The beam s, [Fig. 3403], for working the air, circulating and feed pumps, is driven from the cross head of the intermediate cylinder.
The boilers are of the Scotch pattern that is usually employed for high pressures, as 160 or more lbs. per square inch, and have Fox corrugated furnaces and stay tubes.
Each cylinder requires a starting valve (which is sometimes called an auxiliary valve or a bye pass valve), which is used to warm the cylinder before starting the engine, and also (when there is no vacuum in the condenser) to admit high pressure steam when the high pressure piston is on the dead centre, in which case, there being no vacuum and no admission of steam to the low pressure cylinder, the engine would not have sufficient power to start.
In some cases the high pressure cylinder has no starting valve, the reversing gear being used to admit steam to one end or the other of the high pressure piston, and the starting valve being used to admit enough live steam to the low pressure cylinder to compensate for the absence of the vacuum.
When the vacuum in the low pressure cylinder is maintained while the engine is standing still, its starting valve obviously need not be used, except for warming purposes, before starting the engine; as soon, however, as the engine has started, the starting valve must be closed.
Each cylinder is provided with a relief valve, both at the top and at the bottom, to relieve the cylinder from a heavy charge of water, such as may occur if the boiler primes heavily.
Each cylinder is also provided with drain cocks, to permit of the escape of the ordinary water of condensation in the cylinders when the engine is started, and also for use if the boiler primes.
The low pressure relief valve also prevents the accumulation of too great a pressure in the low pressure cylinder, which, from its large diameter, is not strong enough to withstand high pressure.
The oiling apparatus for the cylinders is arranged as follows:
In some cases pumps, and in others automatic or self-feeding devices are used. Oil is fed to the steam pipe of the high pressure cylinder, and this lubricates both the valves and the cylinders, but in many cases it is also fed to the steam chest, so as to afford more perfect lubrication to the valve.