Pressure Drop and Velocity
Strainers are selected on the basis of allowable pressure drop and not pipe size. Where the strainer or filter is to be installed on the suction side of a pump handlingwater,the recommended pressure drop is usually in the region of 0.5 PSIG. A pressure drop as high as 2 PSIGmay be acceptable for some applications. For water service, a velocity of nominally 7 feet per second generally produces a reasonable pressure drop for simplex or duplex strainers. For oil service, lower velocities may be required due to the higher viscosity of the oil. Normally, pressure drops for oil service are desired within the one to two PSIG range. Velocity may range from 3 ft./sec. downward to less than 1 ft./sec. depending upon viscosity.

Maximum Allowable Working Pressure
The flange rating should not be relied upon as an indication of maximum or design working pressure. Frequently, specifications do not indicate a working pressure but only a flange connection rating such as a strainer having 8" 150 LB ANSI flanges. This flange rating is not indicative of maximum working pressure since the ASA standards permit higher working pressures. An 8" 150 LBANSI flange can operate at a pressure of 275 PSIG at -20 to 100 F. Design pressures of strainers do not conform toASA flange ratings of pressure or temperature. Typically a strainer with 8" 150 LB flanges will be designed for 150 PSIG MAWP. Only by specifying the exact operating pressure and temperature can the correct strainer be selected.

Perforation, Slot or Mesh Size
Basket openings should be selected on the basis of the equipment to be protected. Applications should not be filtered finer than required since frequent and unnecessary cleaning will result. Wedgewire slotted baskets offer better open area and greater resistance to collapse than equivalent perforated plate or mesh lined baskets. They are also easier to clean and have better backwashing capabilities.

Open Area Ratio
Open area ratio is the yardstick for determining the length of time a strainer will operate without cleaning or suffer undue pressure loss. This ratio is the relationship between the internal cross sectional area of the inlet pipe and the total open area of the openings in the basket. A 1:1 ratio would give an unrestricted flow while clean, but as clogging occurs, flow would be inhibited. A 2:1 ratio would still provide full flow, even after the screen was 50% clogged. A 4:1 ratio is normally recommended. It should be noted that automatic self-cleaning strainers will operate quite well with smaller ratios as the automatic cleaning will keep 100% flow area open at all times. Again wedgewire baskets are preferred, since they offer greater open area.

Viscosity
Viscosity is the measure of resistance to flow. Oils, tar, etc. do not readily flow and are called viscous fluids. Viscosity varies inversely with temperature, so it is therefore necessary to know the viscosity and the temperature of the flowing fluid, for a proper strainer selection.