Valves

What are the factors that affect the performance of the Control Valve?

  • In every process industry, the performance of a control valve is an essential parameter.
  • To achieve world-class reliability, & maximizing overall process efficiency the manager or higher authority of process industries realizes that the control valve performance is an essential element.
  • These control valves in the process loop actually move the valve stem to adjust the process performance which is critical.
  • Selecting the right final control element of the control valve is the most essential concept to achieve excellent performance for the required application to maintain its performance over time.
  • The valve performance is influenced by various process parameters such as Flow, Level, Pressure, temperature, and so on.

What are the factors that affect the performance of a control valve?

What are the factors that affect the performance of the Control Valve?1
  • Performance of the Control valve is a complex interaction of these factors, 
  • Careful consideration of each factor is essential to confirm the operation of the control valve as required furnishing process control as desired
  • However continuous monitoring & periodic maintenance are essential to maintain the performance of the valve over time.

Control valve performance is affected by several factors as shown below.

Flow Characteristics: 

  • The selection of flow characteristics must meet the process requirements to establish accurate control.
  • Various control valves betray varying flow characteristics like 
  1. Linear flow characteristics,
  2. Equal percentage flow characteristics, 
  3. Quick opening flow characteristics. 

Valve Sizing: 

  • The size of the control valve with respect to the flow rate of the process fluid is very crucial. Here to achieve rated flow control proper valve sizing is essential.
  • Since a control valve with improper size results in poor control, cavitation, inadequate control range, or excessive pressure drop.

Valve Type: 

  • Various valve types such as globe valves, butterfly valves, ball valves, and plug valves offer individual flow characteristics suited for specific process applications.
  • These valves have well-defined designs, & mechanisms that influence their performance.
  • However, selecting the right valve must include various factors such as flow rate, pressure drop, & control precision which are the most essential for exclusive performance.

Valve Trim: 

The design and configuration of the valve trim, including the size and shape of the plug, seat, and other internal components, can affect the valve’s control accuracy and rangeability.     

Pressure Drop: 

  • Usually, the pressure drop across a control valve is defined as the difference between inlet pressure and outlet pressure.
  • Excessive pressure drop across the control valve arises in reduced efficiency, poor control, & minimized capacity of the system.
  • However, actual consideration of the pressure drop across the system is essential.
  • Excessive pressure drop can lead to cavitation or erosion, while low pressure drop may limit the valve’s control authority.

Fluid Properties: 

  • The properties of the fluid being controlled such as fluid viscosity, density of fluid, temperature, & corrosiveness may influence the performance of the valve.
  • For this purpose, some kind of process fluids need some special valve design and trim material to avoid degradation or erosion of the valve components.

Cavitation and Flashing: 

  • Valve Cavitation and valve Flashing are unacceptable phenomena that exist in a control valve due to quick drop in pressure.
  • Due to these phenomena, the control of the valve may be reduced and might be damaged.
  • Cavitation and Flashing of the valve can be diminished by selecting a proper valve, and accurate sizing.
  • Cavitation in the valve transpires when fluid pressure reduces below vapor pressure leading to the origination & breakdown of vapor bubbles.
  • Valve Flashing on the other hand is somewhat similar to valve cavitation presumes immediate vaporization of the part of the liquid.
  • But Both Cavitation and Flashing phenomena can deteriorate the valve performance, and damage the valve.

Valve Actuator: 

  • The valve actuator is an integral component of the control valve that accounts for positioning the valve stem to open or close in response to the control signal received by the controller. 
  • The speed, precision, and control are the most essential parameters for overall performance.
  • Poor actuator performance arises from stagnant response times and inaccurate control.

Valve Positioner: 

  • The valve positioner is equipped with control valves to improve the control accuracy by adjusting the position of the valve position in response to the control signal received by the controller.
  • Here type of valve positioner & performance are the most essential factors.
  • An electronic controller functions in conjunction with & a valve positioner to regulate the position of the valve with respect to the desired setpoint.
  • The quality and tuning of the valve positioner play a notable part in achieving accurate control.

Instrumentation and Control System: 

The performance of the system is made up of a controller and feedback loop that affect the valve’s quality in order to keep it at the intended setpoint. The total effectiveness of the control valve can be affected by the performance of the control system, including the level of accuracy of the sensors, controller tuning, and component communication.               

Maintenance and Calibration: 

Regular maintenance and calibration of control valves are essential for consistent performance over time and inspection of control valves are essential to ensure they continue to perform optimally over time.

Maintenance and Wear: 

  • Regular maintenance is essential to resume proper valve operation over time.
  • Wear and tear of valve components reduces the performance over time.

Environmental Factors: 

Ambient conditions, like temperature, humidity, and exposure to a corrosive environment influence the performance & life span of the valve.

Piping Layout: 

The positioning of pipelines and necessary fittings around the control valve may impact pressure drops, and flow patterns that can affect the system performance.

Noise & Vibration: 

  • Since Control valves generate irritable noise and vibration, 
  • The management or periodic maintenance is essential to avoid the occurrence of any damage or any issues to nearby field instruments and personal safety. 

Safety and Reliability: 

  • Basically, equipment safety, & reliability are essential and predominant for critical processes.
  • Control valves are designed, & maintained to satisfy safety standards & to reduce the occurrence of failure risks and damage to equipment     .

Operating Conditions:

The control valve used must have the capability to handle the discrepancy of process conditions such as changes in the rate of fluid flow, temperatures, composition and pressures can impact valve performance. Properly designed control strategies can help mitigate these effects.

Material Compatibility: 

The material used to construct the valve should be congruent with various kinds of process fluid to intercept corrosion or degradation.

In applications with abrasive or corrosive fluids, erosion and corrosion can degrade valve performance over time. Proper material selection and maintenance are critical.

Mechanical Stiction: 

  • Usually, Mechanical friction refers to static friction to intercept the valve from smooth movement for a small or low control signal applied. 
  • This can help to manage inaccuracies and oscillations.

Piping Design, Installation, and Orientation:

  • To achieve excellent performance of the valve an accurate installation and orientation in the pipeline is most essential.
  • In most cases, an improper installation and piping design may initiate fluctuation in pressure, miss-alignment, and turbulence which also influences the performance of the valve. 

Tuning of the Control loop: 

  • The tuning of the control loop comprises (PID) Proportional (P), Integral (I), and derivative (D) settings 
  • This loop tuning improves the ability of the valve to establish the desired setpoint accurately.

People also ask

Which valve type offers the highest performance?

Basically, Butterfly valves are considered the best value for their high performance.

How to improve the performance of the control valve?

The control valve positioner offers several performance-related features to enhance the response of the control valve to process set point and load variations 

The performance of the control valve can be improved by accurate positioning of the valve sliding stem, or rotary shaft at the time of actuation.

How does the control valve affect the performance?

Higher or lower valve clearance leads to poor performance & operates at peak efficiency, 

Large valve clearance signifies that the valve may clatter and cause damage to the valves.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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