Basket Strainer
Definition
A basket type strainer is installed horizontally in flowing fluid systems and is designed to removed suspended objects from the flowing medium (usually a fluid). Theory Fluid flows into the strainer casing through the suction port, it then travels through a perforated cylinder and is discharged through the discharge port. The perforated cylinder is called a 'basket' due to its basket like appearance. Suspended object in the fluid that are too large to fit through the perforations (holes/gaps) are retained in the basket and often accumulate in the basket's base. The size of the suspended particles being removed from the fluid depends upon the size of the strainer perforations. The removal of objects from a system allows the system to remain in a constant condition e.g. no detrimental effects to downstream apparatus. Failure to remove objects from a system can lead to common problems such as pump damage, or, a reduction in the heat transfer rate of heat exchangers etc. (there are many other examples!). Construction Construction materials vary depending upon the system served. Systems with corrosive fluids e.g. seawater, will typically have bronze strainer casing's and stainless steel basket's. Systems with reactive or extremely corrosive fluids may use polymer based materials e.g. PVC. Strainer construction materials should be chosen carefully as the wrong choice will often cause the strainer to fail (either suddenly or progressively over time). Basket Strainer Applications Applications are numerous due to the strainers simple design and easy maintainability. The strainer can also normally be repaired locally without requiring expensive equipment or advanced technical 'know-how'. Spares (partial or entire) are normally readily available and cheap. Typical applications would include systems with a high flow rate where fine filtering is not necessary e.g. a cooling water system. Basket strainers are primarily used for the filtering of large coarse objects e.g. rock, seaweed, plastic-bags etc. The shape of the basket allows these large coarse objects to 'fall' into the base of the basket, once in the base of the basket they no longer impair the flow through the strainer. Basket strainers have a tendency to easily block when the strainer is fine. There are three main types of basket strainer, these are coarse, medium and fine. Although very robust in design, suspended objects such as plastic bags or seaweed etc. can cause significant reductions in flow through the strainer as they block many strainer holes and do not sink to the base of the strainer due to the constant flow. Some modern installations have a compressed air fitting to allow blowing of the strainer in a reverse direction, although the approach sometimes works and does loosen entrained objects, it also introduces air into the system and this rarely desired. What is the difference between a strainer and a filter? The two terms are often used intermittently although technically a filter refers to a disposable object which when cleaned is not restored to its original state. A strainer is disposable but can be cleaned in order to restore it to its original condition (and is thus not disposed of!). |