The efficient transport of natural gas in pipelines across the world is critical to modern society. Achieving this, while incorporating high expectations for the levels of inherent safety and reliability that the communities these pipelines pass through, is dependent on reliable and safe, valves and actuators.

Across the whole delivery and distribution network there are many different styles of valve and actuation systems in use. The type of system chosen will depend on the application and will also depend on the required performance and cost. On a trunk pipeline system there will be a mainline valve station located at strategic points in order to isolate sections of the pipeline. Valve stations could be located at compressor, booster and metering stations, at critical points such as river crossings, and at various points in between. It may be necessary to isolate the pipeline for maintenance or for safety reasons, such as line-break control. Valves used for safety purposes are often called Emergency Shutdown or ESD valves. These mainline valve stations provide the critical protection that secures the safety of the environment and communities alongside the pipelines.

Because valve stations are often situated in remote locations, it can be difficult to provide the power needed to actuate them by electrical or pneumatic means. To address this issue, many of the actuators fitted to these valves utilise the high pressure in the gas pipeline as the source of power for the actuators. One such actuator is the Shafer® Rotary Vane Actuator from Emerson Process Management. This actuator has been proven in tens of thousands of critical long-term installations and provides reliable operation over a 50 year life. The Shafer RV-series is powered by hydraulic fluid which has been pressurised by the pipeline gas. Utilising hydraulic fluid as the power media offers significant operational advantages compared to the alternative approach of infusing natural gas from the pipeline directly into the actuators.

‘Gas over oil’ valve actuation

The Shafer RV-series gas over oil actuator uses two external pressure vessels to convert pressurised gas into pressurised hydraulic fluid. When called on to close the mainline valve, the pressurised gas in the tank forces hydraulic fluid into the actuator’s closing port and rotates the valve spindle by a quarter turn. The opposing section of the actuator cylinder is also filled with hydraulic fluid, which assists in damping, or smoothing the torque output. At the end of the valve stroke the pressurised gas is vented to atmosphere. Pressurising the opposing vessel is used to drive the reverse actuator movement, and provide the valve opening sequence.

Direct gas valve actuation

In this alternative actuation configuration, high pressure line gas is directly admitted into the actuator cylinder, filling the static volume. The cylinder pressure generates a thrust which operates the scotch-yoke mechanism of the actuator, moving the valve. The opposing cylinder will sometimes utilise a hydraulic fluid to smooth the actuator operation.

Advantages of ‘gas over oil’ actuation

The ‘gas over oil’ actuation principle has distinct advantages over ‘direct gas’ actuation in critical applications. With the gas over oil system, hydraulic fluid is the only medium to enter the actuator, thereby protecting internal metal parts from corrosion. Making use of hydraulic fluid also prevents contaminants and moisture from entering the actuator. Any moisture in the gas falls to the base of the pressure vessel where it can be drained off. In the direct gas system internal metal parts are exposed directly to pipeline gas, with all the contaminants and moisture contained within it. This can cause long term corrosion and moisture collecting in the actuator can freeze and cause a malfunction.

In the gas over oil system the actuator’s seals are constantly bathed in hydraulic fluid providing lubrication and extending service life. In the direct gas system the actuator’s seals are more prone to dry out and, this can be aggravated by corrosion or dirt on the metal surfaces leading to premature seal failure.

The growing awareness in several industries of the need to properly inspect and certify all pressure vessels containing high pressure gas has led to restrictions on the use of non-uniform pressure vessels such as actuator cylinders. The gas over oil operating system restricts the high pressure gas to the specifically designed pressure vessel, with the actuator housing only being pressurised with hydraulic fluid.

Gas over oil actuators work smoothly, with no jerky motion, because the non compressible hydraulic fluid provides precise speed and movement control. It also allows for easy installation of hydraulic hand pumps for manual valve operation when needed. Direct gas actuators without hydraulic dampening systems have no integral energy absorbing device, which is needed to ensure against the ‘stick-slip’ phenomenon. This can occur where valves have been static in one position over a long period and subjected to temperature cycling. When the excess pressure applied by the gas pressure finally frees the valve, it can swing over and impact on the end stop, with consequent potential damage to the valve structure.

Expert advice for your application

These views reflect Emerson Process Management’s experience with valve automation in relation specifically to actuators intended for critical applications in natural gas pipelines. With a combined 100 plus years of experience, and an installed base of more than 100,000 actuators on gas transmission lines operating safely in many different environments around the world, the company’s expertise is considered impressive.

The concerns expressed about the use of direct gas actuation are based on the high potential or long term deterioration effects of corrosion, combined with eventual seal embrittlement and the prospect of fatigue from cyclic loading. Valve Automation has a 60-year history, with no pressure vessel accidents, associated with tens of thousands of gas/oil actuator installations in critical applications.