July, 1946
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  REPORT 2G-21
  1.  The valves in this type of vessel have been in part redesigned for higher pressures.  The changes are most pronounced on the hull valves.  
  2.  The different types and sizes of hull valves given in the hull closure book (Bordabsperrungen - U Boote Typ XXI) are as follows:  
Angle opening out
                       in (note 1)
Globe opening out
(note 2)
                       in (note 3)
Mushroom opening out (note 4)
Flapper opening out (note 5)
(note 6)
Cock  (note 7)
Note 1: Hull valves opening inward are all special service valves, and are used on gauge lines, high pressure blow lines, horn, oxygen system and torpedo tube flood lines.
Note 2: The outward opening back-up valves are in the exhaust gas blow system, and in vent ducts as drains.
Note 3: The inward opening hull valves are on hydraulic piping to external operating units such as turret turning and bow plane operating gear.
Note 4: These are the air intake and exhaust valves, and the negative tank vent valves since removed.
Note 5: These are the exhaust gas valves, and the voice tube valves which last were formerly cocks.
Note 6: These are the ballast tank emergency vent valves.
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  REPORT 2G-21
Note 7: The hull valve is on the sanitary tank overboard discharge.  The back-up valves are three types on grease lines, one on the sanitary tank discharge, one three-way cock on the WRT flooding line, and one special cock combining the function of cable cutter and hull valve on cable leads through the hull.
  3.  Of interest is the general reversion to the earlier practice of having hull valves close with pressure.  Additional points of interest are the absence of gate valves in connection with hull closures, the increase in the completeness of back-up valve installation, and the substitution of flapper valves for cocks in the voice tube installation.  An added note of precaution is the installation of cocks on piping from grease cups to hull fittings.  
  4.  Other evidence of change are apparent in the character of the flange employed to connect hull valves to hull fittings, in the appearance of the yoke on back-up valves, and in the abandonment of shouldered spindles with gaskets on air valves.  
          a.  The flange connection which on earlier types of vessel was the common welding flange with multiple bolts, is here changed to the type of flange used in American commercial practice on ammonia lines.  The two halves of type have a mating and rather narrow gasket face, outside of which the adjoining (but not fraying) surfaces are beveled away from one another.  In plan the flange has a square shape, and the two halves are joined and secured by means of four bolts; one at each corner.  Provisions are made to offset the effect of minor misalignment on bolt and nut stressing by means of washers which are spherical segments.  
          b.  The yoke on back-up valves is still cast integral with the bonnet, but projects out from the stuffing box, up and back in an oval shape to the spindle nut.  It would appear that the type of yoke introduces the possibility of unnecessary concentrations of stress at points on the spindle nut and the stuffing box.  
          c.  Hull valves on air lines in this vessel have the normal gland nut, separate gland, and packing found on valves for other systems.  This appears to be a retrogression in design, considering the merits of the  
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  REPORT 2G-21
  type previously employed.  
  5.  The piping arrangements within the vessel are notable for the reduction in the number of manifolds.  Instead, multiple individual valves are employed, connected to give the desired combination of suction and drain connections.  Manifolds are retained only on the air systems (including blowing arrangements, torpedo air arrangements and the oxygen system) and on the regulating tank gauge piping.  It would be interesting to know whether the absence of manifolds was caused by scarcity of casting facilities or by lack of trust in manifold behavior under shock.  
  6.  The air manifolds remain unchanged in type from earlier vessels.  The shouldered spindle, glandless valve is still used.  
          Points of interest are the reversion to hull valves closing with pressure, the reduction in number of manifolds and miscellaneous changes which generally are a return to older practice.  Valves have been made sturdier to withstand the greater submergence depth of this vessel as compared with that of previous vessels, but no new design details have become evident locally.  
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