Archive for year: 2020

Air separation unit is a series of equipment that converts the air into liquid by means of deep freezing through the compression cycle and then separates the inert gases such as oxygen, nitrogen and argon through the distillation process. It is widely used in metallurgy, coal chemical industry, large scale nitrogen fertilizer, gas supply and other fields. The coal chemical industry puts forward higher requirements on the system performance and processing capacity of the air separation unit.

The air separation unit mainly provides high pressure and high purity oxygen and nitrogen. Oxygen with a purity of 99.6% is used as a vaporizing agent in the coal vaporization unit to react with coal and water under high temperature and high pressure in the vaporization furnace. The resultant syngas (CO+H2) is the raw material for the production of alcohol, ether, olefin, coal-to-oil, coal-to-natural gas, hydrogen and ammonia, etc., or for IGCC. Nitrogen with different pressure levels of 99.99% purity is used as an emergency shutdown safety nitrogen plug, raw material nitrogen, inert protection gas, pneumatic conveying gas and purge gas.

The large air separation unit consists of air compression system, air pre-cooling system, molecular sieve purification system, air pressurization system, pressurized turbine expander system, distillation system and heat exchange system, of which matched valves are directly related to safety, system performance and cost. The valves commonly used in the air separation plant are oxygen globe valve, eccentric butterfly valve, ball valve and high-pressure special relief valve.

 

Oxygen globe valve

Oxygen pressure can be divided according to the different gasification process and gasification fuel, one is 4.5 ~ 5.2MPa (medium pressure oxygen), the other is 6.4 ~ 9.8MPa (hyperbaric oxygen). Hyperbaric oxygen pipeline general choice oxygen with cut-off valve. The valve body chooses the good flame retardant performance, the friction impact will not produce the spark copper-based alloy or the nickel base alloy, the sealing material also chooses the difficult to burn or the flame retardant. Valve cavity runner needs to be polished smooth to avoid wrinkles; Valve should be degreased and tightly packed to prevent contamination; Large diameter oxygen globe valves also need to be set with small pressure bypass valve to ensure the safety of the open valve. For DN25 ~ DN250mm, pressure PN10MPa, and temperature from -20℃ to 150℃.

 

Hard seated butterfly valve

The liquid nitrogen reflux system and molecular sieve purification system in the distillation column mostly use three – eccentric type butterfly valve or three stem butterfly valve which is characterized by convenient operation, without friction opened and leakage, long life. The tri-eccentric butterfly valve is widely used in the expander system of air separation equipment because of its advantages of abrasion resistance, long service life and good sealing performance. Three-stem butterfly valve is a type of cut-off valve mainly used in thermal systems, power plants, steel mills and air separation units, which is suitable for clean gas medium (such as air, nitrogen and oxygen, etc.) and impurity gas containing solid particles. For DN100 ~ DN600mm, pressure PN6-63Mpa, temperature -196℃ ~ 200℃.

 

High-pressure special safety valve

To ensure the safe operation of the equipment, a safety valve can be installed on the pipeline as overpressure protection device. The valve opens automatically to prevent further increase of equipment When the pressure of equipment increases beyond the allowable value. When the pressure is reduced to the specified value, timely closing the valve can protect the safe operation of the equipment. Special safety valve is the safety protection device of hyperbaric oxygen pipeline, it can discharge the excess medium which may be produced in the system, its performance directly affects the safety and reliability of the equipment. For DN40 ~ DN100mm, pressure PN10MPa, temperature -20℃ ~ 150℃, opening pressure 4 ~ 10MPa, sealing pressure 3.6 ~ 9MPa, discharge pressure 4.4 ~ 11MPa.

 

In addition to the valve type, the material is also crucial for the chemical valve. Full port trunnion mounted ball valve can also be used in the molecular sieve system. The maximum temperature of polluted nitrogen after heated by the steam molecular sieve reaches 250℃ and the bidirectional sealing rings of DN200 and DN150 ball valves are made of high-temperature carbon fiber reinforced PTFE which can withstand 250℃.

Spring return valve refers to the valve that can return to the original starting position under the action of internal spring. It is suitable for 1/4 rotary handle operation of the ball valve, is generally composed of two/three pieces of the ball valve and a spring lever or handles unit to return the valve to the fully open position, also known as spring automatic return ball valve or spring self-closing ball valve. The spring return ball valves can be offered to include socket weld, butt weld and flanged, they are used in applications where positive return to the closed position is required following momentary or short periods of operation for food, pharmaceutical, oil, chemical, metallurgical, mechanical process and other industries. In addition, the spring return design has been used for gate valves and globe valves.

 

 

Details of the spring return ball valve

Size: Up to DN50

Pressure: Up to Class 600

Standards: API 608/API 6D

Test standards: API 598

Nominal diameter: DN15 — DN100 (mm)

Connection: Thread, flanged

Temperature range: ≤-180℃

Body material: Cast steel WCB, Stainless steel 304/316

 

Features

• Manual back to started position quickly and avoiding the wrong operation;
• Two-piece or three-piece structure is simple and easy to maintain, full port and low flow resistance.
• Stainless steel ball material, reduce the parts wearing and extend the service life.
• PTFE seat/packing rod offers good sealing performance, not easy to be medium corrosion or friction damage when fully opened or fully closed.

 

In past articles, we introduce “What caused the leakage of valve” and “The leak rates standards of the industrial valve”, today here we will continue to discuss the valve leakage class and classification.

ANSI FCI 70-2 is an industry standard for control valve seat leakage, specified six leakage classes (Class I, II, III, IV, V, VI) for control valves and defines the test procedure, and superseding ANSI B16.104. The most commonly used are CLASS I, CLASS IV and CLASS Vl. Metal-elastic seal or metal seal should be selected in engineering design according to the characteristics of the medium and the opening frequency of the valve. Metal seated valve seal grades should be is stipulated in the order contract, the rates I, Ⅱ, Ⅲ is used less due to request a lower level, generally choose Ⅳat least and V or Ⅵ for higher requirements.

 

Classifications of control valve seat (ANSI/FCI 70-2 and IEC 60534-4)

Leakage class	Maximum leakage allowable	Test medium	Test pressure	Test rating procedures	Valve type
Class I	/	/	/	No test required	Metal or resilient seated valves
Class II	0.5% of rated capacity	Air or water at 50-125 F (10-52C)	3.5 bar, operating differential whichever is lower	Lower of 45 to 60 psig or maximum operating differential	Commercial double-seat control valves or balanced single-seat control valves with a piston ring seal and metal-to-metal seats.
Class III	0.1% of rated capacity	As above	As above	As above	Same as class II, but a higher degree of seat and seal tightness.
Class IV	0.01% of rated capacity	As above	As above	As above	Commercial unbalanced single-seat control valves and balanced single-seat control valves with extra tight piston rings or other sealing means and metal-to-metal seats.
Class V	0.0005 ml per minute of water per inch of port diameter per psi differential	Water at 50-125F (10-52C)	Max service pressure drop across valve plug, not to exceed ANSI body rating.	Maximum service pressure across valve plug not to exceed ANSI rating	Metal seat, unbalanced single-seat control valves or balanced single-seat designs with exceptional seat and seal tightness
Class VI	Not to exceed amounts shown in following table based on port diameter.	Air or nitrogen at 50-125 F (10-52C)	3.5 bar(50 psig) or max rated differential pressure across valve plug whichever is lower.	Maximum service pressure across valve plug not to exceed ANSI rating	Resilient seating control valves either unbalanced or balanced single-seat with “O” rings or similar gapless seals.