Live Loaded Packing vs. O-Ring Stem Seals
LIVE LOADED PACKING
Fugitive emissions in piping systems are usually a result of leakages at the valve stems. A simple way to avoid leaks is using a packing material between the bonnet and the valve stem. Stud bolts exert compressive forces on the packing material, generating sufficient mechanical stress on it. This stress causes the packing material to tightly seal against the valve stem and the bonnet’s interior wall.
Constant packing stress has to be retained for the sealing to remain effective. It gradually decreases due to creep, thermal expansion or vibrations. Live loaded packing guarantees sealing efficiency despite the recurrence of these operational variations.
Live loading uses conically shaped Belleville spring washers between the gland follower and the fasteners of a valve with packing. The spring provides a means for controlling the intensity of the packing stress by compression or deflection. Under this setup, the follower continuously exerts pressure against the packing material, even after the packing volume is lost or withdrawn. Tensioning of the disc springs is achieved by compression to a specific load at flat. The pre-tensioned springs supply substantial force required to retain the packing stress at a level which is higher than the minimum sealing stress, preventing any leaks.
Live loading is beneficial for use in valves that remain unused for extended periods of time, such as isolation valves. These valves lose initial packing stress, increasing their susceptibility to leakage upon activation. Live loaded packing enhances long-term performance of frequently cycled valves, whose packing tends to wear faster around the valve stem. They are resilient to high pressure and higher temperature operations.
Live loaded valves require little maintenance, making them the perfect candidates for critical or frequently cycled applications, as well as for valves in inaccessible locations.
O-RING STEM SEALS
O-rings are versatile sealing components, and most can operate over a wide pressure range, up to 1500 psi. The sealing between the valve stem and the body is a function of the elasticity of the O-ring material (fluoroelastomer). The sealing process begins as the pressure of the working media rises or falls. The seal created is either static or dynamic. In a static seal, the O-ring does not move and is primarily used to maintain upstream pressure or create a vacuum within the pipeline. For dynamic sealing, the O-ring sits inside a groove between rotating surfaces, providing a leak-tight seal.
An increase in fluid pressure causes the O-ring to deform, exerting extra pressure on the valve stem. When fluid pressure drops, the compressive stress that resulted in the deformation of the O-ring gets withdrawn. As this happens, the O-ring relaxes and fills the space between the stem and the bonnet. Since they’re highly elastic, O-rings require backup mechanisms such as Polytetrafluoroethylene (PTFE) to prevent expansion beyond the retaining glands.
O-rings are preferable for their reliability, low cost, ease of maintenance and replacement. They are commonly used to seal spool valves, pistons in pneumatic cylinders as well as some ball valves and butterfly valves.
OTHER SEALING OPTIONS
Valve sealing is not limited to live loaded packing and O-ring stem seals alone. There are several other methods employed to prevent fugitive emissions and unwanted media leaks. The most common sealing method is the use of a graphite-based gasket between flanges. Other seals are made from die-formed rings, which are pre-stressed to meet a valve’s operational parameters. Die-formed sealing rings improve the packing life cycles and minimize consolidation, a common cause of leaks.
VALVE SEAL SELECTION CRITERIA
Leakage tightness
Leakages and fugitive emissions are expressed as flow rates (cu ft/sec or parts per million (ppm). Leak tightness for different industrial valves is defined using industry standards such as API 624, API 641 or ISO 158248. For example, according to API 624, the allowable fugitive emissions limit for rotating stem valves should not exceed 100 ppm.
When selecting a valve seal, it is salient to identify the intended leakage class (categorized as AH, BH and CH). Metal bellows provide sufficient leak protection for stem valves classified AH. It is mandatory to use live loaded packing seals for class BH leakage classification.
Temperature and pressure
Valve seals should perform well over a wide range of pressure and temperature. Cryogenic valves control flow at temperatures as low as -238o F, with others operating at temperatures above 752o F. Process temperature determines if the seal to be used is fabricated from a nonmetal or metal.
Pressure conditions are vital for predicting the permeability and compressive resistance of the seals over time. Live loaded packing stem seals, reinforced with graphite, will be preferable for a frequently cycled process operating at over 401o F, and pressure above 1450 psi.
Service-specific requirements
Ideally, valve seals should control emissions. Their leak protection capabilities decrease as the working cycles increase. The presence of volatile compounds in the system equally lowers their performance. Different standards exist to qualify valve seals for fugitive emission service.
Steam service valve seals are the most difficult to select. Facilities such as power plants, which have steam as the working medium, operate under high temperature, high pressure and are subject to frequent valve cycles. Such plants use live loaded packings that compensate for the thermal strain and are fitted with oxidation inhibitors.
Hydrogen service applications use a combination of live loaded packings and O-ring stem seals. Hydrogen gas is unstable and permeable. Valve seals for hydrogen service shouldn’t contain any PTFE component, as it evaporates when heated. Combining that with the highly combustible nature of hydrogen could lead to fire accidents.
O-rings and non-metallic packings are commonly used in oxygen service processes. Their selection is based on the ability to inhibit ignition and the proliferation of fire in case of accidents.
SUMMARY
Suitable seals that meet operational requirements, while enhancing the durability of valves, should be selected. Limiting fugitive emissions not only improves safety, but also enhances the compliance of a facility or process to statutory regulations. A valve seal should have excellent resistance to aging, low ignition sensitivity and the ability to operate over a wide pressure and temperature range. Selection criteria differ depending on the nature of the working media.
