Air and steam pressure reducing valves: Learn about the working of valves used for pressure reduction on ships

What are Pressure Reducing valves?

Pressure reducing valves are used commonly on all types of ships. As the name suggests, these valves reduces the pressure of a system and keeps a check on the same. Generally used in boilers and compressed air systems, they have a self actuating mechanism which activates due to increase in pressure. This valves are often confused with the relief valves, which are used for a totally different purpose. A relief valve comes to rescue when the pressure of a system rises above a set limit. Thus relief valves work only when the pressure reaches the highest point or the set pressure.

Unlike relief valves, pressure reducing valves are used to maintain the pressure of a system at an optimum level. These valves are subjected to a minor change in the pressure level, thus allowing to maintain an equilibrium pressure in the system.


Being self-actuating, pressure reducing valves are also of the type of spring loaded valves. In a system that handles high pressurized steam or compressed air, it is extremely essential to provide them at a pressure lower than that of the boiler or compressed air reservoir. This is done to maintain a downstream pressure within defined limits over a range of flow, despite any changes in the supply pressure. This prevents any kind of damage to the system or the pipeline.

The construction of the valve is similar to that of any normal spring loaded valve. In this type, the valve and valve seat comes in direct contact with the fluid that is being controlled. For this reason, valves are coated with protective material to prevent corrosion and wear-down. The spring is attached to the bottom of the valve with the help of an adjusting nut which can be used to fix the tension of the spring. Both, top and bottom area of the valve comes in direct contact with the steam or compressed air. A reciprocating piston, controlled by the steam pressure, maintains the position of the valve. The steam that gets condenses in the valve gets accumulated in a condensation leg, which also plays an important part in maintaining adequate pressure.

On the top side of the piston, a rubber diaphragm is attached with the help of a nut. The pressure entering the valve will also act on this diaphragm. The main valve along with the rubber diaphragm will maintain an efficient control of the spindle and thus help in opening and closing of the valve.


As shown in the figure, when the inlet pressure P1 is higher, it will act in the upward direction on the main valve and at the same time in the downward direction on the diaphragm. When the outlet pressure P2 is less than the inlet pressure, the spring will push the spindle in the upward direction, opening the valve. When the output pressure P2 increases, it will exert a force on the top area of the valve and closes it.

When the pressures on the top and bottom side of the valve are equal, a balance will be created which will maintain an optimum pressure in the system.

pressure reducing

Special configuration

The self actuating mechanism of the valve can be replaced with a remote operating system. This type of mechanism is used for fluid pressure control fucntions in unmanned machinery spaces. This can be done by carefully selecting the valve’s trim ( The internal parts of the valve that come in direct contact with the fluid) and adjusting the set pressure.


Marine Auxiliary Machinery , 7th Edition ,1995 by McGeorge

Image credits

Marine Auxiliary Machinery , 7th Edition ,1995 by McGeorge