I. Needle Valve Overview
Needle valves are a kind of valve that can be adjusted precisely, widely used for various purposes, such as cutting distance in flame cutting, the knob for adjusting the flame temperature is a needle valve. Instrument needle valves are an important part of instrument measuring pipeline system, mainly including globe valves and ball valves, whose function is to open or cut off the pipeline path. The needle valve core is a very sharp cone, like a needle inserted into the valve seat, hence the name.
Compared with other types of valves, needle valves can withstand higher pressures and have better sealing performance, so they are generally used to seal gases or liquid media with smaller flow rates and higher pressures. Needle valves are best suited for use with pressure gauges, and the general shape of needle valves is made with threaded connections.
II. Needle valve parameters
1. Nominal Pressure:
PN2.5 PN4 PN6.4 PN16 PN32Mpa
2. Nominal Diameter:
DN5～DN25
3. Applicable medium:
Oil, water, gas and other non-corrosive or corrosive media
4. Applicable temperature:
-20℃ to +440℃ -70℃ to -240℃ ≤540℃ ≤570℃ etc.
5. Manufacturing material:
20#, 1Cr18Ni9Ti, 304, 316, 316L, 12Cr1Mov, etc.
6. Manufacturing Standard:
GB ANSI JIS
III. Anticorrosion Methods
1. The main materials selected for the valve body are stainless steel and carbon steel. The difficulty in selecting valve body materials lies not only in considering corrosion issues, but also in considering the pressure and temperature resistance capabilities, whether it is economically reasonable and whether it is easy to acquire, among other factors.
2. Adopt coating measures such as coating with lead, aluminum, engineering plastics, natural rubber and various synthetic rubbers. If average conditions permit, this is indeed a cost-saving method.
3. Under low pressure and temperature conditions, the use of non-metallic materials for the valve body can often be very effective in preventing corrosion.
4. In addition, the outer surface of the needle valve body is also subject to atmospheric corrosion, and general steel materials are protected by painting.
4. Installation Precautions
1. The installation position, height and directions of inlet and outlet must meet the design requirements, paying attention to the direction of the medium flow must be consistent with the direction of the arrow marked on the valve body, and the connection must be firm and tight .
2. Before installing the valve, an external inspection must be carried out and the valve nameplate must be in accordance with the current national standard “General Valve Markings” GB12220.
3. For valves with a working pressure greater than 1.0MPa and acting as shut-off valves in the main pipeline, a strength and tightness performance test must be carried out before installation, and they can only be used after passing the test. During the endurance test, the test pressure is 1.5 times the rated pressure, lasting no less than 5 minutes, and the valve body and gasket must be free of leaks to be considered qualified. During the tightness test, the test pressure is 1.1 times the rated pressure; The test duration meets the requirements of GB50243.
V. Needle valve selection principles
1. For piping systems and devices with low temperature media, it is advisable to use low temperature needle valves with bonnet.
2. For the piping systems of catalytic cracking units in refining devices, lifting rod type needle valves can be used.
3. In devices and piping systems with corrosive media such as acids and alkalis in chemical systems, it is advisable to use all stainless steel needle valves made of austenitic stainless steel with polytetrafluoroethylene as the valve seat sealing ring.
4. In metallurgical systems, power systems, petrochemical devices and district heating systems with high temperature media, needle valves with metal-to-metal seals can be used.
5. To regulate the flow, needle valves with V-shaped openings, driven by helical gears and operated pneumatically or electrically, can be used.
6. For the main oil and natural gas transportation pipelines that need to be cleaned and buried underground, fully welded and full-bore needle valves are used; for those buried above ground, welded or flanged ball valves are chosen.
7. For transportation pipelines and finished oil purchasing and storage equipment, flanged needle valves are chosen.
8. For urban gas pipelines and natural gas pipelines, flanged and internally threaded needle valves are chosen.
9. In oxygen piping systems of metallurgical systems, needle valves with strictly degreased flange connections are advisable.
