Strainers are used to remove any sediment that may be entrained in the fluid stream of the water, brine, or steam system. Strainers are commonly installed at the main system pumps in water or brine systems and also upstream of sensitive components, such as control valves and pressure regulating valves. Strainers used in commercial HVAC systems typically have a Y-pattern body23 and are constructed of bronze with soldered or threaded ends for pipe sizes 2 in. and smaller. For pipe sizes 2½ in. and larger, strainers are constructed of cast iron with flanged ends. Sediment is trapped within the strainer by a 20-mesh stainless steel screen.24 The water pressure drop through a clean strainer depends upon the pipe size and the fluid flow rate and is generally in the range of 1 psi. A water pressure drop of about 2 to 4 psi should be included in the pump head calculation for the main system strainer to account for some loading of the strainer.
Strainers must be flushed periodically to blow-down the trapped sediment. Small strainers (1 in. and smaller) are typically furnished with a plugged blow-down outlet. The blow-down outlet of larger strainers (1¼ in. and larger) is commonly specified with a ½- or ¾-in. blow-down pipe that is equipped with hose-end drain valve.25 Thus, for small strainers, it is necessary to close a shutoff valve upstream of the strainer before removing the blow-down plug. Then, the shutoff valve can be slowly throttled open to flush the sediment from the strainer. For larger strainers, the blow-down valve enables the strainer to be flushed without shutting off the equipment.