Loss of Head — Simplified

1 In our line of work, we field questions from contractors and tech-nicians concerning repairs, installations, and general backflowprevention practices. We d like to share some questions we receiveand our answers. Everyone has different opinions on these subjectsand we would like to hear yours. Contact us with your questionsand ideas via email at: or mail us atAmerican Backflow Products Co., Post Office Box 37025,Tallahassee, FL 32315. Mark Inman and Jason GreggQUESTION I will be installing a 6-inch reduced pressure principleassembly on an existing system. Can you tell me what the pressure loss or CV factor is for that particular assembly?Mark -This is probably the most common question that isasked by plumbing contractors, fire-line contractors, andeven engineers for new or retrofit backflow preventionassembly installations.

2 Obviously, anytime a backflow pre-vention assembly is specified for an installation, it is veryimportant to know what the pressure loss is for thatassembly. The problem we find many times is that theanswer can be easily misunderstood. For example, whenwe call our supplier and ask this question, normally theywill look it up and find a flow curve not a specific can be confusing, so many times they may guess, orgive an estimate of the overall pressure loss . It is impor-tant that we understand how it is represented on a flowcurve, so that pressure loss can be calculated correctly. - JasonBasically, pressure loss is present in any plumbing sys-tem. It normally starts with the friction of the wateragainst the interior walls of fixed orifice items such aspipe, valves, and fittings.

3 They are called fixed orificeitems simply because the pathway that the water flowsdoes not change, so it is easy to calculate the pressure lossfor that item. The pressure loss for a fixed orifice item isidentified as the CV factor. A backflow prevention assem-bly is not a fixed orifice item so it cannot have a CV fac-tor. The check valves within the assembly are force loadedto the closed position. This means that the orifice orpathway that the water flows through is constantly chang-ing (As the flow of water increases or decreases, the checkvalves are opening and closing). This opening and closingof the check valves changes the amount of friction withinthe assembly. Mark -With that in mind, the best way to show the flow char-acteristics of a backflow preventer is by a pressure lossverses flow rate curve.

4 This type of flow curve will showthe pressure loss of the assembly throughout its full rangeof operation not just at a single point. So, to determinethe pressure loss , we need to know at what flow rate theassembly will be operating. Then it is just a matter ofreading the curve. We know that as the flow rate varies,the pressure loss through the assembly will vary as example, you could have a 6-inch reduced pressureprinciple assembly that has a pressure loss of 12 PSIat 140 GPMand then a pressure loss of PSIat 840 GPM. page 26 october 2003 dw&bpLoss of Head Simplified - JasonIt is important to remember that the values weget from a flow curve are for the assembly itselfand does not include any of the piping and fittingsrequired to install the assembly. There have alsobeen some questions concerning pressure losstables that are published by approval tables are used to show the maximumallowable pressure loss requirement for a specificsize and type of backflow prevention , these tables show the standard by whichassemblies are evaluated by that agency or lab.

5 Tofind the pressure loss for a particular model ofbackflow preventer, you will normally have to referto the manufacturer s specification october 2003 page 27