Valve torque and actuator

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Valve torque refers to the valve the required twisting force when the valve is opened or closed, which is one of the main parameters for choosing the valve driving actuator. Close the valve between the opening and closing parts of the seat sealing surface to form a sealed pressure, but also to overcome the stem and packing, stem and nut thread, stem end support and other friction parts of the friction force, where the need for a certain opening force, its maximum is in the final moment of closure or the initial moment of opening. The opening torque of the manual valve should not exceed 360N•m, if exceeded, appropriate driving actuators such as electric, pneumatic and hydraulic should be considered. Valves should be designed and manufactured to reduce the opening/closing force and torque.

The opening torque is also known as the operating torque and can be obtained by calculation or measurement, or by actual measurement by the tools like a torque wrench. Electric and pneumatic actuators are available for 1.5 times the valve torque. When the valve opening torque is too large, the gear or worm gear transmission can be used for driving. The torque of different types of valves is different. There are three types of friction to be considered when calculating the torque of ball valve: ball and seat friction torque; Friction torque of packing on the stem; The friction torque of the bearing on the stem, how to calculate the torque of the ball valve? The total stem torque of the ball valve.

M=M1+M2+M3

M1: Friction torque between the ball and sealing surface of valve seat.

M2: Friction torque between packing and stem due to medium pressure.

M3: Friction torque on top of the stem.

 

In addition, comprehensive consideration of medium, material and other parts of the friction factor when calculating valve torque. Because there are so many types of disc, seat, and packing, they all have different friction, contact surfaces, compression, and so on. The calculated valve torque differs from the actual measured value and cannot be used as a guide. The exact value needs to be corrected in combination with the valve manufacturer’s test results.

The type of safety valve

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The safety valve is also known as the relief valve, when the pressure or temperature of the medium in the system or pipeline increases beyond the specified value, the safety valve discharges the medium outside of the system to protect the pipeline or equipment from exceeding the specified value. It is widely used in steam boilers, liquefied petroleum gas truck or liquefied petroleum gas tanker, oil Wells, steam power generation equipment of the high-pressure bypass, pressure pipelines, pressure vessels.

 

Classification of safety valve

The safety valve can be divided into a lever safety valve, spring safety valve, static weight safety valve and pilot safety valve according to the overall structure and loading modes. Spring safety valve refers to the valve of which disc seat seal by the spring force; Lever safety valve is operated by the force of the lever and heavy hammer; the Pilot safety valve is designed for large capacity, composed of the main valve and auxiliary valve.

 

Lever safety valve

The heavy hammer lever safety valve uses a heavy hammer and lever to balance the force on the disc. According to the principle of lever, it can use the weight of a smaller weight to increase the action of the lever to obtain a greater force and by moving the weight position (or changing the weight of the weight) to adjust the opening pressure of the safety valve.

Advantages: simple structure, convenient and accurate adjustment, the load will not be greatly increased due to the rise of the disc, suitable for higher temperatures, especially for higher temperature boilers and pressure vessels.

Disadvantages: Heavy structure, easy vibration and leakage of loading mechanism; low Return seat pressure and difficult to close and keep tight after opening.

Spring safety valve

The spring safety valve uses a compression spring to balance the force on the disc. The amount of compression of the coil spring can be adjusted by the opening pressure of the relief valve through the nut.

Advantages: Compact structure and high sensitivity, unrestricted installation position, can be used for mobile pressure vessels because of the small sensitivity to vibration.

Disadvantages: The load will change as the valve opens, that is, as the disc rises, the amount of spring compression increases, the force on the disc also increases. This is detrimental to the rapid opening of the valve.

The safety valve will reduce the elasticity due to the long-term high temperature, so the spring used in high or low-temperature situations should be fully considered the temperature of the spring on the deformation and the spring material creep or cold brittleness. In order to ensure the stability of the spring when working for a long time, the spring should be subjected to strong pressure treatments, and issued a strong pressure treatment, material, heat treatment report. Generally appropriate to use a cylindrical spiral compression spring, to ensure that the safety valve is fully open, the deformation of the spring is equal to the maximum deformation of the spring of 20%-80%, the maximum shear stress of the spring design is not greater than the allowable shear stress of 80%.

 

According to the way of medium discharge, the safety valve can be divided into fully closed, semi-closed and open safety valve.

Fully enclosed safety valve

The gas is discharged through the exhaust pipe and the medium cannot leak out when the fully enclosed safety valve exhausts. It is mainly used for the container of toxic and flammable gas.

Semi-enclosed safety valve

Gas from the semi-enclosed relief valve passes partly through the exhaust pipe and partly through the gap between the cover and the stem. It is mainly used for the container of gas which will not pollute the environment.

Open safety valve

The bonnet is open to allow the spring chamber to communicate with the atmosphere, which is conducive to reducing the temperature of the spring, mainly suitable for the medium of steam, as well as the atmosphere does not produce pollution of high-temperature gas containers.

 

According to the ratio of the maximum height of disc opening to the diameter of the relief valve port, the safety valve is mainly divided into low lift safety valve and full lift safety valve.

Low lift safety valve

The opening height is less than 1/4 of the diameter of the flow channel, usually 1/40 -1/20, proportional action process, mainly used for liquid and sometimes for small emissions of gas occasions.

Full lift safety valve  

The opening height is greater than or equal to 1/4 of the port diameter and the discharge area is the minimum cross-sectional area of the seat. Two-stage action process, it must rely on a lifting mechanism to achieve full open and mainly used in the gas medium.

 

The valve for air separation plant

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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℃.

What’s mud valve?

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Mud valve is a type of angel- globe valve controlled by a hydraulic actuator, used in sedimentation tank bottom for city water or sewage treatment plant sludge and wastewater discharge. The medium for mud valve is primary sewage less than 50℃ and its working depth is less than 10 meters. The mud valve is for low-pressure applications only and composed of the valve body, actuator, piston, stem and disc, which also can be controlled by the solenoid valve from a distance.

The mud valve supplied by PERFECT CONTROL is made by cast iron body, cover, and yoke, bronze seats with a resilient seat which forms a bubble-tight seal that won’t leak, even when minor debris obstructs the valve. The stainless steel stem is to prevent corrosion from years of submerged services. Mud valve can be generally divided into hydraulic mud valve and pneumatic angle mud valve according to the actuator. Double chamber diaphragm drive mechanism to replace the piston without movement wear. The hydraulic cylinder drive disc lift valve body channel open or closed to achieve fluid on and off.

The mud valve offers a lot of advantages: Cover with screw can be directed by the handle for shallow water; Tin-bronze sealing surface offers good corrosion resistance and better wear-resistant or use in submerged installations; The cast iron coating is corrosion resistant and safe for potable water applications; Hydraulic relief slots of plug stem allows any sludge to drain out so your valve won’t jam.

The mud valve is installed in the place where the discharge of sediment in the pipeline and the discharge of sewage during maintenance is needed, that is, the discharge tee at the lowest position of the pipeline and tangent to the sewage flow, and the impact of sewage erosion on the accessories shall be considered.

What’s spring return ball valve?

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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.