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How do you select for ball valves for alternative fuel applications?

Peter Ehlers of Swagelok looks at ball valve considerations for alternative fuel service.

When choosing a shut-off ball valve for natural gas vehicles (NGV) or compressed natural gas vehicles (CNG) applications, the first consideration should be certification. Is the ball valve you're considering certified in the locations that your product will be operating? If not, you should consider a different ball valve. Certification represents the minimal requirements for a given application.

Most certifications are given for either low cycle life ('service') or high cycle life ('manual'). You will need to specify which category your application requires. The lower number is intended for applications where the valve will be shut off or cycled only when maintenance is required, while the higher number is intended for applications where the valve will be shut off or cycled during normal operation of the product. Most certification cycle requirements - also referred to as industry standards - are considerably lower than actual market requirements as determined in the field.

Beyond certification, there are at least three critical design issues that anyone selecting a ball valve for alternative fuels applications should understand. These issues concern the design of the stem, the ball, and the seat. In addition, there are numerous choices to be made concerning the materials of construction. The objective is to match design choices in these areas with the application. Failure to do so may result in leakage, which can be both costly and environmentally damaging. 

In a ball valve, there must be some means of ensuring that the fuel does not leak from the stem seal. The most basic and primitive technology is a one-piece packing that encircles the stem. As a packing bolt is tightened down on the stem, the packing is crushed, filling the space between the stem and the housing. The tightness of the packing bolt must be calibrated for the maximum rated pressure, for example 69 bar. Because the packing material creeps over time, the packing bolt may have to be tightened periodically. 

In addition, if pressure exceeds the original usage pressure, the packing bolt may have to be tightened to create a seal. With all the occasional retightening, it is possible that the packing bolt will bottom on the valve body, at which point the packing will need to be replaced. This basic packing technology requires frequent inspection and adjustment. Unfortunately, to the untrained operator, it is not always clear when adjustment is required. Also, pressure from the packing bolt requires greater actuation force so long handles are the norm on ball valves employing this packing technology. 

Manufacturing tolerances
While this packing technology has some limitations, it is relatively inexpensive because it requires less critical manufacturing tolerances. Packing made from PTFE or thermal plastics can withstand chemically aggressive conditions but they do not perform well in applications where pressures may vary as part of normal system operation.  

The alternative to a crushed packing is an O-ring. Like the one-piece packing, the O-ring fits around the stem, filling the space between the stem and the valve body. However, unlike the one-piece packing, an O-ring does not require excessive pressure from the stem's packing bolt. Rather, the O-ring is energised by pressure in the gas stream. As pressure in the gas stream increases, the O-ring deforms and the tightness of the seal increases. Conversely, as pressure in the gas stream decreases, the O-ring relaxes. Because it is elastic, the O-ring changes shape to make the necessary seal. The O-ring provides flexibility for applications requiring high pressure, low pressure, or a broad pressure range, such as a cylinder on a natural gas vehicle, where pressure may drop from 310 bar when full to 34.4 bar as it nears empty. 

A proper stem design with an O-ring configuration requires a back-up ring or some other mechanism that will contain the O-ring under high pressure. If the O-ring is permitted to extrude beyond its specific bounds, the O-ring may be sheared during actuation; it will make the handle of the valve difficult to turn and the valve may leak. 

O-rings and back-up rings are made of different materials and have different properties. O-rings are elastic. They should deform easily under pressure but spring back to their original shape when pressure is relieved. By contrast, back-up rings are harder and should carry a greater load. Typically made of a fluoroplastic, such as PTFE, a back-up ring should deform under pressure but less so than an O-ring, and it may not resume its original shape after the pressure is relieved. The objective of the back-up ring is to fill space and thereby define the boundaries of expansion for the O-ring. Both the O-ring and the back-up ring should be made of lubricious materials so the stem may move with ease during actuation. 

Since the O-ring must remain elastic under all operating conditions, special attention should be given to the upper and lower temperature ranges. When selecting the proper material, consider fluorocarbon FKM for temperatures up to 232°C. It is resistant to many chemicals and displays excellent high-pressure capability. For the lower temperature range (down to -53°C, consider Buna; however, Buna is sensitive to some contaminants in natural gas. Newly developed materials - ultra-low temperature fluorocarbons - are based on fluorocarbon but provide low-temperature performance down to -53°C; they are stable and resistant to chemical attack. 

Ball valves employ either a floating or trunnion ball design. In a floating ball design, the ball is not fixed inside the housing but, rather, floats between two seat seals. In the shutoff position, it seals against the seat on the low-pressure side, pushed downstream by a positive pressure differential. If the pressure differential increases, the ball will move farther downstream, increasing the effectiveness of the seal. The position of the ball is variable depending on pressure and temperature. Such is the floating ball valve's principal advantage: In most conditions, it adopts a position advantageous to an effective seal.   

