When Reusing An Existing Open-Loop Water System
An engineering checklist (regularly updated and improved on) can help to simplify these design installations.
How often does an existing open condenser water system, or some other type of open water system, get reused as part of a building renovation project and/or mechanical system upgrade? It happens probably at least monthly somewhere, and yet I can’t say I have ever seen a quality control engineering checklist to standardize the process, to maximize the reuse of the pipe distribution, and to reuse or replace existing pump(s). Hindsight is always 20/20, so why isn’t there a checklist to avoid past errors and omissions, as well as to improve on system design from site-specific lessons learned?
To start, the design engineer should document the existing basis of design and review it with the existing facility personnel to get a sign-off on the original criteria and design assumptions. As the new design of the open-loop system is being finalized, the engineer should meet with the facility personnel to get sign-off that the new basis of design also includes the addressing of past deficiencies, proposed energy conservation measures, and O&M design intent.
With the preliminary design, there are a number of checklist tasks to consider as the engineer gives the existing water system a renewed lease on O&M life:
- Use as-built drawings to initiate the basis of design and have the design engineer and an O&M technician walk the site confirming the existing conditions.
- Pipe distribution things to look for:
- At the terminal units: are the coils piped correctly (e.g., supply and return water connections should be counter-flow and not parallel flow)?
- Are there adequate shut-off valves as well as drain valves?
- Is there adequate air-control?
- Does the existing condenser water pump have adequate inlet net positive suction head to satisfy the pump curve requirement?
- Does the condenser pump have duplex basket strainers to allow cleaning one strainer while flow is directed thru the parallel second inlet filter?
- Original system design
- What were the original design parameters (e.g., antiquated fixed supply-to-return temperature difference; 10ºF for centrifugal chiller, 15ºF for steam absorption chiller)?
- Was the existing pump head conservatively selected, resulting in the discharge balancing valve set 15% or more closed? Note: analogous to having one foot on the gas and the other foot on the brake.
- Are there chronic operating parameters that need to be addressed (e.g., cooling tower sump overflows when condenser water system is shut off) and corrected?
- Complete a hydraulic model of the closed-loop system, especially if the condenser water system serves several units of process equipment.
- Complete a water balance to record pertinent flows and head pressures to document existing conditions versus design criteria conditions.
- Reassess existing conditions and consider improvements (e.g., would one common indoor sump provide a better reservoir solution versus multiple cooling tower sumps for the new design?).
- Based on the adjusted existing condition hydraulic model (computer simulation with actual water readings), add in the new engineered system to come up with the final open-loop system design for optimum design.
- Based on the new system hydraulic design, can the existing pump be reused (e.g., review pump curve and especially the net positive suction head criteria)?
- If existing pump can be reused, does it need a more energy efficient motor?
Incorporate predictive O&M with planned maintenance.
- Compare pump run-hours versus a benchmark to determine the optimum time to perform pump maintenance.
- Consider differential pressure drop transmitter across the pump strainer(s) to generate a maintenance work order to clean the strainer.
- Other considerations
- Are seismic supports needed with the new design?
- Is there a personal protection rail system installed around the roof-mounted cooling tower?
- Create master list of energy conservation measures associated with open water systems.
The draft of this designer checklist can easily be increased to cover a broad range of issues, concerns, requirements, and compliance. Once completed, the checklist can always be continuously improved upon based on lessons learned and other people’s input, so that reusing an open-loop system will be a cost-effective solution. For more on developing open-loop water system checklist tasks, read last month’s reusing of a closed-loop water system. ES