ROB NUMBER - RELATIVE RATING SYSTEM FOR SAFETY SCREENS

Reprinted in part from 2 articles published in Machine Design, submitted by R. Kolp, Vice President, Research and Development, The Lee Company.

The Problem
The need to protect orifices, relief valves, and other sensitive hydraulic components from contamination is widely recognized, but contaminant size has a lot to do with the type of protection needed. Critical components are often relatively immune to low levels of small-size contaminants, but a single large particle can cause sudden failure - possibly with catastrophic effects. While filters maintain fluid cleanliness during operation, large particles, which are built into the system or introduced later, require a different approach. Here, safety screens provide an added level of protection.

Unfortunately, there are no standard methods to compare the performance of one safety screen to another. This is in contrast to the well-established design and selection methods available for rating filter performance, including beta ratings and bubble point testing. As a result, hydraulic system designers often must resort to costly testing programs or haphazard trial and error methods to select the proper screen.

The Solution
To bridge this technology gap, a "resistance to blockage" factor has been developed to explain screen design. This factor relates, without complex analysis or elaborate testing, the roles that the size and number of holes in a screen play in overall performance.

For example, deciding whether a screen with 2000 holes, 0.008" in diameter will last longer than one with 1000, 0.015" holes can cause a lot of headaches. To answer such questions, the Resistance to Blockage (ROB) factor was developed.

The blockage factor is calculated from a volume of fluid with known contamination required to block a screen. Because most safety screens have holes ≥ 0.004" (100 µ), contamination specifications such as NAS 1638 and ISO 4406 are of no use (the largest particles they deal with are 100 µ). Thus MIL-STD 1246, which deals with large particle contamination, was chosen as the baseline for comparison. Using MILSTD 1246 Class 200 fluid, the blockage factor is defined.

and d = hole size, µ, and N = number of holes in a screen.

A master screen with ROB = 1 is defined as having 1000 holes all 100 µ in size. Taking a ratio of any other screen to the master gives a comparison of the Resistance to Blockage. Because a ratio is involved, a lot of errors cancel. Factors such as the flow rate and mode of clogging may be unknown, but both screens are affected in the same relative manner. For example, if comparing two screens and the fluid is cleaner than Class 200, both will last longer than with Class 200 fluid, but they will still clog at the same ratio. About the only data needed are the size and number of holes.

In all systems, the more known about the fluid environment, the more accurate a design can be. However, lacking detailed knowledge of a system, the ROB factor is a lot better than guessing because it offers a ranking system independent of many system variables.

ROB numbers provide a reliable means of comparing one screen design to others. The table below gives a single orifice ROB number based simply on the diameter of the hole. When multiplied by the number of holes in a screen, the screen ROB number is obtained.

Screen ROB # = Single orifice ROB # x # of holes.

The screen ROB number is an indication of the relative resistance to blockage of the screen, independent of the type of contamination or the flow rate. A higher number indicates a more blockage resistant screen.

Using the preceding example, it can be seen how easily the ROB number table or graph allows comparison of various screens.

Example: Which screen will be more resistant to blockage? A screen with 2000, 0.008" diameter holes or a screen with 1000, 0.015" diameter holes?

Solution: From the table look up an 0.008" diameter hole. Its ROB number is 0.0169. Multiplying by 2000 yields the screen ROB number of 34.

Referring to the table again, for a 0.015" diameter hole the single orifice ROB number is found to be 0.291. Again multiplying by the number of holes (1000) gives the screen ROB number of 291. Therefore, in this case, the screen with fewer, larger diameter holes has a higher resistance to blockage.

Alternatively, the Safety Screen Numbers graph below may be used. Simply locate the intersection of the number of holes and hole size to determine the ROB number.

Example: Which screen will be more resistant to blockage? A screen with 2000, 0.008" diameter holes or a screen with 1000, 0.015" diameter holes?

Solution: From the graph look up an 0.008" diameter hole. Its ROB number is 0.0169. Moving right to 2000 holes yields the screen ROB number of 34.

Referring to the graph again, for a 0.015" diameter hole the single orifice ROB number is found to be 0.291. Again moving right by the number of holes (1000) gives the screen ROB number of 291.

Therefore, in this case, the screen with fewer, larger diameter holes has a higher resistance to blockage.

Safety Screen ROB Number

ROB Chart

Swipe to the right for more table information

TABLE OF SINGLE ORIFICE ROB NUMBERS
FOR SAFETY SCREEN ANALYSIS
Hole Size Single Orifice
ROB Number
(Inches) (Micron)
0.002510.0000979
0.003760.000376
0.0041020.00106
0.0051270.00248
0.0061520.00511
0.0071780.00963
0.0082030.0169
0.0092290.0281
0.0102540.0448
0.0112790.0687
0.0123050.1023
0.0133300.1482
0.0143560.210
0.0153810.291
0.0164060.397
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