Between the boiler and the process, a steam line carries a supporting cast of small iron: valves that isolate, valves that stop backflow, screens that catch dirt, vessels that dry the steam, glasses that let you see, and one humble check valve that stops a vacuum crushing your plant. None is expensive; each has a right and a wrong application; and most plants carry at least one of each installed wrong. This lesson is the selection walk — what each accessory is for, and the mistake to avoid with each.
Isolation: gate, globe and ball
Gate valves isolate: full bore, low pressure drop, a clean open/shut. Their sin is throttling — a half-open gate's disc flutters in the flow and wire-draws (the seat erodes in a fine groove that never seals again). Open, or shut; nothing between. Globe valves regulate: the flow path turns through a seat whose opening tracks the handwheel, so they throttle honestly and drop pressure even fully open. The manual bypass on a control station is a globe, never a gate. Ball valves give quarter-turn convenience and a bubble-tight shutoff — excellent for smaller bores and frequent operation, with one steam-specific caution: a closed ball traps a slug of condensate in its cavity, and reopening it against pressure fires that slug downstream. Steam-service balls with relieved cavities exist; cheap water-spec balls on steam are how mystery hammer enters small lines (lesson C5).
Protection: check valves, strainers, separators
Check (non-return) valves permit one direction. Steam duty prefers the spring-loaded disc pattern — swing checks slam on pulsating flow. The two mandatory addresses: after every trap discharging into a common return (one failed trap must not feed the whole return backwards through another — lesson D6), and on the boiler feed line (lesson B8's stop-and-check pair). Strainers — a Y-body holding a fine screen — stand guard before every trap, control valve, PRV and meter, because mill scale and weld beads travel for years after commissioning. The one installation rule from lesson C3 bears repeating: on steam, mount the Y-leg sideways, not hanging down — a downward basket fills with condensate, corrodes, and hands its rust to the valve it was protecting. Separators — a baffled vessel that spins and drops entrained droplets out of the flow, drained by its own trap — are the cure for wet steam at the point of use: before PRVs, meters, and any process that pays for dryness in product quality (lessons A9 and C3). A separator without a working trap on its drain is just a wide spot in the pipe.
Seeing and breathing: the quiet three
Sight glasses put a window in the line — most usefully just downstream of a steam trap, where the difference between a trap cycling condensate and one blowing live steam is visible to anyone (the testing methods of lesson D5 all get easier with a window). Vacuum breakers and air vents — lesson C8's pair — finish the bench: one lets air in when cooling equipment pulls vacuum (protecting thin jackets from collapse and letting condensate drain out of a stalled coil), the other lets air out when steam arrives. Small heated vessels that gurgle and refuse to drain after shutdown are asking for the breaker.
Buying and installing without regret
Three disciplines cover most of it. Rating: every accessory on an IBR line carries the line's flange rating and its certification (lesson B10) — one water-rated ball valve in a steam line is the cheapest component and the likeliest failure. Orientation: most of these devices care which way is up and which way flow goes — the arrow on the body is not decorative, and the strainer rule above is the classic miss. Maintainability: an accessory that cannot be reached will not be maintained; a strainer that has never been cleaned is a pressure drop, then a blockage, then a mystery.
- Walk one steam line end to end reading only the small iron: arrows pointing the right way? strainer legs sideways? gates being used as throttles (listen — a throttled gate sings)?
- Open the strainer nearest the boiler at the next shutdown. What you find inside is your system's dirt census, and it sets the cleaning schedule for the rest.
- Fit a sight glass after the trap you distrust most; arguments about that trap end the same week.
- Every ball valve on steam: confirm steam-rated with cavity relief. If the paperwork can't say, the valve can't stay.
- Gate isolates, globe regulates, ball is quick — and each fails when asked to do another's job.
- Checks after traps and on feed lines; strainers before everything precious, Y-leg sideways.
- A separator is a drying machine only while its trap works.
- Flow arrows, flange ratings, IBR certs — the boring trio that prevents the exciting failures.
- Vacuum breaker in, air vent out: opposite fittings for opposite moments.
The Y-strainer's screen tells the biography of a plant. Commissioning fills it with weld spatter and mill scale; year two brings rust flakes from whichever line skipped its lagging; a sudden harvest of resin beads means the softener downstream has a cracked lateral (lesson B7's slip, caught in a screen). Old-hand fitters photograph the basket contents at every clean-out and read them like tea leaves — the cheapest condition-monitoring programme in industry, running quietly inside a ₹2,000 casting.