1. Valve Live Loading Using Belleville Springs

What is Live Loading?
Live loading is the application of a spring load to the gland follower of a packed valve. A Belleville spring is fitted between the gland follower and its fastening studs and nuts. This maintains a controlled amount of packing stress. How much of this packing stress can be controlled depends on the size of the Belleville spring and how far it is compressed or deflected. Many different packing materials can and have been live loaded, but the graphite-based materials are the most commonly used.

In a live-loaded packing system, the gland follower continues to push against the packing, even when packing volume is lost (due to friction, extrusion, consolidation, etc.) The spring load reduces slightly 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.

Selecting the right valve for Live Loading
To choose the appropriate valve for live loading, there are a few questions that need to be considered:

• 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 any of these questions is a"yes," then you've found the right valve for the process.

Other valves that are good candidates for live loading are those that have been leaking continuously and those that are inaccessible. Live-loaded valves require less packing maintenance and the controlled load on the packing set may just help those chronic leakers.

Leakage Issues in Valves
Some valve leakage issues can be overcome by live-loading. Although, the most common problems like the ones listed below can be prevented by regular maintaining the valves and materials used in the application.

• Wrong packing material used 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 step may be done anywhere between 5-20 times.
  
  Besides, 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 has not been used for an extended period of time tends to leak when opened and closed. This is because, the packing has consolidated over time and the initial compressive load is reduced. Also, when the valve is actuated, additional compressive load is lost. This reduction in packing stress falls below the minimum seal pressure, which results in leakage. Valves that fall into this category are not typically live loaded, but adding another single set of springs can prove to be handy.
  

• 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. To overcome this issue, the gland follower bolts need to be frequently tightened. Live loading can lessen this stress reduction and remove 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.

EPA Regulations
The Clean Air Act has made everyone take a closer look at packing leaks. If a valve has to be monitored, it must meet as well as maintain the EPA standard leak rate 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 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 than others, but all of them work more or less the same way: Belleville springs are used to maintain the load on the packing set.

2. Linear Actuators– Crucial Devices for Automating Gate Valves

What is a Linear Actuator?
Typically, multi-turn electric actuators have been used when automating gate valves. However, most folks in the valve industry are familiar with operating globe valves using compressed air and a linear actuator. Besides, a linear actuator is simple and reliable with just one moving part.

Attaching a linear actuator to a gate valve
It’s quite straightforward. First, remove the handwheel and drive nut to expose the threaded stem. Then, use a coupling to connect the stem to the piston rod. This allows the linear actuator to move the stem up and down directly. The actuator will produce "thrust" not "torque” which is a critical specification for linear actuator design.

As it turns out, the size of the cylinder is a function of the required valve thrust and the available supply (pressure x area = force). The cylinder is larger for a higher required thrust and a lower supply pressure. Since the actuator price increases as the cylinder size increases, engineers must verify that the supply pressure given is the highest available at the project site.

Moreover, if the calculated thrust is based on the maximum differential pressure rating across the valve (as defined by ANSI) and not actual operating design conditions, the specified thrust might be much higher than actual, which would require a larger cylinder and thus a higher price. Therefore, it is best to specify thrust based on actual design conditions to get the best price.

Benefits
Linear actuators can be an effective automation solution for gate valves. As automation increases, so should our choices. That the linear actuator has proven reliable in many applications outside of the valve industry is testament to the enduring design of the pneumatic and hydraulic cylinder.