the max service pressure and for flow rates you're looking at anything between 5 - 20 meters per second depending upon if the flow medium is a liquid or a gas. For liquids, the flow rate is actually gonna be less so you're looking in the 5 meters max per second range and for gases you're looking up to maybe 20 meters per second. We're gonna look at all these aspects of the gate valve later in the course and we're gonna go into a lot more detail. So let's now go to the next lesson and we can have a look at the next valve design.

- [Instructor] In this video, we're gonna look at the gate valve. The gate valve's quite interesting because it's one of the most common types of valve, and it's also one of the simplest. So let's take a cross-section so we can actually have a look at the insides of the valve. So here we have a gate valve, I'll push the play button, and then we can watch the gate valve operating or actuating. So now it's in the closed position, and it's moving up to the open position. You can see that when the valve is fully open, there is very little resistance to flow passing through the valve. Now this is very good because this means we're gonna have a very low pressure drop. So as the fluid or the gas is flowing through the valve, it's going to encounter very little resistance, and we'll have a correspondingly low pressure drop. Now gate valves are normally used in-line,

that means they are used just to pass flow through, but occasionally you might need to close the valve and isolate the system, and that is what a gate valve is for. Gate valves are only used for open and close; they're not used for throttling or regulating the flow. I'll show you why. If we look down here at the bottom of the valve, you can see it's fully closed now; the disc is pressed against the seat. I'll pause it. Now it is only half open, so this is in the throttled position. The problem with this is that, firstly, the amount of flow through the valve is not proportional to the amount that this gate has traveled. So, the disc, this section here, has traveled up, and it looks like approximately halfway, but the flow through the valve is not gonna be half of the max flow capacity. The flow through the valve is not linear or proportional to the amount the valve is open.

This means that when you actually open the valve halfway, you're gonna get a lot of flow between the disc and the body here, and this is gonna be proportionally quite a large amount of flow with a high velocity. This high velocity often leads to seat and disc wear. The other problem, apart from the high velocity, is the turbulence. As the flow passes through this small gap between the disc and the body, there is a lot of turbulence, and this also creates a larger pressure drop, which again is what you don't want; this is resistance to flow. So, gate valves are not very well suited to throttling; in fact, they should either be only fully opened or fully closed. Typical service temperatures are gonna be between about -20 to 60 degrees Celsius, pressures up to approximately 16 bar. These valves are not suitable for very high pressures because you're actually going to blow the packing out or cause the packing to leak, so 16 bar would be

How Gate Valves Work

How Gate Valves Work

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