The importance of stem seal configuration in ISO 15848-1 certified valves

Successfully sealing a valve depends on three major pillars:

• The use of a high quality and well selected stem seal is required

• The valve condition needs to be suitable for containing the seal

• The valve seal needs to be installed correctly

These three pillars are equally important and not meeting one of these three requirements will result in failure to meet

the low emissions requirements. The first step in the certification process is to use a stem seal that is able to seal according to these low emission requirements. Low E packings have special fillers and blocking agents to make them practically impermeable for the smallest molecules such as helium molecules. They are often graphite based for use at higher temperatures. Graphite is an excellent seal when it comes to relatively large molecules, such as water molecules (in steam applications) but to seal hydrocarbons and gases like helium the gaps in the graphite structure have to be filled with other materials. PTFE packings have excellent sealing properties but are limited to lower temperatures up to 260°C which also limits their use in applications where fire safety is a requirement.

Spring energized seals are polymer seals that are energized by internal system pressure and are equipped with a spring to make them seal at lower pressures. They can be made from different polymers with PTFE as an option. Sometimes back up rings are required to increase their reliability and pressure capabilities. The used springs are manufactured from corrosion resistant materials to withstand the extreme conditions that they often have to operate in. Spring energized seals are often used on cryogenic applications using special materials for the spring and the seal jacket because compression packings tend to be a less suitable solution.

In the case of compression packings the sealing stress is created by a gland and gland follower. This force is transduced in the packing material to a radial force. This radial force makes the packing seal and also gives stem friction. The higher the gland load, the higher the sealing force and the lower the leakage. So to make low E packings perform they need a relatively high stress to compress them well. Traditional valves often don’t have the structural strength to deal with these requirements therefore modifications need to be made to the stuffing box configuration such as the gland bolts and the gland. An upgrade to bolt materials with higher yield strength might have to be made. The downside of a higher sealing force is increased stem friction. This is something that needs to be taken into consideration when designing a hand wheel or lever in a manual valve, or the actuator or motor in an air or motor operated valve.

Stuffing box clearances often allow extrusion of the packing materials, especially at the higher sealing forces that emission packings require. Extrusion of packing material leads to loss of the integrity of the packing material and to volume loss, which in its place means loss of sealing force. This means that stuffing box clearance needs to be kept to a minimum while maintaining correct operability of the valve. Critical areas are the stuffing box bottom, the gland follower inner diameter and the gland follower outer diameter. Some other design features such as chamfers and fillets on equipment components must be evaluated in this process to eliminate precursors for extrusion.

It is a challenging task to certify valves according to modern emissions requirements. However it is not an impossible task. Since the stem seal is the most critical component in this respect it is crucial to select the correct stem seal, ensure that the valve meets the requirements for the seal to optimally function in and make sure that the seal is installed in the correct way.