A block valve is technically any valve that has the capacity to block movement in one or more directions. In practice, most people refer to a block valve as a valve that can prevent motion or allow motion to happen without restriction. This means that the valve must have no effect on movement in the off position and totally block movement in the on position. This definition separates it from some standard valve types and completely removes one-way valves from the grouping. The most common type of block valve is the simple gate valve although there are hundreds of different variations.
The gate valve has casings slightly larger than the pipe to which it is connected and a control stem that sticks off the side. There is a gate built into the valve that may be raised and lowered through the external control source. This type of valve is common in many different kinds of machinery and can be operated manually or with an automated system connected directly to the valve.
In the broadest of terms, nearly any valve type could be a block valve. All the valve needs to do is prevent motion from at least one direction some of the time, and it qualifies. Since that is essentially any valve, there is a tendency to narrow the term down to a more select grouping.
With the more restrictive meaning, the valve must do two things. When the valve is open, it must not have any effect on the movement of the flowing material. When it is closed, it must totally prevent material from moving through the valve.
By allowing material to flow unchecked through the open block valve, certain common valve types are excluded from the group. For instance, a globe valve typically has two chambers, and the material flows through them. Even when open, the flowing substances are disrupted by the process of moving from one chamber to another. The second restriction removes check valves and other one-way valves from the list since the material is only blocked from one direction.
This definition is more of a rule-of-thumb than an actual specification. Since the true meaning of block valve is so wide, different industries and processes have their own specific meanings for their equipment. As a result, it isn’t uncommon for one industry to call a specific valve type of block valve while another does not.