From the ALCO Manual:
"VACUUM AND RELIEF VALVES.
In the high pressure steam chests or some other convenient place which is in communication with the steam chests, are located vacuum valves. The function of these valves is to admit free air into the steam chests when the locomotive is drifting so as to avoid a vacuum and give a moderate flow of air through the cylinders.
The low pressure cylinders are equipped with combined vacuum and relief valves which in addition to having functions similar to the vacuum valves of the high pressure cylinders also regulate the steam pressure in the low pressure steam chests. These relief valves are set at 45 per cent of the boiler pressure.
As these valves relieve any excessive pressure in the low pressure cylinders, they should be tested occasionally to see that they are correctly set.
From the previous description of the intercepting valve it will be seen that when the locomotive is working compound the packing rings of the high pressure valves and pistons alone separate the boiler pressure from the pressure in the receiver and low pressure cylinders. Consequently if there was a blow in these packing rings, the pressure in the receiver would be increased, causing the relief valves in the low pressure steam chests to blow off. Therefore, if these valves rise from their seats frequently when the locomotive is working compound, it might be due to the fact that there was a blow in either the valves or the pistons of the high pressure cylinders, and these should be tested.
To test for blows simply throw the emergency operating valve in the cab to the simple position, namely, with the handle pointing to the rear. Spot the locomotive and test the same as a simple locomotive."
LP valve
HP valve
