Using I/P Transmitters in Natural Gas Applications

Until recently, there was no other choice but to rely on I/P transmitters that were not agency approved for use in hazardous 'classified' natural gas applications. Fortunately, a few have become available for this type of service

By Gus Elias

When conditions do not permit the use of instrument air, look to use natural gas to power valves and actuators. But do it very carefully.

A major challenge facing the natural gas production and transport industries is that sites are often located in extremely remote locations. Natural gas gathering stations, compressor stations, pipeline feed applications, and booster stations all involve the operation of pneumatic control valve actuators that require a pneumatic supply for operation.

At many of these remote locations, the air demand is not large enough to warrant the significant costs of installing, operating, and maintaining an on-site air compressor to produce instrument air. What to do?

The traditional answer is to use natural gas from the pipeline to provide the operating pressure required by the pneumatic valve's current-to-pressure (I/P) transmitter. The I/P transmitter (also referred to as an I/P transducer or converter) converts an electrical signal, typically 4-20 mA from a DCS or PLC, to a proportional pneumatic signal, typically 3-15 psig, for valve actuator control, as shown in Figure 1.

Off-the-Shelf I/Ps

Off-the-shelf I/P transmitters are not approved, designed, or intended to use natural gas as their pneumatic supply. While they are often called upon to perform this duty, they are dangerous because inappropriate seals on their electrical conduit wiring fitting can permit natural gas to migrate into the conduit, leak to atmosphere, or leak into non-hazardous areas where the presence of gas could cause a potential fire or explosion hazard. Improper venting of an off-the-shelf I/P's exhaust gases, often caused by inappropriate pneumatic connections, can also lead to dangerous conditions.

Until recently, there was no other choice but to rely on I/P transmitters that were not agency approved and designed for use in hazardous "classified" natural gas applications. This presents a huge liability problem and, worse, a severe safety problem.

Fortunately, a few I/P transmitters have become available for this type of service. These I/Ps are CSA, FM, and KEMA (ATEX) approved to be used with natural gas as the pneumatic supply. Such I/Ps can be used with sweet natural gas consisting of up to 20 ppm of hydrogen sulfide. Approvals allow installation in Explosion-Proof and Intrinsically Safe applications covering Class I, II, III, Division 1, Groups A-G. One model, for example, is suitable for installation in Class I, Division 2, Groups A-D hazardous "classified" locations with IP66 and NEMA 4X environmental protection ratings.

A gas-approved I/P incorporates special conduit seal and gas venting methods. The nipple and seal assembly prevents natural gas from traveling into the electrical conduit or into the atmosphere. To accomplish this, the seal must be designed in a way that absolutely prevents natural gas from migrating into the instrument's wiring conduit. This includes gas traveling through the small space between the copper wires and its insulation as well as gas escaping on the outside of wires into the conduit. The seal must also positively prevent gas from escaping into the atmosphere from the conduit connection.

Most I/P transmitters have small built-in vents that are used to release the air that powers the unit into the atmosphere. In a natural gas application, such as an enclosed compressor station, this is, of course, unacceptable. The gas must be released into a controlled area, often far away from the application. An I/P designed for natural gas solves this problem by providing a vent port where piping can be connected to transport the explosive gases to an area where they can be safely vented as required.

Indoor and Outdoor Installations

Indoor natural gas operations are typically monitored to maintain safety conformance within Lower Explosive Limits (LEL) and Upper Explosion Limits (UEL). Placement of the transmitter should be such that detection and alarming surround any critical connections between the transmitter and the natural gas process, as shown in Figure 2.

Recommended ventilation for an indoor system should consist of a leak-proof connection from the exhaust of the transmitter to a process vent. The process vent should already be dedicated to natural gas operations and should conform to all standards for flaring, after-burn, or flame arrest, as dictated by local environmental and safety regulations. If the installation site maintains no monitoring capabilities, additional considerations must be made to ensure the operating area is well ventilated and the transmitter can safely be exhausted to a process vent.

For an outdoor system, ventilation should consist of a weather-proof connection between the transmitter exhaust and a riser, six feet above the transmitter and control valve assembly. The riser should be shepherd-crooked to prevent rain or incident water from accumulating at the base. In accordance with local safety regulation, an inline flame arrestor should be applied to the riser to prevent flash-back to the transmitter from an external spontaneous flame source, as shown in Figure 3.

Moreover, when applying any instrument, such as an I/P, in general or hazardous areas, it is vital that proper installation practices be observed in order to avoid unsafe and dangerous circumstances.

Gus Elias is a senior certifications engineer at Moore Industries-International Inc. More information is available by visiting www.miinet.com.