Why Valves Matter

A plant is held together by valves

A process plant moves liquids and gases through pipes. Pumps and compressors push the fluid. Vessels store it and separate it. But valves decide where the fluid goes, how much flows, and when it stops. A medium-sized plant can have many thousands of valves, from small instrument valves to large pipeline valves that need a gearbox to turn.

Every valve does one of three jobs:

• Isolate — stop the flow completely, or allow it fully.
• Regulate — set the flow, pressure or level somewhere between fully open and fully closed.
• Protect — act on its own to stop reverse flow or release excess pressure.

The rest of this course builds on these three jobs. When you meet a new valve, the first question is always: which job is it here to do?

Case: the drain valve left open

During a shutdown, a small drain valve on a pipe was opened to empty it. The job finished and the line was put back into service. Nobody closed the drain. When the pump started, product sprayed from the drain onto a hot surface. A small fire started and was put out quickly. The investigation found that the valve list used to restore the plant did not include small drain and vent valves. The fix was simple: every valve moved during the job goes onto the restoration list, however small it is.

Case: the wrong valve for the job

A gate valve was used to control flow to a tank because it was the only valve available. It was left half open for months. When maintenance later needed to isolate the line, the valve would not seal. The seat had been cut by the fast flow. A job that should have taken one shift needed an extra day to fit a blind.