Operations personnel are well advised to become familiar with the basic electronic systems built into the gas measurement and detection instruments used by gas utilities.

Familiarity with these field devices, their instrumentation, applications and how they function is especially vital for those tasked with installing and maintaining electronic devices for gas delivery.

It also is important to have the right tools on hand and to be familiar with troubleshooting techniques.

Electronics

For gas utility operations personnel, electronics have become a part of everyday life. Gas utilities are installing more devices every day. Customers demand it; downsized organizations rely on it. Timely consumption and pressure information is quickly becoming crucial to everyday operations.

The foundation of electronic circuits begins with Ohms Law. This simple equation relates the voltage (E), current (I) and resistance (R) as R x I = E. A good analogy to explain this relationship exists in the flow of gas through pipes, exhibited as:

  • Pressure = voltage (E, electromotive force, in volts);
  • Flow = current (I, in amperes); and
  • Friction = resistance (R, in ohms).

Picture the inside of a pipe with a smooth steel surface. There is some friction as the gas flows through, but little changes from day to day. If it is a loop of pipe (or a circuit) with steady resistance, then increasing the pressure in that loop will increase the gas flow, proportionally, from R = E/I. To keep R the same while increasing E requires an increase in I.

When engineers design a compressor station, an analog study is performed. This study uses electrical components arranged in a circuit to mimic the piping layout. Resistors and inductors are used to represent pipe segments, with higher resistance for smaller diameter pipe. Voltage is applied to check the pressure and flow characteristics and fine-tune the design. This simple equation can help technicians fine-tune their instruments as well.

The most common instrument is a pressure transmitter. The mechanical portion uses a diaphragm, bellows or bourdon tube to convert the pressure into a linear motion. This movement then is converted into an electric current proportional to the pressure. One method of producing current is by sliding a magnetic rod through a coil of wire. By precise control of the motion and employing some amplifiers and filters, an exact current is transmitted. A transmitter typically is powered by 24 VDC, and the return ground wire carries a current of 4 to 20 ma. So for a 0- to 100-psig range, 12 ma, the midpoint, would represent 50 psi. Many times this current signal needs to be converted to a voltage to be recognized. A 250-ohm load resistor commonly is used to convert to a 1- to 5-volt range. Going back to the math, if a 12-ma current goes through a 250-ohm resistor, what is the voltage drop across that resistor? Manipulating the equation gives:

  • E = I x R;
  • Volts = amperes x ohms;
  • Volts = 0.012 x 250; and
  • Volts = 3.0.

Next up in math class is the power, the consumable resource, the energy. The relevant equations are:

  • P = I x E;
  • Power = current x electromotive force; and
  • Watts = amperes x volts.

Electrical kilowatt energy is mathematically related to dekatherms, or gas energy. For that matter, mechanical energy, horsepower, can be directly converted to watts. Using the previous gas pipe analogy, the pressure times the flow equals the volume, or the amount of energy, the power.

Just like sizing a meter for the correct volume, a circuit, or electrical installation, must consider the power requirements. A 60-watt light bulb, for instance, at 120 VAC uses half an amp, 60 watts = 120 volts x 0.5 amperes. So a typical 20-amp circuit breaker could theoretically power 40 60-watts bulbs. In practice, however, the National Electric Code limits this value to about half of that, based on wire size and installation concerns. Consult NEC Article 500 for details.

Gas measurement devices use much less energy. A transmitter at 20 ma and 24 volts requires only 0.48 watts. The latest battery-powered correctors and pulse accumulators use even less. Remote terminal units (RTUs), alone, can operate at less than 1 watt. Modems and radios can consume from 2 to 10 watts, especially when transmitting, but modems are increasingly using the phone line’s power to operate. Smaller installations are fused for 2 amperes, while more complex sites use 10-amp fuses for 12 or 24 volts. A 60-watt solar panel at 12 volts can accommodate 5 amperes when charging. Using a 30 amp-hr, deep-cycle battery to provide enough backup power can help get through the nights and cloudy days.

