What is the purpose of the check valve in Figure A.8.17.5.2.2(b)?

8.17.5.2.2*  The following restrictions shall apply:     
(1)     Each connection from a standpipe that is part of a combined system to a sprinkler system shall have an individual control valve and check valve of the same size as the connection.
(2)     The minimum size of the riser shall be 4 in. (102 mm) unless hydraulic calculations indicate that a smaller size riser will satisfy sprinkler and hose stream allowances.
(3)     Each combined sprinkler and standpipe riser shall be equipped with a riser control valve to permit isolating a riser without interrupting the supply to other risers from the same source of supply. (For fire department connections serving standpipe and sprinkler systems, refer to Section 6.8.)

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