Globe Valve Internal Parts: Trim Design and Maintenance

Globe Valve Trim Components

Globe valve trim refers to the internal flow-controlling components that are exposed to the process fluid. Proper trim design determines flow characteristics, shutoff capability, and maintenance frequency. Understanding trim components helps maintenance teams perform correct disassembly, inspection, and reassembly procedures.

Key Trim Parts

• Plug (disc): closure element that moves toward or away from the seat ring to control flow

• Seat ring: stationary seating surface, typically screwed or welded into body

• Stem: connects plug to handwheel or actuator, transmits force to seat closure

• Backseat bushing: provides emergency stem seal if packing is removed under pressure

• Stem nut and packing gland: compress packing material around the stem to prevent leakage

• Guide bushing: maintains plug concentricity and prevents lateral stem deflection

Seat Design Variations

Flat-faced plugs against flat seat rings are simple and effective for soft-seated valves. Tapered cone plugs provide enhanced metal seating force distribution. Needle plugs are used in small metering valves for precise flow control at low flow rates. Parabolic plugs provide equal percentage flow characteristics. The choice of plug and seat geometry is critical for achieving the specified flow characteristic and leakage class.

Trim Material Selection

• Carbon steel plug and seat: standard service, erosion-limited above moderate velocity

• Stainless steel (316): improved corrosion resistance, widely used as base material

• Stellite (Co-Cr alloy) hard facing: excellent erosion and galling resistance for abrasive service

• Tungsten carbide: extreme wear resistance for slurry and abrasive erosion service

• Ceramic (alumina, zirconia): highest hardness for abrasive slurry at moderate temperature

• PTFE or PEEK soft inserts: soft-seated leakage class VI for clean fluid service

Maintenance Procedure Overview

Globe valve maintenance typically involves removing the bonnet, extracting the plug and stem assembly, replacing or re-lapping the seat and plug surfaces, renewing packing, and reassembling in the correct order. Seat lapping uses lapping compound and a lapping tool to achieve a mirror-finish metal-to-metal seal. Torque values for bonnet bolting and packing gland nuts must follow manufacturer specifications to avoid damage.