Safety valve calibration method and its advantages and disadvantages
To fully evaluate the technical performance of a safety valve, a complete performance test is essential. The fundamental requirement of such a test is to simulate actual operating conditions, and ideally, even exceed them. This type of testing requires specialized equipment such as high-temperature and high-pressure vessels, along with rapid measurement instruments capable of capturing various parameters. Additionally, it necessitates a reliable source of high-temperature, high-pressure steam with a large flow rate, which makes the process both complex and expensive.
Calibration is an integral part of the safety valve testing process and serves as a primary task during factory testing. Before leaving the factory, the set pressure and sealing tests are typically performed using a standard temperature calibration table for air media. However, this normal temperature calibration table is limited in its application and can only be used for setting the opening pressure and conducting sealing tests.
(1) Under what circumstances should a safety valve be verified?
1) When the valve has been stored for a long time or before its first use
2) During regular inspections
3) If the valve is seriously damaged or corroded
4) If the nameplate is missing
5) If the sealing mechanism is damaged
(2) Meaning and method of adjusting the reseating pressure
According to national standards, the reseating pressure refers to the point at which, after the safety valve has reached the discharge state, the medium pressure drops to a certain level, and the valve disc recontacts the valve seat, resulting in zero lift. The static pressure at the inlet and the reseating pressure must be carefully balanced. If too low, it may lead to medium loss and energy waste. If too high, it might prevent the valve from discharging properly, causing unnecessary oscillation. The goal is to increase the reseating pressure while minimizing medium loss and energy waste, ensuring efficient operation when the valve is able to discharge effectively.
(3) Adjustment methods
The reseating pressure is adjusted by modifying the gap between the valve components. A smaller gap increases resistance during discharge, requiring more force to return the valve to its seat. Conversely, a larger gap allows the valve to return more easily, thus increasing the reseating pressure. For safety valves equipped with a single lower adjustment ring, raising the ring decreases the reseating pressure, while lowering it increases it. For valves with both upper and lower adjustment rings, reducing the distance between them lowers the reseating pressure, whereas increasing the distance raises it.
(4) Calibration methods and their advantages and disadvantages
There are two main methods for calibrating a safety valve: on-site calibration (in-service verification) and calibration table verification. Whenever possible, on-site calibration is preferred because it better reflects real-world operating conditions, making it more accurate and reliable.
Advantages of on-site calibration include the ability to verify welded safety valves, measure reseating pressure accurately, and ensure precise results. However, it is time-consuming, requires repeated system pressurization, is costly, and poses some risks. It also cannot be used for sealing tests.
Advantages of the normal temperature calibration table include its ability to set and detect leaks in valves operating at temperatures below 250°C, reduce the error range in opening pressure, save adjustment time for newly installed valves, and lower labor intensity and work risk. However, it has limitations, such as not being able to measure reseating pressure due to temperature differences affecting spring stiffness.
Jinan Sett's safety valve calibration tables come in digital display and computer-controlled versions. The digital model can collect lifting and reseating pressures, set holding times, pressure levels, and pressure differences, and choose clamping force, enabling semi-automatic testing. In contrast, the computer-controlled version uses NI data acquisition software, offering advanced features, stable performance, and the ability to automatically complete the entire testing process on a computer, displaying pressure curves and allowing flexible parameter settings like test pressure, hold time, and acceptable pressure drop.
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**Labels:** Safety Valve Calibration Methods and Their Advantages and Disadvantages
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