R. Bhattacharya, MCPI Haldia, West Bengal, India
Control valves play a critical role in manufacturing units as a final control element. Each valve’s performance impacts production quality and quantity. However, it can also have detrimental effects on a plant’s environmental footprint, as poor control valve maintenance can lead to the escape of fugitive emissions. An undersized or oversized control valve not only limits the valve’s operation, but it also becomes responsible for the loss of mechanical/electrical energy produced in the plant. An optimally designed control valve is of paramount importance for an instrumentation engineer. It is also important to adopt a good maintenance strategy and maintain an optimum level of spare inventory to ensure continuous, uninterrupted operations.
Valve maintenance strategy. All valves installed in a unit must be scheduled for periodic maintenance. The strategy for periodic maintenance of valves is generally classified into the following types:
Preventive maintenance (minor): Valve stroke checking over the full range, and attending to noninvasive-type maintenance jobs, among others
Preventive maintenance (major): Complete overhaul of the valve’s body and actuator
Predictive maintenance [look, listen, feel (LLF)]: Physical observations of a valve’s condition during a plant patrol
Shutdown valves used in safety integrity level (SIL) applications: As per SIL guidelines of the International Electrotechnical Commission (IEC).
The type of maintenance to be selected for a particular valve and its frequency depend on valve criticality and application/service. The criteria for deciding valve criticality may have the following bases:
Supercritical valves:
Valves used in SIL-2 and SIL-3 applications
Valves used in frequent operations in areas such as pressure swing adsorption (PSA) and continuous catalyst regeneration (CCR) units, and polymer service, among others
Valves used in hydrogen service, high-pressure steam service or any corrosive/abrasive service
Critical valves:
Valves used in a plant’s safety instrumented system (SIS), other than SIL-2 and SIL-3 applications
Valves operating with a pressure drop of more than 20 kg/cm2
Cylindrical actuators of furnace dampers
Valves used in supercritical machines
Semi-critical valves:
Valves used in critical control loops (i.e., loops that cannot be operated in manual mode or that cannot be taken in bypass mode for prolonged periods) in which a field bypass line or handwheel operation facilities are not available
Valves that operate with a pressure drop of more than 10 kg/cm2
Non-critical valves: All other valves that are not included in the above categories.
General guidelines for which maintenance strategy to adopt for a particular valve are detailed in TABLE 1.
Detailed activities during maintenance. The following are tips when undertaking maintenance:
Preventive maintenance (minor): Check the valve fail-safe action and the performance of the air filter regulator (AFR), and set the pressure indicated in the control valve’s data sheet.
Procedural steps
File appropriate work permits and get the necessary approvals.
Inform the operations engineer/panel controller regarding the control valve stroke check activity that will be completed during preventive maintenance activities.
Inform the operations engineer/panel controller of any interlocks related to the valve, and force/bypass the related logics following the appropriate procedure.
Check the pressure in the AFR and confirm it matches the air pressure listed in the valve’s data sheet.
Using a snoop liquid leak detector (or any other leak detector), check the air tubing and all connections for air leakage.
Open the control valve positioner cover and check the tightness of all cable connections. If an I/P converter is used, check this also.
Check the supply voltage.
If the feedback to the valve positioner is through the mechanical link, check its condition. If the mechanical link is corroded, then replace it. In addition, check the other valve accessories’ conditions (e.g., the air volume booster, trip valve, quick exhaust valve).
Isolate the instrument’s air supply, and vent the remaining air in the supply line to check the control valve’s fail action.
Restore the instrument’s air tubing after returning the valve to normal operations, and check the fail action after removing the signal cable.
Ask the control room operator to give commands from 0% to 100%, with increments in the steps of 0%, 25%, 50%, 75% and 100%.
Similarly, check the valve stroke and decrease it from 100% to 0% in steps. If there is no facility to provide input from the control room, then connect the HART communicator or mA source to the control valve to give commands from the field directly.
Record the values in a table (see TABLE 2).
If the valve command vs. travel and feedback is not satisfactory, then calibrate the control valve.
Apply a calibration sticker on the control valve with the appropriate due date.
Clean the control valve.
In the case of a solenoid valve connected to an emergency shutdown (ESD) valve, the resistance of the DC supply solenoid coil should be checked and compared with an original equipment manufacturer (OEM) manual to detect any possible degradation in prolonged operations. Similarly, the switch operation/action should be verified during preventive maintenance.
Liquidate all abnormalities of the valve observed during plant patrol inspection rounds that could not be completed during plant operating conditions.
Normalize the logic if forced and close the work permit.
Fill up the calibration record.
Preventive maintenance (major): The following are steps regarding valve overhauling and servicing:
Dismantle the valve body and actuator. Change the valve trim wet parts (if they are beyond refurbishing) and the actuator’s soft parts.
Hydrotest the valve’s body after assembly.
Conduct a valve seat leakage test.
Calibrate the valve.
Valve LLF: TABLES 3–5 should be used for notations during scheduled field patrols. Each column should be filled with a “Y” (OK) or an “N” (not OK).
The maintenance practices and strategies specified above do not always provide a guarantee of uninterrupted valve operation. Certain sudden failures of valve components and accessories that can lead to an outage may be encountered due to aging. It is recommended to replace the key components of aging valves before they start to malfunction. TABLE 6 provides a guideline on replacement frequencies.
At times, a lack of a valve’s spare inventory can affect plant operations. A well-defined process for valve spare parts maintenance must be established to manage an optimum supply of valve spares to ensure that plant operations are not affected. TABLE 7 provides some general guidelines to ensure adequate control valve spares are on hand, if needed.
Takeaway. The recommended maintenance practices listed in this article are to be treated as guidelines. Many production units use asset health management software packages supplied by the valve manufacturer. These applications collate many process inputs from the field and infer a real-time signature related to valve health parameters. These signatures are then compared with the baseline data from the manufacturer to predict the early deterioration of the valve. This system may be a good proactive maintenance assisting tool for control valves. However, for older facilities, the above-mentioned guidelines for valve maintenance may be useful. The same should be optimized based on the nature of the plant. The treatment for SIL-compliant valves should be guided by the standards provided by the IEC or by the term set by local regulatory bodies. HP
Ranjan Bhattacharya is a Technical Advisor for MCPI (West Bengal, India), a producer of purified terephthalic acid in India. He has 40 yr of experience in plant maintenance, project engineering and workforce development. He is also the former Head of Instrumentation for the naphtha and gas cracker complexes of Reliance Industries Ltd. and Haldia Petrochemicals Ltd.