The trunnion design employs a ball, but it is not free floating. Affixed to the upper and lower parts of the ball are vertical cylinders, which are the trunnions. The unit fits into a space in the valve body and cannot move along the flow axis. As the ball rotates to the open and closed positions, it glides on the trunnions, which are fitted with a bushing or bearing. The ball still seals against a seat on the low-pressure side, but the trunnions bear most of the load and protect the seat. As a result, the trunnion design can perform well in high-temperature, high-pressure applications. Under these same conditions, the free floating ball design could transfer too much load to the seat, causing damage. On the other hand, trunnion designs fall short in very cold, low-pressure applications because the design is not free to compensate for contraction and stiffness in the seat.

If designed well, both the free floating and trunnion designs will have low actuation forces. In the trunnion design, bushings fitted to the trunnions distribute bearing load evenly over a large area. If the bushings are made of a hard, lubricious (low friction coefficient) material, such as PEEK, turning the handle will be virtually effortless, even under high-pressure conditions. In the free floating ball design, the seat is both the seal and the bearing. Ease of actuation depends largely on the lubricity of the seat material and the surface area. A large seat area ensures that the load will be widely distributed and actuation will not be difficult.

For a seat to be effective, the materials of construction must be consistent with the application. One very effective seat material is PEEK or filled PEEK. It demonstrates greater stability and consistent characteristics over the full temperature range (-53°C to 232°C ). It is harder and tougher than other seat materials, and recovers well from damage that may be caused by cycling, throttling, or contamination. In addition, it has a low friction coefficient for ease of actuation.

Seat designs also differ according to the method used to apply pressure to the seat. While the fluid stream itself provides the primary force to generate the seal, additional force may be needed to allow for a wider range of applications. This force comes from added components used in the seat design. In the crush-type design, loading force comes from the end screw. When it is tightened and set in place, the end screw squeezes the seat against the ball. There are several inherent difficulties in this design for alternative fuel applications. First, pressure from the end screw is static; there can be no means of increasing the pressure once it is set. 

The crush-type design depends on the load created by the original set conditions. In alternative fuel service situations, the crush-type design must depend on the elasticity of the seat - its capacity to rebound toward its original shape - to make the seal, which may not be sufficient for an effective seal. Without manual adjustment to the end screw, there is no way to increase sealing force.

Correcting for damage
A second difficulty is that the crush-type design will not apply enough pressure to correct for damage to the seat. Therefore, when the media is aggressive, as in a CNG application, cycles may be limited to one or two before the seat will need to be replaced. In alternative fuel service applications, seats are exposed to aggressive media at high velocities, so it is imperative that valves contain mechanisms for compensating for the inevitable damage. Damage to the seat may result from: debris in the gas stream; hydrates, which are crystalline structures in the gas stream caused by pressure drop; throttling, which occurs when the valve is neither fully closed nor open; and cycling the valve under high pressure, which may cause an irregularity on the seat. 

Pressure is the main means of compensating for damage to the seat. If the seat is made of PEEK, filled PEEK, or another moldable material, it will tend to further deform and reseal under sufficient pressure from the ball pushing downstream. 

The energised seat design constitutes a significant improvement over the crush-type design. It employs either a spring or O-ring - inserted between the end screw and the seat - which becomes a ready source of dynamic energy and supplies an augmenting force that is generated by the differential pressure across the ball. In the case of a high- pressure seal, the O-ring or spring will provide pressure in addition to the gas stream's, ensuring a better seal under these tough conditions. And in the case of a damaged seat, the dynamic spring or O-ring provides extra load to create a seat seal, enabling the valve to accommodate a few more cycles before the seat will have to be replaced. 

While it will compensate for moderate changes in temperature and pressure, the energised seat design does not perform well on the outer limits of the temperature and pressure range for natural gas applications.

The live-loaded design is an improved version of the energised design. Working under the same principles, the live-loaded design employs both a spring and an O-ring to create load on the seat. It also uses the differential pressure across the seat to generate more sealing force. Live loading provides more performance over the energised designs and is highly effective in compensating for wide variations in temperature and pressure. On the outer limits of the temperature and pressure range for natural gas vehicles, the live-loaded design performs admirably. Below -29°C, the spring may lose its elasticity but the larger force generated by the differential pressure across the O-ring provides sealing force to the ball or the seal and enables a load to be transferred to the seat. At high temperature and high pressure the O-ring and the spring work together to provide adequate pressure to make this difficult seal. 

Regardless of the design, seats must be replaced periodically. The cost of seat replacement should figure into your choice of a ball valve. 

In reviewing key issues with regard to the design of the stem, ball, and seat, keep in mind that the primary failure mode is always leakage. Other matters may come into play, but what is most important is a leak-tight seal.
 
Swagelok Ltd

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