BACKFLOW PREVENTION 1

[cher122976]

I am pulling a 12" fire line off a 18" domestic line with a Double Check Valve (backflow valve) installation on the fire line. Do you need a seperate gate valve from the DCV for line shut down? What goes into design something like this?

Also, does anyone know a good learning resource for Water (or mech pressure lines in general) Systems that would teach pipe design, metering, fittings, analysis, etc etec etc? I have been a site civil engineer for almost 20 years at a high level but have always had a countpart to do mech pipe system design for me. I know the ins and outs of these systems but want to know everything about them in depth at this point. Any help would be appreciated.

 

[stanier]

Haestad published a book Advanced Water Distribution Modelling and Management. This was availble as a give away CD vrsion at water industry exhibitions. I would email but it is 35Mb. I suggest you try someone local or Bentley to see if you could get a copy.

http://www.pumpsystemsmatter.org

has some intersting information.

AWWA would be a good start.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[CPENG78]

Hi Cher122976,
Based on the provided information, it appears you would have a 18"x18"x12" tee installed or if this is an existing line you may want to do an 18"x12" tapping sleeve. Right after the tee or tapping sleeve I would install a 12" gate valve to isolate the fire service. Your DCV would be downstream of this probably along your project frontage. In order to provide more recommendations downstream of the DCV, I would need additional information such as the length of service from the DCV to the building or buildings. A 12" fire line is a substantial size for a fire line so have you conducted hydraulic calculations. The layout of equipment beyond the DVC would get affected by what you are looking to serve with this 12" line. These would include post indicator valves, fire department connections, check valves as needed and additional gate valves depending on your layout. I hope this helps.

 

[cher122976]

Thanks for the info stainier, it's crazy how hard it is to find good literature on water systems.

 

[cher122976]

Thanks CPE, 12" is the required size for our site and the client. Downstream of the DCV we will simply have piv's at each building connection with the fire guys taking care of adding fdc's on the building. Can you think of a situation we would need additional check valves outside of the DCV?

My main concern is the DCV, I am completely unfamiliar with them. It looks like the local jurisdiction has a detail of one we can use. Seems like they are just a standard assembly?

 

[CPENG78]

Not a problem Cher,
I would recommend that you do your homework and ensure that the 12" service is sufficient. This can be either via a hydraulic study prepared by a separate consultant or even just a quick rough model that you develop to determine you are delivering sufficient flow and pressure to the site.

In response to your first question, this is what I meant by needing more information about your site. For example, a 12 inch service is once again as I mention a large fire service, so I don't know if you are responsible for only the service up to the DCV or are you preparing an entire underground fire delivery system onsite. Without making too many assumptions on my side and based on what you mentioned, you would potentially need additional check valves if the length of your service from the DCV to the fire department connection is too long. What is too long? It would be the length determined by either your hydraulic calculations and/or your coordination with the fire department. You may find out that the FDC is 200 ft from the DCV and when the fire truck connects to the FDC, now the pumper truck is trying to push water both into the building with some loss pushing back towards the DCV. There is nothing magical about the 200 ft number that I mentioned, this is just a number.

With respect to the DCV, you are correct to start with the local jurisdiction. In fact in many of the installations that I have done, the local agency will own the water service from the main to the right of way, just before the DCV. While they do not own the DCV, they do dictate which model is allowed. A few items to note, check on their detail. Sometimes their detail is too generic so you will need to ask them more questions. For example some agencies have an already approved list of DCVs already on file. Another question to ask, is if the pressures in the main are high enough or additional requirements that may call for the need of a reduced pressure principle as part of the DCV? Some agencies make this a requirement for additional protection to their system. If an RP component is required, you will need to provide some sort of splash box directly below it since water will drip from this unit. Manufactures such as watts, wilkins, febco and others have information on the different models, as well as CAD drawings that you can use for both vertical and horizontal placement. A 12" DVC device will have a good size footprint so many times the the agency standard is not enough to place these onsite.