Body Styles and Trim

Body styles

• Globe — a plug moving against a seat. Precise, repeatable, available in many trim designs, and the general-purpose control valve. Higher pressure drop and cost than rotary types, with single-seated designs giving tight shut-off and cage-guided designs offering better stability and easier trim change.
• Angle — a globe valve with a ninety degree body, used in erosive service and where flashing or coking would block a conventional body.
• Butterfly — a disc rotating in the flow. Compact, cheap in large sizes, low pressure drop when open, good for large flows of gas and low-pressure liquid. High-performance and triple-offset designs extend the control range and provide shut-off.
• Ball — segmented or V-notch ball valves give a wide rangeability, tolerate fibres and solids, and are common on slurry and pulp service.
• Eccentric plug and rotary globe — combine rotary simplicity with globe-like control characteristics.
• Diaphragm and pinch — for corrosive, abrasive and sanitary duties.

Trim

The trim is the plug, seat, cage and stem — the parts that actually do the throttling and take the wear. Trim selection determines the characteristic, the capacity, the noise and cavitation behaviour, and the service life.

• Standard trim for ordinary service.
• Hardened trim — stellite or tungsten carbide facing for erosive service, sand-bearing fluids and high differential pressures.
• Anti-cavitation trim — multiple stages or tortuous paths that take the pressure drop in steps so the pressure never falls below the vapour pressure.
• Low-noise trim — many small passages that shift the acoustic energy to frequencies that attenuate quickly and reduce the peak velocity.
• Seat materials — metal for high temperature and long life, soft seats for tight shut-off, with a defined leakage class stated for the application.

Packing

The stem seal is where control valves leak and where friction is born. PTFE packing gives low friction and good sealing at moderate temperature; graphite handles high temperature but has higher friction; live-loaded packing uses spring washers to maintain load as the packing relaxes, which improves both emission control and consistency of friction.

The trade-off is direct and important: tightening packing to stop a leak raises stem friction, and stem friction is the single biggest cause of poor control valve performance. Repack rather than over-tighten.