This page contains an article about a company who successfully saved substantial sums with a fairly simple surge suppression installation.
On the next page is another example from a major manufacturer you're probably familiar with. They estimated they showed a return on their investment in less than a year....and reduced some of their maintenance costs more than 65%.
Matt Esmacher, P.E., Washington Gas, Springfield, Virginia
Washington Gas has experienced a substantial reduction in maintenance expenses and overtime service costs by installing surge protection to safeguard equipment at all comp any plant facilities and gate stations.
The surge protection devices are used to protect instrumentation, power lines and data communication systems from induced surges and transient voltages.
Today’s modern process control and high-speed communication systems rely on sensitive high-performance electronic components. Any surge and transient conducted on signal, communication and power cables have a significant impact on operations, causing downtime and high repair and/or maintenance costs.
Since installing the protection devices, the company has not realized any equipment failures at remote distribution sites from natural causes, such as lightning.
Washington Gas sells and delivers natural gas to nearly 850,000 metered customers in the metropolitan Washington, D.C., and adjoining areas in Virginia and Maryland. Gas distribution facilities consist mainly of 32 unmanned transportation gate stations, more than 500 regulator stations, and four peak shaving plants.
A gate station is a critical point, where gas purchased by the company is delivered and where the gas pressure is reduced from as high as 1,250 psig down to as low as 190 psig. The gate stations filter, meter, heat, throttle (reduce pressure) and odorize the natural gas to condition it for delivery into the company’s distribution system. Once in the distribution system, the regulator stations reduce the pressure to 60 psig, or less, and then deliver the gas to the customer, where the gas is further reduced to 2 psig, or less.
Gas is purchased from four natural gas transmission companies. The company uses aboveground and underground storage facilities, storing high-pressure natural gas (nearly 2.1 Bscf stored at 2,500 psig) and propane (about 15 MMgal, equivalent to 1.3 Bscf of gas).
The propane storage plants use the propane to make propane air, a substitute for natural gas. This stored energy provides an efficient way to balance the company’s energy supply with widely fluctuating, seasonal, daily and hourly market requirements. The high-pressure natural gas and the propane are purchased and placed into storage to ensure continued supply during heavy demand periods.
Certainly, flow measurement is a critical concern. At each transfer gate station, Washington Gas and its supplier analyze the gas to ensure that the gas volume purchased from the pipeline is measured accurately. This is done by determining the static pressure, temperature and gas flow through the pipelines.
Gas flow is measured using a flowmeter. The type of meter used is determined by flow conditions. The flowmeter transmits a signal corresponding to the gas volume passing through it. However, the gas volume varies greatly with varying pressure and temperature. Therefore, the flow signal is compensated with the pressure and temperature measurements to calculate the equivalent gas volume transmitted at standard temperature and pressure conditions (60°F, 14.7 psia).
The co mpany uses this calculated volume of gas transmitted and compares this volume to the supplier’s measurement. The devices that measure the static pressure, temperature and flow are critical to the proper transfer of gas between Washington Gas and its suppliers.
Additionally, the gas purchased by the company has no smell or color. Consequently, accurate gas measurement also is essential to determine the proper amount of odorant to inject into the gas to give it the desired odor. Too little odorant causes a safety concern, too much odorant results in an overload of leak calls.
Surge protection. In the past, the company experienced equipment damage at its remote gate stations when lightning struck and power surges destroyed expensive instruments used to measure the gas and odorant flow. Maintenance technicians traveled long distances to these remote sites at all hours of the day and night to make the necessary repairs, often taking a full day and costing the company thousands of dollars.
At remote gate stations, the control rooms are connected to the transmitters by long power and signal cables that are mounted on overhead racks or buried underground. The primary cause of damage from a lightning strike (more than 99%) is not due to a direct hit, where virtually nothing survives. Instead, the damage is caused by secondary effects of the strike, where the high energy is resistively coupled through wiring into equipment with different ground potent ials.
Lightning discharges large currents into the ground which cause the ground plane to rise at the strike point, equally affecting all nearby equipment. The equipment voltage rises with the initial strike and falls as the ground potential falls back to earth ground, usually with no damage from the secondary effects of the strike.
The problems occur when these large voltages are coupled over cables to distant equipment that is at earth ground. This differential voltage can cause significant equipment damage. Field transmitters contain low-power semiconductor devices that can be damaged by over-voltages of only tens of volts. The longer the cables, the more frequently this difference in ground potential can occur, and the more likely remote instruments are to suffer from over-voltages and component failures.
