Electrical design considerations for WWTP headworks buildings

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electrical room
Another important consideration is to provide a “physically separated” electrical room.

By Syed Q. Raza

Preliminary treatment is the starting point of a typical wastewater treatment plant. NFPA 820 defines it as “the conditioning of wastewater as it enters the wastewater treatment plant. Preliminary treatment removes materials that might be harmful to or might adversely affect the operation of the treatment plant.”

The process is generally carried out in a headworks facility which comprises screen and grit removal facilities. Screen facilities remove large material like lumber, rags, stones, plastic, etc., while grit facilities remove finer particles like sand, silt and grit from the incoming raw sewage.

Class 1 Division 1 (Zone 1) installation

In Canada, both screening and grit removal are generally carried out in enclosed structures. Therefore, both screen facilities and grit removal tanks are classified as Class 1 Division 1 locations as per National Fire Protection Association (NFPA) 820 Table 5.2.2, Row 2 Line-a and Row 5 Line-a respectively for enclosed spaces when continuous ventilation is less than six air changes per hour. This means that all equipment installed must be approved to meet the requirements of Class 1 Division 1 (Class 1 Zone 1) applications.

Is reclassification to Class 1 Division 2 (Zone 2) worth it?

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To declassify a headworks building as Class 1 Division 2 (Class 1 Zone 2) requires continuous ventilation of 12 air changes per hour to be provided, with provision to monitor power failure and provision to connect to an alternate source of power. This method allows use of Class 1 Zone 2 rated equipment, but, over time, the increased energy cost of heating and ventilation will become significantly higher than the initial saving. Therefore, this option is generally not used for new installations.

Wiring methods

The Canadian Electrical Code (CEC) Part I Section 18 describes the requirements of electrical wiring in classified areas. The most common wiring method in Class 1 Zone 1 (Division 1) locations is to use rigid metal conduits with conduit seals. Conduit seals are installed in order to prevent the passage of hazardous gases and explosions from one portion of the conduit system to another.

After installation of cables, these seals are filled with a conduit sealant compound which will prevent any additional cable installation through them in the future. Therefore, special consideration should be paid if an addition to the facility is envisioned. It usually helps to specify a minimum of 20% spare control cables for any immediate future requirements.

PVC coated steel conduits
PVC coated steel conduits with seals.

Although the Code allows the use of standard rigid steel conduits, design engineers may often specify PVC or epoxy-coated rigid steel conduit. The PVC/epoxy coating on steel conduit provides an additional level of protection from corrosion, since the headworks building environment is comparatively humid. Special consideration is required for installation of these conduits, especially at the coupling area, for trouble free operation.

It is recommended that the installing contractor arranges training of their staff by the manufacturer of PVC/epoxy-coated rigid steel conduit. This can be ensured by including this training requirement in the contract documents.

Another important consideration is to provide a “physically separated” electrical room to house the power distribution equipment, starters and control panel. This enables the use of economical, unclassified equipment. It is also easier to maintain the equipment in an environmentally controlled, unclassified electrical room. A local operator station suitable for a Class 1 Zone 1 location may be provided for the operation staff if required.

A common wiring method is to use approved hazardous location cables with associated cable glands. These cables are generally installed on cable trays and hence eliminate the requirement of conduits seals.

Conclusion

When considering the electrical design of a headworks building, specify equipment approved for a Class 1 Division 1 (Zone 1) area, and use PVC/epoxy-coated steel conduits in the headworks building, with special attention to installer training. Electrical rooms should be physically separated, with provision for viewing windows and local controls inside the screened room.

Syed Q. Raza, P.Eng., is with R.V. Anderson Associates Ltd. This article appears in ES&E Magazine’s June 2019 issue.

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