Precautions for selecting valves for various working conditions

Release time:

2024-01-02 09:00

1. Matters needing attention in the selection of conventional valves

Valve use requirements

①Ordinary gate valves, ball valves, and globe valves are strictly prohibited for adjustment according to their structural characteristics. But in process design, it is generally used for mediation. Due to adjustment and use, the valve seals are in a throttling state for a long time, and impurities in the oil wash out the seals and damage the sealing surface, resulting in lax closing or because the operator has damaged the sealing surface to achieve the seal, causing the valve to pass or open. .

②The valve installation position is unreasonable. When the used medium contains impurities, no filter or filter screen is installed at the front end of the valve, so that the impurities enter the valve and cause damage to the sealing surface, or the impurities are deposited on the bottom of the valve, causing the valve to be closed loosely. Leakage occurs.

Considering from the perspective of process requirements

①For corrosive media, if the temperature and pressure are not high, non-metal valves should be used as much as possible. If the temperature and pressure are high, lining valves can be used to save precious metals. When choosing non-metal valves, economic rationality should still be considered; for media with higher viscosity, smaller flow resistance is required, and direct-flow stop valves, gate valves, ball valves, plug valves and other valves with low flow resistance should be used. Valves with small flow resistance have low energy consumption; when the medium is a special medium such as oxygen or ammonia, the corresponding special valve for oxygen or ammonia should be selected.

② Directional valves should not be used for dual-flow pipelines, but non-directional valves should be used. For example, after the heavy oil pipeline of an oil refinery stops running, the pipeline should be purged with steam in the reverse direction to prevent the heavy oil from solidifying and blocking the pipeline. Affect the efficiency of the valve, and it is better to use a gate valve.

③For some media with crystallization or deposits, stop valves and gate valves should not be used, because their sealing surfaces are easily worn by crystals or deposits. Therefore, it is more appropriate to use a ball valve or a plug valve; a flat gate valve can also be used, but a jacket valve is best.

④In the selection of gate valve, the single gate with exposed rod and the double gate with concealed rod are more suitable for corrosive media; the single gate is suitable for medium with high viscosity; the adaptability of double wedge gate to high temperature and deformation of sealing surface It is better than the wedge-type single gate, and will not cause jamming due to temperature changes, especially better than the rigid single gate.

⑤The valves on the general water and steam pipelines can be cast iron valves. However, if the steam is stopped in the outdoor steam pipeline, the condensate will freeze and the valves will be damaged by freezing. Therefore, in cold regions, the valve should be made of cast steel, low-temperature steel or effective heat preservation measures.

⑥For highly dangerous highly toxic or other harmful media, a bellows structure valve should be used to prevent the medium from leaking from the packing.

⑦Gate valves, globe valves and ball valves are the most used valves among the valves, and comprehensive consideration should be given to them when selecting them. The gate valve has strong flow capacity and low energy consumption of the conveying medium, but has a large installation space. The shut-off valve has a simple structure and is convenient for maintenance, but has a large flow resistance. The ball valve has the characteristics of low flow resistance and fast opening and closing, but the use temperature is limited. For petroleum products and other media with high viscosity, considering the strong flow capacity of the gate valve, gate valves are mostly used; and for water and steam pipelines, stop valves are used, and the pressure drop is not large, so the stop valves are used in water, steam and other medium pipelines. There are many applications, and ball valves can be used under the conditions permitting.

Considered from the point of view of ease of operation

①For large-diameter valves and long-distance, high-altitude, high-temperature and high-pressure occasions, electric and pneumatic valves should be used. For flammable and explosive occasions, explosion-proof devices should be used. For safety and reliability, hydraulic and pneumatic devices should be used.

② For valves that require quick opening and closing, butterfly valves, ball valves, plug valves or quick-opening gate valves should be selected according to the needs. General gate valves and stop valves should not be used. In occasions where the operating space is restricted, the exposed stem gate valve should not be used, and the dark stem gate valve should be used, but the butterfly valve is better.

Considering from the accuracy of regulating flow

When it is necessary to accurately adjust the flow rate, a regulating valve should be used. When the accuracy of a small flow rate needs to be adjusted, a needle valve or a throttle valve should be used. When it is necessary to reduce the pressure after the valve, a pressure reducing valve should be used, and when the stability of the pressure after the valve is to be maintained, a pressure regulator valve should be used.

