What are Thread Valves?
A thread valve is the valve with the thread and can be classified into the internal thread valve and the external thread valve. This kind of valve is connected to the pipeline with the threaded connection. The valve, generally, has the small diameter and is used for engineering projects under normal pressures and low pressures. At the same time, under high pressures, the thread valve is required to be operated with the welded connection or the flange connection.
Common types of thread valves include thread gate valves, thread ball valves, thread globe valves and thread check valves.
The valve body of the thread valve can be made of forged steel, stainless steel and cast steel. The maximum operating pressure of the thread valve is from 1.6MPa to 32MPa (American Standard: from 150LB to 2500LB). Moreover, its working temperature is from -20 to 455℃, and its diameter is from DN10 to DN50.
Differences between internal thread valves and external thread valves
Owing to the price and the volume requirement of the operating environment, in daily practice, the internal thread valve is applied more than theexternal thread valve.
In view of the connection of the thread, with the same diameter, the flow rate and the nominal diameter of the external valve is smaller than the internal thread valve’s, and the flow velocity of the external thread valve is higher than the internal thread valve’s.
In general, the thread of the internal thread valve is the pipe thread and the thread of the external thread valve is the taper thread. Moreover, the external thread of metal ferrules belongs to the pipe thread.
With same materials, the external thread valve has better pressure resistance and the thicker valve wall than the internal thread valve. Meanwhile, the smaller the nominal diameter is, the thicker the valve wall will be. This property is the reason that valves with high working-pressures are always external thread valves.
The sealing of thread valves
1. Direct sealing
Both external and internal thread are used for sealing. To ensure no leakage, lead oil, PTFE sealing tapes, etc. are usually used as the packing of valves. The PTFE sealing tape is applied increasingly. The reason is that, with good anti-corrosive properties and excellent sealing performance, PTFE is much better than lead oil since it is convenient to use and store. When the valve is disassembled, the PTFE tape can be removed as a whole because the tape is a thin film without adhesion.
2. Indirect sealing
The power of the screwing thread is delivered to gaskets between two surfaces, which makes the gasket play the role of sealing.
1. The international standard ISO 228/1, DIN259, external and internal parallel, with the symbol of G or PF (BSP.F)
2. The German standard ISO7/1, DIN2999, BS21: external tapered and internal parallel
3. The British standard ISO7/1, BS21, external and internal parallel, with the symbol of PT, SBP.Tr or Rc
4. The American standard, ANSIB21, external and internal tapered, with the symbol of NPT
Internal Thread gate valves
The valve most widely used is the internal thread gate valve, which is one of the valve sealed by force. With the pressure of the valve stem, sealing surfaces of the valve clack and the valve seat have contact with each other tightly and then the internal gate valve can block the flow of media.
This kind of gate valve obtains properties of the simple structure, excellent sealing performance, high fluid resistance and poor regulation performance, etc.
With the internal thread connection, the internal thread gate valve can be run with manual operation, electric operation and pneumatic operation. At the same time, the internal thread gate valve has the working pressure from 0.1MPa to 2.5MPa, the valve diameter from DN1 to DN50 and the working temperature from 20 to 245℃. Furthermore, the internal thread gate valve is usually connected by the flange when the valve is up to DN50 and the working temperature of the valve varies with different materials of sealing elements and valve bodies.