Steam Valves: Isolation Valves and Control Valves
A steam system uses two families of valve. Isolation valves are either fully open or fully shut, to cut off a section for work or safety. Control valves sit part-open and move continuously to regulate flow, pressure or temperature. Using one for the other’s job wears it out.
You'll learn
- Tell isolation and control valves apart by purpose
- Choose an isolation valve type for steam
- Understand control valve characteristics and sizing
- Recognize the faults of each
Before this: Sizing steam pipes
Two jobs for valves
An isolation valve is a door: open for normal running, shut to separate a section of pipe, a piece of equipment, or a boiler from the main. It should seal tightly when shut and offer little resistance when open. A control valve is a tap: it sits partly open and moves continuously, under the command of a controller, to let through just the right amount of steam.
An isolation valve left half-open to throttle flow is eroded by high-speed steam across its seat, and soon will not shut tight when it is needed to.
Isolation valve types
| Type | Character | Notes |
|---|---|---|
| Gate valve | Full bore, low pressure drop | Not for throttling; slow to operate |
| Globe valve | Good shut-off, can throttle | Higher pressure drop; common on smaller lines |
| Piston valve | Seals on packing rings around a piston | Tight shut-off, low maintenance; popular on steam |
| Ball valve | Quarter-turn, full bore | Fast; seat material must suit steam temperature |
| Butterfly valve | Compact, quarter-turn | Used on larger lines; high-performance designs for steam |
Leaking glands are a common, wasteful fault on steam valves. Bellows-sealed valves replace the gland packing with a sealed metal bellows around the stem, eliminating gland leakage, and are worth specifying on valves that are hard to reach or operate often.
Control valves
A control valve has a body, a plug and seat shaped to give a chosen relationship between opening and flow — its characteristic — and an actuator that moves it: pneumatic, electric, or self-acting from the process itself.
- Linear characteristic: flow proportional to opening. Suits applications where the valve’s pressure drop stays roughly constant.
- Equal percentage: each step of opening increases flow by the same percentage of the current flow. Suits most heat exchanger temperature control, where system pressure drops change with load.
- Quick opening: most of the flow in the first part of the travel. Suits on-off duties.
Control valves are sized by flow coefficient (Kv or Cv) for the flow and pressure drop at the design condition, not by line size. A valve sized to the line is usually oversized: it then controls with only a small fraction of its travel, hunts, and wears its seat. A control valve is normally smaller than the pipe it sits in.
Key takeaways
- Isolation valves are fully open or shut; control valves regulate continuously.
- Never throttle with a gate valve; choose globe, piston or ball valves to suit the duty.
- Bellows-sealed valves stop gland leaks.
- Size control valves by Kv or Cv, not line size, and protect them with a strainer and separator.