pb-2013-102_Balgobin

MEASURING UP TO THE STANDARD: UPGRADING A GRANDFATHERED SCREEN ROOM TO MEET CODE P. Balgobin, P.Eng., R.V. Anderson Associates Limited, M. Krynski, P.Eng.,

Regional Municipality of Durham

For facilities that are older than ten years, the chances are high that at least one of the design codes used in the original design have been revised. If no substantive modifications to the facility are made then the facility does not need to be upgraded to meet the revised code(s) and the facility is considered grandfathered. In the municipal wastewater sector the issue of updating grandfathered facilities to meet current codes is of particular importance for raw sewage collection systems and the headworks area of wastewater treatment plants.

The Harmony Creek Water Pollution Control Plant (WPCP), located in Oshawa, ON, in the Regional Municipality of Durham, is one such facility. This WPCP consists of two plants. Plant 1, which was constructed in 1950s and is now decommissioned, and Plant 2, which was constructed in two stages in the early 1970s and still in operation. When the Region of Durham needed to replace Plant 2s forty year old, main bar screen it was recommended that they upgrade the room where the bar screen is installed to meet current codes, including Ontario Electrical Safety Code (OESC), the Ontario Building Code (OBC), and the National Fire Protection Association (NFPA) 820 Standard for Fire Protection in Wastewater Treatment and Collection Systems, introduced in 1990.

While the NFPA 820 standard is not a Canadian design code, both the OESC and OBC reference the NFPA standards for fire protection. Section 6.2.1.1 (Good Engineering Practice) of the OBC states with, respect to heating and ventilating systems, that those systems are to be designed, constructed, and installed to conform to good engineering practice appropriate to the circumstances such as described in the NFPA Fire Codes. The OESC notes that special conditions exist in sewage treatment plants and specifically references NFPA 820 for hazardous area classification.

Thus, while NFPA 820 is not an Ontario design code, its reference by the OESC and OBC make it a matter of good engineering practice to follow it with the same level of diligence as if it were a design code. In Ontario, most engineers follow this practice and for new construction projects it is usually easy to adjust design elements, such as HVAC configuration, door locations, new equipment electrical ratings, etc. to adhere to the guidelines set out by NFPA 820. However, bringing an existing facility to, retroactively, meet NFPA 820 can be challenging,

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especially when separating areas/rooms designated by the NFPA as Class 1, Division 1 (classified/hazardous areas) from un-classified areas.

The Class 1, Division 1 designation indicates that hazardous gasses that can lead to a fire or explosion are present on a continuous basis and measures need to be in place to prevent a fire or explosion. Harmony Creeks main and by-pass bar screens are installed in the Screen Room of Plant 2s Screen/Boiler Building. The channels that convey raw sewage run beneath the Screen Room floor and the openings in the floor around the bar screens allow gases from the raw sewage channel to enter the Screen Room continuously.

Table 5.2 of the NFPA 820 standard indicates that for Coarse and Fine Screen Facilities where there is no ventilation or ventilation below 12 air changes per hour and the area where the screening facility is located is enclosed, that area is designated as a Class 1, Division 1 area.

The Screen/Boiler Building also houses two boilers and an MCC on the ground floor and grit tank blowers and digester sludge heating equipment in the basement. Refer to Figure 1 for a layout of the Screen Room.

MAINENTRANCE


Figure 1: Layout of the Screen Room Prior to Modifications

The Screen Room is separated from the Boiler Room by the main entrance hallway and staircase that lead to the basement where the other equipment is located. On opposite sides of the entrance hallway are two doors: one leading to the Screen Room and the other leading to the Boiler Room.

For Class 1, Division 1 areas, NFPA 820 provides the following guidelines for safeguarding against fire and explosion hazards present in the area and ensuring that staff and equipment are protected:

OPTION 1: Continuously ventilate the area at 12 air changes per hour.

OPTION 2: Ventilate at less than 12 air changes per hour and ensure that:

i) All electrical equipment, devices, conduits, wiring, etc. in the classified area are to be rated for service in a Class 1, Division 1 area.

ii) Air from the classified area does not migrate to other areas in the building of a lower classification (i.e. not Class 1, Div 1);

iii) Openings, such as doorways or exhaust louvers, in the classified area are installed a minimum of three meters from a doorway/opening that leads to an area of lower classification.

At first glance, the choice between the two options for Harmony Creeks Screen Room seemed simple. Using controls and electrics that are rated for Class 1, Division 1 service is very expensive and is generally avoided as much as possible, making Option 2 undesirable. Option 1 would require the least amount of modifications to the room and minimal change to its electrics. But, it was not to be that simple.

By code, the Screen Rooms ventilation system must be connected to the facilitys emergency stand-by power system, such that if there is a power failure the area will continue to be ventilated thus preventing the accumulation of hazardous gases. Connecting the Screen Rooms increased ventilation system to the plants stand-by power system would not only increase the demand on the stand-by power system, but would also create a demand associated with heating the room at this elevated ventilation rate, both of which would be significant. Analysis of the emergency stand-by power system revealed that it did not have sufficient


capacity to support these additional loads. The Region preferred not to modify the emergency stand-by power or incur the significant operating costs associated with heating the Screen Room. Option 1 was discarded.

