Swimming Pool Construction Expert Witness, Consultant, Designer and Contractor Paolo (Paul) Benedetti discusses defective hydraulic design and plumbing deficiencies.
Getting it from Here to There... Safely & Efficiently
Hydraulics is the science and physics involved with the properties and movement of liquids. In swimming pools, this involves the movement of water through piping, pumps, equipment and fittings.
Swimming pool are the most common form of construction that is almost entirely "design & build." That is, the person who sells the product also designs and builds it.
And therein lies the problem. A pool salesman has absolutely no business specifying pipe and pump sizes during a sales call. Nor is it proper for the plumbing subcontractor to define the pipe sizes during installation.
Engineered Systems
The hydraulics of a swimming pool must be properly designed in order to operate efficiently, quietly and safely.
The line velocities (the speed at which the water travels through the pipes) must be maintained within defined parameters. Too slow and debris will settle in the pipes. Too fast and the system develops excessive resistance (dynamic head), noise, water hammer, premature equipment failure, pump cavitation and an unsafe bather environment (excessive suction).
Whether the system is designed by a mechanical engineer or through the use of hydraulic design software (created by an engineer), strict adherence to the standards must be maintained.
The national standards quantify the absolute maximums for line velocities. This does not mean that this is the goal... it is the speed limit, the maximum speed that the water can travel.
In conjunction with the line velocities, other factors such as gallons per minute (gpm), flow rates, turnover rates, equipment and fitting head pressures and the total dynamic head (tdh) of the complete system must be accounted for.
Restrictions to flow
Every length of pipe, pipe fitting, valve, filter, apparatus and trim fitting contributes to resistance in the system. Smaller diameter pipes have a lower maximum flow rate than larger pipes. The hydraulics of the system must be defined, before the exact pipe size can be specified.
The Correct Design Process
Starting with a scaled site map (project layout), the plumbing is diagrammed and laid out. The lengths of pipe and the pipe fittings are added up. The required system flow rate is extracted from the turnover rate.
A "guesstimate" is made from flow rate charts as to the appropriate pipe size. The filter(s) is chosen based upon the filter's allowable flow rates. The dynamic head (resistance to flow) of the other system components are also added to the system design calculation.
The system's TDH is then calculated from all of these variables. Finally, looking at manufacturer pump curves, a pump size is chosen. The TDH and required flow rate should fall within the center of the pump's operating curve.
Sometimes the TDH of the system may be to great. This is easily adjusted by increasing the diameter of the system's plumbing. This drops the restrictions and therefore the TDH.
Arbitrarily selecting pipe size and pump horsepower is asking for trouble. Pumps that are forced to operate outside of their operating curves are less efficient, noisy (cavitation) and prone to premature failure.
Graeme Baker Safety Act
The GBSA was the governments response to horrific and tragic entrapment accidents. These accidents were a direct result of poorly designed hydraulic systems.
Almost every system involved in suction entrapment accidents had excessive line velocities at the suction points (drains). The easiest means to decrease the velocity at a drain, is to install multiple drains. In addition to lowering the line velocities, the alternate suction points allow for the diversion of the suction in the event of obstruction of any single drain. Properly spaced and placed, blockage of multiple drains is virtually impossible.
Drain Grate Flow Rates
Split drains have been the minimum standard for years. For years prior to the GBSA, the IAPMO and NSPI (now APSP) Swimming Pool Plumbing Standard specified a minimum of 2 suction points for each pump. But swimming pool contractors have ignored this published standard repeatedly.
How many single skimmer pools with a single main drain exist in the US? When the skimmer becomes blocked, the main drain becomes the SOLE SUCTION POINT for the pump. This is hardly 2 suction points!
The grates for main drains also have a maximum flow rate. The flow rating is lowered when the grate is mounted in a horizontal position (pool wall or spa bench). This means that 2 drains in the floor may suffice. However, when one drain is located horizontally, the system may require 3 drains (2 on the horizontal & one on the floor).
This is a contractor induced system design flaw that is still occurring.
As a goal, the suction at the drain inlets should be a minimal as practical. This is especially important for vertical inlets, as bathers with long hair may become entangled in grates with excessive flow rates.
The solution to these issues is a properly designed, engineered, specified and installed hydraulic system.
