When designing a process system, certain standard guidelines are used to determine pipe size and pipe fluid velocity. Typical pipeline velocities are in the 6 to 12 ft. per second range. The more viscous the fluid, the lower the velocity. Strainer sizes should be selected on the basis of allowable pressure drop, not pipeline size. Suction service usually requires lower pressure drops than discharge service.

Where strainers are used for high viscosity fluids, pressure drops will increase and strainers that are larger than normal may be required to keep pressure drops within reason. Oversized units may be specified depending on the amount of solids to be removed in parts per million (PPM), and the allowable pressure drop across the strainer. When designing strainers, the critical velocity is the screen velocity. This is the velocity of fluid through the filtering medium. Screen velocity is usually kept between 3 ft./sec. and 8 ft./sec. depending on the type of strainer specified. The higher the velocity, the higher the differential pressure drop.

A common term used in strainer design is “Open Area Ratio” (OAR). This equals the total open basket area divided by the internal cross-sectional area of the inlet pipe. It is a measure of relative debris-holding capacity and is usually between 2 to 4 times the inlet pipe area. By knowing the pipeline velocity, differential pressure drop and open area ratio, you can get a good overall picture of the appropriate strainers for the application being specified.

Open Area Ratio = Total basket open area /
Inlet pipe cross-sectional area (in. squared)

Inlet Velocity V ft./sec. = .4085 x GPM / ID squared

Screen Velocity Vs ft./sec. = V / OAR

To determine the anticipated maximum differential pressure (DP), it is common to request the DP at 75% blockage. At 75% blockage, there is a 75% reduction in flow area compared to clean, and a DP equal to approximately 4X that of the clean DP Value. This is based on a proportional analysis with open basket area being inversely proportional to DP. This logic does not hold true once the open screen area becomes less than that of the cross-sectional area of the pipe. At this point, orifice factors prevail and both the DP and flow restriction can increase dramatically.

Reliable basket strength is critical to strainer performance. The best basket strength data is obtained from companies such as TATE ANDALE who have been designing baskets for years. During the basket selection process, it is common to see specifications for the straining element’s burst pressure. While this serves as a benchmark for catastrophic failure, it does not truly satisfy the engineer’s intention of protecting the downstream equipment. Cylindrical baskets may deform, allowing bypass of contaminated fluid well before the calculated burst pressure is reached. A more useful number is the maximum allowable differential pressure. To determine this, a modified American Society of Mechanical Engineers (ASME) Sect. VIII Div. 1 cylindrical shell formula is used. The modified formula uses the concept of equivalent strength of materials outlined inASME Sect. II. Since the straining element will never see total line pressure (this would mean zero flow or total blockage), it is wise to design the basket strength for Maximum Allowable Differential Pressure (MADP) in the 10-25 PSI range. In viscous applications and at line pressures above 75 PSI, it is wise to specify heavy duty baskets with a MADP in the 50-75 PSI range. Wedgewire baskets are then the most suitable since they offer greater open area and resistance to collapse.

Strainer designs are chosen based upon the most severe pressure and temperature that will be experienced. These are usually expressed as design limits and they typically exceed the anticipated systems operating pressure and temperature. Strainers are specifically excluded from the requirements ofASME Section VIII Div. 1, under paragraph U1. Nevertheless, due to the similarity of pressure boundary parts, today’s quality manufacturers use the ASME design for guidelines. In fact,most of the strainers cast or fabricated can be “U” stamped pressure vessels.