IanVG
Mechanical
- Jan 21, 2022
- 76
Hey everyone, I am trying to understand and then build an excel sheet to help me select control valves (just regular pressure dependent control valves).
The kinds of systems I work with have already been built a long time ago and are fixed flow at the moment. There are several projects coming up where we will begin to add control to allow the pump modulate and add control valves for the reheat coils at terminal units/AHU's to modulate flow based on the demand of the terminal unit. Fairly simple. Now, several sources I have seen/read guide me on selecting a control valve guide me first on finding the pressure immediately upstream of the control valve and immediately downstream of the control valve. My system does not yet have a control valve. This is confusing for me, because if I were to go to the trouble of calculating the pressure drop starting at the outlet of the pump through all the components to where I intend to put the control valve and then the negative pressure drop (pressure gain) starting at the inlet of the pump working backwards to where I will put the control valve- I get the same pressure!
Do these guides just fail to mention that I need to assume/guess a control valve that I will put in the system that causes some arbitrary pressure drop? That delta P just comes out of nowhere in my opinion, without any adequate explanation. One website here, does a fairly good explanation of the topic, but still defines the pressure drop across the control valve, as what the pressure drop across the control will valve. Feels like circular reasoning? Other engineers in my office have told me to just stick to a Cv of roughly half the design flow (which doesn't feel like a robust design method) which means (based on the Cv equation) assuming a pressure drop across the control valve of 4 psi. Why 4 psi? Don't I need to take valve authority into account and find a control valve, which once installed, drops around 25-50% of the total circuit pressure drop? And on the matter of circuits, that is a part of the system that starts out by branching off from a common supply line and ends when it ties into a common return line, correct?
What am I missing here? I can provide more resources, if that will aid anyone.
Edit: Typos & grammar.
The kinds of systems I work with have already been built a long time ago and are fixed flow at the moment. There are several projects coming up where we will begin to add control to allow the pump modulate and add control valves for the reheat coils at terminal units/AHU's to modulate flow based on the demand of the terminal unit. Fairly simple. Now, several sources I have seen/read guide me on selecting a control valve guide me first on finding the pressure immediately upstream of the control valve and immediately downstream of the control valve. My system does not yet have a control valve. This is confusing for me, because if I were to go to the trouble of calculating the pressure drop starting at the outlet of the pump through all the components to where I intend to put the control valve and then the negative pressure drop (pressure gain) starting at the inlet of the pump working backwards to where I will put the control valve- I get the same pressure!
Do these guides just fail to mention that I need to assume/guess a control valve that I will put in the system that causes some arbitrary pressure drop? That delta P just comes out of nowhere in my opinion, without any adequate explanation. One website here, does a fairly good explanation of the topic, but still defines the pressure drop across the control valve, as what the pressure drop across the control will valve. Feels like circular reasoning? Other engineers in my office have told me to just stick to a Cv of roughly half the design flow (which doesn't feel like a robust design method) which means (based on the Cv equation) assuming a pressure drop across the control valve of 4 psi. Why 4 psi? Don't I need to take valve authority into account and find a control valve, which once installed, drops around 25-50% of the total circuit pressure drop? And on the matter of circuits, that is a part of the system that starts out by branching off from a common supply line and ends when it ties into a common return line, correct?
What am I missing here? I can provide more resources, if that will aid anyone.
Edit: Typos & grammar.