Hazardous Area Classification for Pumping Stations
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One of the first, (and most important consideration) in system design is area classification. Area classification, will at a minimum, dictate the required components necessary to meet code. NFPA 820 along with NEC defines the classifications for Wastewater Treatment and Collection Facilities. It is important to note NFPA 820 covers “Pumping Stations” which is a broad term and is inclusive of most waste water pump systems. For this reason NFPA is pertinent in nearly all in ground waste water pumping systems.
In general, residential collection systems serving 5 or less dwelling units are “Unclassified” and thus fire and explosion hazard is not applicable. NEC still applies for general guidance in these systems. For systems larger than 5 residential units sewers are categorized by usage type. The type categories are:
• Storm Water Pumping Wet Well – serving storm sewer system.
• Residential Wastewater Pumping Wet Well – serving primarily residential wastewater.
• Wastewater Pumping Station Wet Well – serving a sanitary sewer or combined sewer.
A combined sewer is defined as a system containing domestic, commercial, industrial and storm wastewater. the, Wastewater Pumping Station Wet Well category, is the most diverse classification and usually applies to municipal wastewater pumping stations.
The area classifications for the above categorized pump station are as follows:
• Storm Water Pumping Wet Wells are classified as Class 1 Division 2, Group D.
• Residential Wastewater Pumping Wet Wells are classified as Class 1 Division 2, Group D without venting and can be declassified with venting creating 12 air changes per hour.
• Wastewater Pumping Station Wet Wells are classified as Class 1 Division 1, Group D without venting and can be classified as Class 1 Division 2, Group D with venting creating 12 air changes per hour.
It is not generally practical to ventilate a wet well at 12 changes per hour therefore in most all cases NEC article 500 applies to all the above wet well installations.
Class 1 areas require FM rated pumps and intrinsically safe electrical sensing equipment, further, Class 1 Division 1 requiring explosion proof equiptment. It is important to consider space classification in Electrical Design and selection of the wet well components.
Pl provide hazardous areas name for different process services of pumps
How do you normally handle the reuirement for 30 ac/hr when the wet well is not ventilated (as per 10 states)? Do you know the history of thsi requirement?
From 10 States Standards (2004)
42.75 Wet Wells
Wet well ventilation may be either continuous or intermittent. Ventilation, if continuous, shall provide at least 12 complete air changes per hour; if intermittent, at least 30 complete air changes per hour. Air shall be forced into the wet well by mechanical means rather than solely exhausted from the wet well. The air change requirements shall be based on 100 percent fresh air. Portable ventilation equipment shall be provided for use at submersible pump stations and wet wells with no permanently installed ventilation equipment.
The scope and definitions in 820 are ambiguous about what is considered a “collection system” or “collection facilities”. I do not believe that 820 was intended to apply to stormwater collection systems or facilities that DO NOT feed into wastewater treatment systems or facilities. A typical facility that should not fall under 820 would be something like a pump plant that collects stormwater only from a roadway and discharges it to a stormwater system that does not flow to a wastewater treatment facility. This ambiguity is causing many municipalities to waste money to get a minimal reduction in a risk that may never be realized.
I would like to hear opinions of others in the industry about the applicability of NFPA 820. Tight budgets should force us back to making sound engineering decisions instead of ones based on excessive safety merely to avoid a lawsuit.