Pressure Regulating Valve (Cla-Val) - How to Determine Pressure at Discharge Outlet 2

[HernandezL]

I am running a hydraulic water model and I am trying to determine what I should set the discharge pressure at the pressure regulating valve (PRV) to ensure that the source is constantly delivering 3 cfs to the distribution system. The system is dynamic depending on demand whereas the PRV will open and close when there is change in pressure. I need the PRV to constantly remain open to deliver a constant flow. Does anyone have any tips or advice on to how to calculate or determine a theoretical pressure at the PRV to deliver a constant flow downstream?

 

[1503-44]

Use a flow control valve and the software should calculate the pressures up and downstream of the valve. You need to reconfigure the valve to open and close based on the signal coming from a flow indicator, not some pressure indicator. The same flow indicator that reads 3cfs right now. The valve's PID controller will calculate the control valve's position based on a flow rate you input into the PID and send the required position value to the valve's actuator, which moves the valve to that position.

Your software should have the capability to model either a pressure controlled valve or a flow controlled valve. There is no functional difference between the valves themselves. What makes them perform different functions is the signal that their PID controller receives. If the PID is comparing its calculation result to a signal from a flow indicator, the valve is called a Flow Control Valve. If the PID is comparing its calculation result to a signal from a pressure indicator, the valve is called a Pressure Control Valve. And yes, there are other control valves. If the PID is comparing its calculation result to a signal from a temperature indicator, the valve is called a Temperature Control Valve, etc. Etc.

Your software should allow you to specify which sensor the PID gets its signal from. Tell the PID to get the signal from the flow indicator that is reading 3cfs, Give the PID the control value you want to have, the value of 3cfs. The valve should then move to the proper position to do that. You should then be able to print or somehow see the pressures up and downstream.

Valve pressure drop is calculated from its valve coefficient, Cv, adjusted for the value at any given position it may have between full open and closed.

 

[bimr]

Cla-Val has many types of valves. You need to present all of the details if you want an opinion.

 

[LittleInch]

Please add a diagram.

However your description seems confused.

If you want a constant flow then you need to control on flow and not pressure.

Do as Mr 44 says.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Control valves can be controlled by devices other than PIDs, such as pneumatic mechanisms, or just a simple spring, or a tank level switch, however no matter what the mechanism is, your software should allow you to model a CV and automatically set its position based on whatever input variable you give it. It will compare that input value to the set value and output a corresponding valve position.

The software I use has some 15 different types of built-in input variable sensors
Press
Flow rate
Temperature
Density
Tank Level
Pump Speed RPM
Fluid composition, such as a percent of a treatment chemical in the flow stream
General Input Sensor, that one reads any variable that is tracked in the model, such as a pump running status, or even something like a pump vibration amplitude that is calculated at every time step by some specific formula I enter separately.

I can model system control logic with relays, which send a signal based on any number of variables' value, such as an AND, or OR relays. Those can be arranged to close a valve, if the pump is off, or open a valve if the pump is on and discharge pressure is greater than some set value. Or signal when a rate of change of some variable crosses a limit, or the ESD button is tripped signalling everything to go to shutdown position. I can group any number of signals and output a 0, 1 or one x other when all or none are true.

With all of that one can build a complex system that accurately mimics all control logic, with many automatically controlled, diesel driven pump stations, each with multiple pumps, that all run at the speeds that each pump needs to deliver 3cfs, hold some PCV at 1000psig discharge pressure and stay running until tank 101 reaches its high level. The limits of what you can model are only limited by the capability of your software package and how good the operating manual is.

What software are you using?

 

[danschwind]

This can only work if downstream from the valve your system remains fixed, i.e. at a constant pressure.

If that is the case, a simple pressure drop calculation with the desired flowrate from the downstream pressure up to the valve pressure sensing point (which can be in the valve body itself, or not) will result in the pressure needed to achieve that fixed flowrate.

If pressure upstream of the valve changes, then the valve will modulate to keep the pressure at its sensing point (which is downstream of the valve) constant, and therefore flowrate will remain constant.

But, as other have pointed out, if flowrate is the desired variable, then measure and control directly the flowrate.

Daniel
Rio de Janeiro - Brazil