Specialized Designs for Pipeline Valve Actuation

A number of specialized designs of valve actuators have been developed over the years to cope with specific valve control demands that are found in the pipeline industry. Examples include gas-over-oil, high-pressure gas and electro-hydraulic actuators, as well as specialized hydraulic actuators designed for subsea valves.

While the fundamental principles of design for these actuators are well established, rapidly advancing technologies, increasing demands for reliability, reduced maintenance and automation from end-user industries continue to present manufacturers with new challenges. In addition, the expansion of oil and gas production into increasingly remote and previously unreachable areas, combined with the growth of strategic pipeline distribution networks across remote and inhospitable landscapes, places further pressure on the manufacturer to provide rugged and reliable valve actuation solutions.

Testing for Reliability in the Extreme

The importance of testing cannot be over emphasized, not only to prove the long-term performance and reliability of products, but also to gain specific approvals from end-user markets without which it would be difficult or even impossible to bid on projects.

Some major equipment manufacturers’ testing policies are designed to reassure the customer that the quality of its product range is first class through the achievement of internationally recognized quality assurance approvals for design and manufacture.

Running in parallel with these assessments, equipment manufacturers also subject their products to the test procedures required for end-user approvals in specific markets and industrial disciplines. In these areas there have been significant market-led increases in activity in recent years.

In addition to the hyperbaric testing of subsea actuators and performance verification of gas-over-oil actuators mentioned later in this article, testing programs in recent years have expanded to include specific tests such as:

• IP66M/67M, dust, water jet, immersion and deluge testing of pneumatic actuators
• Shell qualification for hydraulic and pneumatic actuators
• Transco qualification for hydraulic actuators
• HIPPS (high integrity pressure protection systems) qualification for control systems
• GOST Russian Certificate of Conformity and Federal Use Permit
• ABS (American Bureau of Shipping)

These are in addition to ATEX explosion-proof certification, PED (European Pressure Equipment Directive) approval and SIL (safety integrity level) assessments that also apply to pipeline actuation products.

Subsea Actuators, Testing In-depth

Subsea valve applications, which often involve vital fail-safe and associated safety-related duties, represent an area of technology demanding the very highest levels of quality and product integrity to ensure reliable long-term performance. It may be prudent to not only test to API (American Petroleum Institute) qualification but also to achieve third-party-witnessed hyperbaric approval to the international qualification procedure ST-028.

In one API test performed by a major equipment manufacturer, a scotch-yoke hydraulic actuator with ROV (remote-operated vehicle) override was operated in a hyperbaric chamber under an external pressure of 3600 psi (240bar-g) to represent the submerged qualification depth of 2,400 meters over a period of six weeks. The actuator was selected as a representative sample of all the company’s subsea actuators for the purpose of the testing, which was witnessed by a third-party inspector. The severity of the testing reflects the crucial nature of subsea actuator duties and the increasing depths at which actuators are expected to perform as exploration moves into new, more remote offshore fields.

In the ST-028 hyperbaric test a similar hydraulic actuator fitted with a limit switchbox was subjected to cyclic testing to prove its suitability for submerged installation at a depth of 45 meters. The witnessed testing included pressurizing the switchbox with air at 75 psi (5bar-g) without any sign of leakage or bubbles, static torque and cycle testing of the actuator at ambient and 75 psi (5bar-g) external pressure (1,090 cycles at 2400 psi [160bar-g] on each test), final static torque monitoring, strip down and inspection. This was a much shorter test, taking only three days to complete, although the combined total operating test, amounting to over 2,000 cycles, is representative of a much longer working life.

The real life application for this equipment involves units with pressure compensators and manual controls for divers, supplied for Phase One of the Ashrafi Gas Lift project on the Red Sea coast. The project involves onshore facilities, gas lines and metering systems for 10 gas wells and associated platforms in the Gulf of Suez and is the continuation of a development that has been producing gas since the 1990s. The subsea hydraulic actuators are operating 8-inch (200mm) Class 900 ball valves.

In common with the stringent demands of all contracts involving subsea equipment, these actuators have to meet specific project requirements. In this task the company can also draw on the experience of an installed base of designs encompassing pressure-compensated rack-and-pinion, scotch-yoke and linear actuators for both double-acting and spring-return duties, equipped with ROV and/or diver-operated facilities.

Actuator companies often work with gear manufacturers—particularly ones with subsea experience—which produce a range of subsea gearboxes with diver or ROV-operable overrides. The combination of products, engineering knowledge and test facilities provided by the actuator and gear companies provides a comprehensive subsea solution for both retrievable and non-retrievable applications. For example, on retrievable applications such as some SSIV (subsea isolation valve) duties the designs provide for interchangeability between the actuator and a ROV-operated gearbox by means of transition spools and quick release methods.

