NCR XXXXXX Engineering Assessment

Date: 02 November 2012Subject: Leaking Tank for Lot XXXXXX (NCR XXXXXX)Author: KH

SUMMARY: No root cause was identified for the loss of sealing integrity in the tank. Several adverse factors were identified that could compromise sealing. They include assembly procedures and properties of materials.

INSPECTION: The leaking 750 liter tank (418-1125) was inspected on October 26th. The tank remained unopened and evidence of leakage from the sealed manway was visible in the form of dry albumin “runs” at several locations on the tank flange circumference. The torque on the bolts securing the manway lid to the tank flange was checked and found to be within 90% of the specified assembly value (a single bolt was 88%). Neither the silicone O-ring seal nor the machined groove in the tank flange displayed visible damage or defects. The lid was subsequently re-torqued to the tank flange with the O-ring in place per the tank assembly SOP XXXXXXXX. The clearance between the flanges was measured around the circumference at both the outer edge of the flange and at the middle of the flange face in the vicinity of the O-ring groove. These measurements showed a gap variation of about 0.003 inches (25% about the mean) between different locations. The gap at the O-ring groove was consistently larger than at the edge of the flange indicating the torque applied was imparting a slight (approx. 0.3 degree) concave warp to the lid assembly. The results are given in the “flange-to-flange clearance” worksheet of the attached Excel “calculations” workbook.

OPERATOR INTERVIEWS: All the 750 liter tanks are pressure tested following assembly and, again, after autoclaving. Pressure loss is investigated by “bubble testing” the tri-clamp connections and the manway flange using a surfactant solution. Tightening or retorquing fasteners is the usual effective corrective action. Most pressure test failures occur post assembly. Post autoclave pressure test failures are rare – one operator estimated one failure in a hundred tanks. Tri-clamps account for the majority of pressure leaks. When the manway seal leaks the location is usually in the vicinity of the spring-hinge. In the past hinges also induced misalignment between the lid and tank flanges making it difficult to seat the sealing surface and apply uniform torque. The worst offending tanks have been removed from service.

ANALYSIS:1) A brief review of the theory, and recommended design practices, for low pressure static O-ring

seals shows that the current design is conceptually sound. This is borne out by the reliable service these tanks provided when they were new.

2) A force / moment analysis of the joined tanks shows that there is insufficient force – even during a panic stop – to lift and rotate the rear tank about its joining pin and bang it into its partner tank. Thus, eliminating this type of mechanical trauma as a likely cause for the sealing failure.

3) The difference in the linear coefficients of expansion between silicone and stainless steel is an area where operational activities might adversely influence sealing integrity. The silicone gasket expands with temperature at about 10 times the rate of stainless steel. The dimensional changes accompanying the range of temperatures from the autoclave to the cold room yield a

difference in expansion between the silicone O-ring and the stainless steel groove of about 1.4 inches. Because the O-ring is restrained within the groove, mechanical expansion (or contraction) energy is manifested as internal stress in the O-ring. The result can be squirming or buckling of the O-ring or separation of the O-ring from the outside edge of the sealing groove. While neither occurrence would necessarily cause leakage at the low tank pressure in question, these effects are adverse and could increase the probability of leakage when combined with other known, or unidentified, adverse effects.

4) Tank preparation procedures necessarily create conditions that may antagonize seal integrity. In addition to the above temperature excursions the tank is subjected to a vacuum prior to initial torquing. This draws the O-ring to the inside edge of the sealing groove. The tank is then pressure-hold tested at 15psig where forces act to displace the O-ring toward the outside edge of the groove. Any movement resulting from these forces, while not necessarily compromising the seal, would be considered an adverse influence.

DISCUSSION: During the tank inspection a Tri-clamp was discovered whose tightening lugs were bent to the point of contact. Clamps with such visible wear should be removed from service.

Regarding the tank under investigation, the manway seal design is appropriate to the mission. While the tank preparation and handling procedures introduce conditions that may be adverse to seal integrity, historically these do not seem to have resulted in any identifiable failure pattern. Nothing was uncovered in the life cycle of the lot to suggest the leak resulted from an operational error or accident. Examination of the tank revealed no damage or defects in the seal system or any condition that could point to a root cause of the leak. However, the concave deformation of the lid flange under torque suggests further increasing torque on the flange attachments will have limited utility as a corrective action.

All the 750 liter tanks have likely experienced a degree of mechanical deterioration resulting from the repeated application of torque, from thermal cycling, from modification-related stresses and from ordinary wear and tear. The ability of a given tank to mitigate the conditions above (and additional adverse elements not yet identified) is not quantifiable. To the extent that tank deterioration contributed to the loss of sealing integrity, it would be categorized as a (statistically) random failure.