Pipe reduction in chilled water modulating valves 1

[MedicineEng]

Hi guys:

I´m currently involved in the construction of a building and the contractor is installing all the AHUs. There was a detail in the installation that caught my attention and I´m struggling to understand the reason for it:

In the chilled water pipe inlet to the coil there´s a modulating valve, as it would be expected. Now what is puzzling me is that in all AHUs the pipe comes at 1", then there´s a reduction to 1/2" where the modulating valve is installed, and then a reduction back to 1" to enter in the AHU coil. In each side of the valve there is around 30cm of 1/2", so the reduction is not right on the valve.

I´m struggling to find a technical/operational reason for the fact. I only found 2 "non-technical" resons and none is very pleasant:
a) The contractor is trying to save some bucks in a smaller valve;
b) The contractor made a mistake ordering valves and now is installing those anyway.

Any other idea? What is the operational impact of this setting?

Thanks a lot for your help.

 

[RandomUserName]

2-way or 3-way valves are usually two-position (on/off) or modulating.

We prefer to size two-position valves with very little pressure drop (line size) as they do not control anything and only contribute to pumping energy costs.

Modulating valves are different. If your chiller system is a variable flow system, 2-way modulating valves will be selected for a high pressure drop in the line so that it is the main contributor of pressure drop in that line. This is called "branch authority". Typically modulating valves are selected for 2-3x the pressure drop of your chilled water coil in the AHU, but not high enough to cause cavitation. In order to achieve such a pressure drop, the control valve is typically a few pipe sizes smaller than the line it is attached to.

Exact same scenario in a constant volume system with 3-way modulating valves only with a small change. The bypass line will have a balance valve in it, usually set to the same pressure drop as your AHU chilled water coil. The 3-way modulating valve is still selected with a 2-3x more pressure drop than the chileld water coil, requiring it to be smaller than line size.

It is important from the standpoint of flow stability in a system using three-way valves to control coil flow, to select the valves for a substantial portion of the available pump head. As in two-way valves used for this purpose, the pressure drop across the valve increases as the valve closes down. This causes an increase in flow through the valve, making it necessary to close off still further to compensate for the increase in flow. If the valve is selected for a low pressure drop (line sized), the pressure drop ratio necessary to throttle to a given flow will be very large and the valve will have to “ride” its seat to achieve control.

If you would like to brush up your hydronic skills, Bell & Gossett have a plethora of technical manuals available for free. Read all of this and you will be much more knowledgeable.

http://bellgossett.com/training-education/training-manuals/

Specifically, try TEH-908A, page 9 and page 25 (of the PDF).

 

[MedicineEng]

Thanks a lot for your reply.

On Monday I´ll go back to site and try to take a picture of the setting and post it here.

Is there a way to give you more than one star?

 

[MedicineEng]

RandomUser Name:
Just to close this issue, this is one of the pictures that I took on site ,so according with your explanation, all should be be good.

Thanks a lot for your help on this.