Pressure Reducing Valves for Steam

Mechanism for Countering Outlet Pressure Drop

• If the outlet pressure drops, the force applied to the large coil spring by the diaphragm weakens.
• As a result, the large coil spring expands and the diaphragm pushes the pilot valve down.
• The pilot valve thus opens wider and allows a greater volume of steam to flow to the head of the piston.
• This pushes the piston down, causing the main valve to open wider and let more steam pass through. The increase in steam volume makes the outlet pressure increase and restores original pressure levels.

Mechanism for Countering Outlet Pressure Increase

• If the outlet pressure increases, the force applied to the large coil spring by the diaphragm increases.
• As a result, the large coil spring contracts and the pilot valve moves upward and shuts tighter.
• A smaller volume of steam then flows to the head of the piston making the force pushing the piston down weaken, so the piston is pushed upwards by the small coil spring.
• This causes the main valve to shut tighter and let less steam pass through. The decrease in steam volume makes the outlet pressure decrease and restores original pressure levels.

Mechanism for Countering Inlet Pressure Drop

• If the inlet pressure drops, the difference in pressure between the inlet and outlet of the valve decreases, and so the force acting to push up and close the main valve weakens and the main valve opens wider.
• This will make the volume of steam flowing to the outlet side of the valve increase, causing the outlet pressure level to increase.
• When the outlet pressure increases, the force applied to the large coil spring by the diaphragm strengthens.
• As a result, the large coil spring contracts and the pilot valve moves upward and shuts tighter.
• The tightening of the pilot valve makes a smaller volume of steam flow to the head of the piston.
• This makes the piston move up, causing the main valve to shut tighter and let less steam pass through. The decrease in steam on the outlet side makes the outlet pressure drop, restoring original pressure levels.

Mechanism for Countering Inlet Pressure Increase

• If the inlet pressure increases, the difference in pressure between the inlet and outlet of the valve will increase, and so the force acting to push up and close the main valve increases and the main valve shuts tighter.
• This will make the volume of steam flowing to the outlet side of the valve decrease, causing the outlet pressure level to drop.
• When the outlet pressure drops, the force applied to the large coil spring by the diaphragm weakens.
• As a result, the large coil spring expands and the pilot valve moves down and opens wider.
• A larger volume of steam then flows to the head of the piston and pushes the piston downward.
• This causes the main valve to open wider and let more steam pass through. The increase in steam volume makes the outlet pressure increase and restores original pressure levels.