|Content Type||Conference Paper|
|Title||Copper Grounding Systems Have a Negative Effect on Cathodic Protection in Production Facilities|
|Authors||Earl L. Kirkpatrick, ELK Engineering Associates Inc.; Mohammad Shamim, Union Texas Pakistan Inc.|
|Source||CORROSION 2000, March 26 - 31, 2000 , Orlando, Fl|
|Copyright||2000. NACE International|
The common bonding of underground ferrous structures to massive copper grounding grids creates problems for corrosion engineers and their attempts to cathodically protect the ferrous structures. Conflicts between copper and ferrous underground systems are discussed and alternatives are presented. A specific case history concerning production facilities in Sindh Province, Pakistan is presented. The case history includes the initial engineering evaluation and recommended system changes as well as the results of the recommended changes on overall performance of the cathodic protection systems in multiple production facilities.
Traditionally, most underground structures have been electrically bonded in common to reduce hazardous voltages associated with lightning and man-made fault currents or induced currents in the earth. A common grounding system provides a more economical and a lower resistance to remote-earth than does the individual earthing connections.
The U.S. National Electrical Code (NEC) ~ does not require copper grounding; instead, it requires that a permanent metallic earthing electrode and conductors must be used for earthing connections. Cathodic protection is routinely employed to overcome soil instigated corrosion cells on crude oil and natural gas production and transportation facilities. When such a system is directly connected to a bare copper earthing system, current demand may increase by several orders of magnitude. This creates a conflict between cathodic protection engineering design and electrical safety design. 2 Acceptable alternatives do exist to the use of bare copper conductors and bare copper or copper clad groundrods. 3
DESCRIPTION OF PAKISTANI PRODUCTION FACILITIES
At the time of the study reported on in this paper, the company operated six (6) major oil and gas production facilities and a number of smaller facilities in the Badin Block, Sindh Province, Pakistan. Exploration began in 1977 with the first producer discovered at Khaskeli on June 16, 1981. 4 The production facilities separate oil and gas, knock out water, ship natural gas via pipeline and store oil and distillate production in aboveground tank farms for later transport to refinery by tanker truck. Electrical power is obtained from on-site generator sets with diesel or natural gas prime movers. Since all of the production facilities in Sindh Province are essentially similar, varying primarily in overall size and in the geometry of the layout, we will primarily discuss in detail, only two of the production facilities in this paper. These production facilities (P.F.), designated as P.F.-1 and P.F.-2, handle both crude oil and natural gas. P.F.-1 was brought on-line about 1988. P.F.-1 is a natural gas production facility that contains two aboveground oil storage tanks and a variety of process vessels. There are a total of seven incoming flow lines in the intake manifold. The P.F. contains a series of separator vessels to knock out water and to separate distillate from the natural gas. There is one outgoing gas transmission line.
The original electrical grounding grid was constructed utilizing PVC coated stranded copper conductor ranging in size from 16ram 2 to 95ram 2 conductor. A 16ram 2 conductor is used for equipment grounding. Grid conductors are 70ram 2 or 95ram 2 and are connected to driven copper clad groundrods at 200 (61m) to 400 (122m) foot spacing around the perimeter of the plant with supplemental ground rods at major equipment. Cathodic protection is supplied to the plant by Transformer/Rectifier (T/R) Number 1, a 25 volt 75 ampere unit powering a distributed vertical anode-bed around the two production tanks. T/R Number 2 is a 25 volt 75 am
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