1. Decide whether a globe or angle valve will best fit your installation. Keep in mind that it is always best to install any control valve "bonnet up," particularly in sizes 8" and larger.

2. Begin with a line-sized valve.

Calculate the pressure drop from the formula.

3. The pressure drop calculated is that for a wide-open valve and would be true for an exhaust-to-atmosphere valve (115-1 or 115-4) regardless of flow rate. On the other hand, a valve exhausting to downstream (e.g., 115-2) may not be wide open. Refer to the "wide open at" column of the table. If the flow rate is less than this figure, the pressure drop of the valve can be 2-3 psi higher than the value calculated in Step 2. If the flow rate is higher than the figure given, the valves will be wide open and will have a pressure drop equal to the echaust-to-atmosphere valve.

4. Check to see that the flow velocity does not exceed 20 ft/sec. If it does, or if the pressure drop is excessive, consider using the next size larger valve.

5. Finally, if an exhaust-to-atmosphere valve is selected, make note of the diaphragm chamber discharge. This quantity of water will be discharged to atmosphere each time the valve opens or closes. Provision should be made to drain or otherwise dispose of this water.