Sugar Factory Tubes Sheet Ligaments

The ligament on a tube sheet is the material between two tube holes. In a perfect world the all the ligaments on a tube sheet will be exactly the sime size. However due to the variation in hole size due to tolerance and also due to mistakes in hole centre positioning there may be ligaments which are smaller than the theoretically calculated ligament. Now if it is found that on a particular tube sheet one or more of the ligaments is smaller than the others are we to reject the tube sheet and all the work done on it? If the tube sheet is rejected, the manufacturing programme will be delayed, and costs incurred.

The Tubular Exchanger Manufacturers Association (TEMA) has guidelines on minimum allowable ligaments on tube sheets. Unfortunately their recommendations only go up to 2 inch OD tubes, and so some extrapolation for 4 inch tubes is required.

tube dia [inch] tube pitch [inch]     maximum hole diameter [inch] nominal ligament [inch] minimum permissible ligament [inch]
dt p p/dt p-dt dh p-dh  
0.250 0.313 1.250 0.063 0.259 0.054 0.025
0.250 0.375 1.500 0.125 0.259 0.116 0.060
0.375 0.500 1.330 0.125 0.384 0.116 0.060
0.375 0.531 1.420 0.156 0.384 0.147 0.075
0.500 0.625 1.250 0.125 0.510 0.115 0.060
0.500 0.656 1.310 0.156 0.510 0.146 0.075
0.500 0.688 1.380 0.188 0.510 0.178 0.090
0.625 0.781 1.250 0.156 0.635 0.146 0.075
0.625 0.813 1.300 0.188 0.635 0.178 0.090
0.625 0.875 1.400 0.250 0.635 0.240 0.120
0.750 0.938 1.250 0.188 0.760 0.178 0.090
0.750 1.000 1.330 0.250 0.760 0.240 0.120
0.750 1.063 1.420 0.313 0.760 0.303 0.150
0.750 1.125 1.500 0.375 0.760 0.365 0.185
0.875 1.094 1.25 0.219 0.885 0.209 0.105
0.875 1.125 1.290 0.250 0.885 0.240 0.120
0.875 1.188 1.360 0.313 0.885 0.303 0.150
0.875 1.250 1.430 0.375 0.885 0.365 0.185
1.000 1.250 1.250 0.250 1.012 0.238 0.120
1.000 1.313 1.310 0.313 1.012 0.301 0.150
1.000 1.375 1.380 0.375 1.012 0.363 0.185
1.250 1.563 1.250 0.313 1.264 0.299 0.150
1.500 1.875 1.250 0.375 1.518 0.357 0.180
2.000 2.500 1.250 0.500 2.022 0.478 0.250

Standard Ligaments

96% of ligaments shall be greater than

ligstd = p - dh - (0.0032 t / dt in + 0.030 in)

where t = tube sheet thickness

Minimum Permisible Ligament

No ligamant shall be less than

ligmin = -0.0010465 + 0.510467 · (p-dh)

The factors in the above formula come from a linear regression of the data in the table above.

Example 1

In this example we consider a 2 in kestner evaporator type tube

tube plate thickness
t = 25 mm
tube outside diameter
dt = 2 in
dt = 50.8 mm
maximum tube hole dia
dmax = 51.48 mm
tube hole pitch
p = 70 mm
Drill drift tolerance
toldd = 0.0016 in · t / dt
toldd = 0.02 in
standard ligament
lstd = p - dmax - (2 · toldd + 0.030 in)
lstd = 17.72 mm
minimum ligament
lmin = 0.0010465 mm + 0.5067383 · (p - dmax)
lmin = 9.38 mm

Example 2

In this example we consider a 4 in continuous vacuum pan type tube

tube plate thickness
t = 25 mm
tube outside diameter
dt = 4 in
dt = 101.6 mm
maximum tube hole dia
dmax = 102.63 mm
tube hole pitch
p = 120 mm
Drill drift tolerance
toldd = 0.0016 in · t / dt
toldd = 0.01 in
standard ligament
lstd = p - dmax - (2 · toldd + 0.030 in)
lstd = 16.59 mm
minimum ligament
lmin = 0.0010465 mm + 0.5067383 · (p - dmax)
lmin = 8.78 mm