Model GCM Gradient Control Mat by Dairyland Electrical
Personnel working near pipelines can be subject to electrical hazards such as lightning, AC fault current/voltage, and steady-state AC induction. "Ground mats" have typically been used at above ground pipeline structures where these hazards exist, but past designs do not provide adequate protection against lightning conditions.
DEI now offers an engineered Gradient Control Mat (GCM) to address lightning and AC fault current conditions, backed by analysis from experts in lightning protection and with a full review by Correng Consulting of the cathodic protection guidelines of this mat design. Step and touch potentials are now limited with the DEI grid-type mat, in a much lower cost design. DEI also recommends decoupling gradient control mats to improve CP on the pipeline, and offers the new, affordable Solid-State Decoupler (SSD) line for use in conjunction with the Gradient Control Mat (GCM).
Features & Characteristics-
Low cost design.
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Galvanized steel grid, using 3" x 3" squares.
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Low inductance design limits step and touch potentials due to lightning.
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Decoupling the mat offers long life and improved CP on the pipeline.
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Data available on design life and supplemental anodes.
Detailed Description
The gradient control mat by DEI is designed to limit step and touch potentials due to lightning because such design will also keep step and touch potentials well below that required by IEEE 80 for power frequency voltages. The gradient control mat is made of 0.135” diameter steel wire with a 3” x 3” wire spacing with all wire intersections welded. After welding, the mat is hot dip galvanized. Mats are available in 4’ x 8’ sheets and are shipped flat on pallets. Each mat weighs 15 pounds (6.8 kg). For larger mats, these sheets can be joined together to form any size mat required. Provision is made for thermit welding adjacent mat sections together to form an electrically continuous mat and for thermit welding lead connections to the mat.
Design Life
To assure long life of the galvanized steel gradient control mat in all soil conditions, DEI has commissioned Correng Consulting to provide guidelines for cathodically protecting the mat using readily available magnesium anodes. The design life of the anodes protecting the mat is 20 years, as documented in Correng Report COR-05-9508D “Guidelines for Cathodic Protection of Galvanized Steel Gradient Control Mats.”
Installed Costs
Although auxiliary anodes and a decoupler are recommended, the total installed cost of the galvanized steel mat will normally be less than a pure zinc mat of any construction, particularly when an 8’ x 8’ or larger mat is required.
Safe Touch and Step Potentials
Specific technical data is provided for the touch and step potentials for the most difficult voltages to control, namely, voltage caused by lightning. Refer to Table 1 in DEI Application Note 8: Gradient Control Mats in Pipeline Applications for more information. To limit the touch potentials to safe levels, it is essential to make the connections between the pipe and the mat (whether directly or through a decoupler) with the shortest possible lead length, and preferably with multiple leads, to reduce lead inductance, and thereby reduce the inductive voltage drop in the leads.
Decoupling
While the DEI gradient control mat may be directly connected to the pipeline or decoupled, it is recommended that the mat be connected to the pipeline through a new, low cost Solid-State Decoupler (SSD) developed by DEI for this purpose as illustrated in Figure 1. A decoupled gradient control mat offers several distinct advantages:
- The galvanic potential of the mat material is irrelevant when it is decoupled from the cathodically protected pipeline.
- CP readings can be taken in the vicinity of the gradient control mat.
- Stray DC currents are prevented from accessing the pipeline, thereby preventing a corrosion problem where stray DC currents would exit the pipeline.
- Any interaction between the CP system and the gradient control mat functions are eliminated.
- When the mat is directly connected, the service life of the anodes is reduced by approximately 36% for the H1-Alloy magnesium anodes and 28% for the Hi-Potential magnesium anodes; hence, considerably longer anode life is achieved with a decoupled mat.
Ordering Information
1. Gradient Control Mat (4’ x 8’): Order #GCM4-8. Specify quantity.
2. Anodes Required: Order from Farwest Corrosion. Typically, one or more 17 or 32 pound magnesium anodes are sufficient for control mat application.
3. Decoupler: For most applications, the SSD model suggested is SSD-2/2-1.2-75 which blocks ±2.0 volts (hence, polarity is not an issue when installing), has a 1.2 kA fault current rating at 30 cycles, and a 75 kA lightning current rating (4x10 µsec). Decouplers with higher AC fault and lightning current ratings are also available - See the Farwest SSD web page for additional model and information. Specify model number and quantity required.
For large station mats, it is suggested that connections to the gradient control mat be made at the incoming and outgoing pipelines. Other pipe segments within a station may also need to be referenced to ground through additional decouplers.
