The safety screen hole size should be based on the minimum passage size, such as a valve opening or clearance, or orifice diameter. Typically, the safety screen hole size is chosen to equal 30 to 70 percent of the minimum passage size. For example, a 0.012" diameter orifice might be protected with a safety screen having 0.004" or 0.008" holes, used downstream of a system filter with a 15 µm rating. It is good design practice to make certain that the smallest passageways of critical components are always maximized. The Lee Company offers a full range of restrictors and valves with optimized clearances that are presented in the Lee Safety Screens Product Selector. Lee multi-orifice restrictors, such as the Lee Visco Jet®, provide a high level of restriction to fluid flow using a tortuous flow path and large minimum passage sizes. Lee Visco Jet® minimum passage sizes range from 0.005" to 0.062" as compared to 0.0015" to 0.026", respectively, for the equivalent single orifice restriction, greatly reducing contamination sensitivity.
The hole size of the safety screen should also be chosen to be larger than any upstream filter, to prevent clogging by fine particles normally removed by these filters. There may often be three or more levels of filtration or "tiers" that fluid must flow through before reaching a critical component. For example, in a typical hydraulic system, the system fluid will first flow through a main system filter (first tier) then branch off to a control valve which has an inlet safety screen (second tier). The holes sizes of each subsequent tier should be coarser than the previous one to prevent the accumulation of contamination at any one level.
STANDARD LEE HOLE SIZES
Etched Safety Screens are available from The Lee Company in four standard sizes: 0.004", 0.006", 0.008", and 0.015" nominal. HI-BAR screens are available as standard items with nominal hole sizes of 0.002", 0.003", 0.004", 0.006", 0.008", 0.015", and 0.020". Hole sizes as small as 0.0015" and as large as 0.050" are also possible for special applications.
The term "open area" is a useful measure of contamination holding capacity. When comparing screens of similar hole sizes, open area gives a quick indication of the dirt holding capacity of one design versus another. It is simply the product of the number of holes times the area of a nominal hole. The ratio of open area to total area is called "percent open area". A high percent open area indicates a potentially weaker screen design that is less able to withstand high differential pressures.
When a Safety Screen is used to protect an orifice, the open area of the screen is compared to the open area of the orifice. By taking a simple ratio of the screen open area to the orifice area one can gauge the potential pressure drop across the screen relative to the orifice.
Open area should only be used when comparing screens of similar hole size and shape. To compare screens with different hole sizes, ROB numbers should be used. For example: If two screens have the same open area, one screen with 0.004" holes and the other with 0.008" holes, the screen with the larger holes will have 3.9 times the resistance to blockage as the smaller hole screen!
All Lee Safety Screens have a maximum flow rate specified except HI-BAR screens. These Safety Screens are not limited by flow and are specifically designed to withstand full system pressure fully clogged up to 7500 psid. This means HI-BAR Safety Screens can be designed into a 5000 psid system and withstand the proof pressure of 7500 psid without collapse or burst while fully clogged in either direction.
The flow capacity of a Safety Screen during cold start must also be considered. A cold start refers to the unusually high pressure drop that occurs through all components including safety screens when a system is first started up and the fluid is cold and viscous. The system designer should realize that due to the effects of viscosity, a Safety Screen during cold start may have a pressure drop many times greater than operation at normal temperatures. Therefore, a maximum allowable pressure drop across the Safety Screen at the rated flow for this condition must be specified.
Typical performance of a 20 Lohm Safety Screen.