Internal leakage test of control valve in air separation plant

Internal leakage test of control valve in air separation plant

Commonly used control valves in air separation plants include normal temperature valves (mainly used outside the cold box), low temperature valves (mainly used in the cold box and low-temperature liquid pipelines outside the cold box), and high-temperature valves (mainly used in vaporizers, steam turbines, and steam injection). The steam valve at the inlet of the valve), the instrument engineer chooses a reasonable sealing method according to the sealing level required by the process. In many sites, the control valve has internal leakage. Internal leakage refers to the leakage at the contact between the opening and closing parts and the sealing surface of the valve seat. We often say that it is not tight. Leakage refers to the fluid flow through the valve under the specified test conditions and when the valve is closed. Leakage is an indicator to measure the internal leakage of a valve. The main causes of internal leakage are mostly as follows:

　　 (1) When designing: the actuator torque/thrust is too small, and the selected gasket is not suitable; the seal is deformed by temperature; the valve is working for a long time with a small opening, and the flow rate is too high, which will cause damage to the internals.

　　 (2) When assembling: the valve seat sealing surface does not require assembly; the valve seat is loose.

(3) During installation and commissioning: the soft sealing surface of the valve is burnt during installation and welding; impurities in the pipeline adhere to the sealing surface of the valve seat during purging, making the sealing surface not smooth; or the sealing surface is scratched by impurities The sealing surface is recessed.

　　 (4) During operation: the sealing surface is worn or corroded; the soft sealing material hardens after long-term use; it is damaged by high-speed fluid when the opening is small.

Although there are many reasons for internal leakage, in principle, internal leakage should be avoided when using a regulating valve. It is hoped that when the valve is closed, the leakage of the sealing surface between the valve core and the valve seat is as small as possible. Some process conditions even require adjustment. The valve must not leak. We should try our best to avoid the above possibilities in the process of design, selection, manufacturing, installation and commissioning. The internal leakage of the valve is discovered after the valve is commissioned and operated. It not only consumes manpower to disassemble and repair, but also affects the progress of the project, so As far as possible, the problem can be found before shipment from the factory. Involving valve internal leakage and our commonly used standards are:

　　1 IEC60534-4 (Industrial-process control valve-inspection and routine testing)

　　 is formulated by the International Electrotechnical Commission; two test procedures are defined:

　　 Test procedure 1: The test medium pressure is 3barg～4barg. If the test pressure is lower than 3.5barg, it should be within ±5% of the z* maximum working differential pressure specified by the buyer.

　　 Test procedure 2: The test differential pressure is within ±5% of the z* maximum working differential pressure before and after the control valve specified by the buyer.

　　2 FCI 70-2 (Fluid controls institute standard control valve seat leakage)

　　 is formulated by the American Institute of Fluid Control; four test procedures are defined:

　　 Test method 1: The test medium is clean air or water at a temperature of 10 to 51°C, and the test pressure is 3 to 4 barg, or ±5% of the maximum working differential pressure of z*.

Test method 2: The test medium is clean water with a temperature of 10～52℃, and the test pressure is within ±5% of the z* maximum working differential pressure, and it does not exceed ANSIB16.1, B16.5 or B16.34 at room temperature. The specified z* maximum working pressure, or some lower pressure according to individual agreements.

　　Test method 3: The test medium is clean air or nitrogen at a temperature of 10～52℃, and the test pressure is 3.5barg, or ±5% of z*max working differential pressure.

　　Test method 4: The test medium is clean air or nitrogen at a temperature of 10～52℃, and the test pressure valve z* is as large as the closing pressure difference or 3.5barg. Choose a smaller value.

