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Valves in a Cement Slurry Line

Valves in a Cement Slurry Line

Basically everywhere you look in modern ...

Triple Offset Butterfly Valves

Triple Offset Butterfly Valves

Since their introduction to the market m...

Digital Valve Control Leads to Increased Plant Availability

Digital Valve Control Leads to Increased Plant Availability

Surge is characterized by fast flow reve...

Cast vs. Forged: The Ongoing Debate Takes a New Direction

Cast vs. Forged: The Ongoing Debate Takes a New Direction

In the valve industry, the cast versus f...

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Industry Headlines

Industry Headlines

VMA Members Among Plant Engineering 2016 Product of the Year Finalists

2 DAYS AGO

Several VMA members are among this year’s Plant Engineering Product of the Year finalists. Emerson has products nominated in four different categories, while Siemens has several products nominated in a total of three different categories. Chesterton and Hunt Valve have products up for awards the...

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MSS Publishes Revised American National Standard for Steel Pipeline Flanges and Receives ANSI Approval

3 DAYS AGO

The Manufacturers Standardization Society (MSS) announces that the substantially revised Standard Practice, SP-44-2016, Steel Pipeline Flanges, has been approved by the American National Standards Institute (ANSI) as a Revised American National Standard (ANS).

The first edition of MSS SP-44 was publish...

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U.S. Chemical Industry Remains Optimistic for 2017

2 DAYS AGO

Moving into 2017, the U.S. Chemical Processing Industry continues to enjoy optimism about future investment, according to Industrial Info's 2017 Global Industrial Outlook . Much of this activity stems from the continued low cost of natural gas liquids (NGLs), a primary feedstock for building-block che...

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Hydraulic Institute Celebrates Centennial with New Logo

3 DAYS AGO
Hydraulic Institute Celebrates Centennial with New Logo

The Hydraulic Institute (HI) will celebrate its centennial in 2017 with a new logo as part of a larger initiative to position HI for the next 100 years of service to the pump industry. The new logo is a key element of HI’s overall brand refresh and redesign process.

The logo includes a hidden &ld...

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U.S. Adds 178,000 Jobs, Unemployment Rate at 4.6%

1 DAY AGO

The unemployment rate declined 0.3% to 4.6% in November, and total nonfarm payroll employment increased by 178,000, the Department of Labor reported today. Employment gains occurred in professional and business services and in health care.

Employment in construction continued on its recent upward trend...

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Third Quarter GDP Revised Up to 3.2% Growth

3 DAYS AGO

Gross domestic product (GDP) in the U.S. increased at an annual rate of 3.2% in the third quarter of 2016, according to the second estimate released by the Department of Commerce. In the second quarter, real GDP increased 1.4%.

This new GDP estimate is based on more complete source data than were avail...

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Valve Live Loading Using Belleville Springs

The "live loading" of valve packing has been the topic of many articles and papers over the past few years. While highly recommended by some, others claim it has no value. The truth is both statements can be correct-it all depends on the individual application.

The problem is determining if a valve is a candidate for live loading. In most cases this can be decided by answering a few questions:

  • Is it a packed valve?
  • Does the valve cycle frequently?
  • Is it motor operated?
  • Is the valve difficult to get at?
  • Is the valve subject to high temperatures and pressures?
  • Is the valve in a critical application?
  • Does the valve have a history of packing leaks?
  • Does the valve have to be monitored under EPA regulations?

If the answer to the first question and any of the others is "yes," there is a good possibility the valve is a candidate for live loading.

Many different packing materials can and have been live loaded, but since the graphite-based materials are the most commonly used in live-load applications, the data referred to is based on these materials.

Live Loading
In its simplest form, live loading is the application of a spring load to the gland follower of a packed valve. A Belleville spring between the gland follower and its fastening studs and nuts provides an effective way to establish and maintain a controlled amount of stress in the packing set. The amount of the packing stress in a live-loaded system can be controlled by the size of the Belleville spring used and how far it is compressed or deflected. In a live-loaded packing system, the follower will continue to push against the packing even when packing volume is lost (by friction, extrusion, consolidation, etc.) The spring load will be slightly reduced as the springs expand, but this reduction in load will be much less than the load that is lost if the packing set was not live loaded. This remaining load allows the packing stress to remain at a level above the minimum sealing stress and enables the packing to remain leak free. There are many reasons why packed valves leak.

The most common problems are listed here, though some cannot be overcome by live loading alone:

  • Wrong packing material for the application.
  • Bent, scored, or pitted valve stems. No amount of spring load can overcome the damage these can cause to a packing set.
  • Improper packing installation. This may be the single biggest reason repacked valves leak. Graphite-based materials are not as forgiving as asbestos. These materials must be installed properly. This includes initially consolidating the packing by cycling the valve and retorqueing. This may take from as few as three to five times, to as many as 15 to 20. Installation procedures can vary from one packing manufacturer to another. Follow the manufacturer's procedures, whether the repacking is being done in house or by an outside contractor. The benefits of live loading are greatest when the packing has been installed properly.

Infrequent Use
A valve that is not used often, like an isolation valve, tends to leak when it has opened or closed after remaining unused for an extended period of time. What has happened is that the packing has consolidated over time, and the initial compressive load is reduced. Also, when the valve is actuated, additional compressive load is lost. If this reduction in packing stress falls below the minimum seal pressure, the valve will leak. Valves that fall into this category are not typically live loaded, but the addition of a single set of springs has been beneficial.

Frequent or High Cycles
When a valve is cycled, the packing around the stem wears. As the packing is lost, the stress on the packing is reduced. If left unchecked, the valve will eventually leak, which becomes very apparent on highly cycled valves, typically control and motor-operated valves. The normal method of overcoming this is frequent retightening of the gland follower bolts. Live loading can alleviate this stress reduction and eliminate the need for constant retightening.

High Temperatures, Pressures and Critical Applications
When valves work at high temperatures and pressures, they are usually in a critical application. Keeping these valves from leaking can be a difficult job. They are subjected to additional factors that make it harder to keep the proper stress on the packing set.

High temperatures can cause the gland follower bolts to creep or relax, which will reduce the stress on the packing set. High pressures usually mean higher packing loads, which can be difficult to maintain. Critical application usually means a need for a higher standard of safety. Live loading can be used to help eliminate these problems.

Maintenance Headaches
Valves that have been chronic leakers and valves that are inaccessible also are good candidates for live loading. Live-loaded valves require less packing maintenance, and the controlled load on the packing set may just help those chronic leakers.

EPA Monitoring

The Clean Air Act has made everyone take a closer look at packing leaks. If a valve has to be monitored, it must not only meet the EPA standard leak rate, but must be able to maintain that leak rate. Also, it must be able to do this without being adjusted. Controlled load on the packing set is essential in attaining this. Live loading may be the most cost-effective way to meet the EPA's standards.

Almost all of the major valve manufacturers, valve rebuilders, packing manufacturers, and packing distributors now have live-loading programs. Some are more complex then others, but all of them work basically the same way: Belleville springs are used to maintain the load on the packing set.

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