6.1 Valve Types and Service Selection
Key Takeaways
- A gate valve is an on/off valve and must not be used for throttling, because a partly open disc chatters and erodes the seat.
- A globe valve is the standard throttling valve; its tortuous flow path gives it a much higher pressure drop than a gate valve.
- Ball, plug, and butterfly valves are quarter-turn valves, which makes them fast to operate and easy to see the position of.
- A swing check valve must be installed in a horizontal line or in a vertical line with upward flow so gravity assists closure.
- A safety valve serves compressible fluids and pops fully open, while a relief valve serves incompressible liquids and opens progressively.
The largest domain on the exam
Domain 5, Valves and Seals, contributes 16 of 115 items — tied for the largest single block. It references 32109 Valves, 32204 Ferrous Metal Piping Practices, 32205 Identify, Install, and Maintain Valves, 32206 Hydrostatic and Pneumatic Testing, and 32308 Installing Mechanical Seals. Chapters 2 and 6 of this guide together cover all five.
The four jobs a valve can do
Every valve exists to do one of four things: start and stop flow, throttle (regulate) flow, prevent reverse flow, or relieve pressure. Matching the valve to the job is the single most tested idea in this module.
| Valve | Primary job | Motion | Pressure drop | Notes |
|---|---|---|---|---|
| Gate | Start/stop | Multi-turn, linear | Very low when full open | Never throttle; wedge or parallel slide disc |
| Globe | Throttle | Multi-turn, linear | High | Seat perpendicular to flow; the throttling standard |
| Ball | Start/stop | Quarter turn | Very low (full port) | Tight shutoff; full port vs reduced port |
| Plug | Start/stop, diverting | Quarter turn | Low | Lubricated or sleeved; multiport versions divert |
| Butterfly | Start/stop and moderate throttle | Quarter turn | Low to moderate | Compact and light; wafer, lug, or double-flanged |
| Diaphragm | Start/stop, throttle | Multi-turn | Moderate | Slurries and corrosives; no packing to leak |
| Check | Prevent backflow | Automatic | Varies by type | No operator; flow itself opens and closes it |
| Relief / safety | Protect from overpressure | Automatic | N/A | Set pressure; discharge to a safe location |
| Needle | Fine throttle, small flow | Multi-turn | Very high | Instrument and sample lines |
Gate valves in detail
A gate valve lifts a flat or wedge-shaped disc straight out of the flow path. Fully open, the bore is nearly unobstructed, so the pressure drop is minimal. Fully closed, the disc seats against both faces.
The failure mode that defines the valve: throttling destroys it. A partly withdrawn disc sits in a high-velocity stream, vibrates against its guides, and the throttled jet erodes the seating surfaces. The valve then will not shut off tight, which is the one thing it was installed to do.
Two stem arrangements:
- Rising stem (OS&Y — outside screw and yoke): the stem rises as the valve opens, so the position is visible at a glance and the stem threads are outside the process fluid. Standard for steam and refinery service.
- Non-rising stem (NRS): the stem turns in place and the disc travels on it. Used where headroom is limited or the stem must be protected, such as buried water service. Position is not visible, and the threads are in the fluid.
Globe valves in detail
A globe valve forces the flow to make an S-turn through a seat that lies perpendicular to the flow path. The disc moves a short distance to and from that seat, which gives fine, stable control of flow and lets the valve seat tightly even after years of throttling.
The cost is pressure drop. A globe valve of a given size passes far less flow at the same differential than a gate valve, which is why globe valves are sized carefully and why they are not used as block valves on long runs.
Most globe valves are installed so flow enters under the disc. That arrangement keeps pressure off the packing when the valve is closed, so the packing can be serviced with the upstream side still isolated, and it makes the valve easier to open against pressure. Always confirm against the flow arrow cast into the body — installing a globe valve backwards causes the disc to slam and the stem to buckle.
Quarter-turn valves
- Ball valves rotate a bored sphere. A full-port ball has a bore equal to the pipe, giving essentially no restriction; a reduced-port ball is one size smaller and cheaper. They give bubble-tight shutoff with soft seats, but the seats limit temperature.
- Plug valves rotate a tapered or cylindrical plug. Lubricated plug valves need periodic sealant injection; sleeved plug valves use a PTFE sleeve instead. Multiport plug valves divert flow between two outlets from one body.
- Butterfly valves rotate a disc on a shaft through the flow. They are light, short face-to-face, and inexpensive in large sizes. Wafer bodies are sandwiched between flanges; lug bodies have tapped lugs so one side can be removed with the line still in service.
Check valves
Check valves have no operator. Flow opens them; reverse flow and gravity close them.
| Type | Closure element | Installation constraint |
|---|---|---|
| Swing check | Hinged disc | Horizontal line, or vertical with upward flow |
| Lift check | Guided disc, like a globe plug | Horizontal for horizontal-pattern bodies; vertical for vertical patterns |
| Piston check | Dashpot-damped disc | Reduces slamming in pulsating service |
| Dual-plate (wafer) | Two spring-loaded half-discs | Any orientation; short and light |
| Ball check | Free ball | Viscous and slurry service |
Installing a swing check in a downward-flow vertical line is a classic error: gravity holds the disc open and the valve cannot check.
Water hammer on pump shutdown is usually a check valve problem. A slow-closing swing check permits reverse flow to build before it slams; a spring-assisted dual-plate or piston check closes before the column reverses.
Relief and safety valves
The distinction is about the fluid:
- A safety valve protects compressible fluid systems — steam, air, gas. It pops to the fully open position at set pressure, because a compressible fluid expands into the huss and needs immediate full capacity.
- A relief valve protects incompressible liquid systems. It opens progressively in proportion to the overpressure, because a small liquid discharge relieves the excess.
- A safety relief valve is designed to function as either, depending on the application.
Never block, gag, or adjust a set relief device in the field. Set pressure is stamped on the nameplate, and adjustment is a shop function with a documented pop test.
An operator has been controlling flow to a heat exchanger by leaving a 6-inch gate valve about one-third open for several months. The valve now will not shut off tight. What happened?
Which check valve type must never be installed in a vertical line with downward flow?
What distinguishes a safety valve from a relief valve?