Plumbing for Critical Facilities' Data Centers
November 18, 2009
By Robert Ioanna, Syska Hennessy Group
Appeared in Plumbing Engineer
What happens if the water supply to the building runs dry and how do I avoid damage from water leakage? These are fundamental questions that need to be analyzed in every component of the plumbing system to ensure no down time is attributed to these components.
The domestic water supply system plays a major role in the overall decision for mechanical equipment, as well as the overall cost. A typical total cost of ownership analysis for the larger critical facilities would reveal a water-cooled chiller plant utilizing cooling tower for heat rejection is the lowest cost. Cooling towers require city water make-up to replenish the water that evaporates from the system during heat rejection. For example, a typical 5,000-ton chilled water plant could require as much as 200,000 gallons per day depending on water quality.
So what happens if the water runs dry in this case? The risk of a municipal water shutdown becomes a single point of failure that needs to be closely analyzed. It is recommended to establish two water sources to the building from separate city water mains or from separate sides of the property. Additional levels of protection could be achieved by providing city water storage to the facility for multiple day’s worth of run time; this can be accomplished by a simple storage tank on site or an enlarged basin underneath the cooling tower. The remote possibility of digging a well on site can be investigated. Based on this perhaps the total cost of ownership for the cooling system changes, the capital cost for these items needs to be added to the initial cost and the cost of heating this water to prevent freezing needs to be added to the operating cost.
If the area of the project in question is prone to water supply disruption or the area in general has droughts then perhaps only air-cooled equipment can be considered to truly eliminate a single point of failure. In a total cost of ownership analysis typically the low first cost option would be to use air-cooled chillers. This leads to another option that needs to be analyzed. Provide an air-cooled system as the backup to the water-cooled chiller plant that would be operating under normal conditions. By considering this option the need for a second water feed to the building and any type of storage tank is eliminated. Now on a loss of water pressure or low water in the cooling tower basin, an alarm could be triggered to the BMS to alert the staff to change to air-cooled chiller plant. Depending on the location of the project and cooling load at your project, this approach could be the right one for your facility.
Now that we have addressed the water supply system, we need to address how to avoid damage from water leaks. Do we need floor drains and where? In my opinion, wherever pressurized water exists, i.e. chilled water pipes, condenser water, city water or sprinkler piping floor drains should be provided. If there is a leak and it can be contained properly then a means to drain it quickly without the use of multiple wet-vacs makes for less accidents during the cleanup process. However, having floor drains invites an issue of having to keep the traps primed with water and the possibility of water backing up to the data center.
This can be handled in multiple ways. The first is to eliminate traps all together and install a sump as an air break before discharging to the street sewer. This serves two purposes: it eliminates the possibility of sewer backup to penetrate through the floor drains backup and eliminates the trap primer altogether. Home run right? Not so fast; this cannot be applied in all cases, as the invert elevation of the sewer drain needs to be low enough to accommodate this drainage methodology.
If this can’t be done, the next best idea is to use manual as opposed to automatic trap primers for the watch engineers to manually fill the trap every couple of days as part of a maintenance program. Also installing backwater valves (check valves) in the traps will eliminate any backup of sewer flow into the data center.
Now that we’ve looked at the floor drains, we need to consider the roof drains. The best solution for a data center is to put all roof drains at the perimeter by pitching the roof in such a fashion as to eliminate any interior drains. Often the issue is that data centers are built in existing core and shell buildings and the avoidance of interior roof drains is not possible. Providing a primary and secondary drainage system is not only recommended, but most likely required by code for your particular building type.
In this case the best solution is to place the roof drains and primary/secondary piping risers in the hallways. Placing concrete curbs under the data center wall will help eliminate any leakage from the hallways into the fields. Couple of things to make sure we include is leak or flow detection in the secondary roof drainage system to alert maintenance personnel in the event of a primary roof drain backup. Also, typically in data centers, roof drainage piping in corridors are left exposed leaving it in the path of rolling equipment and carts. Make sure the rises are well protected by beams or by other means.
In conclusion, no matter how much water is stored on site the amount is finite and the building is subject to closure based on extended drought or major municipal water service problems. Also, consider installing a sump in the floor drainage system and move roof drainage systems to the exterior of the building wherever possible.