Syllabus
Governing: AE Part I syllabus, section 1 (Valves). Builds on: AEC 1 and AEC 2 workshop knowledge.
Why valves come first
Most machinery casualties aboard a yacht start or end at a valve: a sea suction that would not shut, a non-return that let the bilge back-flood, a relief valve that lifted into the wrong place. The examiner is testing whether you understand the function of each pattern well enough to choose it and to recognise it failing.
The syllabus patterns
| Valve | How it works | Where you meet it |
|---|---|---|
| Plug cock | Tapered plug with a through-port, quarter turn | Gauge isolations, small drains |
| Ball valve | Bored ball in soft seats, quarter turn | Fresh water, fuel, hydraulic isolation |
| Screw-lift (globe) | Disc screwed on and off a seat; flow changes direction | Throttling, cooling water control |
| Screw-down non-return (SDNR) | Globe body, disc loose on the spindle | Bilge suctions, pump discharges |
| Butterfly | Disc on a shaft across the bore | Large sea-water lines, HVAC chilled water |
| Gate | Wedge slides across the bore | Full-bore isolation only |
| Diaphragm | Flexible diaphragm pressed onto a weir | Chemicals, dosing, grey water |
| Quick-closing | Held open against a spring or weight; remote trip wire or pneumatic | Fuel and lube oil tank outlets |
| 3-way | Plug or ball with an L or T port | Changeover of filters, coolers |
| Valve chest | Several valves in one casting on a common manifold | Bilge and ballast manifolds |
Relief versus safety
The syllabus asks you to differentiate them, so be exact:
- A safety valve is for compressible fluids (air, steam). It is designed to lift fully at set pressure so that volume is discharged quickly, then reseat below set pressure (blow-down).
- A relief valve is for liquids. It opens progressively in proportion to overpressure, because a small liquid discharge relieves a lot of pressure.
A positive displacement pump without a relief valve on its discharge will burst something the moment a valve downstream is shut. You will meet that again in L1.2.
Materials and the fluid
| Material | Strengths | Watch for |
|---|---|---|
| Cast iron | Cheap, damps vibration | Brittle; cracks under shock or freezing |
| Cast steel | Tough, higher pressure and temperature | Rusts in sea water unless coated |
| Bronze / gunmetal | Sea-water resistant | Dezincification if a brass is substituted |
| Brass | Easy to machine | Loses zinc in sea water, becomes porous and weak |
| Stainless steel | Clean fluids, hygiene | Crevice attack in stagnant sea water |
Erosion is as important as corrosion: high-velocity sea water through a partly open valve wire-draws the seat. Galvanic couples between a bronze valve and a steel pipe will eat the pipe. Applied Marine Engineering covers the corrosion theory; here you only need to recognise the pairing is wrong.
WARNING — Brass is not bronze A replacement "bronze" fitting from a chandlery is often a brass. On a sea-water line it can dezincify and fail without warning. Check the specification, not the colour.
The yacht angle
The Red Ensign Group Yacht Code (Part A chapter 7A) requires, for yachts under 500 GT, a fuel shut-off that can be closed from outside the engine space. On most yachts that is a quick-closing valve with a pull wire or a pneumatic trip at the fire control station. Test it in the drill cycle and record it. A quick-closing valve that has not moved in two years will not close when the engine room is full of smoke.
Maintenance points
- Exercise sea valves regularly; a seized sea cock is found when you need it.
- Repack glands with the correct packing, one ring at a time, joints staggered.
- Lap globe seats; replace gate valve seats if scored.
- Check relief valve set pressures against the plate and record them.