The selection of the proper industrial strainer requires knowledge of the system and the types of contaminants that must be retained. Industrial strainers are macro filters that range in particle retention from as large as .500 inch down to 325 mesh (44 micron). Typically we see a range from .250 inches to 200 mesh (74 microns).
Industrial strainers should retain all particles greater than those acceptable to the downstream equipment. Straining too fine may cause operational and maintenance problems due to premature fouling of the straining medium. This can unnecessarily increase the frequency of cleaning and cause flow obstruction to downstream equipment.
The degree of straining is usually decided by the process design engineer. These decisions are usually based on the engineer’s process flow expertise and the recommendations of the manufacturer of the equipment to be protected. The industrial strainer itself is a simple device to operate and, with minimal maintenance, will last for many years.
Industrial Strainer Design Criteria
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 feet-per-second range. The more viscous the fluid, the lower the velocity.
Industrial 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 industrial 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. Over-sized 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 industrial 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 feet/second and 8 feet/second 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 and 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.
Industrial Strainer Basket Strength
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, manufacturing and servicing baskets for decades.
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) Section VIII, Division I cylindrical shell formula is used. The modified formula uses the concept of equivalent strength of materials outlined in ASME Section 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 an 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 of ASME Section VIII, Division I, under paragraph UI. 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.
Industrial Strainer Selection Considerations
Pressure Drop and Velocity – Resistance to flow through a clean strainer is the sum of the resistance due to the strainer medium, strainer hardware and the strainer housing. For a fluid of a given viscosity, the smaller the diameter of the pores/slots within the straining medium, the greater the resistance to flow, that is pressure drop.
Industrial 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 handling water, the recommended pressure drop is usually in the region of 0.5 PSIG.
A pressure drop as high as 2 PSIG may be acceptable for some applications.
For water service, a velocity of nominally 7 feet per second generally produces a reasonable pressure drop for simples 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 ASME flanges. This flange rating is not indicative of maximum working pressure since the ASME standards permit higher working pressures. An 8” 150 LB ASME flange can operate at a pressure of 275 PSIG at -20 to 100 F. Design pressures of strainers do not conform to ASME 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 industrial 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 measured in centipoise.. 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.
Dirt Loading – The percent by weight of particulate contaminant in the liquid stream to be filtered, or the particulate matter, in slurry form, from which the moisture is to be removed.
Flow Rate – The volume of liquid, measured in gallons per minute (GPM), to be filtered.
Particle Size – The mean diameter of the smallest particles to be removed measured in microns or standard US mesh sizes. This should also include the type of particulate (hard, gelatinous) encountered.
Service Temperature – This should include both the service and design temperatures. It is also important to determine the viscosity of a fluid at its operating temperature. (Liquid viscosity generally decreases as the temperature increases.) If the fluid is extremely viscous, it is advisable to preheat the fluid and install a heating jacket on the strainer housing.
Life Cycle Cost – The capital and operating costs spread over the expected life of the unit. One may lean towards buying a strainer because of its low initial cost. However, other features such as frequency of basket cleaning and replacement, and associated labor, disposal and production downtime costs will need to be considered and should impact the final selection.
Limited Downtime – Certain industrial processes by their nature require significantly more downtime in their operation, while others have been designed to minimize downtime. Whether your operation is batch or continuous is another selection factor. Simplex, or single basket, industrial strainers are suitable for batch operations. Continuous industrial processes require the ability to clean the strainer while it is on-line, which requires either a duplex (twin basket) or self-cleaning (automatic) industrial strainer.
Material Selection – Materials of construction vary according to application. Typically it should approximate the material specification of the piping system; however, it is essential to determine if the fluid is acid, alkali, aqueous, oil or solvent based, and if it has additives that affect compatibility. The chemical composition and the thermal range of the liquids you wish to strain will determine what metals and media are suitable for use. All materials that come in contact with the process fluid at operating temperature (baskets, screens, hardware, housing material, gaskets) must be compatible with the fluid.
The most cost-effective materials are carbon steel and gray iron. Engineered coatings are available to protect metals from corrosive process fluids and gases. Tate Andale has decades of experience working with various grades of the following materials and coating systems:
- Stainless steels
- Hastelloy B or C
- SMO 254
- Alloy 20
- Fusion Bonded Epoxy Coatings
- Industrial Enamel Coatings
- Zinc Based Coatings
- Belzona Coatings
Basket Selection – Industrial strainer baskets, also known as the strainer or filter element, are critical components in industrial strainers. We frequently work with the following time-tested component materials:
- Perforated Plate – Perforated plate is the most commonly used media for basket type strainers and is available in perforation sizes 1/32″ to 1/2″ diameter depending on the degree of straining required and size and type of basket. Perforations are normally made on a staggered pattern for maximum open area and strength.
At Tate Andale, we believe in helping our customers find smart long-term engineered solutions for complex industrial problems. Contact us, see our industrial strainer models, or review our custom items for more information