From time to time, we are re-posting well-received or particularly valuable articles that have previously run on VALVEMagazine.com so that those who might have missed them will be able to catch up on the best of the best. This article, “Improving Valve Sealing Performance and Reliability” initially ran on November 17, 2014.
Approximately 300,000 tons of fugitive emissions are released each year in the United States, according to the Fluid Sealing Association, and regulations put in place to lower that number are being added every year.
Combine that regulatory pressure and the stresses of today’s economic environment, and it is obvious that original equipment manufacturers (OEMs) and end users are increasingly seeking solutions that not only reduce emissions but also result in a permanent and noticeable increase in the service life of their products and equipment. Performance, value, reliability, repeatability and safety are all factors that are taken into account when developing and purchasing products to perform in this environment.
With regulations such as ISO 15848-1 and -2, and API 622 and 624, as well as many other government and company-centric regulations, there is no one product that meets or exceeds all of the needs and requirements that exist, so today’s refinery, chemical, petrochemical and fossil power industries need a program that brings each of the equipment components together while allowing users to monitor and improve performance.
Taking into consideration all plant equipment and applications, the greatest contributors to fugitive emissions are valves. These valves come in a variety of sizes and configurations; whether at the valve stem, a flange surface or stuffing box, valves can have multiple leaks through different leak paths—one of the reasons that no single product can reduce or eliminate leakage for the entire valve.
A comprehensive valve enhancement program created to improve sealing looks at the entirety of the valve and addresses performance and reliability issues to not only decrease leakage, but also to improve valve performance and efficiency.
Suppliers of sealing products, live-load springs and valve packing monitoring devices (VPMD) are brought together, and the products are supported by a software database, on-site valve support and technical support organizations. Each of the components and specific operating parameters are evaluated to determine how they enhance valve performance and operations while reducing or eliminating leakage.
Whether it is a mechanical seal or packing, or static seals, the type of materials used to seal a valve is a major factor that affects the level of fugitive emissions that escape to the atmosphere. As advances have been made in machining, braiding and manufacturing technology, more material options are available.
For example, rotary equipment mechanical seals can be produced from a variety of materials, including carbons, carbides (such as tungsten and silicon), aluminum, chromium, stainless steel, bronze and titanium, among many others. For the flexible parts of these components, options include rubber elastomers (such as butyl, ethylene-propylene, nitrile and chloroprene), graphite and polytetrafluoroethylene (PTFE). Rotary and valve mechanical packing can be produced with natural vegetable fibers (such as jute, ramie and cotton) or manmade fibers (such as acrylic, aramid, melamine, carbon and graphite).
With the development of next-generation yarns, which provide improved friction coefficients and sealing capability, long-term sealing is achievable. Several major fluid sealing manufacturers have developed yarns that offer advantages in friction and consolidation level reduction, thermal evacuation and responsiveness. In conjunction with a valve enhancement program, these advancements offer materials certified to API 622, TA Luft and ISO 15848 that meet today’s stringent emissions standards.
Another common sealing option is die-formed rings. Through the process of die-forming, materials are compressed in a die created to meet the application’s dimensional specifications. The materials are compressed to the required height and density based on the internal stuffing box dimensions and the system operating parameters. Die-formed rings are available in many materials and set configurations, and when installed and consolidated correctly, bring improved packing life cycle to any valve application. The die-forming process allows the rings to essentially be preloaded prior to installation, decreasing the level of in-service consolidation and the potential for leaks due to loss of gland load.
To meet the criteria for current regulations including ISO 15848-1 and 2, TA Luft and API 622 and 624, quality suppliers are incorporating the use of new-generation materials such as graphite and carbon impregnated with PTFE or other proprietary lubricants.
The packing and gasket materials, configurations and supportive components are engineered for the environment in which the material is required to perform. In many cases with next-generation materials, plant standardization to a limited number of materials and configurations is possible. In other applications, further evaluation and non-conventional approaches are required to achieve effective valve sealing.
Live loading is defined as the use of a spring(s) mechanism to maintain a calculated load on the gland follower bolts during the deflection capability of the spring stack. Disc springs are placed between the gland and nut to create and maintain a consistent recommended load on the packing. The life cycle or capability of maintaining such load is relative to the design of the stack and the deflection capabilities of the spring. The energy in the springs helps to maintain the load on the gland follower whenever there may be relaxation due to consolidation, thermal shifts or other internal changes within the stuffing box. Proper live loading can reduce leak paths, which cause valves to leak.
The concept of live-loading valve gland follower bolts has been used for many years and has improved valve sealing, particularly in high-cycle valves with short strokes. Due to the low cost associated with adding this enhancement, many plant operators have added the live-load concept to valves that fall outside the category of modulating valve stems.
