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ROB Number - A Rating System for Safety Screens
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ROB Number - A Rating System for Safety Screens

Definition

The ROB Number is a system of rating the relative resistance to blockage of safety screens. It was developed to minimize the guesswork and expensive testing previously employed in screen selection.

The ROB Number helps system designers choose the optimum screen for their conditions relative to other available screen options. It is not used to predict absolute screen life.

The ROB Number system is based on the following assumptions:

  • The same contamination level applies to all screen options.
  • Fluid contamination level is not impacted by the presence of a safety screen in the system.
  • Particles block different-sized holes, in essentially the same way.
  • Particle distribution follows a log-log2 distribution.
  • Best results are achieved when the screen options are of similar geometry and filtration media type.

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

\[\text{ROB}=\frac{\text{N}}{{63.25}\ \text{n}}\]

Where:

n = 10(4.9029 – 0.926 log2d)
d = Hole size (μ)
N = Number of holes in a screen

A master screen of ROB = 1 is, therefore, defined as having 1000 holes — all 100 μ in size.

Table A, shown below, lists 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:

\[\text{Screen}\ \text{ROB}\ \text{Number}=\text{Single}\ \text{Orifice}\ \text{ROB}\ \text{Number}\ \text{x}\ \text{Number}\ \text{of}\ \text{holes}\]

tABLE A
HOLE SIZE SINGLE ORIFICE
ROB Number
Inch Micron
0.0010 25 0.0000133
0.0015 30 0.0000408
0.0020 51 0.0000979
0.0025 64 0.000202
0.0030 76 0.000376
0.0035 89 0.000650
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 0.0079
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
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

The screen safety factor is defined as the ratio of the screen ROB Number to the ROB Number of the orifice it is protecting. It is recommended to choose a screen that provides the highest practical safety factor. The following is an example of selecting the proper screen:

EXAMPLE: Which of the following two safety screens is the better choice to protect a 0.030″ diameter orifice: screen 1, a 0.008″ hole sized screen with 850 holes, or screen 2, a 0.015″ hole sized screen with 450 holes?

SOLUTION: Table A indicates that a single orifice of 0.030” diameters has a ROB Number of 9.7. Table B shows that screen 1 has a safety factor of 1.5 (screen ROB Number 14.4/orifice ROB Number 9.7) while screen 2, with a safety factor of 13.5 (screen ROB Number 131/orifice ROB Number 9.7), is the better choice.

tABLE B
HOLE SIZE NUMBER 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