Considering from temperature and pressure resistance

High-temperature and high-pressure media often use castings of chromium-molybdenum steel and chromium-molybdenum-vanadium steel. For ultra-high-temperature and high-pressure media, the corresponding castings should be considered. The comprehensive performance of forgings is better than castings, and the temperature and pressure resistance is also superior to castings.

From the consideration of cleanability

In the production and transportation of food and biological engineering, the requirements for valves on the process pipeline need to consider the cleanliness of the medium, and general gate valves and stop valves cannot be guaranteed. In terms of cleanability, no valve can be compared with a diaphragm valve.

①Diaphragm valve has a simple structure and various valve body and diaphragm materials. It can be widely used in the fields of food and bioengineering, and it is also suitable for some difficult and dangerous media.

A. Only the valve body and diaphragm are in contact with the flow, and the other parts are all isolated. The valve can be completely sterilized by steam.

B. It has its own cleansing ability.

C. It can be repaired online. Therefore, the diaphragm valve has become the most widely used valve in the food and biological engineering fields.

②Bottom valve.

In the case of strict sterilization requirements, the discharge valve at the bottom of the storage tank has almost no choice. The bottom valve is directly welded to the bottom head of the storage tank during equipment manufacturing. It is very different from the usual method of making a nozzle at the bottom of the tank and then connecting the valve to the nozzle. It is flat with the inner bottom of the storage tank, so it effectively gets along with the dead corners in the tank, so that all the liquid in the tank can be fully mixed during the fermentation process, and the special steam sealing system greatly reduces the product contamination. Possibility.

2. Matters needing attention in the selection of special valves

Special valve selection requires attention. Most of the fluid media used in the processing industry involves oil and gas streams of different corrosive levels. These flow conditions, including medium flow, are considered to be either clean, dirty, or worn (in mud conditions). The main difference lies in the number and amount of solid suspended particles that cause valve blockage or corrosion damage. type. In addition, media streams containing sulfur and other mixtures will help create a corrosive environment when combined with high temperatures. For such a medium flow, careful selection of materials is required to maintain the valve with sufficient service life. Corrosion engineers continue to research and develop materials to solve these problems.

Special valve for oil refining, hydrocracking and coking unit

Refining delayed coking unit is a process of deep thermal cracking of vacuum residue to produce gas, light distillate oil and coke. It is an important means for refineries to increase light oil yield and produce petroleum coke. The process is divided into two parts: coking and decoking. Coking is continuous production, and decoking is discontinuous production. The inlet and outlet of the heating furnace and the coke tower are connected by a four-way valve. The four-way valve is an important channel for switching the heating furnace into the coke tower. It is a special valve used in high temperature applications. Its quality directly affects the production capacity of the device. Most domestically imported four-way plug valves are used in both new designs and old devices, but they are expensive. The domestic four-way valve generally has the problems of unreasonable structure, unstable quality and easy failure.

Oil refinery hydrocracking is one of the main crude oil refining processes. Since the hydrocracking unit operates under high temperature and high pressure, the medium is flammable and explosive hydrogen and hydrocarbons, and the working conditions are special, so the seal must be reliable. Therefore, higher requirements are put forward on the design and structure of the valve. At present, most of the domestic use stainless steel wedge gate valves and direct-flow stop valves.

Valve for oil and gas

In order to control the oil flow, the oil and gas special valve should have the following basic properties: tightness, pressure resistance, safety, adjustability, fluid flow and switch flexibility. For high-pressure, flammable, and explosive oil and gas media, the tightness must be solved first, and the special working conditions of the oil and gas special valve must be considered:

①In the wet natural gas containing hydrogen sulfide and carbon dioxide gas, special requirements are put forward for the material of the valve body;

② There are brine, residual acid and other corrosive media in the wellhead equipment and gathering and transportation system, and the selection of valve body materials and anti-corrosion requirements;

③Dust and solid particles accelerate the erosion and wear of the valve closing parts. Make the seal pair quickly fail;

④Outdoors in plateaus, deserts and high-cold areas, low-temperature brittleness, bending deformation, etc. of valve materials;

⑤ The oil and gas special valve used in long-distance transportation pipelines requires the same life as the pipeline and will not be replaced for decades.

All these show that the special valve for oil and gas is different from the ordinary valve. It must have high reliability under harsh conditions and meet the requirements of high strength and non-leakage.

