Pressure Drop Measurement for Water Softeners
By Rick Andrew
One of the major considerations to be taken into account when installing whole-house water treatment equipment is pressure loss or pressure drop. All equipment that is added to the plumbing in a building on a public water supply, or downstream of the pump on a private well system, will cause a reduction in water pressure under flowing conditions. Each of these pieces of equipment causes additional friction when the water flows through it, which is responsible for this loss of pressure. The higher the flowrate, the greater the friction and the higher the pressure drop through the equipment. And different equipment will have different pressure-drop characteristics. A large diameter, smooth, straight pipe, for example, will have minimal pressure drop associated with it because it causes a relatively small amount of friction for the water flowing through it. Filtration media with a small pore size, on the other hand, causes significantly more friction as the water flows through it and therefore, significantly higher pressure drop.
Water softeners contain two main features that contribute to pressure drop: the control valve and the media bed. Water softener manufacturers are very aware of the pressure drop caused by these features and take it into consideration when designing systems. For example, they may include gravel or other larger particle size media in the media bed to provide a more efficient flow path and reduce the loss of pressure. They may also design the service flow path through the control valve in such a way as to minimize changes in direction of water flow and to maximize the diameter of the flow path, which also reduces the pressure drop of the equipment. Water treatment professionals are also aware of pressure drop. When sizing equipment for an installation, they are careful to size it large enough so that they don’t end up with complaints about low water pressure after the installation. Requirements in NSF/ANSI 44 help them size equipment with confidence when it comes to water softeners.
- Limitations on the allowable pressure drop for conforming water softener systems
- A test method to measure pressure drop
- A method to calculate pressure drop for water softeners that have a different amount of media in them from the tested softener
The standard requires that the pressure drop of the water softener must not exceed 15 psi at the manufacturer’s rated service flow, which must be at least four gpm. By limiting the pressure drop to this value, there is assurance that there will be acceptable line pressure within the building in which the softener is installed. There won’t be problems (such as low water pressure in a shower in an upstairs bathroom), assuming the softener is sized appropriately and the rest of the plumbing system in the building is according to code requirements.
The test apparatus used is relatively simple. It includes a valve for regulating the flow and a pressure regulator, as well as a pressure gauge for the inlet pressure and a pressure gauge for the outlet pressure. The test for pressure drop of a water softener is straightforward. First, the cation exchange resin within the softener is connected to the test apparatus, then conditioned. Conditioning is done by running water with 20 grains per gallon (gpg) hardness through the softener at a flowrate of four gpm per cubic foot of resin until the cation exchange resin is exhausted. The point of exhaustion is defined as the treated water containing 10 gpg of hardness. Then the softener is regenerated according to the manufacturer’s instructions with the minimum recommended salt dosage. This creates a defined and known state for the cation exchange resin, which could affect the pressure drop results if the resin beads swell or shrink due to the state of their regeneration.
The pressure regulator on the test apparatus is then set to deliver water at a temperature of 18 ± 5°C (65 ± 10°F) to the softener at 35 ± 5 psi pressure. Flowrate is initially set to 20 percent of the manufacturer’s recommended service flow; water temperature and inlet and outlet pressure are recorded. The pressure drop at this flowrate is defined as the difference between the inlet and the outlet pressure reading. So, if the inlet pressure is 34 psi and the outlet pressure is 30 psi, then the pressure drop is four psi (see Figure 1 for a graphical illustration of this measurement and calculation). Following this, the procedure is repeated at 40, 60, 80, 100, 120 and 140 percent of the manufacturer’s recommended service flow. As the flowrate increases, the friction of the water through the softener will also increase; therefore, the pressure drop will increase (the pressure downstream of the unit will decrease even if the pressure at the inlet stays the same). The results are then corrected to a temperature of 60°F by applying a mathematical correction factor.
If it is desired to have the ability to calculate the pressure drop of similar softeners with different amounts and/or bed depths of cation exchange resin, then the procedure is repeated with a softener that is identical to the one tested for pressure drop, except that it does not contain any resin. In this case, it is not necessary to pre-condition the unit prior to testing, because there is no cation exchange resin in it. The pressure drop measurements obtained at each flowrate will be lower than the pressure drop measurements for the system with resin, because the resin creates friction with the flowing water. The softener without the resin creates less friction. By taking these measurements, the pressure drop associated with the resin itself can be known. At a given flowrate, the pressure drop of the resin equals the pressure drop of the softener with resin minus the pressure drop of the softener without resin.
NSF/ANSI 44 includes mathematical formulas to allow the pressure drop of similar water softeners with different amounts and/or bed depths of resin to be calculated from this data. This allows for establishing conformance of a family of water softeners without specifically having to test each one.
Providing data to help professionals
Water treatment professionals have many factors to consider when recommending and installing equipment. They need to think about the water quality, what type of treatment is required, the source of the water in terms of public water supply or private well water and they need to size the equipment appropriately, considering the size of the building, number of bathrooms and occupancy, among other things. An important factor in sizing a water softener is pressure drop. Installing undersized equipment can lead to too much pressure loss and ultimately a dissatisfied customer. Fortunately, NSF/ANSI 44 includes requirements for pressure drop and a straightforward, repeatable and reproducible test method to measure it. These requirements allow professionals to have confidence in the manufacturer’s rated service flow for water softeners that conform to the standard. By choosing a softener with a high enough rated service flow to effectively serve the building and knowing that the pressure drop does not exceed 15 psi at that flowrate, any issues with complaints related to low water pressure can be avoided.
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: Andrew@nsf.org