- Published on Monday, 04 August 2014 09:44
- Written by Kevin Burgess and David Yakos
The topic of controlling or monitoring fugitive emissions is an ever-present discussion in the valve community. Regulation standards get tighter and environmental impacts continue to drive innovation as new technology is released. A trustworthy valve or valve actuator that completely eliminates fugitive emissions might be considered the Holy Grail of the industry for particular applications.
Inherently Leak Free
For valves, the term “inherently leak-free” means that no dynamic seals in the valves are exposed to the outside environment. There are still static seals in the valves that keep the fluid from being released to the outside environment, but the number of these seals is kept to a minimum.
Magnetically driven pumps have been around for several decades. Most of these can also be considered inherently leak free because the dynamic seals in these pumps have been replaced by magnetic couplings. These couplings transfer the torque magnetically from the motor to the pump impeller. Without the penetration into the fluid chamber by a drive shaft and the associated leakage, these pumps do not require the use of a dynamic seal to keep the fluid inside the pump from escaping to the outside environment.
The question is: “If magnetically driven pumps have been around for some time, preventing the leakage of hazardous fluids and chemicals, why haven’t valves seen the same breakthrough?”
The answer is because the torque requirements of valves are much higher than that of pumps of the same pipe diameter. This is particularly true regarding torque encountered when first opening a valve and the high torque encountered just before the valve is completely closed. The Achilles’ heel of magnetic couplings is that they can only transmit a limited amount of torque before slippage occurs inside the coupling. Even with magnetic pumps, care must be taken that the torque required to operate the pump does not exceed the torque limitations of the magnetic coupling that was designed for the pump. Otherwise slippage can occur within a magnetically driven pump as well.
Now there are a number of different valve designs and operators that provide for downstream gear reduction. In these designs, the gear reduction takes place after the torque has been transmitted through the magnetic coupling, multiplying the torque prior to its use in opening and closing a valve. The result is that a much smaller magnetic coupling can now do a very big job--successfully opening and closing a quarter-turn valve, and even a valve that has very high-torque requirements. It is actually now possible to provide turning torque to a valve that is equal to the torque provided by traditional valve stems.
This breakthrough makes it possible to introduce magnetic operation for quarter-turn valves used in places such as oil and gas, hazardous fluid handling and cryogenics. One proven application is in the aerospace industry.
Big Horn Valve was asked by NASA to apply this technology to one of their most difficult challenges, high-pressure cryogenic helium. The problem with packings in cryogenics is that the long stems that isolate the packing from the cryogen can be a burden in tight applications. Packings that are designed for cryogenic performance are notorious for their friction loads on the valve stem. not to mention their propensity to leak when chilled down. A valve was developed to meet the challenge.
Now with the new magnetic technology, in cold applications where stem packings have to be positioned outside the cryogenic environment, it is possible to eliminate the packing entirely. This inherently leak- free actuator can function right along with the valve itself, negating the need for long stems.
Retrofitting Existing Valves
End-users typically are happy with the valves they are using for particular applications, so a number of technologies have recently been developed to retrofit actuation of existing valves. Rather than replacing the entire valve, there are a number of actuator designs that encapsulate the dynamic seal (packing) on a valve, thus creating an inherently leak-free operation for any given valve. Leakage from the stem packing is contained within the actuator. Another feature shown in Figure 1 is the provision for position indication. The same shaft that drives the valve to open or closed position contains a magnet at the opposite end that provides accurate position indication, either through manual read-out or by installation of a sensor to pick up the magnetic signal.
In the case of valves that are handling very corrosive or poisonous fluids, a fluid management system, employing a moving piston (Figure 2) within the packing chamber can be added to the actuator. An actuator with this modification would contain a non-corrosive, lubricative fluid at a slightly higher pressure than the upstream fluid pressure to the valve itself. That way, any leakage occurring between the valve actuator and the valve through the stem packing would be leakage into the fluid path, not into the actuator chamber itself. In other corrosive applications, the same materials that allow the valve to handle a given fluid can also be used in the actuator, should the corrosive fluid enter the actuation chamber.
This proven technology lays the groundwork for some exciting new possibilities and enhancements for quarter-turn valves. Rather than monitor fugitive emissions, why not eliminate them all together? In a world where stem and packing maintenance have historically been the primary solutions to reduce fugitive emissions, the introduction of magnetic technology with zero dynamic seals to the outside world is a paradigm shift which addresses all the ongoing struggles with fugitive emissions: safety concerns, loss of valuable product and the environmental impact of leakage.