It  has  already  been  noted
that  impulse  wheels  of  the
Pelton  and  Girard  types
cannot  operate  satisfactor-
ily submerged,  and  must
be  set  at  such  positions
that  they  will  be  above  the
tail  water  at  all  times.  In
many  localities  where  the
Fig.  197.— Boyden  Dlffuser.  variation  in  the  surface  of

tail  waters  is  considerable,
this  means  a  large  relative  loss  in  the  head  utilized  and  that  this
type  of  wheel  will   therefore  not  be  practicable  except  under  high

•  From  Bulletin  No   6.  Abner  Doble  Co.
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3o8  Turbine  Details  and  Appurtenances.
head  conditions  and  where  the  loss  entailed  by  the  rise  and  fall  of
the  tail  water  will  be  inconsiderable.  An  attempt  has  been  made,
however,  to  so  design  a  draft  tube  that  a  vacuum  will  be  established
and  maintained  below  the  wheel,  in  such  a  manner,  however,  that
the  water  will  not  come  in  contact  with  the  wheel.  The  vacuum  is
so  maintained  as  to  hold  the  water  at  an  established  point  just  below
the  wheel,  thus  permitting  the  wheel  to  utilize  the  full  head  except
for  the  small  clearance  between  the  wheel  and  the  water  surface  in
the  draft  tube.  This  arrangement  is  shown  in  Figs.  168  and  171,  as
applied  by  The  Piatt  Iron  Works  Company  to  a  Girard  turbine.
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CHAPTER  XIV

HYDRAULICS  OF  THE  TURBINE.
