_Water pressure engines are machines with a cylinder and piston or ram,
in principle identical with the corresponding part of a steam engine_;
the water is alternately admitted to and discharged from the cylinder,
causing a reciprocating action of the piston or ram. It is admitted at
a high pressure and after doing its work on the piston is discharged.
The water in some of these machines acquires a high velocity; _the
useful work is due to the difference in the pressure of admission and
discharge_, whether that pressure is due to the weight of a column of
water of more or less considerable height, or is artificially produced.
When an incompressible fluid such as water, is used to actuate piston
engines, two special difficulties arise. One is that the lost work in
friction is very great, if the water attains a considerable velocity;
another is that there is over-straining action on the machinery. The
violent straining action due to the more or less sudden arrest of
the motion of water in machinery is termed _hydraulic shock_. For
these reasons the maximum velocity of flow of water in reciprocating
hydraulic machines should generally not exceed 5 to 10 feet per second.
Under high pressure, where there is less object in saving and it is
very important to keep the dimensions of the machinery small, Mr.
Anderson gives 24 feet per second as the limit of velocity. In large
water-pressure engines used for pumping mines the average piston speed
does not exceed 1/2 to 2 feet per second.
The suitability of water for _the transmission of power_ has been
fully recognized in recent years; the facility with which water under
pressure is capable of being utilized, and the advantages that attend
its use in motors have resulted in many practical difficulties being
overcome, which were at first considered insurmountable.
At the outset of the employment of water pressure it was feared that
the water in the pipes and machinery might freeze. This, however,
has been found not to be a difficulty where well-known precautions
are taken. The working parts should, where possible, be placed under
ground, or should be cased in, if they are above ground. _The water
should be run out of all valves and cylinders which cannot be cased
in_, and protected as soon as the working of the machine ceases.
A very small gas jet or lamp placed near the unprotected parts will
prevent freezing.
Experiments have also shown that a mixture of glycerine and water
prevents the effects of frost at a temperature as low as 16° Fahr.,
provided the glycerine has a specific gravity of 1.125, and that it
is mixed in the proportion of one part of glycerine by weight to four
parts of water.
Where water is used over again in the machines (by returning the
exhaust water from the machines to a reservoir), such addition of
glycerine is more easily resorted to. Where moderate risks of frost
have to be dealt with, the proportion of one gallon of glycerine to 300
gallons of water proves effectual. If the water is at a high pressure,
such as 1,500 lbs. to the square inch, it is less liable to freeze than
when used at a low pressure.
Again, it was at first feared that accidents might be frequent from the
bursting of hydraulic pipes and cylinders under high pressure. Such,
however, has been proved not to be the case in practice, and even where
pipes or cylinders do burst, the pressure is at once dissipated, as the
body of water which can escape at the opening is but slight.
It is desirable to use water which is as free as possible either from
suspended matter or from chemical impurity. The former increases the
wear and tear of the packing, and is otherwise inconvenient, and the
latter acts injuriously on the seats and fittings of valves. Sea-water
can be used for hydraulic machinery, but on account of its corrosion,
fresh-water is better.
Water-pressure has sometimes been applied to operate machines which are
worked continuously and not intermittently, and to continuous working
rotary machines. This is unwise, for in applying hydraulic power to
the continuous working of shafting or shop tools, the amount of power
developed by the hydraulic engine cannot be varied to suit the work to
be done, neither can the speed be regulated with sufficient nicety.