Option 2 of upgrading the electrics in the room isolating the Screen Room from the other areas of the Screen/Boiler Building involved the following major tasks:

1. Replacing the by-pass bar screens motor and limit switches with Class 1, Division 1 rated equivalents (the new screen would be design for Class 1, Division 1 service);

2. Replacing the local control panels with Class 1, Division 1 models; 3. Replacing all electrical conduits, wiring, devices, lights, junction boxes,

etc. in the Screen Room with Class 1, Division 1 equivalents;

4. Relocating the raw sewage sampler to an unclassified area (the fridge that stored the samples was not Class 1, Division 1 rated, neither was the sample pump);

5. Isolating the Screen Room from the rest of the building.

Items 1 to 4 were fairly straightforward to implement. As expected, item 5 proved to be the most extensive and challenging of the required modifications. This task involved installing a new, roof-mounted air handling unit to provide the Screen Room with an independent HVAC system. The Screen Rooms HVAC system was linked with the Boiler Rooms via ducting between the two rooms, which meant that the Boiler Rooms HVAC system also had to be modified to compensate for removal of these interconnections. The Boiler Rooms HVAC system was also upgraded to meet current codes (OBC and Digester Gas Code, since the boilers use a dual fuel train with natural gas and digester gas).

The first HVAC interconnection that had to be removed was ducting between the two rooms. The buildings vent shaft brought fresh air into the Boiler Room and an exhaust fan, installed in the Boiler room, transferred air from the Boiler Room, through ducting across the stairwell, into the Screen Room. The ducting and exhaust fan were removed and the openings in the walls blocked up.

The second HVAC interconnection that had to be removed was a duct that transferred air up from the Blower Room (below the Screen Room), through the Screen Room floor, along the west wall and exhausted outside the building. The Blower Room is an unclassified area so air from the Screen Room is not permitted to enter it. The existing ducting was needed to ventilate the Blower Room, but it was not sealed to prevent air from the Screen Room from entering. The duct was


replaced with one that was sealed and incorporated a fire damper at the floor level.

The last HVAC interconnection that had to be removed was the door leading from the Screen Room into the building entrance hallway (refer to Figure 1). When performing daily monitoring and maintenance checks, operators normally enter the Screen/Boiler Building through its west entrance and then proceed to either the Screen Room or Boiler room via the doors in the hallway or down the stairs to the basement. With the Screen Room doorway blocked up, there would be no entrance to this room from the west side of the building and since the operators offices are located in the west side of the plant, entering from the buildings east entrance would be inconvenient.

A new Screen Room entrance was needed on the west side. The only feasible location was directly next to the buildings main entrance because the Grit Tank is adjoined to the west wall of the Screen Room and extends almost the full width of the room. However, installing a door to a Class 1, Division 1 area (Screen Room) that is within 3 meters of an opening/doorway to an unclassified area (the hallway) is contrary to the NFPA 820 standard.

There seemed no way of complying with this NFPA guideline and still providing the operators with a west-side entrance to the Screen Room, until it was noted that the 3-meter separation requirement was based on air dispersion modeling that showed that it took approximately 3 meters of travel for air containing a hazardous concentration of gas to disperse to a safe level. That being the case, if a wall were constructed between the two doors that was 1.5 meters wide and 1.5 meters high, air from the Screen Room door would have to travel a total of three meters (1.5 meters on each side) before entering the door that led to the hallway. In this way the 3 meters of separation would be provided. An architecturally unusual solution, but the only one available. A new Screen Room entrance was installed with a 1.5 meter long wall beside it. Refer to Figure 2 for a layout of the Screen Room after the modifications were complete.

With the Screen Rooms HVAC system now isolated from the rest of the building a new HVAC system was needed for the building, one configured to accommodate the Screen Room and the remainder of the building. A new Air Handling Unit (AHU) was installed on the roof of the Screen Room and an new exhaust fan installed in the Screen Room.

A glycol system was installed in the Boiler Room to heat the air from the AHU when building heating was required. The Boiler Room ventilation system was also altered to include an new exhaust fan a series of motorized intake dampers,


interconnected with the fan and operation of the Boilers. The latter was to comply with the Digester Gas Code.

A new exhaust fan was also needed in the Blower Room, below the Screen Room, along with and intake air damper.


Figure 1: Layout of the Screen Room Prior to Modifications

Operation of the new HVAC system is automatically, thermostatically controlled via a new thermostat, installed in the Boiler Room.

In this grandfathered facility, replacing the main bar screen became an extensive upgrade of the Screen Room and, to an extent, the Boiler Room. However, the area has been raised to the current codes, including the NFPA 820 standard, and equipped with the appropriate devices and features to keep personnel and equipment safe and reduce the risk fire or explosion in the Screen Room.

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