Email this pageTypes of Valves and Their Applications
Different Types of Valves
There are many different types of valves in the world, but the among the most typical manual valves used on steam systems are globe valves, ball valves, gate valves, and butterfly valves.
This article will cover the various types of valves and their different uses.
According to JIS, the definition of a valve is as follows:
A generic name for a device with a moveable feature that allows it to open and close a passageway in order to allow, prevent or control the flow of fluids.'
Valves fall into the following categories when divided by construction and features:
The valving element body 'rotates' in the passageway to stop flow: e.g. ball valve, butterfly valve
The valving element acts as a 'seal or a plug' in the passageway to stop flow: e.g. globe valve
The valving element is 'inserted' into the passageway to stop flow: e.g. gate valve
The passageway itself is 'pinched from the outside' to stop flow: e.g. diaphragm valve
The construction of a gate valve is similar to that of a floodgate. A main feature of this type of valve is the small pressure drop when the valve is fully open. However, the valve body must be raised up completely out of the passageway in order for the valve to be fully open, and this means the handle must be turned many times. The face-to-face dimension of a butterfly valve can be extremely small, making a small pressure drop a main feature of this type of valve as well. These types of valves are often used on applications for water and air.
Let's take a closer look at ball valves and globe valves, which are often used on steam systems.
Ball Valves
Ball valves offer very good shut-off capabilities and are handy because opening and closing the valve is as simple as turning the handle 90°.They can be made to be 'full bore,' which means the valve opening is the same size as the interior of the piping, and this results in a very small pressure drop. Another main feature, the decreased likelihood of leakage from the gland seal, is a result of the valve shaft having to turn only 90°.
It must be noted, however, that this valve is for use only in the fully open or fully closed position. It is not suited for use in a partially open position for any purpose, such as controlling flow rate.
The ball valve makes use of a ring-shaped soft seated valve seat. If the valve is used in a partially open position, pressure is applied to only part of the valve seat, which may cause the valve seat to deform. If the valve seat deforms, its sealing properties are impaired and it will leak as a result.
Globe Valves
The globe valve is suitable for use on a wide variety of applications, from flow rate control to flow shut-off.
When the valve plug is in tight contact with the valve seat, the valve is closed. When the valve plug is away from the valve seat, the valve is open. Thus, flow rate control is determined not by the size of the opening in the valve seat but rather by the lift of the valve plug (the distance the valve plug is from the valve seat). A feature of this type of valve is that even if used in the partially open position, there is little chance of damage to the valve seat or valve plug by the fluid. In particular, the main type of globe valve used for flow rate control is the needle valve.
It must be noted, however, that because the passageway in this valve is S-shaped, the pressure drop is greater than that of other types of valves. In addition, the valve stem must be turned many times in order to open and close the valve, and there is therefore a tendency for the gland seal to leak. Furthermore, because closing the valve requires turning the valve stem until the valve plug presses down tightly onto the valve seat, it is difficult to tell the exact point at which the valve is fully closed. There have been cases where accidentally turning the valve shaft too far has damaged the seating surfaces.
Supplement
Diaphragm valves that stop the flow by 'pinching from the outside' are mainly used on liquid systems, but there is a valve for steam systems that goes by a similar name. It is an automated valve with a diaphragm type actuator. This is often shortened to just a 'diaphragm valve,' so when a valve is referred to by this name, care must be taken to verify which type of valve it is.