What is Control valve stem?

What is a Control Valve?

A control valve is an electrical device that manages or modifies the flow of fluids such as gas, oil, water, and steam. It is an example of a final control element and a critical component of a control loop. Nowadays, the Control Valve is by far the most frequent ultimate control element.

Control Valve Selection Requirements

Parts of Control Valve

Parts of control valves are

  • Actuator Part
  • Body Part

Actuator Part

For valve control, it can be Pneumatic, Motorized, or Hydraulics, but the most popular in our industry is Pneumatic Actuator or controlled by the wind itself. Actuator basic structure for easy maintenance, including the Yoke.

Various components in actuators are

  • Rain Cap: it is used to prevent the flow of water to the control valve.
  • Eye Bolt: it looks like a hook is used to move the valve. It is on both sides of the control valve
  • Spring: The spring is located in the Yoke Actuator Case or, depending on the manufacturer’s design. It will act as a force for the Actuator Stem Cell as well as a powerful wind in the opposite direction of the plate Diaphragm.
  • Diaphragm: The diaphragm is a flexible component of the control valve. Its purpose is to change both the incoming air power and the power delivered to the diaphragm plate in order to cause actuator stem movement.
  • Diaphragm Case: It is referring to the packing components. The diaphragm plate is divided into two sections: upper and lower.
  • Scale Plate: The Scale Plate is based on the position of the valve between 0 and 100%.
  • Stem Connector: Actuator Stem and Plug Stem are connected via a Stem Connector.
  • Yoke: Yoke is a component that connects the Actuator and Valve Body portions.

What are Digital Valve Controller (DVC)?

Body parts:

Part of the Body Valve is included in the Bonnet valve, with which this section is directly exposed to the fluid (fluid), therefore selecting the required qualifications material (material) as well as, Temperature, Pressure, and so on.

  • Packing Flange: Packing Flange is used to compress the stud bolt so that all of the Gland Packing is tight and fluid does not seep out of the neck Bonnet.
  • Packing Follower: Packing Follower is the Packing Flange’s strength. Gland Packing is crushed tightly and will tighten over time.
  • Gland Packing: Gland Packing is necessary to prevent fluid from leaking up to the neck and is in direct contact with the Bonnet Plug Stem. There is a great necessity for the material and kind to fit. The majority of the materials utilized are PTFE or graphite, and they require regular maintenance. The requirement to change the Gland Packing at all times.
  • Gasket: A gasket is a device used to prevent leaks during the iron and steel assembly, as well as between the Body and Bonnet.
  • Guide Ring: In order to align Plug straight up, a Guide Ring is located in the Bonnet. Saving money on maintenance time is another reason to utilise a Guide Ring Bonnet instead of doing everything else. Because this component is constantly exposed to Stem Plug damage, the Guide Ring may be replaced as a complete rather than just the Bonnet Guide Ring.
  • Guide Bushing: Guide Bushing is used to support the Guide Ring once more.
  • Valve Plug: To use the force of fluid flow, a valve plug is required. And choose from Linear, Equal Percentage, and Quick Opening flow qualities.
  • Seat Ring: Seat Ring is a component of the Valve Body that, given the size of the valve’s Rated Cv, supports Plug, and Plug and Seat Ring must be near together. To be able to track Class Leakage as needed.
  • Valve Body: it is a key component of the round and comes into direct touch with the fluid. The pipeline’s link, As a result, the size and material must be chosen carefully.
  • Bonnet: It serves as a mounting platform for the guide and actuator, as well as a passageway for the stem. The centerpiece, packing, packing nut, and guide are all part of it. Between the bonnet and the stem, the packing serves as a fastener. It aids in the prevention of any leakage.

Working Principles of a Control Valve

A control valve actuator is an assembly that gives power to the control valve mechanisms. Actuators in a control valve loop move the plug, ball, or vane in response to a signal from the control system to allow or disallow full or partial flow. A Control Valve has three types of actuators. They’re

  • Pneumatic Actuated
  • Electric Actuated
  • Hydraulic Actuated

Pneumatic Actuated

Pneumatic Actuators provide a modulating control action by using an air or gas signal from an external source. Through a top port, the actuator receives the force of the pneumatic signal. The signal is then distributed across the actuator’s diaphragm. The diaphragm exerts pressure on the diaphragm plate as a result. This causes the valve stem to slide downward, causing the control valve to be stroked.

Electric Actuated

They’re powered by motors. They use an electrical signal to assist in the rotation of the motor shaft. This movement is translated to a linear motion, which aids in the driving of the valve stem for flow modulation.

Hydraulic Actuated

Hydraulic actuators work similarly to pneumatic actuators, with the exception that they employ hydraulic oil as the signal fluid to regulate the valve’s action. When the force required to move the valve stem is high, they are employed instead of pneumatic or electric actuated valves.

What is a Control valve? Purpose of a control valve in a process loop

What is a control valve stem?

The disc is positioned by the stem, which connects the actuator to the disc. Typically, stems are forged and threaded or welded to the disc. For valve designs that require stem packing or sealing to avoid leakage, the fine surface quality of the stem in the area of the seal is necessary. A stem is not commonly thought of as a pressure boundary portion.

The connection of the disc to the stem allows for some rocking or rotation to help with disc location on the seat. Alternatively, the stem may be sufficiently flexible to allow the disc to align itself against the seat. Constant fluttering or rotation of a flexible or weakly linked disc, on the other hand, can destroy the disc or its attachment to the stem.

Observation allows you to tell the difference between these two sorts of stems. When a rising stem valve is opened, the steam rises over the actuator. This happens because the stem is threaded and matched with the bushing threads of a yoke that is either an intrinsic component of the bonnet or is affixed to it.

A non-rising stem design has no upward stem movement from outside the valve. The valve disc is internally threaded and mates with the stem threads in the non-rising stem configuration.

How do Sliding Stem Valves Work ?

A sliding-stem valve body has moving elements that slide in a linear manner. Here are some examples of sliding-stem valve body designs:

The majority of sliding-stem control valves are direct-acting, which means that the valve expands as the stem is moved out of the body. A direct-acting valve, on the other hand, shuts off (closes) when the stem is pushed into the body. A reverse-acting valve body, on the other hand, would open up as the stem is pushed in and close up as the stem is taken out.