[Fig. 241] is an example introduced to show the method of lettering a descriptive mechanical drawing.
The figure shows a “blow-off valve” of approved design drawn in section; the letters designate the several parts; the reading to accompany the lettering is as follows:
A—Inlet communicating with annular passage (C) to admit steam, which blows off scale and sediment from seat (E) before disc (L) comes in contact with same.
B—Plug to permit passage of rod to clean out blow-off pipe.
C—Annular steam passage around casing (D) and communicating with inlet (A).
D—Removable bronze casing, in which plug (L) fits snugly.
E—Removable bronze seat ring, which also holds casing (D) in place.
J—Slot in casing (D) arranged to discharge sheet of steam from C, which, blowing across seat (E), cleans off scale and sediment before contact with disc (L).
K—Non-rotating washer to prevent loosening of locknut (H) when opening valve.
L—Reversible disc, having two Babbitt-metal seating faces (F) (F).