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The assertion that all gate valves have non-rising stems is false because there are two primary types of gate valves: non-rising stem and rising stem. A non-rising stem gate valve has its stem contained within the body of the valve, so it does not extend upward when the valve is operated. This design is often used in situations where space is limited, such as underground installations.

In contrast, a rising stem gate valve features a stem that rises as the valve is opened. This design allows for easy visual indication of the valve's position, making it easier to determine whether the valve is open or closed. Rising stem valves are typically found in applications where space is not an issue and where operator visibility is important.

Thus, while non-rising stem valves are common, they are not the only type of gate valve available, leading to the conclusion that the statement is false.