Since the early days of valve actuator use, there has been a choice between powering the actuator with electricity or pressurized fluid. Sometimes a user industry has a traditional preference for actuator power and sometimes it’s dictated by the circumstances of the application.
Deciding the best power medium for an industrial application depends on many factors such as following what has traditionally been used in the application. But with today’s sharp focus on return on investment and environmental impact, the traditional solutions now warrant closer examination.
For plants that have instrument air systems, the choice of actuator power is more flexible because electricity is usually available to power the instrument air supply. Either fluid or electric-powered actuators could be used. But there are many installations, such as well heads, pipelines or irrigation systems where an electric power supply is not available.
Where both power modes are available, then the choice comes down to other criteria and the traditional choice varies by industry and region.
In the earlier days of the power industry, many boilers were controlled by instrument air controllers such as the Bailey controller. Boiler valves were controlled by pneumatic controllers and positioners using spring diaphragm and piston actuators, mainly on the smaller valves. Larger valves such as the main steam-stop valves became electrically actuated. Modern plants use proportionally more electric actuators, but electronically controlled and positioned pneumatic modulating valves still are used extensively.
The early designs of filter plants in the U.S. water industry used pressurized water as a power medium for piston actuators. These have been superseded by electric actuators in the majority of new plants around the world, but some plants being built are using pneumatic actuators for filter control.
Oil and Gas
Oil and gas production has a long history of using pneumatic actuators offshore and onshore, likely because of the mechanical simplicity of piston operators and their straightforward maintenance. Also, an instrument air supply poses no sparking hazard to the explosive environment on an oil rig, although any solenoid control valve has to be hazardous-area rated.
Some oil and gas companies are using electric actuators offshore for certain valves, particularly where there are weight or space restrictions.
The functionality of a fluid-powered actuator has an advantage over the electric actuator in its capacity to provide a simple mechanical fail-to position function. The cylinder or diaphragm of a fluid-power actuator is easily opposed by a spring that can move the valve to an open or closed position on loss of fluid power pressure.
For process shut-down or safety shut-down, the fluid-power actuator is the traditional choice, particularly when high levels of safety and integrity are required. There are some technologies that can provide integral backup power for electric actuators. These are becoming a viable alternative to the traditional fail-to position function for process valves but are limited to smaller valves at present.
Gas pipelines often use the pressurized pipeline gas to power shut-off valves for line break sectioning and general shut down. These fluid-powered units often use oil as an interface between the pipeline gas and the actuator. However, many countries are restricting the use of gas-powered actuators due to the environmental impact of the exhausted gas. One replacement solution is the electro-hydraulic actuator, which can provide shutdown capability using a spring-return hydraulic actuator. For remote areas, these units can be solar powered, using a DC-motor-powered hydraulic pump to drive the reset stroke after a shutdown.
For smaller process control valves, the traditional actuator is the pneumatic spring diaphragm unit. This is a simple but effective device with few moving parts. To make a complete automated valve, it is usually coupled to a positioner, often a “smart” device. Most process control valve makers manufacture their own diaphragm actuators and package them with the valve and positioner as a complete unit.
The relative capital cost of electric actuators when compared to fluid power units of equivalent output is usually greater. The electro-hydraulic designs are the exception as they combine components of electric and fluid power units.
The running cost of actuators does not always figure prominently in evaluations, but for frequently modulating valves, this could be significant. Electric actuators look good in these scenarios as they only use energy when moving. Fluid power units using instrument air have a constant energy draw from positioner bleed and system leakage. The carbon footprint is lower for electric units.
Actuator manufacturers of both types of actuator are constantly improving their products. From the control room or maintenance shop, there is the ability to monitor and diagnose issues with actuators so that planning maintenance can be streamlined and plant downtime reduced to a minimum. The smart controls on actuators are available for both fluid and electric-powered actuators so the actuator power supply is transparent to the control room.
The general trend appears to be an incremental encroachment on the traditional applications for fluid-powered actuated valves by electric actuators in certain niche areas. However, there is no doubt that there is a robust and diverse demand for fluid power actuators both now and into the future.
This article first appeared in Valve User magazine, the magazine of the BVAA. All Rights Reserved.