Pool Systems Introduction

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.

Surge Tank

The surge tank fundamentally supports the gutter system, which captures surface water and absorbs of waves formed within the pool. The water on the surface has the potential to be the dirtiest in the pool. Debris (such as leaves, pollen), algae spores and oils (from skin, sun screen lotion) collect on the surface. The sun's UV rays break down disinfectant (chlorine) most at the surface. It is important that this water be collected with priority.

When the flow from the gutters is not sufficient (due to water level and/or a lack of waves), additional flow from the drain is required to maintain the surge tank level. This is typically handled via a modulation valve on the drain inlet to the surge tank, controlled by a level monitoring mechanism (such as a float). When a modulation valve is not present, flow much be restricted on the drain return and the pool level must be maintained to ensure some amount of gutter return is present.

Low Water Level

When the surge tank is too low, this is typically due to one of more of the following reasons:

• The pool water level is too low (when a drain return modulating valve is not present). Fill the pool.
• The drain return modulating valve is malfunctioning, if present. Service the valve.
• The main pump flow rate is too high. Reduce the outlet flow rate (only after ensuring that other conditions are not present).

When a drain return modulating valve is not present, it is important that the drain return is restricted (the valve is partially closed). Otherwise, flow from the drain will potentially prevent adequate flow from the gutter return. In an ideal case, the surge tank should always have a modulating valve on the drain return.

High Water Level

Typically, high water in the surge tank is not of concern (unless it has nearly reached or exceeded the overflow point, of course). High water is usually the result of:

• A nominal or slightly elevated pool water level in conjunction with a large amount of pool activity (spilling water into the gutters).
• Limited flow into the filter system due to a plugged strainer basket or blockage elsewhere.
• Excess water in the pool due to over-filling or a significant rain event (in this case, some water may be removed via backwash at the operator's discretion).

All three scenarios, of course, assume the main pump is operating. The surge tank will rise to a relatively high level shortly after the main pump is disabled (it will match the pool level).

Filters

Four Pentair Triton II Commercial TR140C-3 silica sand-filled filter tanks operate in parallel to capture particles in the water that passes through. As particles accumulate, flow is restricted. This is indicated by an increase in pressure.

Maintenance

Periodic back-washing is required when filter tank pressure exceeds a threshold. PHTA recommends back-washing when a single gauge pressure reaches 8-10 PSI above the starting value (typically 10 PSI). The backwashing procedure is described here

The filter sand should be replaced every 5-10 years. In a marcite pool, the filter sand should be replaced following the cure period of a new marcite coating (as the process releases a large number of particles, clogging the sand filters).

Chemical Monitoring

This is detailed in the Chemical Treatment: Monitoring section, below.

Boiler

Information about the boiler has moved to a separate page.

Chemical Treatment

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.

Monitoring

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.

Chlorination (Chlorinator)

Calcium hypochlorite (chlorine) is added when the controller has deemed the ORP to be too low. A PowerBase Model 3150 chlorinator is responsible for adding chlorine by dissolving "Accu-Tab" solid tablets in water.

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.

Theory of Operation

Water flows to the chlorinator from a tap on the outlet side of the main pump, i.e. "Unfiltered Water". An inline strainer provides a limited amount of filteration, preventing larger particulatesr from entering the chlorinator. Evidence has shown that without any inline filtering, the chlorinator solenoid in particular is subject to malfunction.

Troubleshooting

Overflow

Overflow in the reservoir (mixing basin) is likely due to the failure of one of two valves:

• Water fill via the solid tablet tank is controlled by a PurAqua PASV150B solenoid valve. This valve is electrically actuated by a a small float switch. Verify that the solenoid opens/closes (listen for an audible click) when the float switch is operated. The valve's purpose is to stop flow of water to the solid tablet tank (and ultimately the reservoir) when the water level has reached the upper threshold. Build-up of contamination on the rubber diaphragm or debris in the housing may prevent the valve from fully closing, allowing water to flow uncontrolled. To clean, remove the 1/2" bolts and separate the two metal parts carefully not to damage the rubber diaphragm. Remove the diaphragm and clean surface contamination with fresh water. Inspect for cracking, tears or other signs of deterioration. If all is deemed to be in good working order, re-assemble (careful not to over-tighten the bolts).

• Make-up water into the mixing basin is controlled by a mechanically-operated plunger valve, opened and closed by a float ball. This valve is a 1-inch Kerick PT100SS (it seems that a Kerick PT100SS may work as well). This valve appears to make up the difference between outflow (via the chlorinator pump) and inflow that would otherwise only pass through the solid tablet tank. The valve may over time become damaged, or blocked by debris. Inspect and clean as needed. The valve can be purchased individually from various sources. Alternatively, a rebuild kit (p/n 9500200) or the whole assembly (p/n 9501056) may be purchased from Aquatic Source or PurAqua Products. Assuming the rod and float ball are in good condition, replacing only the valve (re-using the rod and float ball) is a good balance of cost and ease of repair.

Pump Leakage

The pump shaft seal may have failed. Details to follow.