The control valve is a device that is created or designed specifically to direct, start, stop, mix, or regulate the temperature, flow, or pressure of a process fluid. Mostly the control valve is manufactured by using certain materials like steel, iron, plastic, brass, bronze, etc. So in order to check the ability or quality of the valve we need to do the valve testing. So we should test the valve before it is used in the system, the test would assure that the valve would work properly. Manufacturers do certain tests on control valves in order to check their quality.
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- What are the problems in a control valve and how to do the troubleshooting and maintenance of a control valve?
- What is a single-seated and double-seated valve and how are they different from each other?
- What is a pilot valve and how does pilot valve work
- What is a directional control valve and what are the types of DCV
- How to troubleshoot a control valve and How to do the preventive maintenance of the control valve
Types of valve test
In this test, each valve will be operated more than once and this test will be conducted in a way that we could determine if these valves are suitable for the applications for which it is designed for. In this test, we can determine if all the parts of the valve are functioning properly or not. If the valve is not functioning according to the standard then it must be rejected. The faults in the valve must be cleared and the test should be conducted again till the valve perform properly in the application for which it is designed for.
In this test, the valve will be in the open position and a hydrostatic test pressure will be applied to the valve which will be twice its rated working pressure. So during this process, there shouldn’t be any leakage through the metal, joints, or the stem steals. This test is conducted to determine the ability of the valve during critical conditions. We can determine if the valve leaks or not by doing this test. So in order to do the test, the valve should be mounted on the test bench and the valve body will be subjected to the hydrostatic test. The pressure must not be dropped while conducting the test.
Shell test procedure
- Pressure will be applied to the valve and the valve will be in a half-open position
- Pressure will be increased gradually till the required test pressure is reached
- This test pressure will be maintained for a certain period of time and we must check if there is any leak
- The test pressure must be ambient pressure at a rated temperature
- The holing time of the test must be done according to the testing standard of the manufacturer
- There shouldn’t be any leak in the shell test
- After the completion of the test, the test pressure must be reduced to atmospheric pressure
- The water must be drained from the valve
- Remove the valve from the bench and use hot air to dry it
In this type of test, the test will be done from all the direction and this test will be conducted according to the operating pressure of the valve. This test is conducted to determine the sealing ability of the valve from both the flow direction. There shouldn’t be any leakage from the metal, or through the seats. The check valve seats are tested in the stopped-flow direction.
Seat test procedure
- This test will be done by bolting the test flange on both sides of the valve
- The tightening of the flange bolt must be done according to the manufacturer standards
- The valve must be kept in a horizontal position after fixing the blind flange
- A calibrated pressure gauge must be fitted on the pressurizing side of the flange
- Test pressure must be applied on the upstream side by closing the valve and measure the seat leakage from the downstream side of the valve
- The test medium for the low-pressure seat test should be air
- The valve must be in half-opened condition for the hydrostatic seat test
- The test fluid must be filled in the valve from the downstream flange side so that the air can be avoided. The test fluid temperature must be between 10 and 30 degree Celsius
- Seat test procedure must be done at the rated pressure by closing the valve and this pressure must be applied from the downstream side of the flange and the upstream side will be kept in atmospheric pressure
- If this test is going to do in a ball valve then repeat the above stems on another side too.
- The holding time of this test must be done according to the manufacturer standards
In the seat test for the globe and gate valves it would be done in the normal flow direction of the valve. In the check valves these tests will be conducted in the reverse flow direction of the valve. The seat test can be done in preferred direction for the ball and butterfly valve.
In this test, the valve will be torqued in both the open and closed positions to make sure that there is no stem or seating damage or a seal failure. The amount of torque that will be used for this test will be related to the valve standard.
The hydrostatic test is a combination of shell test and gland test. The gland test is conducted to detect the leakage in the actuator part. In order to check the gland leak in the air to close the control valve, the flange will be fitted on both sides of the control valve body and the air supply is removed and water will be supplied. In the air to open control valve this test will be conducted by fitting the flange on both sides of the body with air pressure and a certain amount of water pressure will be applied and this will be according to the valve standard.
This test should be done for most of the valves and this test is not needed for the bellow seal valve because these valves have the backseat feature. We can do this on those valves by applying the pressure inside the assembled valve with the valve end closed, the valve fully open. If the back seat test is going to be conducted after the shell test then the packing must be installed and we must retighten the packing glands after the backseat test.
Low pressure closure test
This test must be conducted by cleaning the sealing surface and it must be clean from oil, Greece, and sealant.
- The test flange must be installed at one side of the valve, one side will be injected with pressure air and the other side will be filled with test liquid
- The valve must be closed fully
- Air pressure must be injected into the valve after that snoop at the inlet point of the valve
- The valve should be at a vertical position
- Seat sealing surface must be attached at the outlet flange we must check if the bubbles are coming from the closure of the disk or seat. If there is no bubble then the valve is good
High pressure closure test
This test is the same as the low-pressure closure test and the only difference is that, during the liquid test, leakage should be detected when drops and not bubbles.
Fire-test & Antistatic device testing
These tests are special tests and they must be done according to the manufacturer’s agreed procedure.
What are the pressure test procedure?
The valves which are designed to permit the emergency sealant to the seating area must be tested with the injection system empty. If we use the liquid as a test medium then the valve should not have any trapped air. A protective coating is needed such as paint which could mask the surface defects. During the closure testing of the valve, we must not use excessive pressure to close the valve.
How to select the test fluid for the control valve test?
We must choose air, kerosene, inert gas, or water for the shell test, seat test, and also for high-pressure closure test. We should use inert gas or air for the low-pressure closure or for the low-pressure back seat test. When water is used for any test then there should be a water-soluble oil or rust inhibitor.
How can we check if the valve is leaking or not?
The valve leakage can be checked by using a hydrostatic test liquid or a pneumatic test gas. It’s not possible to get zero leakage