When a discharge valve is opened to the atmosphere, what happens to the pressure on the upstream side?

When a discharge valve is opened to the atmosphere, the pressure on the upstream side decreases. This occurs because the upstream side is now connected to an area with lower pressure, meaning that the fluid can escape to the atmosphere. The downstream side pressure does not remain the same; instead, it drops as the discharge valve allows the fluid to exit, resulting in a lower pressure on that side as well.

The decrease in upstream pressure is a direct result of the opening of the valve, which creates a pathway for fluid flow. Accordingly, if the discharge valve was initially sealed, the upstream pressure would be held at a certain level. Opening the valve allows the fluid to escape, effectively reducing the pressure that was contained. Thus, option C accurately reflects that the upstream pressure decreases, while the downstream pressure concurrently drops under the same conditions.