For ROV-operated overrides another option is the ability to fit adjustable extensions between the ISO torque tool receptacle (or “bucket”) and the override input drive shaft located on the actuator. This enables the receptacle to be remotely positioned from the actuator to facilitate ROV or diver access with the tool.

Gas-over-oil Actuators

The basic principle of operation of the gas-over-oil actuator involves taking gas from the pipeline upstream of the valve and storing it under pressure in a reservoir (accumulator). The gas is used to power one side of gas/oil tanks (one for each valve operating direction), controlling the flow of hydraulic oil to and from the actuator cylinder to move the valve.

The gas-over-oil actuator is well suited to respond very quickly to changes in the physical condition of the pipeline—for example, a loss of pressure or increase in velocity denoting a line break—and utilizes the inherent speed of hydraulic operation to immediately close (or open) the valve. Because it uses the pressurized pipeline gas as an independent, self-contained power source, the actuator can perform this duty when mains power is not available or when the loss of mains power triggers the immediate failsafe operation of the valve. Consequently, and combined with its suitability for installation in remote locations, many end users see the main function of the gas-over-oil actuator on a pipeline as being the reliable provision of precautionary and emergency shutdown and isolation duties in the event of a line break or similar event.Emergency shutdown, however, is just one of the many duties the actuator may be specified to perform. Virtually all pipeline applications demand a control package specification that is customized to some degree, so the actuator must be capable of meeting a diverse range of control and instrumentation requirements without excessive re-engineering.

One solution is to engineer a multi-function manifold block at the center of the gas-over-oil system, which integrates both the gas and hydraulic functions in order to meet a wide variety of the control functions that are likely to be specified. These include, for example, altering the speed of valve travel, which may be required very quickly in an emergency but much more slowly to prevent damage to the pipeline during routine operations or partial stroking maintenance programs. Valve operation can also be preset to activate when a set point pressure drop value is signalled, or from other data provided locally or remotely.

In addition, individual gas company approvals may need to be obtained. In Russia the GOST certificate of conformity and federal use permit is required. Similarly, ENAGAS, the Spanish gas pipework network operator, and SNAM, the Italian equivalent, require approvals. In both cases, two examples of actuators are required to be subjected to an arduous program, as this summary of the main SNAM test illustrates:

• Dimensional check of actuator components before assembly
• Pressure test on piston, hydraulic circuit and pneumatic circuit
• Dielectric strength test on the main electrical components
• Stroke time regulation and verification
• Static torque test
• Dynamic torque recording at 195 psi (13bar-g), 375 psi (25bar-g), 750 psi (50bar-g) and 1050 psi (70bar-g)
• Strain test on main components at 1200 psi (80bar-g)
• 5 cycles with remote control at 25%, 50%, 75% and 100% of the maximum torque
• 5 cycles with manual control at 25%, 50%, 75% and 100% of the maximum torque
• Torque Limiting Device verification test (see below)
• Low temperature test at -4˚ F (-20˚ C) (functional test and torque limiting device verification test)

A Customized Solution

The 135-kilometer-long Shoukar-to-Hurghada pipeline is an important strategic extension to the natural gas distribution network in Egypt. Running southwards along the western coastal area of the Red Sea, the 24-inch-diameter pipeline has an operating pressure of 1050 psi (70 bar) and is designed to supply natural gas at a rate of up to 5MMSCMD (5 million standard cubic meters per day) to power stations and consumers in the provinces of Shoukar and Hurghada.

The pipeline is owned by the gas transportation company GASCO, which was responsible for engineering, procurement, construction supervision, commissioning and start-up. Construction of the $60 million project, which began in May 2006, was completed in 2007.

Customized actuators were supplied for block isolation valves, which enable segments of the pipeline to be closed down for maintenance, inspection and safety reasons. For this application the customer specified double-acting valve operation under either local manual control or remote electrical control, with hydraulic manual override. In addition, each actuator is equipped with enough accumulator capacity to complete four “stand-alone” valve strokes. The valve manufacturer also specified the torque output from the actuators should be limited to prevent any risk of damage to the valve stems. For this function an innovative solution had to be developed.

On the pipeline example above a torque limiting device (TDL) was fitted to limit the maximum torque output from the actuator in order to prevent damage to either the valve stem or the actuator itself. Mounted between the gas elements of the manifold block and the gas/oil tanks, it works by measuring and limiting the differential pressure between the gas on the supply side and the oil on the return side of the tanks. As the amount of actuator torque is directly proportional to this differential pressure, damage to the actuator or valve might occur if the differential pressure becomes too great as a result of unexpected resistance from the valve during travel or when the actuator travel reaches the end stop bolts. Clearly, the risk also increases in proportion to the speed of valve operation, so the TLD is pre-set to either the maximum torque supported by the valve stem or the maximum allowable torque produced by the actuator—whichever is the lower value.

