The importance of piping configuration in ensuring optimal UV system performance

by Brian Grochowski

In 2006, the United States Environmental Protection Agency (EPA) issued the Ultraviolet Disinfection Guidance Manual (UVDGM) which provides technical information on the application of ultraviolet (UV) light for the disinfection of drinking water by public water systems.

While the UVDGM has its roots in municipal water treatment, it has found its way into the industrial marketplace over the last few years. For example, parts of the UVDGM can be found in process water treatment specifications at the largest beverage manufacturers and within guidelines of the Pasteurized Milk Ordinance (PMO). It is now being used in the Model Aquatic Health Code (MAHC), a publication produced by the Centre of Disease Control, that looks at best practices for making aquatic facilities healthier and safer.

As the use of the UVDGM and other 3^rd party validations spread into the industrial marketplace, questions will arise from those less familiar with UV technology as to whether the UV system they are implementing matches the guidance being provided in terms of sizing, installation and maintenance procedures.

One question that is increasingly asked regarding conformance to 3^rd party validations, is `how should upstream and downstream piping be installed?’ and `how will that piping affect the performance of the UV system?’

The UVDGM has specific language on how upstream and downstream piping should be installed:

3.6.2 Validation and Installation Hydraulics Recommendations
The inlet and outlet piping to the UV reactor in the UV facility should result in a UV dose delivery that is equal to or greater than the UV dose delivered when the UV reactor was validated. If off-site validation is used, the three preferred options for meeting this condition are presented below.

1.    Minimum five pipe diameters of straight pipe upstream of UV reactor:

The length of straight pipe upstream of each UV reactor at the UV facility is the length of straight pipe used in the validation testing plus a minimum of five (5) pipe diameters. During validation testing, the inlet piping to the reactor consists of either a single 90-degree bend, a “T” bend, or an “S” bend, followed by a length of straight pipe if necessary. See Figure 3.7 for validation and installation configuration options.

2.    Identical inlet and outlet conditions:

Inlet and outlet conditions used during validation match those used at the WTP for at least ten (10) pipe diameters upstream and five (5) pipe diameters downstream of the UV reactor.

3.    Velocity profile measurement:

Velocity of the water measured at evenly spaced points through a given cross-section of the flow upstream and downstream of the reactor is within 20 percent of the theoretical velocity with both the validation test stand and the WTP installation (NWRI 2003). The theoretical velocity is defined as
the flow rate divided by the cross-sectional area.

The most suitable option for a given application depends on specific site layouts, piping constraints and how the validation was conducted.

Option 1. tends to be the most applicable and widely used solution. Using this method, if during the validation, 24” of 4” diameter straight pipe was used upstream of the UV system, the UV system would then need to be installed with 44” of straight pipe upstream (i.e. 24” + (5 pipe diameters * 4” diameter pipe) = 44”).

For Option 2. to be considered, a pipe layout for the specific UV system will be required. This can be provided by the UV system supplier, but they will usually request that a Non-Disclosure Agreement (NDA) is signed.

Example of pipe layout from a validation report

(output), the configuration used during the validation. Computational Fluid Dynamics (CFD) modeling of inlet and outlet conditions can be used to determine if the UV dose delivery at the installation is greater than the UV dose delivery achieved during validation. CFD modeling will take into account the flow rate, ultraviolet transmittance (UVT) and lamp output (intensity) for a given point of water flow in a chamber.

Pipe layout, both before and after the UV system, is critical to achieving the desired performance. Failure to take this into consideration can lead to as much as a 25% drop in performance (even though the UV system’s display may state that it is delivering the required dose and amount of mJ/cm2 inside of the chamber).

It’s important to note that pipe layout is just one of many considerations that must be made when installing the appropriate UV system. As the industrial market continues to adopt the UVDGM and other validation guidelines, it is critical to work with an experienced UV system provider that fully understands the guidelines and the intricacies of 3^rd party validations. This will ensure the UV system performs correctly, protecting consumers and brand reputation.