ntroduction—Seals in an Industrial Valve
Valve seals are a critical component in the overall development of a new valve. In addition, upgrading seals in an existing device with the properly selected retrofit has the potential to enhance valve life and performance while reducing leakage and mean time to repair (MTTR).
Valve seals must be selected to provide enhanced durability and life in the end-use application while meeting increasingly stringent performance criteria in regards to leakage and low emissions. Careful evaluation and analysis is required to ensure that the proper valve seal is selected for a new OEM valve design under development as well as a valve being retrofitted or redesigned in an existing installation.
Several valve components require sealing such as:
- Stem packing or gland seals—The stem provides a mechanical connection between the handwheel or handle and valve. The sealing element around a stem consists of a packing material or a combination of seals, followers, O-rings, bellows, spring energized seals, spacers and packing. The seal prevents leaks while allowing the stem to rotate or slide freely.
- Valve Connection Seals—Depending on the valve port or connection type, a seal can be required. While welded valve connections do not require any seal, a welded connection precludes easy removal for maintenance or replacement. Valves with bolted flange connections or face seals typically require gaskets or O-rings. Threaded connections use PTFE tape, joint compound, O-rings or ring seals. Compression fittings can be sealed with flaring, ferrules, O-rings, sealing washers and/or flare seals.
- Bonnet—Valve Body Seals—The valve bonnet or cover holds the stem and steam seals. Seals, sealing rings, O-rings or gaskets are required between the bonnet and valve body. On larger valves, the bonnet is often bolted to the valve body or larger valve and sealed with a bonnet gasket.
- Valve disk and seat “seals”—Certain valves have a seal material between the plug and seat or seat and body. Some valves use a corrosion and wear resistant alloy to maintain sealing between the plug and seat with aggressive media.
- Sensor or gauge connections—Instrumented valves with integral pressure gauges or other sensors may require a diaphragm seal to protect the instrumentation while reading pressure.
- Valve Seal Functions and Performance
Valve seals have an impact on the main functions of valves, which are to control flow and prevent leakage. Valve seals are critical in preventing the escape of the liquid or gaseous media in the valve such as the fugitive emissions of volatile compounds, release of hazardous or toxic media (gases or liquids), or leak of corrosive compounds into the environment. Bonnet, stem and connection seals also prevent the ingress of contamination into the media flowing through the valve such as bacteria, dust, chemicals, oil or other environmental contaminants. Ingress of contaminants can destroy downstream products and trigger expensive shut downs until the contamination source is found and repaired. The valve seat or plug seals prevent leaks of media past a closed valve seat and in some design-enabled functions of valve mechanisms (e.g., prevent leaks between separate ports or a pressure balancing chamber, diaphragm valve membranes,
gate seals, ball seals).