Selection criteria for valve positioners

Why do we need to understand valve professional terms such as valve working pressure, valve working temperature, and valve test pressure?

Because in our daily valve selection, valve maintenance, valve maintenance, and valve replacement, we need to use the above-mentioned valve related terms. Otherwise, it is easy for us to choose the wrong valve or use the wrong valve because we don't understand the above names and related values, causing the entire equipment and piping system to fail to operate normally.

Is the nominal pressure of the valve suitable for pressure pipes within the nominal pressure range? This is not known to many users, even valve sales staff.

1. The working pressure of the valve

The pressure of the valve in the working state is called the working pressure of the valve. The working pressure of the valve is related to the material and working temperature of the valve. The maximum working pressure that a valve with the same nominal pressure can withstand at different working temperatures is different. The nominal pressure of the valve is usually based on the pressure at 200°C.

Nominal pressure PN (such as 1.6MPa): The reference value related to the pressure resistance of piping system components. For ease of use, the priority number of the R10 series is often used. Nominal pressure refers to the working pressure of the pipe at the secondary temperature (20°C). If the water temperature is between 25°C and 45°C, the working pressure should be corrected according to different temperature drop coefficients.

Nominal pressure PN is a numerically-expressed marking code related to pressure, which is a convenient round integer for reference. All pipeline accessories of the same nominal diameter indicated by the same nominal pressure PN value have the same connection size that is compatible with the end connection form.

2. Valve operating temperature

Before understanding the working temperature of the valve, we must first understand the rated temperature of the valve. The rated temperature of the valve corresponds to a certain pressure level. The temperature refers to the temperature of the pressure-bearing shell. Generally, the rated temperature of the valve should be consistent with the medium in the shell. The temperature is equal.

The temperature of the pipeline medium also has a certain influence on the valve:

1. At high temperatures, when used within the creep temperature range, the flange, bolts and gaskets are loosened, causing the bolt prestress to decrease. The flange connection may also cause this phenomenon under the condition of temperature gradient, thereby reducing The bearing capacity of the flange connection causes the valve to leak. Therefore, when the temperature rises, attention should be paid to the possible leakage of the flange connection.

2. At low temperatures, when the temperature is lower than -29°C, the toughness and impact resistance of some materials will decrease significantly. In this case, the operating pressure cannot be greater than the corresponding pressure value at this temperature.

E.g:

When a carbon steel globe valve with a nominal pressure of 2.5MPa is used on a pipeline with a medium working temperature of 400℃, the maximum working pressure that this carbon steel globe valve can use is 1.6MPa. Therefore, it is not the nominal pressure that the valve can be used in the pipeline of the pressure.

According to the maximum operating temperature and operating pressure of the medium used in the pipeline, if the maximum operating temperature of a certain medium is 370°C and the maximum operating pressure is 2.5MPa, according to the relevant parameters, this valve must choose a chrome-molybdenum steel valve.

3. Valve test pressure

The test pressure of the valve is different from the nominal pressure of the valve. The test pressure of the valve is a pressure specified for the strength test and sealing test of the valve. Valves with different nominal pressures have corresponding valve test pressures.

When the valve shell test has special requirements, the order should be placed according to the on-site technical conditions and related requirements.

The valve sealing and upper sealing test pressure is the test pressure for testing the sealing performance of the opening and closing parts and the sealing pair and the sealing performance of the valve stem and the bonnet.