Actuators & Controls
- Published on Tuesday, 10 December 2013 09:02
- Written by by Ross Wolkart and Daniel Myers
More than 200 pipeline systems in the United States alone transmit natural gas through more than 300,000 miles of intrastate and interstate pipelines. What’s more, 1,400 compressor stations operate in the U.S. to transfer the gas through the transmission pipelines to ultimate distribution points1. As the industry continues to grow, pipeline operators seek alternatives to traditional equipment to ensure pipeline safety and reliability. That includes new types of actuators.
The greatest numbers of valves on a pipeline system are in compressor stations, placed at intervals of 40 to 100 miles along the pipeline. These stations contain banks of as many as 25 (or more) engine-driven compressors fueled by gas taken from the pipeline. Each engine/compressor has a suction line and one or more discharge lines between 10 and 24 inches and one bypass line from 4 to 8 inches; each line in turn has its own valve. These are mostly ball valves with some gates in older installations. In addition, each compressor station will have a mainline valve and a blowdown valve, both designed to blow open or auto-close. They also have pig-launching valves and manifolding valves.
Meter stations will have flow control valves (with isolation ball valves), automatic shutdown valves, manual bypass valves and pressure-regulating valves. Finally, the pipeline itself has shutdown/isolation valves every five to 20 miles. This situation means there are also many actuators doing heavy-duty work.
TYPES OF ACTUATORS
Pipeline operators have a choice of actuator types: gas hydraulic (gas over oil), pneumatic (either direct or air over oil), direct gas motor-operated, electric, hydraulic and electrohydraulic. Each has advantages and disadvantages.
For gas-powered actuators, the pressurized gas in the pipeline provides the power to operate the valve, either directly or via hydraulics. Since pressurized gas is always available, no other energy source is needed to run the actuator; power lines don’t have to be run; and air compressors don’t have to be installed. New problems crop up in populated areas, which are steadily encroaching on pipelines. For example, when a gas-operated actuator cycles, noise levels can alert neighbors not accustomed to the controlled discharge, which may require that mufflers be installed to provide noise abatement.
Some pipeline operators choose to install a compressor and operate valves on compressed air instead of gas. The compressor feeds a storage tank that, in turn, provides the air to run the valve actuators. This requires a reliable source of electric power and does nothing to reduce the operating noise of the valves.
WHEN ELECTRIC IS RIGHT
Electric actuators have several important advantages in this application, and are popular in pipeline transmission and compressor stations. While pneumatic actuators are capable of faster stroke times, electric actuators are generally capable of meeting stroke time requirements for most gas pipeline applications. They are considerably smaller than gas-operated actuators. A gas actuator for a 24-inch rising stem gate valve can be 8 feet long, while an electric actuator will be 12 inches tall and 26 inches long. This compact size makes electric units much less susceptible to vibration created by the engines in a compressor station. The compact footprint keeps the unit closer to its center of gravity and minimizes the effects vibration has on actuator internals and controls. It also means the equipment can be put in tighter locations (such as through a manhole).
Many lines for compressor stations and pipelines date back 50 years. While some infrastructure has been upgraded, older technologies for compression are still in service. Compressors of all types cause vibration, but older reciprocating compressors generate the most because their suction valves take in gas from upstream gathering lines, compress it internally and discharge it into the main transmission pipeline. Valves and actuators operating subject to vibration can separate piping from the bodies, loosen power connections and do irreparable damage, rendering the equipment inoperable. Because of their inherently smaller size and design, electric actuators may be less susceptible to the damages of vibration.
When electrical power is not readily available, options exist. For example, electric actuators can be used when commercial electric power is not available through use of a solar panel to charge a storage battery, which can then power an electric actuator. This is a favorable option when frequency of operation is low but bringing in electric power would be expensive or impractical, or when it is impossible to guarantee sufficient gas pressure at all times to operate a gas-driven actuator. But this method has also found favor in some urban areas. In Denver and Pittsburgh, for example, installations with solar panels and storage batteries are mounted on utility poles and DC-powered electric actuators on plug valves are placed in manholes.
Electric actuators also make sense on new construction of gas compressor stations since electrical infrastructure cost is absorbed in the overall project, and the lower installed maintenance cost justifies the initial expense. For modulating services in areas where environmental concerns are heightened, electric actuators serve as an actuation alternative.
WHEN ELECTRIC IS NOT RIGHT
While electric actuators have many advantages, they are not a panacea. One area where gas-operated or pneumatic spring return valves are used almost exclusively is in emergency shutdown. If a break occurs in the pipeline or another upset occurs to the system, getting the pipeline shut down as quickly and reliably as possible is the first priority.
A disadvantage of electric actuators is that they are dependent on a supply of electric power, so if power is lost, the valve cannot operate—although they are provided with hand wheels for manual operation. This can be less of a problem for pneumatic actuators, which may include a spring (or in some cases, a nitrogen tank) to provide loss of power operation. Still, options exist for smaller electric actuators that can be equipped to run on DC power backed up with batteries.
Electric actuators offer a number of important advantages in natural gas line operations. However, because valve types differ from large high-pressure ball valves to smaller butterfly, globe and plug valves, choosing just one type of actuator use across all valve types would be difficult. For this reason, partnering with an automation supplier who has experience serving the pipeline industry and has the expertise to make recommendations specific to each application is paramount to selecting the right valve actuator.
1Source: Cleveland, Cutler J. (January 16, 2013) Energy Transitions in the United States, retrieved June 3, 2013. www.trunity.net/the-energy-library