Primary Areas for Valve Leakage
“Bubble Tight”
- This is where the disc and liner do not form a tight seal.
- It can be caused from poor quality manufacturing – more details are in the next section.
- There are two main tests for disc/liner leakage:
- Bubble Tight is an API 598 or EN 12266-1 test, where the valve is closed and attached on one side to a flange. Water is put poured onto the open side of the valve, and air pressure is applied from the other side. If air bubbles come through the water, then the valve is not considered ‘Bubble Tight’.
- Spark Test shows the density of the liner. A certain voltage is applied to the liner, and, if it goes through the liner, the density of the liner is not good, and media can permeate it. The higher the voltage test, the better quality the liner is.
- Garlock Spark Tests every single disc and liner at 35,000 volts.
- Valves that are not bubble tight could be leaking process media when the valve is supposed to be closed.
Stem Leakage
- This is where the media leaks inside the valve and comes out the stem.
- It can be caused from a poor quality liner or stem seals (more details in the next section).
- There are two main tests for stem leakage, and one test which shows the overall quality of a valve:
- ISO 15848-1: A global test for stem leakage (fugitive emissions) over 4,000 actuation cycles. There are 3 grades of leakage: A (best), B, and C (worst).
- The GAR-SEAL® valves meets Leakage Rate A.
- TA-Luft: A German pass/fail test for stem leakage. To pass, you must meet x10-4 leakage.
- The GAR-SEAL® valve meets Leakage Rate 5x10-8—10,000 times better than requirement!
- EN 51508: A European test for the overall safety of the valve. It is a very comprehensive and time-consuming test which ultimately shows how likely the valve is to fail. There are 4 Safety Ratings: SIL 1 through SIL 4, with SIL 4 being the best.
- The GAR-SEAL® valve meets SIL 3 (mean probability of failure ≥10-4 to <10-3).
- Stem seals are important because they prevent the media from escaping up and out the stem. If the media escapes out the stem, it can be a safety issue; it can contribute to fugitive emissions, and it can damage actuators or the equipment running the valve.
Valve Manufacturing
There are three main molding processes to create a non-metallic disc and liner: Compression Molding, IsoStatic Molding, and Injection Molding.
Compression Molding
This is a less expensive process which presses the PTFE from the top and bottom. This can result in irregular mechanical properties and uneven densities which affect sealing, reliability, and permeation resistance.
IsoStatic Molding
With this process, the mold is surrounded by water, and the water is pressurized, which creates equal pressure in all directions. The result is a homogenous structure with lower porosity. The product has a consistent density with improved permeability, resulting in a better-sealing, longer-lasting valve. The below pictures show the different methods and microscopic pictures of what the PTFE looks like after molding.
All Garlock PTFE valves are IsoStatically Molded.
Injection Molding
This process is used for PFA disc and liners. PFA is more difficult to mold, and this can result in thinner materials or uneven densities.
PFA has a slightly lower temperature rating than PTFE, 320⁰F vs. 400⁰F (160⁰C vs. 200⁰C), and the chemical resistance isn’t quite as good. Permeation resistance is very close–it will depend on the quality of the molding process.
The quality of the disc and liner will affect how well the disc seals against the liner (bubble tight) and permeation through the liner resulting in stem leakage.
Stem Seal
The weakest spot in a valve is where the stem goes through the liner and disc.
Most valves have multiple ways to seal off this area:
Primary Stem Seal
The disc pressing against the liner.
Alternate Stem Seal: O-Rings
O-rings or encapsulated o-rings. O-rings can wear over time or be abraded if abrasive media gets past the primary seal.
Alternate Stem Seal: Spring-Loaded Seals
Spring loaded (similar to Belleville Washers). Springs press elastomeric seal against either the liner or the stem. The result of this is a thicker neck and base on the valve.
With this stem seal, disc and liner repairs can also be more difficult and expensive. If the spring is damaged, you can lose all of the load on your stem seal; plus, the bottom of the valve is not solid, so if there is stem leakage, it can escape out the bottom of the valve.
Alternate Stem Seal: Dynamic Seals
Dynamically self-loaded three-part stem seal that combines compression, labyrinth, and rotary seal concepts into one unique system. This patented stem seal uses a PTFE carrier ring energized by two Viton o-rings.
Garlock valves use the primary stem seal (disc against liner) plus the patented dynamically self-loaded stem seal.
Additional Seals
Some valves have additional stem seals in the valve neck for added protection. Additional stem seals are important, because they add an extra layer of protection in case the media gets past the primary and secondary stem seals. They also prevent external contaminants from getting inside the valve.
Single O-Ring Seal
One o-ring pressing against the stem and body.
Multiple O-Rings
O-rings in different positions to press against multiple sealing surfaces.
Garlock valves use multiple o-rings in various positions to seal against both the stem and the body.