Chlorine condition

The selection of valves for chlorine-containing working conditions should refer to the "Dry Chlorine Piping System" compiled by the American Chlorine Society. The working condition containing chlorine gas or liquid chlorine is a highly corrosive working condition, especially when this working condition contains water. The HCI (hydrochloric acid) formed by mixing chlorine and water will corrode the valve body and internal parts. Since chlorine has a high thermal expansion coefficient, if liquid chlorine is sealed in the valve cavity, the pressure in the valve cavity will increase at a high speed. The valve used in this working condition should have a reliable cavity pressure relief function.

freezing (low temperature) conditions

Although the valves used in low temperature conditions are based on the ASME B16.34 standard and API standards, these valves also have other design features to ensure that they have a certain degree of reliable operation in low temperature conditions. Such a valve may also include a bonnet extension design that extends the distance between the packing and operating mechanism and the cryogenic fluid, thereby allowing the valve stem packing to be operated at a higher temperature and ensuring that the valve operating device will not be frozen in use . MISS SP-134 provides some details including the extension design of the bonnet.

Hydrofluoric acid condition

Valves used in hydrofluoric acid conditions should only be limited to the types of valves that have been demonstrated in use or can successfully handle such conditions in testing. Valves that generally do not provide opportunities for solid matter accumulation are the preferred valve types. The hydrofluoric acid treatment operation should be carried out by qualified technicians who strictly control the valves on the market.

For these (typical carbon steel valves with special Monel internals or solid Monel internals) the design and material requirements and the details of the internal geometry are very detailed. This valve should be designed to be resistant to hydrofluoric acid Corroded special structure. In hydrofluoric acid operating conditions, valve inspection and testing should be higher than the standards used in typical process valves.

Hydrogen condition

Valves used in oxygen-containing conditions should follow the American Compressed Gas Association standard CGA G4.4-2003 "Oxygen Piping System" when applicable. Valves used in such conditions should be completely degreased, clean and installed under clean conditions, and properly packaged and sealed, because oil and grease are extremely flammable in the presence of oxygen. The relevant guidelines are given in the cleaning equipment for CGA G4.1 oxygen conditions. Proper handling and storage are necessary before installation.

Bronze or Monel valve body and trim materials suitable for oxygen-containing conditions are often used to prevent sparks and fires due to high-energy mechanical collisions. There are specially formulated silicone greases for use in oxygen-containing conditions, because standard hydrocarbon lubricants should not be used in the presence of oxygen.

pulsating or unstable flow

Special consideration should be given to the selection of check valves used in pulsating or unstable flow. For example, check valves used in reciprocating compressors may be opened and closed quickly as the flow changes, which may cause Hammering and damage to the valve. There may be different opinions on the types of valves used in pulsating and unstable flows, but generally butterfly check valves, swash plate check valves and axial flow check valves are recommended for pulsating or unstable flows.

Acid-containing working condition (wet H2S working condition)

The use of valve materials in acid-containing conditions should comply with NACEMR0103 standards. This standard for downstream hydrocarbon processing industries restricts the hardness of all steels; requires solid solution annealing of austenitic steel; prohibits the use of certain materials for pressure-bearing parts (including valve stems); and proposes special requirements for bolted connections, welded valves, etc. Require.

Should pay attention to the user's responsibility in NACE MR0103, which stipulates that the user should specify in detail whether the bolt is exposed to the environment containing H2S. Unless specified by the user, bolts that are not inside the valve, such as bonnet connecting bolts, are often subject to product standards, and sulfur-containing working conditions are not included in such standards. If the bolting material does not directly withstand the process fluid, then the valve body-bonnet bolting does not need to meet the requirements of NACE. If any sulfur leakage of sulfur-containing oil cannot be ruled out or evaporated (such as a shut-off valve), then the bolted connection should comply with NACE standards.

If the materials allowed by NACE are deemed unnecessary, then the bolted connection materials should be of special concern. This imposed hardness requirement will result in a decrease in strength. The strength of the bonnet connecting bolts will be reduced, which may not be suitable for the same design conditions as the standard bolting materials.

Viscosity or curing conditions

Valves used in viscous or solidified conditions, such as liquid sulfur or heavy oil. In order to make the valve operable, steam tracing or steam jackets are often required to maintain sufficient operating temperature. Because the delayed response of the check valve can cause operational problems, special attention should be paid to it.