As an illustration, consider the effects of a lightning strike to a building, housing control equipment that is protected by lightning conductors and ground rods (Fig. 1). The conductor carries the large strike current into the earth and dissipates the charge transfer into the earth’s mass. The effect of this current will elevate the reference potential of the building. As an example, if the strike current is 200 KA and the ground conductor impedance is 0.5 Ohms, then the potential aboveground is 100 KV. Exposed metalwork within the building is bonded to the same reference potential, and so only small voltage differences exist, posing little risk to personnel.
Fig. 1. Illustration shows damage potential at remote transmitter caused by lightning strike.
As lightning discharges into the earth, the ground potential rises to the value determined by the amount of current and the amount of resistance of the earth. Moving further from the strike point, the ground acts as a dropping resistor, creating voltage rings of lesser potential though still at tens of thousands of volts at increasing distances from the strike. This difference in ground potential is the primary cause of equipment damage, even when the remote equipment is grounded.
The field transmitter is mounted on the pipeline away from the control building but connected to it by long signal cabling. In this case, the building’s ground is at a much higher voltage than the ground at the instrument. Yet, the signal line on the field transmitter and, therefore, the electronics of the transmitter, will develop the same energy as the ground swell, because it is connected directly to the building. The transmitter case is held to the voltage potential that is developed with its local ground reference. Most transmitters incorporate some level of isolation from ground, typically 500 V. The isolation level now has to withstand the transient voltage between the new building reference potential (up to 100 KV) and the transmitter’s earth potential (0V). This large momentary difference in earth ground will damage the remote device, even if the transmitter is grounded.
If lightning were to strike near the remote transmitter, the effects of this example would be reversed. The large surge current could damage the equipment in the building, due again to the large difference in ground potential. If lightning were to strike in the middle of the cable run, it could damage both the building’s equipment and the remote transmitters.
Surge protection devices (SPD) are used to protect equipment from the potentially destructive effects of high-voltage transients by diverting the excess current and voltage until it subsides, while in normal opera tion they pass AC and DC signals with little or no attenuation. These devices operate instantaneously to divert a surge current to ground with no residual common mode voltage presented at the equipment terminals. Once the surge current has subsided, the SPD automatically restores normal operation and resets to a steady state to receive the next surge.
For transmitter protection, it is usually necessary to protect both ends of the loop, since surge protection devices can only provide local protection. SPDs control voltage and current relative to their local ground, requiring that "remote" devices have their own individual protection.
Solution. After repeated lightning losses costing thousands of dollars, Washington Gas decided to use surge protection devices to protect equipment at its remote sites, including all signal lines that run outside the control buildings. The company wanted a surge protection product line that could be easily installed in both retrofit applications and new installations.
Each site has three or four buildings, as well as outside instrumentation that is mounted on the pipeline. The company wanted each external signal to be protected on both ends of the long cable run. In the control room, where space is at a premium, they wanted a small, DIN rail-mounted product that would protect the input / output (I/O) system while taking up little space. A protector with a replaceable fuse for loop disconnect and trouble shooting was desired to ease system startup and maintenance.
The protector for the remote transmitters had to be field mounted and rugged to withstand the harsh environmental elements. The company wanted the protector to mount directly to the field transmitter through the unused conduit opening in the transmitter.
Washington Gas chose MTL Inc.’s Telematic products for its instrumentation surge protection needs. The compact DIN rail-mounted SD Series surge protection devices were selected to protect the control room instrumentation, while the TP48 transmitter protector was used to protect the field transmitters. These devices were quickly installed in 10 remote gas distribution sites and have provided cost effective surge protection.
Quick payback. Shortly after installing the surge protection equipment, the company was able to deliver an immediate payback on its initial investment. Lightning hit near a storage plant in West Virginia, about a two-and-a-half-hour drive from company headquarters, and the three expensive transmitters measuring gas flow in the pipeline were protected. The company’s supplier had redundant transmitters for their own gas flow measurement and did not use surge protection, but instead had all of their transmitters grounded in an effort to divert any surges to ground. Most of the grounded transmitters were destroyed by the lightning strike, while none of Washington Gas’ transmitters were damaged, saving the company thousands of dollars in equipment, repair time and downtime.
This application, where all transmitters were mounted from the same pipeline in the same area, presents a compelling argument for surge protection. The three instruments with surge protection suffered no damage, while most devices used by the gas supplier that were only grounded were severely damaged.
Conclusion. The decision to install surge protection at all of its plants and gate stations has been beneficial. Maintenance costs have been greatly reduced by protecting remote gate station equipment. The labor costs to send a technician to the site also have been greatly reduced. The company has eliminated over 90% of the overtime costs associated with servicing gate station equipment after hours or weekends, in conjunction with lightning storms. The company plans to protect all of its plants and gate stations within the next two years to ensure further safe, reliable and cost effective natural gas supply to its customers.
Copyright © 1999 Pipe Line & Gas Industry
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Last Updated: 07 Jun 2004
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