Valves leak. There’s no getting around it. In fact, industry sealing experts have cited that 60% of all fugitive emissions are from valve leaks. In addition, it is estimated that 75% of these valves leak at the stem. Improvements in packing materials and design have led to significant reductions in emissions in all industries. However, any stem seal needs stress to be effective, and packing stress fails over time for a variety of reasons.
Valve live loading is effective at reducing the loss of packing stress. In its simplest form, valve live loading is the application of a spring load to the gland follower of a packed valve.
Determining if your valve application is a candidate for live loading can best be decided by answering “yes” to any of the following questions:
- Is it a packed valve?
- Does the valve cycle frequently?
- Is it motor-operated?
- Is the valve difficult to access?
- Is the valve subject to high-temperature or high-pressure cycles?
- Is the application critical or a safety concern?
- Is there a history of packing leaks?
- Is the valve subject to environmental safety concerns or EPA regulations?
The more “yes” answers, the greater the risk that the valve may leak or that any potential leak would be costly.
Belleville springs, compression washers, flange washers, Belleville disc springs, disc springs or conical washers are all different names for the conical-shaped disc that will deflect (flatten) at a given spring rate. This spring rate is usually low relative to the rate of stretch of the gland studs. The design of a Belleville spring can produce elastic deflection at high loads in tight spaces and have a higher working load than other options.
Because they offer efficient use of space, high spring forces and elasticity, no setting or fatigue under normal loading conditions, and may be stacked in various combinations to produce the required load properties, Belleville springs could be a good choice for valve live loading.
They are also available in a variety of materials that can handle temperatures ranging from -400°F to 1100°F (-240°C to 593°C). Material selection should be based on application considerations, such as bolt material, temperature and environment. It is important to note that in certain environments, 17-7PH stainless steel is the most frequently used material for valve live loading; stress level and heat-treated condition should be taken into consideration when used in coastal applications.
By combining Belleville springs and washers in a variety of stack/series configurations, it is possible to achieve a variety of different levels of load and deflection to sustain heavy loads in small or tight spaces. There are four types of stacking arrangements (Figure 1):
- Single: One Belleville spring/washer
- Parallel: All Bellevilles stacked with the orientation in the same direction
- Series: Belleville springs stacked in alternating directions
- Parallel/Series: A combination of parallel sets in alternating orientation
To make a recommendation for a valve live loading stack, it is helpful to know:
- What is the stud diameter?
- What is the torque or aim preload? This is usually calculated by the supplier of the packing.
- What is the radial clearance (R.C.) of the stud? Or, what is the maximum outside dimension (OD) of the Belleville?
- What is the axial clearance (A.C.)? Or, what is the maximum height of the stack of Belleville’s (including flat washers)?
- What is the maximum estimated temperature at the studs?
- Is the valve in a corrosive atmosphere? If so, what are the corrosive elements?
- What is the expected consolidation of the packing over a given unit of time? (This is optional and would be used to calculate the stack height needed to maintain packing stress over that same time.)
- What is the required residual stud load over that same time as above? (This is optional and would be used to calculate the stack height needed to maintain packing stress over that same time period).
If the packing supplier does not provide the stud load or torque, the following information will help make that determination:
- What is the valve stem diameter or packing ID?
- What is the stuffing box diameter or packing OD (S.B. DIA.)?
- What is the stuffing box depth (S.B. Depth)?
- What is the number of studs? (typically 2)
- What is the packing style/type? Who is the manufacturer, if known?
Based on the application requirements, a comprehensive risk analysis will provide an optimized solution to increase the value of the design by identifying and communicating recommendations with evidence and justification such as:
- Springs and spring arrangements
- Assembly techniques
- Best practices for live-loading design
- Cost-saving opportunities
To consider whether live loading is a good choice for the valve and to design a live loading system, many more factors must be considered. To learn more, download this technical white paper.