This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The action of pressure reducing valves has been taken up quite fully in "Boiler Accessories," and need not be repeated here. When the reduction in pressure is large, as in the case of a combined power and heating plant, the valve may be one or two sizes smaller than the low pressure main into which it discharges. For example - a 5-inch valve will supply an 8-inch main, a 4-inch a 6-inch main, a 3-inch a 5-inch main, a 2 1/2-inch a 4-inch main, etc.
For the smaller sizes the difference should not be more than one size. All reducing valves should be provided with a valved by-pass for cutting out the valve in case of repairs. The connection is usually made as shown in plan by Fig. 53.