By Rick Andrew

Pressure drop is an important consideration when sizing and installing POE water treatment equipment. If the equipment is undersized, several problems can occur. Lack of capacity, frequent filter changes and low water pressure are among them. Low water pressure results from equipment that has too much pressure drop at the flowrates demanded by consumers. The dynamics and causes of pressure drop are relatively straightforward. Pressure drop is caused by friction between the flowing water and pipes and fittings, and increases in elevation throughout the piping network. Keeping these causes in mind, water professionals are careful to make sure pipes and equipment are not undersized. Helping these professionals are modern code requirements for sizing of pipes and equipment, even to the point where some believe modern codes go too far in this area. In order to size and evaluate equipment, there must be a measure or rating for pressure drop. This allows professionals to ensure that equipment will have adequate flow capacity without causing undo pressure drop in a given installation. Facilitating these measurements are standardized tests for pressure drop ratings, included in the NSF/ANSI DWTU Standards.

Pressure drop measurements
The NSF/ANSI DWTU Standards require that POE systems have their pressure drop measured. To perform this measurement, highly calibrated pressure gauges are installed immediately upstream and downstream of the system. Care is taken to ensure that pipes used on the measurement apparatus are sufficient diameter so as not to influence pressure drop measurements. Prior to conducting the measurement, the media must be conditioned. Because media can swell with wetting, pressure drop measurements on systems with unconditioned media can be different from those with conditioned media. For all POE systems except water softeners, the media is conditioned according to manufacturers’ instructions. Cation exchange water softeners are conditioned by introducing water containing 20-gpg hardness at a flowrate of four gpm per cubic foot of cation exchange resin until the point where the softened water contains 10-gpg hardness. Conditioning is completed by regenerating the softener at the manufacturer’s lowest recommended salt setting.

Figure 1. Measurement of pressure drop
Pressure drop = 35 psig – 26 psig = 9 psig

The inlet pressure for the measurement is set using a pressure regulator designed to have low pressure drop and installed upstream of the equipment being evaluated. For all POE systems except water softeners, the inlet pressure is set to 30 psig and for water softeners, it is set to 35 ± 5 psig. The flowrate for the measurement of pressure drop is set using a valve located upstream of the equipment being evaluated. The flowrate for measurement for all POE equipment except water softeners is the rated service flow. For water softeners, measurements are taken across a range of flowrates from 20 to 140 percent of the manufacturer’s recommended maximum service flowrate. Additionally, pressure drop data is gathered at these flowrates on specially prepared softeners that do not include any ion exchange resin. The pressure drop of the ion exchange resin itself is calculated as the difference between the pressure drop of the complete softener and the pressure drop of the softener without the ion exchange resin at each flowrate used for measurement. This pressure drop data across the range of flowrates for water softeners enables calculation of pressure drop for similar but untested softeners. Figure 1 describes how the difference in pressure readings on gauges at the inlet and outlet of the POE system is the pressure drop.

Pressure drop limits
Pressure drop increases with flowrate. The higher the flowrate, the more friction there is between the water and the piping network. The NSF/ANSI DWTU Standards require that pressure drop of POE systems shall not exceed 15 psig at the rated service flow. Codes will dictate the rated service flow required for a given size of house, depending on the number of bathrooms, etc. Code requirements vary by jurisdiction; be sure to know the local code requirements. So, the code dictates the required flowrate, and POE equipment should be selected such that it will have no more than 15 psig pressure drop at the flowrate required by appropriate code.

Pressure drop calculations for water softeners
It is recognized that many water softener models can be built using the same control valve, but varying in tank size and cation exchange resin amount. Bearing this in mind, NSF/ANSI 44 for residential cation exchange water softeners contains a section that allows calculation of pressure drop for water softeners using the same control valve, such that not all of them must be tested. There are limits on how much different in size the softeners can be for the calculations to be valid. Figure 2 describes these limits.

Figure 2. Limitations on calculating pressure drop of softeners based on similarity to a tested softener

  • The control valve must be identical.
  • The distributor must be identical, with the exception that it may vary in length of distributor tube to account for differences in resin tank height.
  • The resin tank diameter must be no more than 200 or no less than 50 percent of the tested softener.
  • The bed depth must not be less than 75 percent of the tested softener.
  • The bed depth of underbedding or adjunct media must be within 75 to 150 percent of the bed depth of the tested softener.
  • The maximum service flowrate per unit of cation exchange media must be no more than 120 percent of the tested softener.
  • The slowest rinse flowrate per unit of bed cross sectional area must be 50 to 120 percent of the tested softener.
  • The total rinse volume must be at least 15 gallons per cubic foot of cation exchange media, or at least 90 percent of the volume per cubic foot of cation exchange media of the tested softener if it is less than 15 gallons per cubic foot. Also, the total rinse volume cannot exceed 200 percent of the total rinse volume per cubic foot of cation exchange media of the tested softener.
  • The salt dosages per cubic foot of the softener must be within the range of the tested softener.
  • The cation exchange media must be within 10 percent of the particle size, crosslinking and exchange capacity of the cation exchange media in the tested softener.

Pressure drop measurements – helping professionals size equipment correctly
All professionals want to make sure their work complies with regulations. When it comes to installing POE water treatment equipment, professionals must ensure that the equipment is sized sufficiently to allow the flowrates required by code. By describing detailed pressure drop measurement procedures, the NSF/ANSI DWTU Standards facilitate the generation of ratings that allow these professionals to select equipment with confidence.

About the author
Rick Andrew is the General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols), and Biosafety Cabinetry Programs. He has previously served as the Operations Manager, and prior to that, Technical Manager for the program. 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,


















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