Live-load springs are commonly installed on valves that experience a decreased gland load due to in-service packing consolidation. There are two forms of consolidation that can create potential leakage issues: initial and in-service. Initial consolidation, or lack thereof, occurs during the installation phase when insufficient valve stem stroking prevents the packing rings from consolidating. In-service consolidation occurs during the life cycle of the packing and the operation of the valve, and is the result of stem cycling while the valve is operational.
If live load springs are included as part of a valve enhancement program, a spring set design is required. Data for the valve and application, including dimensions and parameters for the valve and system, are required.
Without live loading, there may be increased valve leakage and personnel may need to complete more frequent gland adjustments, especially in modulating valves. When incorporated as part of an overall valve enhancement program, the live-load springs work to help maintain the sealing capability and extend the service life, and offer the added benefit of decreasing maintenance and monitoring by plant personnel.
Valve Packing Monitoring Device (VPMD)
A VPMD is a combination of live-load springs and a visual indication device that is based on valve type and application, and gland loads. The same data is required and provided as with live-load springs. The VPMD provides an easy-to-read visual indication of the status of the mechanical packing. When the VPMD is installed, it is set to maintain a specific load. As changes to the packing occur—whether through packing consolidation, thermal changes or other effects—the level can be monitored with a quick visual check. This avoids the need to use tools such as a torque wrench to physically check and confirm that each gland stud load is adequate.
The visual indicators provide personnel with confirmation that the packing is maintaining sufficient load or when in-service consolidation has occurred and the gland load is decreasing. When the indicator reflects loss of load conditions, valve stem leakage can be expected. This early load loss indicator notifies personnel before a leak occurs so the packing can be re-torqued, repacked, repaired or replaced, as necessary.
A remote monitoring capability device is currently in development. This device permits monitoring of valves that are equipped with the monitoring device from a remote location (such as the control room). Remote monitoring allows for better management of staff hours to inspect valve packing loads, improves safety for valves in inaccessible areas and can be used in geographically large areas.
Unlike other concepts that can monitor the level of leakage as it occurs, VPMD is designed to provide advance warning of potential valve leakage. Such devices are available for a large percentage of valve types and sizes, and are limited only by the actual dimensional limitations.
To make the most of a valve enhancement program, the actual valves should meet the requirements of API 624, API 622, ISO 15848, TA Luft and other VOC emission standards. With a variety of valve designs and styles available, sealing solutions are available that meet the most stringent applications and criteria.
Monitoring each component is an essential part of any program designed to improve performance. Each component is tracked from manufacturing to installation and throughout its installed life. For each product, required specifications are maintained, inspected and provided to the end user. After installation, the product’s performance is monitored and tracked through the database to ensure it meets its warrantied life. Any maintenance or repairs performed are tracked and trended. The resulting reports can be filtered to give the end user a picture of how individual products in the program are performing as well as the overall health and operation of equipment.
On-Site Valve Support
One of the main challenges related to effective fluid sealing is the existing valve population throughout a given plant, which has been operating for many years. These valves have been exposed to severe operating conditions and numerous packing removals and installations. They’ve also experienced packing leaks that in some cases, due to steam, created stem and stuffing box steam cuts. And in many cases, valves have been neglected over time.
With the new stringent regulations, the industry is facing challenges because no differentiation is made between new equipment and equipment that has been operating in facilities for many years. A control valve that has been operating in the plant for the last decade is held to the same leakage standards as recently installed valves. Until older valves are replaced, there will be leakage issues and challenges to overcome. Whether it is damaged, bent or pitted stems, bolted joints with irregular sealing surfaces or stuffing boxes in less than optimum condition, all of these items may affect performance of the valve and lead to excessive leakage. An effective program has to address this issue and older valves in questionable condition.
To make a valve enhancement program effective, qualified valve support companies must be in place to perform site work. Technicians can perform examinations, determine root cause and repair or replace equipment. All work is tracked through the database, which allows for the analysis of trends developing with a particular piece of equipment or product. The valve refurbishment process—from the disassembly, refurbishment and reassembly process—can be completed on-site by experienced valve teams supported by mobile valve repair units brought to the site. Valves can be repaired and components installed at the time of repair. Following repair, each valve becomes part of the program and tagged to reflect the components installed, date of completion, valve ID number, contact information and loading information. For those valves not requiring this service, inclusion in the program occurs when valve components are installed in the specific valve.
Engineered solutions through well-developed fluid sealing programs that track all components and service are effective tools for reducing leakage and improving performance.