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MIL-DTL-24688 Insulation, Thermal And Acoustic Absorptive, Cellular Polyimide FoamThis specification covers thermal and acoustic absorptive cellular polyimide foam insulation for use on bulkheads, overheads, and ventilation ducting. This specification also covers preformed thermal pipe insulation for use on piping operating at temperatures up to 600 degrees F in shipboard applications \(see 6.1\).

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MIL-DTL-24688A
3.13.2 Type III, classes 3 and 4. Compressibility of Type III, Class 3 (unlagged) preformed pipe shall be a
minimum value of 26 lb/in² at 20 percent deflection when tested in accordance with 4.4.11.1. Compressibility of
Type III, Class 4 (prelagged) preformed pipe shall be performed on the unfaced foam backing only with a minimum
value of 26 lb/in² at 20 percent deflection when tested in accordance with 4.4.11.1.
3.13.3 Type III, classes 5 and 6. Compressibility of Type III, Class 5 (unlagged) preformed pipe shall be a
minimum value of 28 lb/in² at 10 percent deflection when tested in accordance with 4.4.11.2. Compressibility of
Type III, Class 6 (prelagged) preformed pipe shall be performed on the unfaced foam backing only with a minimum
value of 28 lb/in² at 10 percent deflection when tested in accordance with 4.4.11.2.
3.14 Sound absorption (type I and type II, class 2). When tested as specified in 4.4.12, the acoustic absorptive
board shall have coefficients of absorption that are equal to or greater than those shown in table IV.
TABLE IV. Minimum sound absorption coefficients.
Board
Frequency, Hz
Type,
thickness
class
125
250
500
1000
2000
4000
(inches)
I
½
0.04
0.05
0.15
0.40
0.55
0.55
I
1
0.06
0.20
0.45
0.65
0.65
0.65
I
2
0.15
0.40
0.75
0.75
0.75
0.70
II, 2
½
0.05
0.15
0.35
0.50
0.60
0.60
II, 2
1
0.07
0.25
0.70
0.80
0.75
0.70
II, 2
2
0.25
0.70
0.90
0.85
0.75
0.75
3.15 Thermal conductivity.
3.15.1 Type I and type II, class 1 only. The thermal conductivity of Type I and Type II, Class 1 material shall
not exceed 0.32 British thermal units (Btu) inches per hour per square foot at an average mean temperature of 75 °F
when tested in accordance with 4.4.13.1.
3.15.2 Type III. The base pipe insulation (unlagged) shall not exceed the maximum thermal conductivity
specified in table V when tested in accordance with 4.4.13.2.
TABLE V. Thermal conductivity (unlagged) (maximum).
Type III
Type III
Type III
Type III
Type III
Type III
Mean temp.
Class 1
Class 2
Class 3
Class 4
Class 5
Class 6
(°F)
(Btu)
(Btu)
(Btu)
(Btu)
(Btu)
(Btu)
25
0.27
0.27
50
0.30
0.30
75
0.32
0.32
0.24
0.24
0.24
0.24
100
0.35
0.35
0.25
0.25
150
0.27
0.27
200
0.48
0.48
0.30
0.30
NOTE:
1.
British thermal unit (Btu) measured in inches per hour per square foot at the average mean temperature
shown.
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