If you cannot remember the last time you tested your valves for leakage, make that time now. A leaking valve can invite media (liquid or gas) into a chamber or outside the valve where it can cause serious damage. If that media should build up in the chamber, it can lead to more than valve damage – it can lead to hazardous leaks and significantly increase risk of explosions.
Valve Leakage Classifications
In order to detect and stop valve leakage, it is necessary to understand the most common forms of leakage. The American National Standards Institute and the Fluid Controls Institute have six different valve leakage classifications.
Class I Valve Leakage – Considered to be the least dangerous of leaks, it is known as a dust tight leak and often refers to metal or resilient seated valves.
Class II Valve Leakage – This classification is intended for valves with double port or balanced single port that use a metal piston ring seal and metal to metal seats with a leakage of 0.5% in full open valve capacity.
Class III Valve Leakage – This classification uses the same types of valves as shown in the above but at 0.1% in full open valve capacity.
Class IV Valve Leakage – This type of classification is in regards to single port and balanced single port valves that have extra tight piston seals and metal to-metal seats with a leakage of 0.01% in full open valve capacity.
Class V Valve Leakage – This classification is the same as Class IV but for when the test liquid is at 100 psig. Some leakage is allowed but limited to 5 x 10-4 ml per minute per inch of orifice diameter per psi differential.
Class VI Valve Leakage – Known as soft seat leak, it involves valves where the seat and/or shut-off disc are made from resilient material like Polytetrafluoroethylene. Often the test fluid is some kind of gas like nitrogen, and leakage can be from 0.15 up to 6.75 ml per minute.
Valve Leakage Testing and Detection
The best way to reduce and prevent leakage is to constantly test for it or have a detection method in place. Many valve tests are performed with water or air at 3.5 bar, unless looking for a Class V leak. In this case, the valve would be tested at its maximum operating pressure.
American Petroleum Institute (API) standard 598 outlines the testing requirements for many types of valves, including ball, check, gate, globe, plug and butterfly. The standard covers acceptable leakage rates for liquid along with gas testing. All valves that have been built to an API standard are required to meet API-598 criteria for leakage prior to delivery from the supplier or manufacturer.
Valve Leakage and Safety
Valves that contain volatile gases such as natural gas or other combustible media can have a huge impact on safety when leaking. In fact, the largest industrial disaster in history is suspected to have occurred because of a gas leak. The Bhopal disaster occurred in India and killed thousands of people when lack of routine maintenance caused a backflow into a tank. It ended up releasing a toxic methyl isocyanate gas into the plant and throughout the surrounding town. Another well-known disaster is the BP Deepwater Horizon oil spill in 2010, which is also thought to have been the result of the failure of two valves tasked with halting the flow of oil and gas.
In addition to being a source of tremendous industrial accidents, valve leakage can also lead to wasted consumption of energy and other operating losses. Valve leaks can also cause a drop in system pressure that forces other air tools to function less efficiently and negatively impact production.
How to Establish a Valve Leakage Prevention Program
Setting up and following through on a good leak prevention program must include the following:
- Identification of valve leaks (including tagging)
- Tracking of leaks
- Swift repair of leaks
- Verification that valve leakage has been repaired
- Employee encouragement to look for and repair leaks implemented at all levels from managerial to custodial.
Facilities and plants who use compressed air systems should be especially aggressive in establishing a valve leakage reduction program. This can include hiring out leakage experts, conducting test and repairs in-house, or implementing a combination of the two. Once a valve leak or leaks have been found and repaired, the entire system should be re-evaluated.