Pool Systems Introduction
This page is presently being overhauled, as much of the relevant information has been moved onto other sub-pages.
Systems for pumping, filtering and chemical treatment ensure that pool water is sanitary, clear and warm.
As illustrated in the Process Flow Diagram, water exits the pool via the drain and/or gutters. A surge tank balances the flow from both sources. An intake pipe near the bottom of the surge tank feeds a strainer basket that is used to catch large debris (such as leaves) prior to entering the main pump. The main pump forces water into a bank of sand filters. Filtered water then proceeds to enter multiple "loops" for chemical monitoring, heating (via the boiler) and chlorination. Finally, water is returned to the pool via small outlets at the base of the gutter tray.
Items below are listed in water flow order.
Details moved to another page.
Information about the sand filters is available on this page.
This is detailed in the Chemical Treatment: Monitoring section, below.
Information about the boiler has moved to a separate page.
The pool is treated with calcium hyprochlorite (chlorine) and hydrochloric (muriatic) acid automatically according to measurements.
When requested, chlorine is added via the chlorinator loop and acid is added via the acid pump. Both of these are added post-boiler to minimize chemical interaction (corrosion) with the heat exchanger.
Why acid? pH will almost never naturally drop in an outdoor pool. pH rises due to:
- Removal of carbon dioxide from the water. Refer to the chemical reaction for carbonic acid for why carbon dioxide reduces pH. This is be caused by:
- Splashing, which accelerates off-gassing via bubbles.
- Algae, which consumes carbon dioxide (photosynthesis).
- A general desire to be at equilibrium; the carbon dioxide concentration in the water approaches that of the air.
- The addition of non-stabilized chlorine.
- Chemical interaction with the cement-based liner (i.e. marcite).
The pH will almost certainly rise faster when the pool is heavily occupied. Decomposition of leaves and acidic rain can lower pH, but these are considered minor contributors.
The acidity and amount of chlorine in the pool are monitored by pH and ORP (oxidation reduction potential) probes, respectively. When insufficient levels of either exists, the controller requests the addition of the respective chemical. Ideally, pH should be between 7.4 and 7.6. Proper pH prevents eye/skin irritation, maximizes chlorine effectiveness and minimizes corrosion. ORP should typically be between 650 mV and 750 mV for proper water disinfection.
While ORP is a means for verifying the level of pool chlorine, it is not a direct measurement of chlorine. ORP measures chlorine's ability to oxidize pool contaminants. Care should be taken to minimize cyanuric acid (stabilizer) in the pool, as it may reduce ORP and affect chlorination control.
Water for the probes is sampled at a point just beyond the filters, prior to the boiler and chemical feeds. This is identified as the Chemtrol Unit Monitoring Loop in the process flow diagram. A flow meter identifies whether adequate flow is present; insufficient flow indicates that water in the flow cell assembly may not accurately reflect the composition of the pool water. Water from the monitoring loop is returned to the surge tank.
Addition of Acid
Hydrochloric (muriatic) acid is added when the controller has deemed the pH to be too high. A electrically actuated peristaltic dosing pump transfers acid from a reservoir into the pool water return. The relatively small amount acid flow relative to the flow of the return ensures significant dilution.
Calcium hypochlorite (chlorine) is added when the controller has deemed the ORP to be too low.
It has been observed that the chlorinator has a tendancy to over-shoot. Therefore, the ORP setpoint on the controller is set lower than otherwise might be expected to minimize this over-shoot. The net result is a proper amount of chlorination in the pool.
Static Line (Water Level Monitoring)
A static line (water pipe) can be used to monitor pool water level from a remote location. In the past, a 2" PVC water line for this purpose was run between the pool and the pump house. The "inlet" is located on the west side the dive well, just below the gutter trough. It is run into the pump house, at which point an electrical probe was used to establish water level for automatic fill.
The electrical probe and associated electronics were removed from the pump house in 2020, having been (apparently) out of service for some time.
The static line is abandoned, no longer usable as described in the history section below.
An investigation spanning 2020-2021 determined that at least two prior events have rendered this pipe unusable.
In 2020 the pool closing contractor was asked to perform a pressure test on the pipe. Upon pressurization, water and air bubbled from the ground just outside of the pump house entry door. The site was later excavated by hand. Observed damage to the pipe suggests that it was crushed during installation of a fence post footing (2014). Further digging revealed that damage extends well under the concrete pool deck. It is not clear whether the fence contractor (and B&G) knew that this pipe was already abandoned at the time.
Despite a poor attempt at capping off the static line at the pool, water was observed to flow out of the pool into the ground (shortly after pool opening in 2021). This raised the urgency of the repair as a not-insignificant amount of treated/heated pool water was being continuously lost into the ground. It is assumed that this had been happening for many years.
Plumbing and concrete contractors were brought in during July 2021 to make repairs. At this point, it was assumed that damage was relatively contained, and a full repair would be feasible. Unfortunately, it was found that damage to the pipe is extensive, extending well beyond that caused by the fence installation. It is believed that many years of water expansion (and ground heave) led to widespread cracking; the pipe was not drained. Given the situation, a decision was made to cut out a small section of concrete by the pool entry point (near the gutter) to find a portion of "good pipe" to cap. Priority shifted to strictly stopping water leakage; the static line would be abandoned. Shockingly, it was discovered that the pipe was broken off within a couple feet of exiting the pool, with no apparent effort made to cap. A repair downstream would never have been possible. It is not clear when this break was made, but it seems possible it extended back into the late 1990's during concrete replacement.
As of July 2021, the pipe is capped off near the pool to eliminate all leaks. Concrete has been re-poured. A portion of pipe has been left in the event that a new static line is ever run. Due to budget constraints, no new static line was run; what is left in the ground is abandoned. The 2" pipe in the pump house will be capped off and labeled.
With no static line, automatic fill is still possible by monitoring the level of the surge tank (at least for now). The difficulty of doing so may increase when/if a modulating valve is installed on the floor drain return.