Process Valves

One of the most common pieces of equipment affected by these standards is the process valve.

Commonly used types are control, globe, gate, plug, ball, relief, and check valves.

All except the relief valve and check valve are activated through a valve stem, which may have either a rotational or linear motion, depending on the design.

The valve stem requires a seal to isolate the process fluid inside the valve from the atmosphere.

The possibility of a leak through this seal makes it a potential source of fugitive emissions.

Since a check valve has no stem or subsequent packing gland, it is not considered a potential source of fugitive emissions and is not subject to the standards. The stem can be sealed to prevent leakage by using a packing gland or O-ring seals. Valves that require the stem to move in and out with or without rotation must use a packing gland.

Professional Achivements

Conventional packing glands are suited for a wide variety of packing material. The most common are various types of braided asbestos that contain lubricants.

Other packing materials include graphite, graphite-impregnated fibers, and tetrafluorethylene polymer. The packing material used depends on the valve application and configuration.

These conventional packing glands can be used over a range of operating temperatures, but at high pressures, these glands must be quite tight to obtain a good seal.

Elastomeric O-rings also are used for sealing process valves. These O-rings provide good sealing, but are not suitable if sliding motion occurs through the packing gland.

These seals are used rarely in high pressure service, and operating temperatures are limited by the seal material.

Bellows seals are more effective for preventing process fluid leaks than the conventional packing gland or any other gland-seal arrangement.

This type of seal incorporates a formed metal bellows that makes a barrier between the disc and body bonnet joint.

The bellows is the weak point of this type of system, and service life can be quite variable.

Consequently, this type of seal normally is backed up with a conventional packing gland and often is fitted with a leak detector in case of failure.

A diaphragm may be used to isolate the working parts of the valve and the environment from the process liquid.

The diaphragm also may be used to control the flow of the process fluid.

In this design, a compressor component pushes the diaphragm toward the valve bottom, throttling the flow. The diaphragm and compressor are connected in a manner so that separating them is impossible under normal working conditions.

When the diaphragm reaches the valve bottom, it seats firmly against the bottom, forming a leak-proof seal.

This configuration is recommended for fluids containing solid particles and for medium-pressure service.

Depending on the diaphragm material, this type of valve can be used at temperatures up to 205oC and in severe acid solutions. If the seal fails, however, a valve using a diaphragm seal can become a source of fugitive emissions.


Flanges and other connectors

Flanges are bolted, gasket-sealed junctions used wherever pipes or other equipment such as vessels, pumps, valves, and heat exchangers may require isolation or removal.

Connectors are all other nonwelded fittings that serve a similar purpose to flanges, which also allow bends in pipes (elbows), joining two pipes (couplings), or joining three or four pipes (tees or crosses).

Connectors typically are threaded.

Flanges may become fugitive emissions sources when leakage occurs because of improperly chosen gaskets or poorly assembled flanges.

The primary cause of flange leakage is thermal stress, which causes deformation of the seal between the flange faces.

Threaded connectors may leak if the threads become damaged or corroded or if tightened without sufficient lubrication or torque.