What Is The Difference Between A Pressure Control Valve and A Flow Control Valve

09 Jan.,2023


What Are Flow Control Valves?

The flow control valves are used for controlling and adjusting the fluid or gas flow through the system.

The flow control valve (FCV) is exactly the same except it gets its signals from a flow transmitter (FT). Just like the PT, an FT is basically a flow meter that is able to communicate with a related valve. Again, like the PCV, the FCV opens or closes based on the required flow and the signals it receives from the FT.

closed-loop control system

What Are Pressure Control Valves?

The pressure control valve is used to regulate the pressure of a fluid passing through the pipe.

The difference is exactly how it sounds. A pressure control valve (PCV) opens and closes based on the setpoint predetermined by an engineer. This is usually controlled using a pressure transmitter (PT). The PT is basically an electronic pressure gauge that sends signals to the PCV and tells it whether or not there is enough pressure entering the equipment and the PCV opens or closes to compensate.

control valves for boiler6

Ultimately, pressure and flow are not independent characteristics of fluid transport. You can’t change one without changing the other. But what you can do is monitor and control one, subjugating the other to follow. Pressure and flow control valves can look almost indistinguishable sometimes, except for the sensing mechanism, which is monitoring the variable you need in controlling.

A pressure control valve opens and closes depending on the setpoint foreordained by a specialist. This is normally controlled utilizing a pressure transmitter. The flow control valve is actually the equivalent with the exception of it getting its signs from a flow transmitter. Much the same as the pressure transmitter, a flow transmitter is essentially a flow meter that can interact with a related valve.

Pressure Control Valves V.S Pressure Regulators

How to Adjust Pressure Control Valves?

During the installation of pressure control valves, the engineers need to adjust control valves on-site. This is a significant step for commissioning. So we need to understand the various parts of the mechanical positioner and the working principle of the mechanical positioner. The following words will tell you all about the information on adjusting control valves.

Parts of Linear Mechanical Positioner for Globe Control Valve

ep positioner parts principle of linear valve positioner

When INPUT SIGNAL is supplied to the positioner to open the valve, power is generated
from ① the torque motor and pushes ② the flapper to the opposite side of ③ the nozzle.
The gap between ③ the nozzle and ② the flapper becomes wider and from the inner part of ④
the pilot left to ⑤ the spool, air inside ⑨ the chamber is exhausted through ③ the nozzle.
Due to this effect ⑤ the spool moves to the right. Then, ⑦ the seat which was blocked by
⑧ the poppet pushes the poppet away and supplied pressure (air) goes through ⑦ the seat
and OUT1 Port and enters into ⑩ the chamber of the actuator. Then ⑩ chamber’s pressure
will increase and when there is enough pressure inside the chamber to push the actuator’s
⑪ spring, the actuator’s ⑫ stem will start to go down and through the feedback lever, the stem’s
linear motion will be converted to span ⑭ lever’s rotary motion. This span ⑭ lever’s rotary
motion will then once again rotate ⑮ the span and pull the spring. When the valve’s
position reaches the given input signal, the span ⑯ spring’s pulling force and ① torque motor’s
power will be balanced and move ② the flapper back to its original position to reduce the gap
between ③ the nozzle. The amount of air being exhausted through ③ the nozzle will reduce
and left to ⑤ the spool ⑨ the chamber pressure will increase again. ⑤ Spool will move
back to its original position on the left and ⑧ the poppet will also move in the same direction
blocking ⑦ the seat to stop the air coming into the ⑩ chamber through the SUPPLY. As a
result, the actuator will stop operating and the positioner will return to its normal condition.

Wiring Diagram of Valve Positioner

wiring diagram of valve positioner

Linear Positioner Installation

Pneumatic globe type pressure control valve usually assembles linear positioner for diaphragm actuator or piston actuator.

installation of the electro-pneumatic positioner

Step 1: Please refer to the above wiring diagram of the electro-pneumatic positioner to connect power cables to the terminal.

Step 2: Check the positioner’s feedback lever position, it should be vertical to the valve stem at 50% of the valve stroke, same as the following picture.

The feedback lever for the control valvefeedback lever is vertical

Step 3: Connect air power to the diaphragm actuator, usually, it’s 2.4 bar, then send a 4mA input signal to see if the valve is closed, and 20mA signal valve is fully open. (Failure position valve close, air to open) When we set zero and endpoints positions are set, then the rest of the position will be automatically calibrated. (25%, 50%, 75% position)

If you want to know more about installation problems that happened on-site when you are commissioning or maintenance you can read the below post.

Industrial Valve Symbols

industrial valve symbols

Valve Actuator Symbols

valve actuator symbols

Control Valve Symbols by Function

control valve type

Common Valve Symbols

We provide various industrial valve symbols, including ball valves, gate valves, check valves, plug valves, diaphragm valves, safety valves, pressure regulators, solenoid valves, butterfly valves, and control valves. In case you will need it during the engineering fluid process.

common industrial valve symbols

Pressure Control Valve Symbol

pressure control valve symbol

Self Actuated Pressure Control Valve Manufacturers

Self-actuated pressure control valves are used to keep the adjustable set-point pressure within a certain range in tanks or processes. It doesn’t request any auxiliary energy to operate valves but by the medium energy itself.

Since the self-acting pressure control valves do not need air supply or electric power to operate, so it’s kind of the most cost-effective pressure control valves to achieve a simple process of pressure control.

steam pressure reducing station

Final Thoughts

Learn from this post, you already know the difference between a flow control valve and a pressure control valve. THINKTANK is a reliable control valve manufacturer who experiencing over 30 years in manufacturing control valves. So if you have any questions, just feel free to contact us. Also, you can read more posts about control valves as below.