Motion components for oil and gas: ATEX actuators, motors and slip rings

In short:Oil and gas motion components are chosen by hazardous-area certification first and performance second: a choke actuator, motor or slip ring in Zone 1 must carry ATEX, UKEX or IECEx certification for the right equipment category, gas group and temperature class, such as Ex db IIB T4 Gb. Downhole parts add temperatures to 200 °C or more and pressures to 1,700 bar.

Typical requirements

Machinery we specify motion for

• Managed pressure drilling chokes
• Choke and kill valve control
• Topside valve actuation
• Rig cable reels and spooling
• Drilling equipment and top-drive services
• Downhole tools and MWD
• Pipe-crawler inspection
• Explosion-proof process machinery

Why hazardous-area certification comes first

On drilling rigs, production platforms, FPSOs, terminals and refineries, much of the plant is classified as a hazardous area. In Great Britain, HSE explains that two frameworks apply: DSEAR (implementing ATEX Directive 99/92/EC) requires employers to classify zones, and the Equipment and Protective Systems Regulations 2016 (implementing Directive 2014/34/EU) govern the equipment that goes into them. They apply to electrical and mechanical equipment alike. Certified equipment carries the “Ex” symbol, and most categories need third-party certification by a conformity assessment body.

For a motion component that means the first question is not force or torque but where the part sits and what certification it carries. A brilliant actuator with no Ex certificate cannot go in Zone 1.

Decoding zones, gas groups and temperature classes

R. STAHL’s Ex marking guide sets out the mapping:

Temperature classes set the maximum surface temperature: T1 450 °C, T2 300 °C, T3 200 °C, T4 135 °C, T5 100 °C, T6 85 °C. The equipment’s class must be below the auto-ignition temperature of the gases present. Flameproof enclosures (d) are used for motors and switchgear, increased safety (e) for terminals and squirrel-cage rotors, intrinsic safety (i) for sensors, pressurisation (p) for cabinets and large motors.

Internationally, the IECEx Certified Equipment Scheme certifies equipment by type testing and factory quality assessment, with ongoing surveillance. If a product is not on the IECEx online certificate system, it is not IECEx certified, so check the certificate number rather than a brochure.

Where motion components sit in oil and gas

Electric choke actuators for managed pressure drilling

In managed pressure drilling, the choke controls the pressure of mud flowing back to surface. Tolomatic describes its ServoChoke SVC as an electric rod-style choke valve actuator for precision control of that pressure, also used in choke and kill installations, and certified for Class I Division 1, ATEX and IECEx. The same page positions its RSA rod actuators for valve actuation and for replacing pneumatic and hydraulic cylinders. The general case for electric chokes is precise, repeatable positioning, simple remote control and no hydraulic power unit; see our hydraulic to electric actuator guide.

Slip rings for rigs, reels and FPSOs

A flameproof slip ring in our range is marked Ex d IIB T4 Gb. Read against the tables above, that means a flameproof enclosure for Zone 1 (and Zone 2), group IIB and IIA gases, and a maximum surface temperature of 135 °C. It is not suitable for hydrogen or acetylene (IIC) atmospheres. It carries up to 100 A per circuit at up to 50 rpm with an operating range of -45 to +135 °C, which suits rig reels and slow rotating process equipment.

Moog’s slip ring product guide shows the extremes. FPS swivels for FPSO turrets use Ex d, Ex de or Ex p protection and reach 145 kV on high-voltage units. Downhole model 303 slip rings are built for high-temperature, high-pressure service, generally oil-filled, rated 200 °C standard and up to 230 °C, at 25 kpsi (1,700 bar), with up to 600 rpm on the miniature-bore version. Moog describes its oil and gas work as motion control, surveillance and data communication for long life in extreme and hazardous environments. See rugged slip rings, high-current slip rings and slip rings for cable reels.

Safe area, protected enclosure or certified part?

There are three ways to put motion into a hazardous plant, and the choice shapes the whole design:

1. Keep it out of the zone. Mount drives, controllers and standard motors in a safe area or a pressurised room and transmit motion mechanically, or move the equipment boundary. Often the cheapest route when it is possible.
2. Put standard parts inside a certified enclosure. A flameproof or pressurised housing around a standard actuator or slip ring. The certification then belongs to the assembly, which must be assessed as a whole.
3. Use certified components. Ex-certified motors, actuators, slip rings and sensors installed directly in the zone. Simplest to install, but the choice of parts is narrower and lead times are longer.

Downhole tools: heat, pressure and oil-filled designs

Downhole tools and measurement-while-drilling equipment run at temperatures and pressures far beyond surface plant. Moog’s downhole slip ring figures, 200 to 230 °C and 1,700 bar, show the scale. Moog describes its downhole slip rings as generally configured for an oil-filled environment, and motors, encoders and electronics are built from high-temperature materials. Envelope is tight, because everything must fit inside a slim tool string. Ask suppliers for rated temperature and pressure, the fill fluid, life at temperature, and shock and vibration test levels.

Worked example: sizing an electric choke actuator

Assume a choke with a 25 mm stem, a worst-case unbalanced pressure of 34.5 MPa (5,000 psi) and 2 kN of packing friction. All figures are illustrative.

1. Stem area: π × 12.5² ≈ 491 mm².
2. Pressure force: 491 mm² × 34.5 N/mm² ≈ 16.9 kN.
3. Total force: 16.9 + 2 ≈ 18.9 kN. Add a 1.5 margin for pressure spikes and wear: about 28 kN.
4. Speed: decide the full-stroke time the pressure control loop needs; that sets the screw lead and motor speed.
5. Certification: if the choke manifold is in Zone 1 with group IIA gases, the actuator needs at least category 2G, IIA and a temperature class below the gas’s auto-ignition temperature, for example Ex db IIA T4 Gb or better.
6. Fail-safe: define the position on power loss (hold, open or close) and how it is achieved.
7. Feedback: absolute stem position, inside the certified enclosure or as a certified sensor.

Typical pitfalls

1. Specifying from a datasheet without the zone drawing. The area classification decides the certification.
2. Wrong gas group. IIB equipment is not acceptable where hydrogen is present.
3. Ignoring temperature class at full load. Motor and brake surfaces heat up with duty.
4. Assuming mechanical parts are exempt. The EPS Regulations cover mechanical equipment too.
5. Long motor cables from a safe-area drive without checking voltage reflection and motor temperature.
6. Certificates not checked on the IECEx system or with the notified or approved body.

What to send an engineer

Send the zone, gas group and temperature class, required certification, force, stroke, speed and duty (or circuits, speed and bore for a slip ring), process pressure and temperature, ambient range, ingress rating, supply, fail-safe position and design life. Our specialist engineering and supply partners can then propose certified or safe-area components that fit, and say clearly which is which.

Standards to have in view

• Directive 2014/34/EU (ATEX equipment)
• Directive 1999/92/EC (ATEX workplace); in Great Britain DSEAR
• Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 2016 (Great Britain)
• IEC 60079 series (explosive atmospheres), including IEC 60079-0 general requirements and IEC 60079-1 flameproof enclosures
• IECEx Certified Equipment Scheme
• IEC 60529 (IP codes)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.