NFPA 25 13.6.2 on backflow preventers

[ContractorDave]

Hello

I sent this off to NFPA25@nfpa.org:

Hello

I’m trying to understand 13.6.2

13.6.2.1 speaks to the forward flow testing of backflow preventers. A13.6.2.1 starts off by discussing this, then ends with “The tests required by 13.6.2 typically test only for operation of the device under backflow
conditions. Forward-flow test conditions are required by other portions of this standard. “ … which seems rather contradictory. 13.6.2.1.1 says ” For backflow preventers sized 2 in. (50 mm) and under, the forward flow test shall be acceptable to conduct without measuring flow, where the test outlet is of a size to flow the system demand. “ But how do you know for sure if the flow is meeting system demand just because the test outlet is sized for it?
Both 13.6.2.1.3 and 13.6.2.2 say the exact same thing and doesn’t help clarify anything here.

Anyone care to take a shot at clarifying this for me?

Thx
Dave


Regards
D

A thing moderately good is not so good as it ought to be
Thomas Paine

http://www.ykfireprevention.ca

 

[cdafd]

what edition of 25 are you in?

from 2011::

A.13.6.2.1

A.13.6.2.1 The full flow test of the backflow prevention valve can be performed with a test header or other connections downstream of the valve. A bypass around the check valve in the fire department connection line with a control valve in the normally closed position can be an acceptable arrangement. When flow to a visible drain cannot be accomplished, closed loop flow can be acceptable if a flowmeter or sight glass is incorporated into the system to ensure flow.
The tests required by 13.6.2 typically test only for operation of the device under backflow conditions. Forward-flow test conditions are required by other portions of this standard.


If this is your question::::"""But how do you know for sure if the flow is meeting system demand just because the test outlet is sized for it?"""

you can calculate the flow through the size of the opening? You know the demand of the system, you know the static, and you can always pitot the flow when you are testing it?