Field equipment

Two types of field devices exist, primary and secondary. A primary device is the RTU, corrector or pulse accumulator. These include primary support devices that may be in the enclosure – the modem, power supply, surge protection, intrinsic barriers and displays. Secondary devices connected to the primary device are known as inputs/outputs (I/O) and include such things as transmitters and pulsers.

Today’s RTU is a powerful device, derived from programmable logic controllers of the past. They can perform the complex equations of AGA 3 orifice metering and manipulate I/O in any conceivable way. Electronic correctors incorporate pressure and temperature inputs within the unit, and some have the modem and batteries in the same compact device. They also can do AGA 3, AGA 8 and AGA 7 calculations, but are generally less flexible and have fewer I/O options.

In hazardous areas, intrinsically safe barriers are installed in the circuit between the primary device and a secondary device. The barrier prevents the possibility of a spark occurring in the gas house by shunting any excess current or surge to ground. Barriers act like a resistor in the circuit, and a voltage loss will occur across them. Be sure to select the proper barrier for the application, and test it in the circuit to be certain it will not affect the normal I/O operation.

Four types of secondary devices exist: analog inputs (AI), analog outputs (AO), digital inputs (DI) and digital outputs (DO). Some secondary devices are listed in Table 1.

An analog signal is a 4- to 20-ma circuit, powered by 18 to 24 VDC. This is a control industry standard. A digital signal is a switch on or off, a change of state. The voltage level is not standard, but typically is low-level direct current (DC). Some common ranges are 0 to 10 VDC, 1 to 5 VDC or 0 to 24 VDC.

Often the actual voltage is not critical; only a change needs to occur within a deadband. For instance, going over 8 volts will turn on the input, and falling below 2 volts will turn it off. Most digit outputs will have a current limitation of about 500 ma or less. This could not power a motor or high-current device, but may be enough to switch a relay that has high current contacts to power the motor.

Applications

Using an RTU’s programmable features allows personnel to perform many impressive tasks with the I/O (Table 2).

Common failures

One of the most common failures in the primary device is loss of power. The cause can be as simple as a tripped circuit breaker or depleted battery. Lightning strikes or power line surges can trip surge protectors or damage them out of service. Fuses from the power supply, at the battery or integral to the primary device, are another culprit. Power supplies can fail, but these usually begin to undercharge or overcharge the backup battery, and a fuse may blow.

Communications losses are as common as power failures. Telephone lines can surge, especially with lightning strikes. Customers change or disconnect phone lines occasionally. Voltage levels can be off too. Typically, an analog phone line is about 45 to 50 VDC for a dial tone, and drops to 5 to 15 VDC when ringing. Some older phone systems operate at about 90 VDC, and some may even have alternating current (AC) voltage. When surges occur, the surge protector should take the hit, but often the modem fails as well. Radios are more reliable, but their antennae can get damaged or become misaligned. They can act like lightning rods, too. Separate radios are more reliant on power, and are thus more susceptible to surges.

Secondary devices can be victims of lightning as well. Lightning can travel miles on a pipe, and when the pipe comes up at the meter or regulating station, the lightning will try to escape the pipe from rough edges or protrusions, like a pressure transmitter. Many times a surge will travel back on the wires or conduit to the primary device, but the barrier will stop the surge from damaging the RTU.

Improper installation or replacement is another nemesis. One result can allow rainwater to damage primary and secondary devices. Moisture can find its way into correctors and pulsers, and then corrode magnetic reed switches or interfere with internal optical sensors. Water in conduit eats away at any exposed part of the wire, like a nick in the insulation, until a short-circuit occurs. Incorrect replacement parts can draw too much power and damage a primary device. Any failed secondary device can damage the primary if a short or power surge occurs with the failure.

