Sep.19.2024
Part 1: Globe Valve vs. Needle Valve
(1) What is a Globe Valve?
The globe valve, also known as a forced-sealed valve, is a type of valve that uses the pressure of the valve core sealing surface against the valve seat to stop the flow of the medium and achieve sealing. When the valve is closed, pressure needs to be applied to the valve disc to forcibly hold the fluid, thereby cutting off the fluid flow. This pressure is usually applied through a screw lifting method: by rotating the valve handle, the valve stem descends, which in turn drives the valve disc. The sealing surface of the valve disc is pressed tightly against the valve seat sealing surface under pressure, thus sealing the fluid flow. This is the basic working principle of a globe valve.
The globe valve has the following characteristics and advantages:
1. Wide range of applications: Globe valves are widely used in various industrial fields.
2. Cost-effective: Compared to other types of valves, globe valves are generally more cost-effective.
3. Good adaptability and control performance: Globe valves can adapt to various media and operating conditions and have good flow control capabilities.
4. Strong pressure adaptability: Due to its structural characteristics, globe valves can easily achieve good sealing effects under low to medium pressure conditions.
Therefore, globe valves are often used in process pipelines, especially in situations where precise flow control or low-pressure conditions are required.
(2) What is a Needle Valve?
A needle valve is a type of valve that can be precisely adjusted and has a wide range of applications. For example, the knob used to adjust the flame temperature in flame cutting is a needle valve. Instrument needle valves are important components in instrument measurement pipeline systems used to control flow.
A needle valve achieves sealing by pressing a conical, tapered, or spherical surface into the center of the valve seat. It has a good sealing effect and is suitable for working under high pressure, low flow rates, and small diameter conditions. Large-diameter needle valves are rare, but they have good flow adjustment performance and are prone to blockage. In cases of large temperature changes, the valve disc and valve seat are prone to seizing. They are widely used in instruments, meters, and control equipment.
A needle valve is a type of globe valve. The sealing surface of a standard globe valve is flat, while the sealing surface of a needle valve is conical.
Summary:
1. The working principle of a needle valve is similar to that of a globe valve but with a smaller flow passage, usually not larger than DN10. Depending on the requirements, it is used for smaller flow rates and is generally thread-connected. In general, a needle valve is a type of globe valve. Needle valves are used for small instrument pipelines, while globe valves are used for larger process pipelines.
2. The core of a needle valve is needle-shaped, whereas the core of a globe valve is cylindrical. Moreover, needle valves generally have pressures of 3000 PSI, 6000 PSI, and 10000 PSI, whereas globe valves do not.
3. It can also be distinguished by the valve disc. Whether the core (valve disc) is mounted on or inserted into the valve seat can be a distinguishing factor, especially for needle valves with tapered valve discs, which are similar to globe valves.
Part 2: Installation Reminders for Globe Valves and Needle Valves
(1) Installation Reminders for Globe Valves
Globe valves adjust the flow rate or shut off the medium passage by changing the gap between the valve disc and the valve seat, i.e., changing the size of the channel cross-section. When installing a globe valve, attention must be paid to the direction of fluid flow.
The principle that must be followed when installing a globe valve is that the fluid in the pipeline passes through the valve hole from bottom to top, commonly known as "low in, high out". Reverse installation is not allowed.
(2) Installation Reminders for Needle Valves
1. The installation position, height, and inlet and outlet directions must meet the design requirements. The direction of medium flow should be consistent with the arrow direction marked on the valve body, and the connection should be firm and tight.
2. Before installing the valve, a visual inspection must be conducted, and the nameplate of the valve should comply with the corresponding production standards.
3. Strength and tightness performance tests must be conducted before installation for valves with working pressures greater than 1.0 MPa and that act as main pipelines. After passing the tests, they can be used. During the strength test, the test pressure is 1.5 times the nominal pressure, and the duration is not less than 5 minutes. The valve body and packing should have no leakage to be qualified. During the tightness test, the test pressure is 1.1 times the nominal pressure, and the duration meets the corresponding requirements.
