3 replies to this topic

[datjohn]

Just seeking clarification, 

 

Please, how different is an FCV (Flow Control Valve) from a PCV (Pressure Control Valve) on a liquid service.

 

I mean, an FCV regulates fluid flow rate by adjusting the flow obstacle (i.e. valve orifice). But can this be achieved without the fluid pressure being affected?

Most certainly not! Any restriction to flow, contributes to pressure loss.

So, does this make an FCV also perform the work of a PCV as well? 

 

Also, can a PCV adjust pressure without affecting the flow rate of the fluid?

 

Basically, how different are these two valves? Because it seems that the controlling parameters in both valve is the same i.e, flow and pressure at the same time though nomenclature separates them (FCV, PCV).

 

 

I need expert clarification.

Thanks.

 

[IGC]

What about a temperature control valve

 

You could have the same valve body but the function be different depending on what the input is.  You could re-use a valve from flow control service and put it into pressure control service for instance, it would be up to an engineer to decide if the opening% vs flow coefficient is applicable or not.  

 

For a control loop you need an input (sensor), a controller (to compare between input and process set point), a response (output from controller) and a control element (valve adjustment).

 

A flow control valve would take information from a flow transmitter and the controller would open or close the valve in order to move the flowrate towards the required setpoint.  The pressure will adjust accordingly as the valve position has changed.

A pressure control valve is the opposite.  It looks to move the pressure to the required setpoint and the flow will adjust due to the increase/decreased opening of the valve.

[datjohn]

What about a temperature control valve

 

You could have the same valve body but the function be different depending on what the input is.  You could re-use a valve from flow control service and put it into pressure control service for instance, it would be up to an engineer to decide if the opening% vs flow coefficient is applicable or not.  

 

For a control loop you need an input (sensor), a controller (to compare between input and process set point), a response (output from controller) and a control element (valve adjustment).

 

A flow control valve would take information from a flow transmitter and the controller would open or close the valve in order to move the flowrate towards the required setpoint.  The pressure will adjust accordingly as the valve position has changed.

A pressure control valve is the opposite.  It looks to move the pressure to the required setpoint and the flow will adjust due to the increase/decreased opening of the valve.

 

What I make out of this:

 

1. Control valves (FCV, PCV, TCV, LCV, etc) are not practically different by mode of valve operation (The valves are all same in construction).

 

2. The functional difference between say an FCV from a PCV comes from the loop in which the valve is in (A random control valve in a pressure loop becomes a PCV, and it could as well be a TCV if it's in a temperature loop).

 

3. The control valve ITSELF does not specifically alter the control variable alone. Other variables are indirectly affected in consequence (eg, an FCV maneuvers Flowrate directly by impacting restriction along the flow path while Pressure is indirectly affected due to pressure drop as a result of the restriction).

 

4. In the case of a TCV or an LCV, the control variable (T or L) is not directly adjusted at the valve point. Rather, its the consequence of meddling with flowrate/pressure at the valve point that affects the control variable at a different point.

 

 

Many thanks for the clarification.

Edited by datjohn, 14 January 2021 - 06:00 AM.

[IGC]

https://www.spiraxsa...rature-controls

 

The examples on there for both temperature and pressure will help to explain it further. 

 

The self-acting valve is a small control loop - you still have an input (sensor), a controller (adjustment knob), response (via capillary tube) and control element (spring loaded actuator).