The API 600 trim table lists common material combinations which are frequently specified by the oil and gas industry for quick specification and references of gate valves from 1 to 21. A reorganized reference table can be found here for easy printout.
What are trims?
The trims of a gate valve is officially defined in API 600 as the stem, the body seating surface, the gate seating surface, the backseat bushing, and the small internal parts that normally contact the service fluid. Alternatively, trim can be understood to be parts of the valve exposed to the service fluid, which needs to resist wear and corrosion to maintain the basic functions of the valve; containing and sealing the flow media.
The stem of a gate valve is responsible for allowing control of the gate. It must be strong enough to not buckle or break under the required thrust and torque for operating the valve while resisting corrosion and wear of external element and internal media and maintaining a seal at the packing box. The stem is considered a major trim component, and one of the few trims which traceability may be maintained due to adequate size.
Body/Gate Seating Surfaces
The surfaces of the body and gate which comes in contact is considered the important trim of the API 600 valve and are selected to resist the wear and media corrosion. It is important to understand that the trim is only the contact surface where the parts seals against each other, and not the entire components. Overlay of trim material directly on the main components is allowed. Additionally,for corrosion resistant alloys, such as 316 stainless valves, monel, and other exotic materials, it is acceptable for the seating surface to be “integral” to the main components, such as the body, by direct machining of the body to final dimension for trims such as 12 and 10.
As the seating surface is the workhorse of a valve, it is important when specifying the proper seating surface trim material to arrive at proper balance of cost to longevity. Depending on material, hardness requirement exist and are specified in API 600. Finally, some differential hardness between the body and gate surface is desirable to reduce chance of galling damage.
Additional details may be found in API 600 Section 5.3.
The backseat bushing is installed at the top of the bonnet and is considered a seating surface for emergency sealing of the valve stuffing box. The bushing material per API 600 is only required to be better in corrosion resistance to the body/bonnet material. However, most manufacturers will provide material similar to that of the stem and overlay the seating contact surface per API 600 trim. As any corrosion of the backseat bushing can drastically reduce the reliability of the valve.
Small Internal Parts
Small internal parts of the valves, such as spacers in the stuffing box, springs in a double disc gate valves, are considered critical pieces which should not corrode. They should match the stem trim material. If necessary, they may require to be upgraded to better material due to strength requirements, such as monel or 316 trims.