Wiring can cause intermittent or total failures. Connections not screwed down tightly or incorrectly terminated can result in an intermittent signal or loss of a signal months after installation. Improper grounding or ground loops can disrupt normal operation at any time and go away later. Incorrect wire size or type may be a concern. Shielded wire grounded at the source or primary device and floated (unconnected) at the secondary device is a best practice. Sometimes industrial machinery nearby causes failures that may be eliminated with proper shielding or grounding. Other times, even proper grounding and wiring practices cannot entirely eliminate stray voltage problems.

Tools and techniques

Besides a common screwdriver or Leatherman, the best tool is a multimeter. This allows for the measurement of AC or DC voltage, milliamp current, resistance in ohms and even frequencies. Some can even simulate 4- to 20-ma inputs. When using a multimeter, be sure to note the various test lead locations. Some meters will chirp if the leads are in the wrong terminals.

Be especially careful when testing milliamp circuits. It is easy to damage the meter or blow a fuse when this is not done properly. To test current, the meter must be put in the loop. This usually is done by disconnecting a wire and completing the circuit again with the meter. Some pressure transmitters have test points to allow current sampling without disconnecting.

When testing resistance, be sure not to touch the leads. This can corrupt the readings. When checking wires for continuity, expect only a few ohms for wires up to 100 ft long. Large amounts of resistance on a wire indicate a short somewhere along its length. To check a loop, twist one end of the pair together and meter them at the other end. Check each wire to ground individually, and do not forget to check them to the shield as well.

In many cases, troubleshooting is a process of elimination. Think of all the failure possibilities, then rule each out. Simulation is a good way to check the primary device and eliminate the secondary component. A paper clip or small jumper wire can be used to create pulses at the RTU. A D-cell battery can be wired to input a small voltage, which can simulate either an AI with a load resistor or a DI. A more advanced technique uses a process meter, like a Fluke 787, which can simulate a pressure transmitter and output the full 4- to 20-ma range quickly and precisely.

The next best things to carry are spare parts. The fastest way to solve a problem is by replacing parts. A little meter testing is recommended prior to swapping components. This helps prevent damage to the replacement part. Spare fuses, barriers and batteries are easy to keep on hand. Power supplies, pressure transmitters, solenoids and pulsers may be more difficult to manage, but can be invaluable to solving a problem and moving on. A good suggestion is to have a box full of equipment and parts, replenishing supplies as they are used.

A notebook computer may be necessary to fully troubleshoot an electronic gas measurement or supervisory control and data acquisition (SCADA) site. The correct software and connecting cables are essential elements. Some programs have restart or warm start procedures; in some cases, that may be all that is needed. Other RTUs and correctors have reset buttons that can restore them to operation. Still others require a program to be completely reloaded to restart a unit, or at least reconfiguration of portions of the operation to get the site back in operation after a reset. Consult the proper manuals for the correct procedure. Notebook or laptop computers are not designed to operate in natural gas environments. It is not recommended to start or use a laptop inside a meter house. One computer manual advises against operating the device below 20°F or above 110°F. Keeping a 50-ft cable in the vehicle helps keep the laptop warm and dry.

Safety should be the first thing to consider when working in the field. Be sure the site is safe upon arrival, while at work and upon departure. Gas leaks or smells that are evident upon arrival should be addressed immediately. Sometimes weather or abuse can leave power wires dangling at RTU sites. In such cases, get a professional electrician to repair unsafe wires. Rain or water in a conduit may cause an unsafe condition when working with wiring. Be sure to turn all power off to a device when repairing or replacing it.

Field service work can be satisfying and frustrating. Good communications between field technicians and office support helps reduce tension. Knowing when and where a problem occurs as well as the exact symptoms helps determine the proper course of action. Eliminate multiple trips to repair the same site by having the correct tools and equipment to do the job right the first time. Think about what needs to be done to protect field personnel, the equipment and facility so that the job can be done efficiently and safely, and the information can begin flowing again.

Acknowledgment

This article is based on a paper presented at the 61st Annual Appalachian Gas Measurement Short Course, Moon Township, Pa.

Author

James P. Reinmann is an instrument, measurement and controls engineer at Dominion-East Ohio in Cleveland, Ohio.