ROB Number

Definition

The ROB Number, a system of rating the relative resistance to blockage of safety screens, has been developed to minimize the guesswork and expensive testing formerly involved in selecting a screen.

The ROB Number was conceived to aid system designers in choosing the optimum screen for their conditions relative to other screen options available. The ROB Number is not used to predict absolute screen life.

The ROB Number system is based on a few assumptions.

  • The same contamination level applies for all screen options.
  • Fluid contamination level is not affected by having a safety screen in the system.
  • When particles block holes, the manner in which they block different size holes is essentially the same.
  • Particle distribution follows a log - log2 distribution.

A fluid contamination level per MIL-STD-1246 Class 200 was chosen as the basis of comparison with the ROB Number defined as;

Swipe to the right for more table information

where n =
  d = hole size (µ)
  N = # of holes in screen
       
  A master screen of ROB = 1 is therefore defined as having 1000 holes all 100 µ in size.

The following table gives the ROB Number for single holes of a given diameter. To determine the ROB Number for a screen, multiply the single hole value by the number of holes in the screen.

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

The screen safety factor is defined as the ratio of the screen ROB Number to the ROB Number of the orifice it is protecting. You should choose a screen to provide the highest practical safety factor.

EXAMPLE: Which safety screen would be the better choice to protect a 0.030 diameter orifice: A 0.008 hole size screen with 850 holes, or a 0.015 hole size screen with 450 holes?

SOLUTION: From the table below, we find that a single orifice of 0.030 diameter has a ROB Number of 9.7.

Again using the table for 0.008 and 0.015 holes and multiplying by the respective number of holes gives the following results:

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  HOLE SIZE # OF
HOLES
ROB
NUMBER
SAFETY
FACTOR
  (in) (µ)
Orifice 0.030 762 1 9.7 1
Screen 1 0.008 203 850 14.4 1.5
Screen 2 0.015 381 450 131 13.5

The results show screen 2 to be the better choice.

Single Orifice ROB Numbers

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Hole Size SINGLE ORIFICE
ROB #
Inch Micron
0.0010 25 0.0000133
0.0015 38 0.0000408
0.0020 51 0.0000979
0.0025 64 0.000202
0.0030 76 0.000376
0.0035 89 0.00065
0.0040 102 0.00106
0.0045 114 0.00165
0.0050 127 0.00248
0.0055 140 0.00361
0.0060 152 0.00511
0.0065 165 0.00709
0.0070 178 0.00963
0.0075 191 0.0129
0.0080 203 0.0169
0.0085 216 0.0220
0.0090 229 0.0281
0.0095 241 0.0357
0.010 254 0.0448
0.011 279 0.0687
0.012 305 0.1023
0.013 330 0.1482
0.014 356 0.210
0.015 381 0.291
0.016 406 0.397
0.017 432 0.533
0.018 457 0.706
0.019 483 0.922
0.020 508 1.19
0.021 533 1.52

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Hole Size SINGLE ORIFICE
ROB #
Inch Micron
0.022 559 1.93
0.023 584 2.42
0.024 610 3.01
0.025 635 3.72
0.026 660 4.56
0.027 686 5.56
0.028 711 6.73
0.029 737 8.10
0.030 762 9.71
0.031 787 11.6
0.032 813 13.7
0.033 838 16.2
0.034 864 19.1
0.035 889 22.3
0.036 914 26.0
0.037 940 30.3
0.038 965 35.1
0.039 991 40.5
0.040 1016 46.6
0.041 1041 53.5
0.042 1067 61.2
0.043 1092 69.9
0.044 1118 79.5
0.045 1143 90.3
0.046 1168 102
0.047 1194 116
0.048 1219 130
0.049 1245 147
0.050 1270 165
0.051 1295 185

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Hole Size SINGLE ORIFICE
ROB #
Inch Micron
0.052 1321 207
0.053 1346 231
0.054 1372 257
0.055 1397 286
0.056 1422 318
0.057 1448 352
0.058 1473 390
0.059 1499 431
0.060 1524 476
0.061 1549 525
0.062 1575 578
0.063 1600 635
0.064 1626 698
0.065 1651 765
0.066 1676 838
0.067 1702 917
0.068 1727 1002
0.069 1753 1094
0.070 1780 1.2E + 03
0.071 1800 1.3E + 03
0.072 1830 1.4E + 03
0.073 1850 1.5E + 03
0.074 1880 1.7E + 03
0.075 1910 1.8E + 03
0.076 1930 2.0E + 03
0.077 1960 2.1E + 03
0.078 1980 2.3E + 03
0.079 2010 2.5E + 03
0.080 2030 2.7E + 03
0.081 2060 2.9E + 03

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Hole Size SINGLE ORIFICE
ROB #
Inch Micron
0.082 2080 3.1E + 03
0.083 2110 3.4E + 03
0.084 2130 3.6E + 03
0.085 2160 3.9E + 03
0.086 2180 4.2E + 03
0.087 2210 4.5E + 03
0.088 2240 4.8E + 03
0.089 2260 5.2E + 03
0.090 2290 5.6E + 03
0.091 2310 5.9E + 03
0.092 2340 6.4E + 03
0.093 2360 6.8E + 03
0.094 2390 7.3E + 03
0.095 2410 7.8E + 03
0.096 2440 8.3E + 03
0.097 2460 8.9E + 03
0.098 2490 9.5E + 03
0.099 2510 1.0E + 03
0.10 2540 1.1E + 04
0.11 2790 2.0E + 04
0.12 3050 3.4E + 04
0.13 3300 5.8E + 04
0.14 3560 9.4E + 04
0.15 3810 1.5E + 05
0.16 4060 2.3E + 05
0.17 4320 3.4E + 05
0.18 4570 5.0E + 05
0.19 4830 7.3E + 05
0.20 5080 1.0E + 06

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