There are many types of control valves and these valves are used for many industrial purposes. In a piping system, the control valves are inevitable and this valve can be used to start or stop the flow in a pipeline, it can also adjust the amount of flow. So in a piping system, the valve should work properly without any problem otherwise it could interrupt the process, so in order to keep the valve working without any problem we need to check the valve and test it and the valve must be maintained. So maintenance of a control valve should be done properly so that it would last longer and the process won’t be interrupted.
What is a valve leak and how can it be prevented?
A leak in the valve will be mostly a stem leak, or bonnet and flange leak. The stem leak can be mostly fixed by tightening the packing nut or gland. Mostly the bonnet and flange leaks are created by the over-tightening of the bolts or it could also happen due to the loosening of the bolts during the service strain. So the leakage is an important problem in a control valve and these problems can be avoided by testing the valve so a valve is required to be tested. Certain testing is required for control valves such as shell tests, seat tests, high-pressure tests, etc. All these tests are performed in a control valve to determine the quality of the valve and we could also know if there is a leakage in the valve during these tests. So before selecting a control valve for a certain application the valve testing must be done and it should be done according to the manufacturer standard.
How should we select a control valve for an application?
There are different types of control valve and each of these valves are used for different applications so we should select a control valve for the application which it is capable for other-wise there would be lot of problem in the control valve and frequent maintenance would be needed.
We need to consider certain data while selecting a control valve for the application
- Fluid temperature and operating condition must be checked
- The viscosity of the fluid
- The specific gravity of the fluid
- It must be selected according to the flow capacity
- We should consider the inlet and outlet pressure
- Pressure drop
- Shut off pressure difference must be considered
- Maximum permissible noise level
Why should we do the maintenance of a control valve?
The process plant profit is depended upon the efficiency of the control valve, so a control valve can be considered as a final control element so if it is not working properly then all the control system operation has gone in vain. The control valve should operate properly no matter how complex is the automation system without the proper operation of the valve it is not possible to achieve good quality products and profits. In order to achieve the proper valve efficiency, we must do certain factors and they are
- The valve must be selected according to the application
- Proper storage and protection must be done
- Installation must be properly done
- Proper maintenance
How to improve the reliability of the control valve?
Apply bearing oils at the top and bottom and this would prevent the wear and also provide good valve reliability. The valve bearings should be oiled once a month and this oil must be applied between the stopper and the bearing gap.
How to do the control valve maintenance?
The pipeline must be checked for any foreign material, so if there is any foreign material in the pipeline then it could damage the valve seating surface. This foreign material would affect the valve operation too. So in order to avoid this problem, we should clean the pipeline before installing the valve. The pipe flanges should be checked for a smooth gasket surface. Before installing a control valve it must be inspected properly for any damage. Also, we need to check inside the valve body for any foreign material.
Proper installation of the control valve
The control valve shouldn’t be installed in any position, mostly a control valve is installed with the actuator vertical and above the valve body. If horizontal actuator mounting is required then we should use vertical support for the actuator. Make sure the valve is installed properly so that the fluid flow will be according to the indicating flow arrow. Install the pressure taps upstream and downstream of the control valve so that the flow capacity and the pressure drop can be easily checked.
What are the types of valve maintenance ?
The control valve should be maintained according to the manufacturer’s instruction
Reactive maintenance
Reactive maintenance can be described as the maintenance that is done after any fault happens, the major problem of this type of maintenance is that we won’t be able to determine when this problem could happen. This type of maintenance is the most expensive and has the least efficiency. During this maintenance, if the parts that are required for the replacement then it would be a major problem we wouldn’t know which part is required till it happens.
During this type of maintenance subtle deficiencies will be unnoticed and untreated because there won’t be any clear indication of the problem. So in this maintenance, the fault could be determined while the valve starts leaking or it fails to stroke. We can determine the problem before it could result in a big failure with the help of the production feedback. But sometimes the valves are removed due to suspicion of the malfunction. If the large valves are going to be removed from the process then it could take a lot of time and if the fault is not in the valve then the removal time of the valve is for nothing.
Preventive maintenance
In this type as the name indicates a problem is prevented before it happens and this maintenance will be done by considering the failures that happened earlier. So this maintenance is conducted before any failure and this will be good for the control valve and the valve would work efficiently. But sometimes in this type of maintenance certain control valves will be repaired which doesn’t need it. So this must be done carefully if we do the regular inspection of a control valve then we could determine when the failure could happen to any of the control valve parts so this maintenance is very efficient and it is not costly when compared to reactive maintenance. The preventive maintenance must be done as recommended by the manufacturer
Predictive maintenance
This maintenance process is done in certain steps
Detection of the fault – Most of the time that is used for the maintenance is to detect the fault, so if the fault is detected then the next step is to discriminate the fault.
Fault discrimination – In this process the valve parts are checked to determine the cause of the fault and if it is detected then proper action is taken to repair it.
Process recovery – This step is done to fix the defect
Validation – this is the last step, in this, the valve will be checked according to the new condition and now the valve can be used in the process.
Corrective maintenance of the control valve
Corrective maintenance is done after the damage of the machine or the equipment. This type of maintenance is also called breakdown or run-to-failure maintenance. This maintenance is mostly done in order to overcome the failures or damage that is found during the period of preventive maintenance.
Corrective maintenance procedure
In order to do a corrective maintenance certain tasks must be done
Valve disassembling
The valve stud must be loosened and removed, during the removal of the studs, we must check for nicks and burrs on the threads which can cause a faulty torque reading. The bolts of the actuators must be removed and pull the actuator away from the stem. During the valve disassembling proper examination is needed.
Maintenance of the actuator
The actuator must be refurbished, according to the manufacturer’s instruction we should disassemble the actuator. The parts of the actuator must be cleaned, try to eliminate the rust and corrosion if it is present. Before reassembly, the actuator must be completely clean from particulates.
Maintenance of the positioner
The valve which has high vibration shouldn’t be mounted with an electronic positioner because it would affect the operation of the positioner. Check for the positioner leakage. We must examine the air leakage and relay replacement and it must be done if needed.
Post time: May-05-2022