Globe valves are one of the most common valves in every industry because of their ability to start, stop, and throttle fluid flow. Generally, they consist of a movable disk/plug and a stationary ring seat, which is in a spherical body. The valve seat is in the middle of the pipe cross-section, with its aperture parallel to the pipe. When actuating the valve, fully lowering the disc shuts off the valve, while fully raising the disc allows flow at its maximum rate. Any position in-between regulates flow in proportion to the vertical travel of the disc, which makes globe valves ideal for throttling. This setup results in a change in the direction of flow across the valve, which is largely responsible for pressure losses in a globe valve.
In this article, we will review what dictates flow direction, the flow characteristics of a globe valve, and its CV values.
How to Dictate Flow Direction in a Globe Valve
Because of its importance, there is always an indication on a globe valve from its manufacturer, showing the flow direction. The direction of flow is dependent on whether its design is a flow to open or a flow to close.
Flow to Open
This is also called standard or forward flow because it is the most common flow direction for globe valves. In this setup, the fluid comes into the valve from under the disc. So it tends to lift it upwards, thus, opening the valve. As a result, this flow direction requires higher closing torque and less opening torque, making it ideal for low-pressure applications. When deploying this system in low-temperature applications, it promotes smooth operation, helps protect the packing, and minimizes erosive action on the disc and seat surfaces. Also, this flow direction helps the disc to fit properly on the seat as the valve closes. This ultimately makes the valve less prone to seat leakage.
Flow to Close
Flow to close, which is also referred to as reverse flow, is the less popular flow direction when using a globe valve. In this setup, the fluid comes into the valve from over the disc, which results in a closing action. This flow direction is ideal for high-pressure and high-temperature applications. For high-pressure applications, it helps in maintaining seat sealing and avoiding leakages. The flow direction means the closing torque is less than the opening torque during valve actuation. While in high-temperature service, this configuration ensures that flow diffuses rather than concentrating on the face of the disc. Thereby, it avoids contraction of the stem during cooling. This process tends to lift the disk off the seat, causing leaks. A potential disadvantage of this flow direction is a reduction in the flow capacity of the globe valve.
The following table summarizes the advantages and disadvantages of each flow direction:
|Flow to Open
|– Less prone to seat leakage <br> – Minimizes erosion on disc and seat <br> – Protects packing
|– Higher closing torque <br> – Lower flow capacity
|Flow to Close
|– Lower closing torque <br> – Higher flow capacity <br> – Avoids stem contraction
|– More prone to seat leakage <br> – Concentrates erosion on disc face <br> – Damages packing
Flow characteristics of any valve refer to the relationship between the flow of the medium and the opening/closing of the valve. When discussing flow characteristics due to only the valve, they are referred to as inherent flow characteristics. However, when considering the effect of the system on the valve, they are referred to as installed flow characteristics.
The most common types of inherent flow characteristics for globe valves are linear, equal percentage, and quick opening. Linear flow characteristics mean that the flow rate is directly proportional to the valve opening. Equal percentage flow characteristics mean that the flow rate increases exponentially with the valve opening. Quick opening flow characteristics mean that the flow rate increases rapidly at the initial stages of the valve opening, and then levels off.
The choice of the flow characteristic depends on the application and the system requirements. Generally, linear flow characteristics are suitable for liquid level control, pressure control, and flow control with constant pressure drop. Equal percentage flow characteristics are suitable for flow control with variable pressure drop, temperature control, and pH control. Quick opening flow characteristics are suitable for on-off service, safety relief, and bypass operations.
The CV value of a valve is a measure of its flow capacity under a given pressure drop. It is defined as the number of US gallons per minute of water that can pass through the valve at 60°F, with a pressure drop of one psi. The CV value depends on the size, type, and design of the valve, as well as the flow direction and the fluid properties.