What Is Valve Trim?
Valve trim refers to the internal components of a valve that come into direct contact with the process fluid and control the flow. The trim determines the valve’s performance, corrosion resistance, erosion resistance, and sealing capability. Selecting the wrong trim material is one of the most common causes of premature valve failure in oil and gas service.
Trim Components
The exact components classified as “trim” vary by valve type, but the general definition per API 600 includes:
| Component | Function |
|---|---|
| Disc (wedge, plug, or ball) | Opens/closes the flow path; primary flow control element |
| Seat ring(s) | Provides the sealing surface against the disc |
| Stem | Transmits actuator/handwheel force to the disc |
| Backseat | Seals the stem-bonnet junction when the valve is fully open |
| Cage | Guides the plug and controls flow characteristics (in cage-guided globe valves) |
| Stem nut | Converts rotary motion to linear motion |
| Other wetted parts | Bushings, guides, pins in contact with fluid |
The valve body, bonnet, and packing are NOT part of the trim. The body and bonnet are pressure-containing components specified separately from trim.
Trim Material Selection
Trim material selection depends on the process fluid, temperature, pressure, and the presence of corrosive or erosive agents. Common trim materials include:
| Material | Composition | Application |
|---|---|---|
| 13Cr (AISI 410) | 13% Chromium martensitic SS | General non-corrosive service, standard for most carbon steel valves |
| Stellite 6 | Cobalt-chromium alloy (Co-28Cr-4W) | High-temperature, erosive service, hard-facing on seats |
| 316 SS (AISI 316) | 18Cr-12Ni-2Mo austenitic SS | Moderate corrosion, chemical service |
| Monel 400 | 67Ni-30Cu | Seawater, HF acid, brine |
| Inconel 625 | Ni-22Cr-9Mo-3.5Nb | Severe corrosion, H2S + CO2 |
| Hastelloy C-276 | Ni-16Cr-16Mo-4W | Aggressive chemical environments |
| PTFE / RPTFE | Fluoropolymer | Soft seats for bubble-tight sealing at low temperatures |
Trim vs Body Material
A common practice is to use a carbon steel body (ASTM A216 WCB) with a corrosion-resistant trim (e.g., 13Cr or Stellite). This reduces cost while protecting the critical sealing surfaces from the process fluid. The body sees static fluid contact; the trim sees dynamic contact with high velocity and turbulence, so it needs superior material properties.
For aggressive services (sour gas, high-chloride, high-temperature acids), both body and trim materials must be upgraded. See the valve materials guide for body material specifications.
How to Specify Trim
Valve trim is typically specified by a trim number on the valve datasheet or in the piping specification. API 600 defines standard trim numbers (e.g., Trim 1, Trim 5, Trim 8, Trim 12) that map to specific material combinations for disc, seat, stem, and backseat. See Valve Trim Numbers Explained for the complete chart.
For more on valve internal components and trim number details, refer to the main valve parts guide.
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