Working of Temperature Control Valve TCV

A brief overview of temperature control valve:

• Temperature control valves (TCV) are known as De-super heaters, Attemperatures, or temperature regulators.
• They are referred to as common control elements used in the process control industry.
• Temperature control valves in process industries are an integral part of various applications, such as power generation units, oil and gas plants, and more.
• The temperature control valves are available in various models with different specifications.
• In every power plant, the temperature control valve is a popular type of valve used.
• TCVs are designed and used to control the temperature fluid in the system for efficient operations in the process industry.
• The temperature control valve controls the flow rate of fluid flowing through it mainly attemperation water and other process fluids,
• TCV also controls various process parameters such as temperature, pressure, and levels. 
• These valves are similar to other control valves however, these valves control process temperatures at a specific level. 
• These valves act as a heat exchanger and are used to control the temperature of the process fluids 
• These TCVs in cogeneration plants control the temperatures and ensure the cooling of the engine.
• Temperature control valves are classified based on the number of ports. A 2-way valve means the control valve has two ports. A 3-way valve means the control valve has three ports.

What types of temperature control valves?

Temperature control valves are of two types:

1. Thermostatic Control Valves or internally sensing valves:

• Thermostatic control valves are a type of valves that are actuated by internally sensing and controlling the f temperature of the fluid.  
• These valves are self-contained without any external power source. 
• These valves use wax to remain in a semi-solid state and are more sensitive to temperature variations.
• As fluid temperature changes, these valves cause the wax to expand or contract and to travel the valve stem up or down, for the opening or closing of the valve ports.     

2. Actuated Control Valves or externally sensing valves:

• An actuated control valve is a part of a complete system.
• These valves sense the change in temperature changes through the external probes.
• An external probe transmits a signal to a control panel for opening or closing of the valve ports through an external power source.
• These types of systems include electric, pneumatic, or a combination of both called electro-pneumatic valves.
• These valves are more accurate and require precise temperature control.
• If operating conditions are varied these valves allow flexible adjustment of the temperature range.

How does a temperature control valve work?

• Before knowing about the working of the temperature control valve,
• Understanding the valve structure is the most important concept.
• The temperature control valve includes four main parts
  1. Temperature detecting element.
  2. Sensor.
  3. Power source, and
  4. Controller.
• The temperature detecting element is responsible for sensing temperature signals and for sending either an electrical or mechanical signal to the actuator.
• The actuator receives this signal to act on the power source to determine the position of the valve.
• The temperature control valve comprises a filled bulb that acts as a temperature sensor.

Working principle of temperature sensor:

• The temperature sensors such as RTD and Thermocouple operate on different working principles such as thermal expansion property or see back, Peltier, or Thomson effect.
• In general, the material expands with rising temperatures.
• This thermal expansion generates stress on the valve actuator.
• The exerted pressure due to generated stress changes the position of the valve to control the flow rate of flowing fluid.

What are the schemes involved in temperature control?

The two popular temperature control schemes are shown below:

1. Mixing a cold and hot process fluid for controlling the temperature of the hot process fluid.
2. Heat exchange between the hot process fluid and cold process fluid.

1. Mixing of cold and hot process fluids:

• Generally, temperature control schemes use two basic process fluids a hot and cold fluid:
• Let us consider T1 as the temperature of the cold process fluid and T2 as the temperature of the hot process fluid.
• Hot process fluid is mixed with cold process fluid to maintain the desired temperature.
• As shown above the diagram, the temperature control valve is positioned in such a way that it can easily pass and mix the cold process fluid with the hot process fluid to maintain the temperature of the hot process fluid.
• This type of temperature control is physical and there must not exist any chemical reaction.

2. Heat  exchange between a hot process fluid and a cold process fluid:

• It is known as the heat exchanger scheme of temperature control.
• The process of heat transfer takes place along with the mixing of both cold and hot fluid process fluids
• The hot fluid is allowed to pass through the shell of the heat exchanger while the cold fluid is allowed to pass through the tube.
• The heat energy from the hot process fluid is transferred to the cold process fluid.
• But the temperature of both the fluids varied accordingly.
• However, the raised temperature of the cold fluid is managed by the temperature controller TC.

Temperature Control Philosophy:

• A temperature sensor and temperature transmitters are installed in the hot region of the process area.
• The temperature sensor RTD or T/C detects the degree of hotness or coldness of the process fluid or body.
• The detected signal is transmitted as a process variable to the temperature controller through the temperature transmitter.
• When the process temperature goes beyond set point SP, the temperature controller compares the received signal from the temperature transmitter TT with the given set point.
• The temperature controller releases an open signal to the TCV for maximum opening so that cold process fluid or water can mix up easily with the hot process fluid to minimize the fluid temperature.
• As long as the temperature of the hot process fluid rises and remains above the SP, the TCV will remain in the open condition to introduce more cold process fluid with the hot fluid until the desired temperature is reached.

How Are They Used?

• Both thermostatic and actuated valves behave the same function for mixing fluids of different temperatures. And for diverting fluids to a cooler, heat exchanger, or radiator.
• Both thermostatic and actuated valves operate in any position,
• These allow us to mount the valves based on the existing pipelines.

1. Mixing Applications:

When these valves are used for fluid mixing,

• Port A the common outlet from the cooler. It is a temperature sensing port where hot and cold fluids are mixed in the correct proportion to obtain the desired temperature
• Port B is the hot by-pass fluid inlet to the cooler
• Port C is the cold fluid inlet from the heat removal cooler.

2. Diverting Applications:

When valves are used for diverting services,

• Port A is the inlet temperature sensing port,
• Port B is connected to the cooler bypass line with
• Port C is connected to the cooler.