A block valve can prevent or allow movement without restriction. Gate valves are a common type of block valve that can be operated manually or with an automated system. The valve must have no effect on flow when open and totally block flow when closed. This definition excludes globe valves and check valves. Different industries have their own definitions for block valves.
A block valve is technically any valve that has the ability to block movement in one or more directions. In practice, most people refer to a block valve as one that can either prevent movement or allow movement to occur without restriction. This means that the valve must have no effect on the movement in the off position and totally block the movement in the on position. This definition separates it from some types of standard valves and removes check valves entirely from the group. The most common type of check valve is the simple gate valve although there are hundreds of different variations.
The gate valve has casings that are slightly larger than the pipe to which it is connected and a control stem that protrudes laterally. There is a gate built into the valve which can be raised and lowered through the external control source. This type of valve is common to many different types of machinery and can be operated manually or with an automated system connected directly to the valve.
In broader terms, almost any type of valve could be a block valve. All the valve has to do is prevent movement from at least one direction at times, and it qualifies. Since this is essentially any tube, there is a tendency to narrow the term down to a more select grouping.
In the strictest sense, the valve must do two things. When the valve is open, it should have no effect on the movement of the flowing material. When closed, it must totally prevent material from moving through the valve.
By allowing material to flow unchecked through the open block valve, some common valve types are excluded from the pack. For example, a globe valve typically has two chambers and material flows through them. Even when open, moving substances are interrupted by the process of moving from one chamber to another. The second restriction removes check valves and other one-way valves from the list as the material is only blocked from one direction.
This definition is more of a rule of thumb than an actual specification. Because the true meaning of the block valve is so broad, different industries and processes have their own specific meanings for their equipment. As a result, it is not uncommon for one industry to define a specific valve type of block valve while another does not.
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