What is Valve Trim?

Valve trim is the collective term for the internal parts of the valve that can be removed and replaced and come into contact with the flow medium. These parts include valve seat(s), disc, glands, spacers, guides, bushings, and internal springs. The valve body, bonnet, packing, et cetera that also come in contact with the flow medium are not considered valve trim.

The disk-to-seat interface and the relationship between the disk position and the seat determine a valve’s trim performance. Basic motions and flow control are possible because of the trim. In designs with rotational motion trim, the flow opening changes when the disk moves close to the seat. An annular orifice appears when the disk lifts perpendicularly away from the seat in linear motion trim designs. Due to the various properties required to withstand various forces and conditions, valve trim parts can be constructed from a variety of materials. Bushings and packing glands do not experience the same forces and conditions as do the valve disc and seat(s).

Flow-medium properties, chemical composition, pressure, temperature, flow rate, velocity and viscosity are some of the important considerations in selecting suitable trim materials. Trim materials may or may not be the same material as the valve body or bonnet.
API has standardized trim materials by assigning a unique number to each set of trim materials.

Trim Material Selection

Understanding the material selection process is critical to engineering any application and/or part design. The choice of materials is the foundation upon which all valve applications are built. The most economical trim material is stainless steel. Available in different grades, this material provides strong resistance to erosive, corrosive, and temperature impacts and provides an excellent all-around choice.
In addition to stainless steel, more expensive materials such as Monel, Alloy 20, and Cu-Ni may be selected. These materials, sometimes referred to as exotic materials, generally are more expensive and have longer lead times than stainless.
Trim selection guidelines from manufacturers should be followed. One must consider trade-offs between the initial cost and the estimated service life. Stellite trim, for instance, may not be the best option if it doubles the price while extending the valve’s service life by 20%. It is essential to consider uptime, labor costs, and dependability. Engineers working on applications shouldn’t try to link operating pressures to expected wear.

Hardened Trim
Most high-pressure applications are suitable for hardened trim. Although solid hard plugs are more cost-effective for valves under 1 inch in diameter, a hard material overlay is more cost-effective for larger sizes. For services like superheated steam, two-phase flow, and temperatures above 600°F (315°C), these overlays are effective.

Primary Functions of Valve Trim

The trim components perform these essential roles: