March 2022

Plant Design, Engineering, Construction and Commissioning

Utilize heavy-duty cable tray systems to improve cable tray designs

This article presents a novel design concept that utilizes a heavy-duty cable tray system to optimize cable tray design, procurement and construction.

Al-Hamaidi, M., Mulla, M., Saudi Aramco

This article presents a novel design concept that utilizes a heavy-duty cable tray system to optimize cable tray design, procurement and construction. Typically, cable tray designs follow the examples mentioned in National Electrical Manufacturers Association (NEMA) standards. In Saudi Aramco’s Tanajib Gas Plant project, a full report for the cable tray and structural steel design—containing detailed design calculations, shop tests witnessed by an outside inspection agency and simulations prepared by Wescosa—was conducted to prove the compliance of the heavy-duty cable tray system with all applicable standard requirements.

One of the difficulties that many oil and gas projects face is the demand for power, which leads to a significant number of cables. In Saudi Aramco projects, the preferred installation method for cables is utilizing aboveground cable trays to enhance the safety, reliability and maintenance requirements. Therefore, aluminum cable trays are the ideal option for projects that require properly designed pipe rack and cable tray supports.

Utilizing heavy-duty cable tray systems will significantly optimize the quantity of supports required for the cable tray without affecting the quality, reliability and safety of the facility. This will also eliminate the difficulties of the design and installation, especially in congested areas where multiple curves and levels coincide. Moreover, this will improve the project schedule related to material delivery and installation.

The NEMA VE 2 standard addresses the shipping, handling, storing and installation of cable tray systems. This article will demonstrate the comparison between the typical NEMA VE 2 design and the improved design according to the manufacturer’s recommendation, which utilizes the application of a heavy-duty cable tray system based on horizontal 6-m cable trays with heavy-duty joint connectors and heavy-duty expansion joint connectors. In addition, the system includes heavy-duty horizontal bend fittings, heavy-duty horizontal tee fittings and heavy-duty horizontal cross fittings.

NEMA VE 2 design

The following details NEMA VE 2 requirements:

  • The expansion joint connector should have two supports located within 600 mm (2 ft) on each side.
  • The horizontal bend fitting should have one support located in the center of the fitting and two supports within 600 mm (2 ft) of each fitting’s edge.
  • The horizontal cross fitting should have four supports under each side rail and four supports within 600 mm (2 ft) of each fitting’s edge.
  • The horizontal tee fitting should have three supports under each side rail and three supports within 600 mm (2 ft) of each fitting’s edge.

Improved manufacturer’s design

The following details the requirements of the improved design according to the manufacturer:

  • The expansion joint connector should have one support located within a quarter point of the expansion joint connector.
  • The horizontal bend fitting should have two supports within 600 mm (2 ft) of each fitting’s edge.
  • The horizontal cross fitting should have four supports within 600 mm (2 ft) of each fitting’s edge.
  • The horizontal tee fitting should have three supports within 600 mm (2 ft) of each fitting’s edge.

Tests and reports

The cable tray manufacturer for the Tanajib Gas Plant project submitted a full report ensuring and validating the compliance of the improved manufacturer design with NEMA, International Electrotechnical Commission (IEC) and Saudi Aramco standards. As per paragraphs 9.4 and 9.10 of Saudi Aramco’s SAES-P-104 standard, the following criteria, including safe working load (SWL) recommendations, were followed:

  • 9.4. The working load for cable trays should consist of the weight of the cables (or tubing, etc.) including future additions (if required), plus a concentrated static load of 90 kg at the center of the span. The working load should not exceed the rated load capacity of the cable tray defined in NEMA VE 1 or NEMA FG 1 (destruction load divided by a safety factor of 1.5).
  • 9.10. Deflection of the cable tray system (several sections spliced together as a continuous beam), when loaded to the working load as defined in paragraph 9.4, excluding the concentrated static load, should not exceed L/100 (L = span length).

Actual load tests

To verify that the heavy-duty cable tray system adhered to the mandated required loading capacity as per paragraph 9.4 of this Saudi Aramco standard, all fittings and multi-span straight cable tray sections were tested locally in the manufacturer’s shop and witnessed by a third-party agency as per IEC 61537 clauses 10.2, 10.3 and 10.7, and NEMA VE 1 Clause 5.2. The following are details from these tests.

Horizontal bend fitting. The horizontal bend fitting was assembled as per the improved manufacturer’s design, using the recommended splicing device, and mounted at the supports. The test procedure followed IEC 61537 Clause 10.2. The horizontal bend fitting nominal data used for the actual load tests are shown in TABLE 1. Load Q is the uniform distributed load applied on the fitting (SWL × Lm). Load Q1.7 is the uniform distributed load applied on the fitting, with a safety factor of 1.7 (SWL × Lm × 1.7).

It was observed that the practical mid-span deflection at Load Q met the required acceptance criteria of (< Lm/100). Moreover, the sample sustained at Load Q1.7 without collapse. Therefore, the testing result for mid-span deflection passed. Detailed values are shown in TABLE 2.

It was observed that the transverse deflection—which is the vertical deflection across the width of the base area, omitting the longitudinal deflection—when mounted horizontally, met the required acceptance criteria of (< (w)/20). Therefore, the testing result for transverse deflection passed. Detailed values are shown in TABLE 3.

Horizontal tee fitting. The horizontal tee fitting was assembled as per the improved manufacturer’s design, using the recommended splicing device, and mounted at the supports. The test procedure followed IEC 61537 Clause 10.2. The horizontal tee fitting nominal data used for the actual load tests are detailed in TABLE 4. Load Q is the uniform distributed load applied on the fitting (SWL × Lm). Load Q1.7 is the uniform distributed load applied on the fitting, with a safety factor of 1.7 (SWL × Lm × 1.7).

It was observed that the practical mid-span deflection at Load Q met the required acceptance criteria of (< Lm/100). Moreover, the sample sustained at Load Q1.7 without collapse. Therefore, the testing result for mid-span deflection passed. Detailed values are shown in TABLE 5.

It was observed that the transverse deflection—which is the vertical deflection across the width of the base area, omitting the longitudinal deflection—when mounted horizontally, met the required acceptance criteria of (<distRT/20). Therefore, the testing result for transverse deflection passed. Detailed values are shown in TABLE 6.

Horizontal cross fitting. The horizontal cross fitting was assembled as per the improved manufacturer’s design, using the recommended splicing device, and mounted at the supports. The test procedure followed IEC 61537 Clause 10.2. The horizontal cross fitting nominal data used for the actual load tests are shown in TABLE 7. Load Q is the uniform distributed load applied on the fitting (SWL × Lm). Load Q1.7 is the uniform distributed load applied on the fitting, with a safety factor of 1.7 (SWL × Lm × 1.7).

It was observed that the practical mid-span deflection at Load Q met the required acceptance criteria of (<Lm/100). Moreover, the sample sustained at Load Q1.7 without collapse. Therefore, the testing result for mid-span deflection passed. Detailed values are shown in TABLE 8.

It was observed that the transverse deflection—which is the vertical deflection across the width of the base area, omitting the longitudinal deflection—when mounted horizontally, met the required acceptance criteria of (<distRT/20). Therefore, the testing result for transverse deflection passed. Detailed values are shown in TABLE 9.

Multi-span straight cable trays. The multi-span straight cable trays (five spans) were assembled as per the improved manufacturer’s design, using the recommended splicing device, and mounted at the supports. The nominal data included the following:

  • Rated load: 149 kg/m at 6-m span
  • Maximum mid-span deflection (Lm/100): 60 mm
  • Total test load: 4,470 kg.

The test procedure followed IEC 61537 Clause 10.3 and NEMA VE 1 Clause 5.2. The total test load is defined by Eq. 1:

Total test load = 5 spans × 149 kg/m per span = 5 × 149 kg/m × 6 m = 4,470 kg                                                            (1) 

It was observed that the average deflection span at 100% total test load met the required acceptance criteria of (Lm/100). Therefore, the testing result for average deflection span passed. Detailed values are shown in TABLE 10.

Simulation validation

The design was validated by simulating several recommended tests and utilizing proprietary software that can validate the actual test performed to be installed in an industrial environment. The results of the validation exercise have been reviewed by Saudi Aramco’s engineering consultation committee, including civil, structural and electrical consultants. The simulations performed included the following:

  • Static structural analysis
  • Thermal structural analysis at maximum temperature
  • Thermal structural analysis at minimum temperature.

All simulations were successful and met the required acceptance criteria.

Recommendations and limitations

As a result of the manufacturer’s report, a set of recommendations and limitations was developed to ensure the design capability and the installation integrity. These recommendations and limitations included the following:

  • The heavy-duty cable tray system can be applied to any length of trays, with proper testing and validation.
  • Installation should be limited to horizontal cable tray runs. Vertical cable tray runs should be installed as per NEMA VE 2 requirements.
  • Fittings should not be installed immediately before or after a heavy-duty expansion plate.
  • Only three types of fittings are considered: horizontal bend fittings, horizontal cross fittings and horizontal tee fittings.
  • Cable tray fitting supports should be located within 600 mm of the cable tray fitting’s end.
  • Cable tray supports should be located within a quarter span of all straight cable trays adjacent to the cable tray fitting.

Takeaway

This article has demonstrated an improved design for cable tray systems by utilizing the heavy-duty cable tray system that will enhance the design and installation of the structural steel and cable tray system. Concerning the design, the heavy-duty system eliminates the design complexity (especially in congested areas where multiple curves and levels coincide) that affects maintenance requirements. In addition, the heavy-duty cable tray system reduces the quantity of material needed for the structural steel, thus improving the schedule of the installation. For this case study, applying the heavy-duty cable tray system in the Tanajib Gas Plant project eliminated more than 15,000 supports of approximately 270 km of cable trays. HP 

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