Every on-off pneumatic valve in a process plant depends on a small electrically operated pilot: the solenoid valve that directs instrument air into or out of the actuator. In a safe area, that solenoid can be a general-purpose unit. In refineries, chemical plants, gas processing facilities and fuel terminals, where flammable gases or vapours may be present, the solenoid coil becomes a potential ignition source and must be protected accordingly.

Specifying the right solenoid for these environments involves more than picking a voltage and port size. This article explains what a flameproof NAMUR solenoid valve is, how it differs from other protection methods, and what to verify before approving one for a project. Engineers looking at flameproof NAMUR solenoid valves for hazardous areas can use the checklist at the end as a starting point for vendor evaluation.

What NAMUR Mounting Means

NAMUR refers to a standardised interface for mounting solenoid valves directly onto the air ports of a pneumatic actuator. Instead of running tubing between a remotely mounted solenoid and the actuator, a NAMUR solenoid bolts straight onto the actuator body. This shortens the air path, reduces leak points and simplifies replacement, since any compliant solenoid will fit any compliant actuator.

NAMUR solenoids are commonly available in 3/2 configurations for single-acting spring-return actuators and 5/2 configurations for double-acting actuators. Some 5/2 units can be converted to 3/2 duty with a plate change, which helps plants reduce spare parts variety.

Flameproof Protection Compared With Other Methods

Electrical equipment in hazardous areas can be protected in several ways. For solenoid valves, the most common are:

  • Flameproof (Ex d): the coil sits inside an enclosure strong enough to contain an internal explosion and prevent flame from escaping to the surrounding atmosphere.
  • Encapsulation (Ex m): the coil is fully encapsulated in compound so that the explosive atmosphere cannot reach it.
  • Intrinsic safety (Ex i): the energy in the circuit is limited so it cannot ignite the atmosphere, which requires low-power coils and suitable barriers.

Flameproof solenoids are widely used where standard control voltages are needed and intrinsically safe infrastructure is not in place. The trade-off is weight and the need for careful cable entry practice, because flameproof integrity depends on certified glands and correctly maintained flame paths.

Certification Must Match the Installation

The most common certification problem is not a missing certificate but a mismatched one. Before approval, check that the certificate covers:

  • The exact model, coil and voltage being supplied, not just the product family.
  • The zone, gas group and temperature class required at the installation point.
  • The ambient temperature range at site, which matters in hot climates such as the Middle East and parts of India.
  • The applicable national scheme, for example PESO approval for installations in India, alongside ATEX or IECEx where specified.

Ask for the full certificate including any schedule of conditions, and compare the marking string on the certificate with the nameplate drawing. Small discrepancies are easier to resolve before despatch than after installation.

The Assembly Is Only as Certified as Its Weakest Part

A certified actuator carrying an uncertified solenoid is an uncertified assembly. The same applies to limit switch boxes, positioners and cable glands mounted on the same valve. When valves are automated in a hazardous area, review the certification of every electrical accessory on the package together rather than item by item. Sourcing the actuator and its accessories as a coordinated package from a single valve automation manufacturer makes this review considerably simpler.

Practical Selection Points

Beyond certification, several practical details affect long-term performance:

  • Body material: aluminium is common, while stainless steel suits corrosive and offshore environments.
  • Manual override: a lockable manual override allows the valve to be operated during commissioning or a coil failure.
  • Coil duty: continuous-duty coils are standard for valves held energised for long periods.
  • Ingress protection: an IP66 or IP67 rating suits most outdoor process areas, provided glands and entries are equally rated.
  • Exhaust protection: breather or exhaust filters keep dust and insects out of the exhaust ports.

Maintenance in Service

Flameproof equipment requires inspection by competent personnel. Flame path surfaces should not be painted, damaged or obstructed, and covers must be refitted with all fasteners. Coil replacements should use the manufacturer's certified parts. Keeping the air supply clean and dry also extends solenoid life, since particulates and water are the usual causes of sticking spools.

Conclusion

The solenoid valve is a small component with a large role in hazardous area valve automation. Selecting a flameproof NAMUR solenoid with certification that matches the actual installation, reviewing the whole valve package as one assembly, and maintaining flame paths correctly help ensure that automated valves operate reliably without introducing ignition risk.