SPECIAL TOPIC : SEALING & PACKING TECHNOLOGIES
The Importance of Valve Sealing and Packing
Conveying a substance from one place to another along a tube would be a simple affair if one never needed to control its flow . A single unbroken piece of pipe can be relied upon to do a effective job conveying material without issue , at least until corrosion or human error betrays it . The need to control flow necessitates a valve . This means finding a way to have a valve mechanism move freely across the boundary of the pipe . Suddenly , one is actively introducing rupture where it seemingly does not belong .
By Justin Yule
only a negligible percentage of the total , these could be tolerated . Packing ring selection was largely a matter of making sure high-pressures did not blow the packing from the valve and cause major loss . Legislation introduced a new consideration . Even financially negligible amounts of spilled process substance and fugitive emissions can create major environmental and safety concerns , and governments had decided to compel industry to bear this in mind . This intervention initiated new developments in sealing and packing technology that would bring leakage down to a bear minimum while still allowing for the smooth functioning of valves in their basic task of opening and closing . Emissions limits were first brought down to 500 ppm , but have since dropped to 100 ppm , and even 50 ppm in some cases . Innovation was needed .
A response to this new challenge has been ‘ live loading ’. The traditional approach to packing involves packing materials that are compressed by bolts against the valve stem . This is called ‘ stationary loading ’ or ‘ jam packing ’. In this set-up , the only elasticity in the system is that of the packing
Sealing and Packing valves is a serious challenge that occupies a position on the front-lines against fugitive emissions and environmental devastation . Ever more stringent environmental regulations , along with demands for high performance and low maintenance , have made the valve packing selection process more critical .
Why Do We Need Sealing and Packing ?
Stem-actuated valves use a ‘ stem ’ to perform its basic function . The stem is a shaft that passes from the interior of the valve , where it does the all-important work of opening or closing the flow of process substance , to the exterior where an operator can actuate the valve , either manually or using an electronic actuator . Whereas other joints in a pipe system can be sealed with a relatively simple gasket , the point of entry of the stem into the valve itself must navigate the cross-purposes of preventing leakage and allowing free movement . As such , much thought has been given to devising mechanism that will solve the conundrum of allowing and disallowing movement of different kinds in the same location .
What is Sealing and Packing ?
Control valves generally fall into two main categories which dictate the nature and movement of the valve stem . Sliding stem valves have a stem that moves perpendicular to the flow of the pipe , while rotary stem valves have a stem that rotates around an axis perpendicular to the flow of the pipe . Whether the stem is rotating or moving in and out , its movement is essential to its proper function . Therefore , the valve stem must be held in place with a material flexible enough that it can be compressed against the stem to create a seal , but that also allows for the relatively frictionless movement of the stem . The sealing mechanism , housed within the valve bonnet , generally consists of packing material formed into rings around the stem and packed tightly into the bonnet . A packing flange is then placed against the rings of packing material and tightened with nuts onto the bonnet . This flange places consistent compression force on the packing rings and creates the seal .
For sliding stem valves , a different type of sealing mechanism is possible , one that makes only secondary use of packing materials . A bellows seal is an accordion-like metal tube that encompasses a portion of the valve stem . One end of the bellows seal is attached the stem , and the other is attached to the valve bonnet . The flexible material of the bellows permits the valve stem to move in and out with very little friction while also presenting no point of vulnerability for leakage – it is an uninterrupted piece of metal . A leak detection port placed above the bellows allows for detection of rupture to the seal , and if this occurs , a secondary sealing mechanism of standard packing is present to prevent full leakage .
The Recent History of Sealing and Packing
Before the introduction of legislation to restrict and control dangerous emissions , like the USA Clean Air Act Amendments by the Environmental Protection Agency , valve packing was an under-theorized and under-engineered area of concern . Financial incentives determined that it was generally in the interests of operators to not regularly lose large amounts of process substance out of the valve aperture , but if there were smaller leaks that represented www . valve-world-americas . net • October 2024 | Valve World Americas 15