When the differential pressure limit is reached, the TLD blocks the gas supply and at the same time vents the pressure from the gas cylinder and gas/oil tanks on the supply side, reducing to a minimum the risk of damaging either actuator or valve.

The Electro-hydraulic Option

The increasingly popular electro-hydraulic option offers a self-contained actuator package for pipeline valves, operated electrically by means of an integral hydraulic power pack and accumulator. Among other benefits, this design enables fail-safe and emergency shutdown duties to be performed by an electrically powered actuator, with the high speed capability inherent in hydraulic operation and with a lower cost of ownership than other solutions.

A number of these actuators have been installed on the recently completed Baku-Thilisi-Ceyhan strategic pipeline, which brings up to 50 million tons per year of crude from the oilfields of the Caspian Sea to the Mediterranean coast of Turkey. The actuators described here are used for block valve and emergency shutdown (ESD) duties along the length of pipeline that traverses Turkey.

This application used electro-hydraulic actuators of the scotch-yoke type, operating ball valves in sizes up to 42 inches. Four model sizes were supplied, the largest having a maximum torque output of 200,000Nm.A modular design of some electro-hydraulic control systems facilitates assembly, operation and maintenance of both standard and customized control logic solutions. Common functions such as local/remote control selection and open/close operation are provided as standard, while optional features are easily integrated.

Generally, valve actuation can be performed in one of four ways:

• Remotely via energization/de-energization of the solenoid valves
• Automatically by control system logic (e.g., low, high or differential pressure detection)
• Locally by either an electrical pushbutton station or the manual overrides incorporated into solenoid valves, or
• Locally by the manual hand pump

The speed of the operation is independently adjustable in both directions. The pump/motor/accumulator provides the hydraulic supply to the solenoid valves and hydraulic pressure is automatically regulated by a pressure switch. The motor runs until the preset maximum pressure is reached and for safety a pressure limitation valve is fitted as standard.

The hydraulic accumulator system ensures operation in the event of primary energy failure and also facilitates fast operating times. A torque control system to protect the valve and actuator from overloading is included as standard in all control configurations.

Actuators supplied to the pipeline were divided into two groups, ESD valves and mainline block valves. In both cases the specification called for stainless-steel control and instrumentation component parts (solenoid valves, etc.), with marine quality stainless-steel solenoid valves specified for the ESD actuators in coastal locations.

The stroke (closing) time for ESD valves is 15 seconds while the stroke time is 7 minutes for the mainline block valves. To meet the block valve time—essential to prevent the risk of pipeline damage—in the various ambient conditions encountered, special temperature and pressure-equalized throttle valves are incorporated.

Each mainline block valve actuator is equipped with a position transmitter with eight position switches. The position transmitter was required to be explosion-proof to EEx IIB T4—which is the enclosure specification demanded for all electrical equipment on the actuators—and to utilize proximity switching. Because a transmitter to this specification was not readily available, the unit was custom designed specifically for the contract and was Ex certified.

The eight switches on the mainline actuators provide fully redundant positional control in four positions: fully open, 80% open, 70% open and fully closed. The 80% and 70% positions enable the actuator to be partially stroked for function test purposes, with position switch redundancy preventing the risk of accidental valve closure and consequential pipeline damage.

Due to the pipeline’s geographical location the actuators are fitted with earthquake-proof solenoid assemblies, each comprising two 90-degree-operated valves to prevent accidental vibration-induced operation. For the mainline valves both normally closed (NC) solenoids must be energized to close the valve, with each under the remote control of a separate PLC. On the ESD valves both solenoids are normally open (NO) and de-energizing either one will cause the valve to close, regardless of the continued presence of an electrical supply.

All the actuators are equipped with accumulators with the capacity to provide three valve strokes without the motor and pump running. In addition, provision is made to attach a mobile pump to operate the actuator in the event of a motor failure, while an integral hand pump is also fitted. All actuator control packages are designed with full cathodic protection and housed within a watertight stainless-steel cabinet.

Conclusion

The activities described above illustrate how the demands for safety, efficiency and environmental responsibility from the contemporary pipeline industry have a significant impact on the valve actuator manufacturer, creating increasingly innovative design solutions, proven and supported by ever-more stringent testing. The constant expansion of oil and gas exploration and production into more and more remote areas can only increase all aspects of this activity for the actuator industry in the foreseeable future.

Mark Clark has worked in the industrial valve, actuation and instrumentation industries in the United States and the United Kingdom for more than 35 years. He is currently a freelance public relations consultant in the UK, specializing in the valve actuation industry, and wrote this article on behalf of Rotork Controls. Reach him at ftpublicity@aol.com.