The Nimrod Review - Official Documents

4.21
4.22
4.23
Chapter 4 – Cross-Feed/SCP Duct
The original 1967 design specification for the Refrasil blanket thermal insulation for High Temperature (HT)
ducts provided for a layer of fibreglass wool insulant ½ inch thick, protected by a thin dimpled stainless steel
skin or sheath. 25 It was designed as insulation capable of providing a thermal barrier at internal temperatures
up to 550oC, allowing an external temperature of no more than 165°C. The 1967 specification clearly indicates
that the blanket insulation was intended to have a dual function: both (a) inhibiting heat radiating out; and (b)
preventing fluids seeping in, and therefore onto, HT ducts. 26 The specification stated, “[b]ecause of its stainless
steel skin it is useful in places where there may be a risk of spillage of fluids”. 27 The specification also provided:
“[w]hen the dimpled s/steel sheath is assembled and welded in place it should present a neat sealed casing
which will prevent the ingress of moisture or fluids”. 28
When brand new Refrasil should be as shown in Figure 4.6.
Example of an uninsulated
expansion joint
(not No .
7 Tank Dry Bay)
Figure 4.6: Newly Installed Section of Cross Feed Duct
Old Refrasil insulation was found on many Nimrods, however, to look as shown in Figure 4.7. The outer stainless
steel covering is compressed almost uniformly around the circumference, has a distinct ‘ribbed’ pattern,
and can no longer perform its function of maintaining the outer surface temperature at 165°C maximum
(see Figure 4.7).
25 “The design consists of a layer of fibre glass wool, acting as a light weight insulant wrapped around the duct, retained and protected by a skin of
dimpled stainless steel of very thin gauge which is welded to retaining rings on the ducts”. (Spec, Ibid, page 1).
26 Hawker Siddeley Aviation Ltd Spec DHA 567 dated 13 September 1967 (BOI Report, Exhibit 63).
27 Spec, Ibid, page 1.
28 Spec, Ibid, paragraph. 2.2.5)
New Refrasil Insulation
47