As a result, the owner will have a safe bathing environment that will operate at peak efficiency and with minimal noise.
Getting it from Here to There... Safely & Efficiently
Hydraulics is the science and physics involved with the properties and movement of liquids. In swimming pools, this involves the movement of water through piping, pumps, equipment and fittings.
Swimming pool are the most common form of construction that is almost entirely "design & build." That is, the person who sells the product also designs and builds it.
And therein lies the problem. A pool salesman has absolutely no business specifying pipe and pump sizes during a sales call. Nor is it proper for the plumbing subcontractor to define the pipe sizes during installation.
Engineered Systems
The hydraulics of a swimming pool must be properly designed in order to operate efficiently, quietly and safely.
The line velocities (the speed at which the water travels through the pipes) must be maintained within defined parameters. Too slow and debris will settle in the pipes. Too fast and the system develops excessive resistance (dynamic head), noise, water hammer, premature equipment failure, pump cavitation and an unsafe bather environment (excessive suction).
Whether the system is designed by a mechanical engineer or through the use of hydraulic design software (created by an engineer), strict adherence to the standards must be maintained.
The national standards quantify the absolute maximums for line velocities. This does not mean that this is the goal... it is the speed limit, the maximum speed that the water can travel.
In conjunction with the line velocities, other factors such as gallons per minute (gpm), flow rates, turnover rates, equipment and fitting head pressures and the total dynamic head (tdh) of the complete system must be accounted for.
Restrictions to flow
Every length of pipe, pipe fitting, valve, filter, apparatus and trim fitting contributes to resistance in the system. Smaller diameter pipes have a lower maximum flow rate than larger pipes. The hydraulics of the system must be defined, before the exact pipe size can be specified.
The Correct Design Process
Starting with a scaled site map (project layout), the plumbing is diagrammed and laid out. The lengths of pipe and the pipe fittings are added up. The required system flow rate is extracted from the turnover rate.
A "guesstimate" is made from flow rate charts as to the appropriate pipe size. The filter(s) is chosen based upon the filter's allowable flow rates. The dynamic head (resistance to flow) of the other system components are also added to the system design calculation.
The system's TDH is then calculated from all of these variables. Finally, looking at manufacturer pump curves, a pump size is chosen. The TDH and required flow rate should fall within the center of the pump's operating curve.
Sometimes the TDH of the system may be to great. This is easily adjusted by increasing the diameter of the system's plumbing. This drops the restrictions and therefore the TDH.
Arbitrarily selecting pipe size and pump horsepower is asking for trouble. Pumps that are forced to operate outside of their operating curves are less efficient, noisy (cavitation) and prone to premature failure.
Graeme Baker Safety Act
The GBSA was the governments response to horrific and tragic entrapment accidents. These accidents were a direct result of poorly designed hydraulic systems.
Almost every system involved in suction entrapment accidents had excessive line velocities at the suction points (drains). The easiest means to decrease the velocity at a drain, is to install multiple drains. In addition to lowering the line velocities, the alternate suction points allow for the diversion of the suction in the event of obstruction of any single drain. Properly spaced and placed, blockage of multiple drains is virtually impossible.
Drain Grate Flow Rates
Split drains have been the minimum standard for years. For years prior to the GBSA, the IAPMO and NSPI (now APSP) Swimming Pool Plumbing Standard specified a minimum of 2 suction points for each pump. But swimming pool contractors have ignored this published standard repeatedly.
How many single skimmer pools with a single main drain exist in the US? When the skimmer becomes blocked, the main drain becomes the SOLE SUCTION POINT for the pump. This is hardly 2 suction points!
The grates for main drains also have a maximum flow rate. The flow rating is lowered when the grate is mounted in a horizontal position (pool wall or spa bench). This means that 2 drains in the floor may suffice. However, when one drain is located horizontally, the system may require 3 drains (2 on the horizontal & one on the floor).
This is a contractor induced system design flaw that is still occurring.
As a goal, the suction at the drain inlets should be a minimal as practical. This is especially important for vertical inlets, as bathers with long hair may become entangled in grates with excessive flow rates.
The solution to these issues is a properly designed, engineered, specified and installed hydraulic system.
As a result, the owner will have a safe bathing environment that will operate at peak efficiency and with minimal noise.
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