the plant.
You'll learn
- Describe what each of the three valves does
- Open a main steam stop valve without causing water hammer
- Explain why the feed check valve is a non-return valve
- Operate a pair of blowdown valves in the right order
Before this: What counts as a boiler
Three valves at the boundary
- Main steam stop valve: on the steam outlet, isolating the boiler from the steam main. It is often a screw-down non-return (stop-check) valve, so a boiler cannot draw steam back from others on a common main.
- Feed check valve: on the feedwater inlet, letting water in and preventing boiler water and pressure from flowing back to the feed pump. An isolating valve beside it allows maintenance.
- Blowdown valves: at the lowest point of the water space, letting accumulated sludge and concentrated water out. They are usually a pair in series.
Between them they control everything that crosses the boiler’s boundary in normal operation: steam out, water in, impurities out.
How to operate them
Main steam stop valve: open it slowly, a little at a time, after the steam main beyond it has been warmed and drained. A cold main full of condensate, hit by a sudden rush of steam, hammers. On a boiler joining others on a common main, open it only once its pressure is close to the main’s.
Feed check valve: keep it free and tight. If it sticks open when the pump stops, boiler water flows back into the feed line; if it sticks shut, the boiler is starved of water while the pump runs. A feed line that is hot well back from the boiler is a sign of a leaking check valve.
- Blowdown with two valves: open the valve nearest the boiler (the quick-opening one) fully first.
- Then open the second (slow-opening) valve to make the blow, for the time set in the plant’s procedure.
- Close the slow-opening valve first, then the quick-opening one. The second valve takes the wear and the cutting action; the first stays tight as the boiler’s isolation.
What goes wrong
Blowdown is a large, fast discharge of water at saturation temperature. Most of it flashes to steam in the blowdown line and vessel, so blowdown must discharge into a blowdown vessel or pit designed for it, never into an open drain or a sewer where flash and hot water can scald.
- Blowing down while the water level is low or unwatched: bottom blowdown removes water quickly. The level must be watched throughout, and two boilers on one blowdown line must never be blown down together.
- Opening a blowdown valve on a boiler that is under maintenance: a common blowdown line can backfeed hot water from a running boiler into one being worked on. Isolate and lock out both valves.
- Stop valve opened against an unwarmed main: water hammer that can break fittings and injure people.
Key takeaways
- Main steam stop valve: steam out; open slowly into a warmed, drained main.
- Feed check valve: water in, never back; a hot feed line points to a leaking check.
- Blowdown pair: open the boiler-side valve first, blow with the second; close the second first.
- Blow down into a proper vessel, watch the level, and never blow two boilers on one line at once.