Anti-stagnation valves can cut water system energy costs

By Dave Alberton and Peter Sucharda

The City of Hamilton, Ontario’s water distribution system is one of Canada’s oldest and most complex. It includes six separate water treatment facilities, 2,031 km of water mains, 144,691 water service connections, 16 pressure-reducing zones, and 145 district-level pressure valves.

Situated at the west end of Lake Ontario, the city has an elevation of 91 m above sea level and is defined by unique geographical features, including the Niagara Escarpment and Hamilton Harbour. A recent pilot project demonstrated that the city could achieve significant energy savings and reduce water demand using anti-stagnation valve technology in its pumping district zones.

The city’s distribution system must maintain a minimum operating pressure of 20 psi (1.38 bar) at ground level at all points under maximum day demand, plus fire flow conditions. The network’s normal operating pressure is 40 – 100 psi (2.76 – 6.89 bar).

This is a difficult undertaking because of the Niagara Escarpment, a steep rock face that runs through the middle of Hamilton across its entire breadth, bisecting the city into “upper” and “lower” sections. The Niagara Escarpment’s vertical wall ascends an average of 100 m and presents a unique challenge in conveying water at acceptable flows and pressures.

Given the elevation change and the city’s sprawling geography, the water distribution system is divided into 25 distinct pressure districts, both open and closed. In an open district, continuous pumping is not required to maintain pressure, thanks to floating storage such as an elevated tank or reservoir. In a closed district, continuous pumping is required to meet the required flows and pressures in that portion of the distribution network.

In areas where a facility such as a reservoir or elevated tank is not present, pumping station discharge head must be enough to overcome system losses and maintain the appropriate hydraulic gradient.

The placement of floating storage within the distribution system provides sufficient amounts of water to equalize demand and translates into energy savings when supplying the network via gravity.

Pressure district zones are interconnected via level valves and an open 20-mm bypass line. Typically, bypass lines allow a continuous flow of water from high to low pressure zones to mix water and maintain an acceptable residual chlorine level, but bypass flows consume significant pumping energy.

Energy-saving measures

Hamilton Water is one of the largest energy users in the city and has started to implement measures to decrease electricity expenditures. In 2017, the utility spent more than $13 million on power costs (6.2% of its total operating budget).

In late 2017, a pilot project was implemented to investigate a significant flow increase from two reservoirs. It was determined that it was acceptable and feasible to significantly reduce water flow through the bypass lines from 24/7 continuous flow to approximately 15 min/day and still maintain water quality, using timer-controlled, anti-stagnation valves. The valves significantly reduced the required water flow from pumping stations.

Each valve is a 20-mm Cla-Val Model 139-10A on/off control valve that can be programmed to operate on a time schedule. The pilot project focused on 37 water distribution level valves and open bypass lines that separate two pressure districts. The valves were installed and operational by the end of June 2018. The previous one-year historical pumping station electrical demand (kW) and energy consumption (kWh) were used as a baseline for the pilot project.

The post-retrofit period (after June 2018) is being continuously monitored using the City of Hamilton Office of Energy Initiatives Energy Management System to gather electrical demand and energy consumption data to verify actual project energy savings and electrical demand reduction. The pilot study results showed that total demand reduction in power for the pumping stations in the two pressure zones was 497 kW. If a continuous, 24 hr/day, 7 days/week operation is taken into consideration, that equates to 4,353,720 kWh per year of energy use reduction.

Retrofit pays off

Based on the product calculations, it was estimated that each valve saves 44,000 kWh of energy, or 1,628,000 kWh per year in total for the 37 valves. This calculation is based on a 185 kW reduction in electrical demand and 24/7 process operation. Preliminary pilot study results have indicated an actual (average peak) electrical demand reduction of 497 kW. In addition, the valve timers were set for 15 minutes, which is shorter than the original time used to estimate savings, providing even greater energy savings.

Also, because electrical demand varies significantly in pumping station processes, the actual power consumption will be compared between the baseline period and post-retrofit data collected for the stations. Savings in the order of $200,000/year are expected for the pilot study’s first phase. With a total installed project cost of under $90,000, the technology’s payback period is less than six months.

Two more phases are now in the process of being developed based on these preliminary findings. The second phase will involve installing an additional 37 valves, providing an energy savings estimated at 400,000 kWh/year, or a cost savings of $56,000. A third phase also will be initiated in which 36 additional valves will be installed.

Dave Alberton is with Hamilton Water, City of Hamilton Public Works Department. Peter Sucharda is with Devine & Associates. This article appears in ES&E Magazine’s October 2019 issue.