From the above IEC60534-4 and FCI 70-2 standards, we can see that there are low-pressure test (3～4bar) and high-pressure test (z*large valve closing pressure). Usually, if there is no special definition when the buyer purchases, the valve is supplied For cost and test safety considerations, the supplier will test the valve for internal leakage at 3.5 bar. However, in practical applications, the medium pressure may be much greater than the 3.5 bar used in the test. For most valves (except for the valves with the upper inlet and the bottom outlet), as the pressure rises, the leakage will definitely increase. Then we can follow IEC60534 Remark 3 in -4 calculates the amount of leakage as a reference, and compares it with the amount of leakage under actual working conditions.

Considering that the control valve leakage standard IEC60534-4&FCI 70-2 only requires the sealing performance of the control valve at room temperature. Even if the valve has good sealing performance at room temperature, the sealing performance at low temperature cannot be guaranteed. It can be used according to the requirements of the process. The supplier conducts low-temperature test to check the sealing performance of the valve at low temperature. The low-temperature test uses -196℃ liquid nitrogen and helium gas is used for the test. Commonly used low-temperature leakage test standards are: BS6364 (British valve standard); EN1626 (Cryogenic vessels-valves for cryogenic service); EN12567 (Isolating valves for LNG); JB/T 7749 (technical conditions for cryogenic valves).

　　3 BS6364 (British Valve Standard)

It is defined in the standard: The valve seat sealing test should be carried out at the temperature of the valve body and the bonnet at -196℃, with helium gas at the rated pressure (1.1*PS) of the full-load seal capping, and the pressure is increased as required during the test. For the metal seal control valve , Z*The maximum allowable leakage should be 100mm3/s*DN; for soft-sealed valves, there should be no visible leakage during the test duration. But the standard does not define the leakage level.

　　4 EN12567 (Isolating valves for LNG)

　　 Although it is a test requirement for the shut-off valve of LNG, the low-temperature internal leakage test standard can be used for reference. When the temperature of the valve body and the valve cover reaches -196℃ and the test pressure of 1.1*PS, the leakage is not more than twice the value in Table 1.

　　Lmax is the z*large liquid leakage mm3/min under the conditions of test pressure and temperature, converted into gas:

　　QG: Gas leakage (under standard conditions), in mm3/min;

　　CEXP: Expansion coefficient (the ratio of gas volume under standard conditions to liquid volume under test conditions)

　　 For high-temperature valves, because of the limited test conditions, there is no standard describing the internal leakage test conditions and qualification standards at high temperatures. In practical applications, the method to determine the internal leakage of the valve is: 4-6 hours after the valve is closed, use an infrared temperature measuring instrument to measure the temperature of the valve stem (near the valve body) or the metal temperature 150mm downstream of the valve body. If it is greater than 70°C, it is regarded as "Internal leakage".

　　 In order to avoid internal leakage problems on site, we can require the supplier to do pressure test or low temperature test according to the actual working conditions, but the high pressure test and low temperature test are often expensive and affect the delivery cycle. The author believes that the following valve internal leakage tests are particularly important:

　　 (1) Valves with cold and heat isolation: If the valve is not closed tightly, cold capacity will be lost.

　　 (2) Involving safety six-stage sealed valves.

　　 (3) Valve for product venting: If the valve is not closed tightly, the device product will be wasted and the user will suffer economic losses.

　　 (4) The front and rear valves of the equipment need to be overhauled frequently: the front and back valves must be sealed and closed during equipment maintenance.

　　 When testing, maintain a certain fluid pressure in front of the valve, measure the leakage after the valve, and pay attention to the following points:

　　(1) Use the pressurized fluid input to the front of the valve to wash away dirt, dust and oil on the sealing surface of the valve seat.

　　 (2) After the actuator or valve positioner is tested with air pressure, the valve is operated several times and the valve is closed.

　　(3) The leakage can only be measured after the test pressure in front of the valve has stabilized.

　　 (4) The oxygen valve in the air separation unit cannot be tested for internal leakage with water pressure.

　　(5) Considering the safety of pressure test, internal leakage test should not be done together with function test.

　　5 Conclusion

　　 In the case of not affecting the use, it is unnecessary to put forward high requirements on the sealing performance of the valve, leading to increased costs and unnecessary waste.