By Rick Andrew
Structural integrity is one of the key design aspects of POU and POE systems that is addressed by the NSF/ANSI DWTU Standards. Although it is not related to public health, structural integrity is critically important to fitness for purpose of products that are connected to a pressurized water supply. Loss of structural integrity means water leaks, which are not only wasteful, but can cause significant damage to houses and other buildings, especially if leaks are ongoing over time or are allowing high flows of water to leak. As such, structural integrity requirements for POU products connected to a pressurized water supply and for POE products are included in Section 5 of each of the NSF/ANSI DWTU Standards. This is true for water softeners, according to Section 5 of NSF/ANSI 44 Residential Cation Exchange Water Softeners.
Pressure rating and design and construction requirements
The standard allows manufacturers to recommend a maximum working pressure. It also requires that water softeners must be designed and constructed to maintain structural integrity at a minimum of 125 psig (860 kPa). The standard allows softeners to be designed and constructed to maintain structural integrity at higher pressures, but not lower. The manufacturer’s recommended maximum working pressure becomes the basis for testing of structural integrity.
Hydrostatic testing
Water softeners are required to undergo a 15-minute hydrostatic test at high pressure. The specific pressure for hydrostatic testing depends on the manufacturer’s maximum working pressure of the softener and the diameter of the resin tank. These pressures are described in Figure 1.
To conduct the test, the softener is attached to a pressure testing rig that allows high water pressure to be applied to the softener under controlled conditions. Water with a temperature of 13 to 24°C (55 to 75°F) is used, with care taken to assure that there is no condensation on the softener that might make it look as if there is a structural integrity failure. There is an option to plug the drain line and/or remove the cation exchange resin. The test unit is filled with water and flushed to purge it of air. It is important to purge the air because entrained air can cause the test unit to explode with force if there is a failure.
The outlet of the softener is then closed and the control valve is placed in the service position for testing. The hydrostatic pressure of the test unit is then raised at a constant rate so that the test pressure specified in Figure 1 is reached within five minutes. The rate of pressure increase must not be more than 100 psi (690 kPa) per second. The test pressure is maintained for 15 minutes. During this time, the test unit is inspected to assure that it is watertight. If there is any leakage out of the unit, the unit has failed the test.
Cyclic testing
Water softeners are required to undergo pressure testing that cycles from low to high pressure repeatedly. The standard requires that this testing cover 100,000 cycles at zero to 150 psi (zero to 1,040 kPa) or maximum working pressure, whichever is greater, as described in Figure 1.
To conduct the test, the softener is attached to a pressure testing rig that allows high water pressure to be applied to the softener under controlled conditions. Water with a temperature of 13 to 24°C (55 to 75°F) is used, with care taken to assure that there is no condensation on the softener that might make it look as if there is a structural integrity failure. There is an option to plug the drain line and/or remove the cation exchange resin. The test unit is filled with water and flushed to purge it of air. It is important to purge the air because entrained air can cause the test unit to explode with force if there is a failure.
The outlet of the softener is then closed and the control valve is placed in the service position for testing. The cycle counter on the test rig is either set to zero or the initial number of cycles at the beginning of the test is recorded. Pressure cycling is initiated. After the pressure rise, the pressure is immediately released, meaning it is held for less than one second. The pressure in the test unit must return to less than two psi (14 kPa) before the initiation of another cycle. The duration of the cycle must not exceed 7.5 seconds for pressure vessels over 33 cm (13 inches) in diameter and must not exceed five seconds for pressure vessels 33 cm or less in diameter.
During the cycling operation, the test unit is periodically inspected to assure that it is watertight. After the cycling is complete, the adjustable timer is stopped so that pressure is on the test unit and the unit is inspected for leaks. The unit must remain watertight throughout the test to pass.
Metallic resin tanks
Testing for softeners with metallic resin tanks requires additional measurements to make sure that the circumference of the tank is not increased by more than 0.2 percent after completion of the test, in addition to making sure that the softener is watertight. If the circumference of the tank is increased by more than 0.2 percent, the unit has failed the test.
A critical design and construction requirement
These structural integrity tests are quite rigorous, including testing well beyond the hydrostatic design pressure and testing extreme pressure cycling for 100,000 cycles. This rigorousness is important, though, because structural integrity of water softeners is an absolute necessity. Given the wasting of water and potential for damage caused by water leaks, it is imperative that water softeners be built to withstand line pressure over a long lifetime. By including these two challenging test requirements, NSF/ANSI 44 helps assure that water softeners are indeed built to last in real-world conditions of pressure variations and opening and closing of valves in the plumbing system.
About the author
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: [email protected]