4. Thermit Welding Molds:
Most gradient control mat installation require multiple 4'x8' ground mats to be joined together to protect a large structure. In these installations, it is necessary to bond the mats together, via the thermite weld process, to provide continuous bonding of the entire grid.
MOLD-6X
The MOLD-6X is designed to join adjacent mat sections together, welding approximately every 18 inches. MOLD-6X uses an industry standard #25 weld metal charge, available from Farwest Corrosion, to weld adjacent mat wires together via "grooves" installed in the mold. This mold can also be used to weld the mat wire to a #6 AWG wire from an anode or Dairyland decoupler, again using a #25 charge. Where smaller anode wires are used, such as #10 or #12 AWG, a #15 charge and protective sleeve can weld these to the mat wire.
5. SSD Decoupler Mounting Options:
(a) Banding may be utilized. The banding is to be purchased by the user. Contact Farwest for banding material sources. This mounting option requires that leads be run from one decoupler bus to the pipeline and from the other decoupler bus to the mat. Two leads to each bus are recommended for a total of four leads. Order two of #MTL-6-12 for 12”(300mm) leads or two of #MTL-6-36 for 36”(900mm) leads. Two sets of leads are provided with either model number; hence, one of either model ordered will provide two leads. #6 AWG copper leads are furnished with terminals attached to one end (plus nuts, bolts, washers, for securing to the SSD bus) with the other lead end unfinished. Order the mold and weld metal in Item 4(b) for welding the leads to the mat. The user is responsible for the lead connection to the pipeline. (Custom molds can be acquired for welding the leads to the pipeline upon request provided the pipe diameter is given and it is specified whether the weld is to a vertical pipe wall or on the top side of a horizontal pipe.) All leads should be cut to the shortest possible length during installation to minimize touch potential.
(b) Attachment via pin brazing M8 studs to a steel pipe is illustrated in Figure 3. The user must have the required pin brazing equipment and consumable items (i.e., the M8-1.25 threaded studs with a 16mm threaded section furnished with two washers and a locknut and ceramic ferrules). Equipment and studs are available from Farwest Corrosion. The desirability of this option is that it virtually eliminates the inductance of one lead connection because one SSD terminal is directly connected to the pipe via the pin brazed stud. For each decoupler installed using pin brazed stud mounting, order one #HCN-M8 (the Hex Coupling Nut plus M8 bolt and washers required) plus one set of leads, #MTL-6-12 for 12” (300mm) leads or #MTL-6-36 for 36”(900mm) leads.
Ordering Information Checklist:
1) Gradient control Mat: GCM4-8
2) Decoupler: Model SSD-2/2-1.2-75 unless other model selected.
3) Thermit Welding Molds/Charges:
• For joining adjacent mats or connecting cables to mat:
- MOLD-6
- #25 weld metal (20/pack)
• Decoupler Mounting Options:
- Leads: MTL-12 or MTL-36
- Banding: Material by others
- Pin Brazing: Equipment/studs/ceramic ferules by others. Order HCN-M8 kit from DEI.
Gradient Control Mat Decoupling
Gradient control mats are installed around above ground pipeline structures to protect workers from potentially hazardous voltages that can be present on cathodically protected pipelines. Pipeline voltages can result from the following conditions:
- Induced AC voltage
- An AC fault in improperly grounded electrical equipment
- A lightning strike directly to or adjacent to the pipeline
The effectiveness of a gradient control mat is determined by the step potential and the touch potential that it allows for the voltage sources described. Unfortunately, no known manufacturer has provided any technical data on the effectiveness of their mat in limiting step and touch potentials. Additionally, single conductor mats such as the spiral configuration type, have been found completely ineffective in limiting step and touch potentials from lightning to safe levels.
Solution: Farwest now offers an engineered Gradient Control Mat (GCM) to address lightning and AC fault current conditions, backed by analysis from experts in lightning protection and with a full review by Correng Consulting of the cathodic protection guidelines of this mat design. Step and touch potentials are now limited with the DEI grid-type mat, in a much lower cost design. Farwest also recommends DEI decoupling gradient control mats to improve CP on the pipeline and offers the affordable Solid-State Decoupler (SSD) line for use in conjunction with the Gradient Control Mat.
A decoupling device such as the SSD is recommended for use in conjunction with the gradient control mat as it allows for several distinct advantages including:
- The galvanic potential of the mat material becomes irrelevant
- Pipeline CP readings can be taken in the vicinity of the mat
- Any interaction with the pipeline CP system is eliminated
- The life of the anodes used to protect the mat is significantly increased
