This blog is going global. OK. Technically, since it is on the internet, it has always been worldwide. But this week, we are going to talk about globe valves.
WHAT ARE GLOBE VALVES?
As alluded to in last week’s article, globe valves are a kind of control valve which allow the operator to increase or decrease flow, usually by turning a hand wheel. Turning the hand wheel rotates the valve’s stem, which raises or lowers a plug into a valve seat.
The distance between the plug and the valve seat determines the amount of flow through the valve. The greater the distance, the greater the flow.
HOW DO THEY FUNCTION?
Globe valves get their name from their body shape, which was once consistently spherical. As technology has changed, so has the body’s exterior shape. Globe valves now come in a number of different looks, not all globe-like.
The internal shape and function, however, is much the same as it has always been.
You can picture a typical straight line, or “Tee” pattern, globe valve’s interior – like the ones that we sell - as a kind of flattened “S” shape. The letter’s center point is the valve seat and the stem descend vertically through it to that point.
The fluid or gas enters through the lower end of the letter and then travels up until it meets the midsection.
If the plug is in place, the flow stops at the midpoint. If not, it continues through at a rate determined by the plug. The globe valve’s ability to throttle a substance’s flow makes it useful as a control valve.
HOW ARE THEY USED?
Globe valves are used in operations which require frequent changes in flow rate. Unlike the gate valve we talked about last week, any wear on the plug or the valve seat should be uniform, making a leak less likely, even after extended use.
In normal operation, the direction of flow comes from under the plug. In high temperature operations, the direction of flow may be changed so that flow comes from the opposite direction. Doing so keeps the stem at a constant higher temperature, which decreases the chance that it will contract and lift the plug off of the valve seat.
One drawback of globe valves is greater flow restriction. The twists and turns of the valve along with a narrowed channel through the valve seat opening conspire to increase flow resistance. That makes pressure drop a factor in some applications.
Check back next week to learn about needle valves, an even more precise type of control valve.