I just saw a post, Article: Proper Valve Stem Sealing Best Practices, in the Valves Forum in the Valves, Actuators & Regulators group in the Emerson Exchange 365 community. The post by Emerson’s Mark Nymeyer recaps an Intech article by Lisa Miller, Proper Valve Stem Sealing Best Practices.
Here is an excerpt of the opening of the post, and I invite you to read the full summary as well as Lisa’s article.
Emerson Automation Solutions Senior Engineering Manager Lisa Miller recently published an article in the May/Jun 2022 issue of InTech. It is titled “Proper Valve Stem Sealing Best Practices” and it describes how control valve stem sealing works, and shows how to select the best option for each particular application. A summary of the article follows.
There are many different methods of sealing valve stems on control and isolation valves. When chosen wisely, a valve stem seal can provide years of reliable service, reduce environmental emissions, and minimize product loss. When chosen poorly, valve stem seals can leak consistently, increasing costs, creating environmental issues, and even placing operating personnel at risk.
This article explains various methods of achieving optimal valve stem sealing, helping end users evaluate the best choice for their application.
What are valve stem seals?
Control and block valves are typically one of two types: sliding stem or rotary. A sliding stem valve has a rod protruding from the body that rises and falls to actuate the valve. A rotary valve has a shaft extending out the side that is connected to a plug, disc, or ball. As the shaft turns, the rotary valve opens and closes. In either design, the valve stem must exit the body and be capable of relatively friction-free movement, yet contain the process media. This is not an easy task, as the author explains:
Valve stem seals must accomplish two contradictory goals. First, they must seal the valve stem completely and reduce – ideally eliminate – any fugitive emissions from the process. Secondly, they must accomplish this feat while allowing the valve stem to move freely and continue sealing, even as the valve stem cycles thousands of times.
Measuring stem seal performance
The three main industrial standards addressing valve stem leakage are TA Luft, FCI 91-1, and ISO 15848. However, the required performance and test methods for each standard varies significantly.
TA Luft is the least comprehensive standard, offering leak rate standards but no test parameter details. FCI 91-1 is more closely aligned to EPA’s LDAR program and uses the EPA Method 21 to “sniff” the valve packing and determine the leak rate (Figure 1). This standard does provide details on how a valve is to be tested, offering various classification ratings based on the leak rate after a specified number of mechanical and thermal cycles.
Read the rest of the post for more on sealing valve stems with packing, sealing valve stems with bellows, and a comparison of the pros and cons of packing versus bellow technology.
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