What should you do when you slightly open a butterfly valve before relieving the line pressure?

When slightly opening a butterfly valve before relieving line pressure, the primary objective is to equalize the pressure on both sides of the valve. This action helps to prevent any sudden pressure release that might cause harm to personnel or equipment, as it allows for a more controlled and gradual equalization of pressure.

By equalizing the pressure, you minimize the risk of shock loading or water hammer effects that can occur when a valve is suddenly opened or closed under pressure. It ensures that the valve can operate safely and effectively, which is essential in maintaining the integrity of the water or fuel system.

In the context of the other options, increasing valve clearance, equalizing static potential, or loosening flange seals do not directly address the immediate safety concern associated with pressure relief in the system, making them less relevant in this specific scenario.