Fastener selection series: selection requirements for petrochemical equipment fasteners

Petrochemical equipment mainly responds. High-pressure vessels, various cylinders, tanks, etc., are mainly characterized by contact with corrosive media such as acids, alkalis, and salts, and they must also withstand the test of various harsh environments. Chemical equipment, including fasteners, must operate in this internal and external environment. It is often required to work under high temperature (such as the design temperature of the hydrogenation reactor up to 500℃) or low temperature (such as the temperature of the liquid nitrogen process as low as -196℃), and it is often accompanied by other media (such as hydrogen, nitrogen, carbon monoxide and carbon monoxide). Uric acid) is strongly corroded. Therefore, people in the industry attach great importance to high-pressure containers and their sealing technology for chemical equipment.

(1) The importance of sealing technology

The U.S. space shuttle Challenger exploded shortly after it took off. The main cause of the biggest tragedy in the history of aerospace was the leak caused by the failure of the O-ring seal at the left side rocket booster connection. It was this small sealing ring that caused all 7 astronauts to die, and the space shuttle worth 1.2 billion US dollars was also wiped out.

According to statistics, in the daily use and maintenance of machinery and equipment, almost 40% to 50% of the workload of the pump is used for the maintenance of the shaft seal. Approximately 70% of the maintenance cost of centrifugal pumps is used to deal with seal failures. Among the reasons for the failure of centrifugal compressors, the failure of lubrication and sealing accounts for 55%~60%, and the sealing system accounts for 20%~40% of the unit price. American sealing technology workers believe that due to the development of sealing technology, only steam turbines can save 300 million US dollars in energy costs each year.

Therefore, preventing the leakage of machinery and equipment is one of the key issues that must be solved for industrial production. In the past century, a new discipline has been formed to study the laws of sealing, the design of sealing devices, and the scientific principles of use, called sealing science. In engineering, it has also developed into a specialized technical sealing technology. .

(2) Performance requirements of steel for high temperature fasteners

1. There must be a sufficiently high yield limit

In order to ensure the tightness of the high-temperature container, a certain pre-tightening force must be given to tighten the bolts and nuts. According to the operation of the thermal power plant

According to the experience of the industry, the pre-tightening stress is generally specified as 250MPa~350MPa. In order to ensure that the bolt does not yield when pre-tightened, it is important to select the room temperature yield strength of the fastener (bolt) steel.

2, there must be a higher relaxation stability

Relaxation stability is one of the important properties of fastener steel. The calculation of bolts uses relaxation stability as the strength calculation index. The requirement of bolt material for high-power UHV units in domestic power plants is 520℃, and the working period is 20000h.

3. There must be sufficiently high lasting plasticity

In order to avoid the brittle fracture of the bolt, the bolt steel is required to have a sufficiently high durable plasticity. It is generally believed that the durable plasticity of the bolt steel after high temperature and long-term operation is less than 3% to 5%, so as to slow down the brittle fracture of the bolt steel.

3, has anti-oxidation properties

It has a certain degree of oxidation resistance at working temperature to prevent seizure between bolts and nuts.