Over-sized valve?

[nickolas]

I have been wrestling with sizing this "chilled water valve" all day, This system will include 100% Outside Air, steam pre-heat,50%glycol, and reheat. Of course the chiller is oversized to allow for 95 degree outside air (85 tons). 12,000 cfm air handler (approx. 30 tons air). The existing pipe size is 4", and the system has approx. 35 ft of head. The submittals list the chilled water flow at 143.75 gpm. I have used every calculation in "the book" and I have determined that the valve should be sized @ 68 to 72 Cv. The problem is:

#1 The existing pipe is 4"
#2 The required (68 Cv) calls for a 2.5" valve @ 5 psi delta P
#3 Cv for a 2.5" valve is 54Cv
#4 Cv for a 3" valve is 83Cv
#5 delta P is 7.7 psi for a 2.5" valve
#6 delta P is 3.3 psi for a 3" valve
#7 reducing from 4" to 2.5" may cause cavitation
#8 using a 3" valve could mean oversized valve and loss of control (hunting)

The submittal also list the pressure drop for the chilled water coil at 19 psi (seems excessive).

What the "!%&*"

Any help sizing this valve will be appreciated, need more information? I got it.

[crab master]

I am not a valve expert by any means but have you contacted a few valve manufacturers to see what they have to say? My thought is an "oversized" valve with a characterized disc to actually get the value you want/need.

Or control it like steam 1/3 and 2/3! ;o)

[orion242]

I would size it up for the 68cv, bumping up the to the next size valve if the 2.5" will not cover it. I would not hesitate to put a 3" with a cv of 83 as your 2" will not meet the 5# drop. However I always thought the control valve should have the bigger pressure drop vs the coil.

It will only be oversized if you botch the selection.....


When in doubt call the valve manufacture.

[orion242]

Something is not right with your numbers.

If the system only has 35ft of head your not going to be able to push water through that coil.

35/2.31=~15psi

That must be the fill pressure, not pump head?

[jdd67]

19 PSI seems excessive for the pressure drop through the coil. The valve needs to be sized to take a larger pressure drop than the coil. If the coil is taking a larger pressure drop than the valve, the coil becomes the "authority" in the system and modulating the valve at the high end of the stem travel will have no effect.

[nickolas]

Something is not right with your numbers.
If the system only has 35ft of head your not going to be able to push water through that coil.
35/2.31=~15psi

That must be the fill pressure, not pump head?

Your are correct, after checking the pump submittals, I found that the pump is designed at 75 TDH and 150 GPM.

And yes I could call someone and have them size the valve for me, but then what would I (and others) learn from that? I have learned already that you must first have accurate information

So here is the latest;

Pump designed at 75 Ft/H2O
Coil pressure drop = 19.27 Ft/H2O (not psi)
designed coil flow = 143.75 GPM
designed pressure drop for valve 5 PSI
Chilled Water 42 degrees
Existing Pipe Size entering valve = 4"
50% ethylene glycol = 1.05 SG
Did the Math and I come up with "65 Cv"

My choices for a flanged valve are
2 1/2" valve with 54 Cv 28 psi closeoff
3" valve with 80 Cv 12 Psi closeoff

Now the question would be which valve would be more suited for this application, and why

Thanks

[jdd67]

I would select the 2-1/2 inch valve for two reasons:

1. The pressure drop is about 7.5 psi which is below, but fairly close to the pressure drop of the coil (8.34 psi). This will yield a larger range of control than the 3 inch valve.

2. The 3 inch valve may not be able to close off against the pump head and will tend to "chatter" when trying to control at the low end of the range during low load conditions.

[jogas]

Your are correct, after checking the pump submittals, I found that the pump is designed at 75 TDH and 150 GPM.

And yes I could call someone and have them size the valve for me, but then what would I (and others) learn from that? I have learned already that you must first have accurate information

So here is the latest;

Pump designed at 75 Ft/H2O
Coil pressure drop = 19.27 Ft/H2O (not psi)
designed coil flow = 143.75 GPM
designed pressure drop for valve 5 PSI
Chilled Water 42 degrees
Existing Pipe Size entering valve = 4"
50% ethylene glycol = 1.05 SG
Did the Math and I come up with "65 Cv"

My choices for a flanged valve are
2 1/2" valve with 54 Cv 28 psi closeoff
3" valve with 80 Cv 12 Psi closeoff

Now the question would be which valve would be more suited for this application, and why

Thanks

Could you please show the complete process/calculations required to size the valve? Complete the lesson so to speak.
Thanks,

[nikko]

One rule of thumb is a 3psi drop for the valve so your calc'd Cv is 82. This valve would be on the large size but if it's a ball valve with a characterized disk it'll control just fine. Sounds like your 3" choice @ Cv80 is the one you want (assuming it has a disk)

Nikko

[bob_scheel]

I would select the 2-1/2 inch valve for two reasons:

1. The pressure drop is about 7.5 psi which is below, but fairly close to the pressure drop of the coil (8.34 psi). This will yield a larger range of control than the 3 inch valve.

2. The 3 inch valve may not be able to close off against the pump head and will tend to "chatter" when trying to control at the low end of the range during low load conditions.

If the chiller is designed for the ahu and solely for that purpose I would think you should have a 3 way valve to prevent cutting down the water flow to nothing. Why do you need to control the water flow if this is the only coil on the chiller? Can't you control the load by raising the setpoint of the water?

[nickolas]

Could you please show the complete process/calculations required to size the valve? Complete the lesson so to speak.
Thanks,

Cv = Q √SG/ΔP

Where
Cv = Flow Coeffecient
Q = Flow Rate in GPM
sg = specific gravity
ΔP = Wide open pressure drop (Psi)


65.87 = 143.75 √1.05 / 5

[nickolas]

If the chiller is designed for the ahu and solely for that purpose I would think you should have a 3 way valve to prevent cutting down the water flow to nothing. Why do you need to control the water flow if this is the only coil on the chiller? Can't you control the load by raising the setpoint of the water?

The chiller is designed for this AHU and coil only, I will be using a 3 way valve to maintain full flow to the chiller under all conditions. The specification requires 55 degree air leaving the chilled water coil, which will then be reheated if needed to maintain space temp, also the chilled water valve will be forced to 100% during the dehumidification cycle causing the reheat coil to maintain space temp.

I have used this sequence before and it works well if controlled properly

[crab master]

Caution! Make sure your valve has the flow characteristics that you need in for both the cooling coil and then when it is in full bypass. We recently had to change out 2 valves, and I don't know all the details as I was just told about it, due to the bypass position not allowing enough flow - anyway one chiller serving two air handlers.

[nickolas]

Caution! Make sure your valve has the flow characteristics that you need in for both the cooling coil and then when it is in full bypass. We recently had to change out 2 valves, and I don't know all the details as I was just told about it, due to the bypass position not allowing enough flow - anyway one chiller serving two air handlers.

Good point; For that reason maybe the " 3 " inch valve would be more suitable in this case. (to insure required flow to the chiller in bypass). I think I have all the infomation necessary to select the proper valve.

One thing that I am sure of is, that the selection has not been "botched"

"Orion242--It will only be oversized if you botch the selection....."

Thanks guys I have learned a lot

[kontrolthis]

I am in no way a valve expert, but have picked up a few things along the way. If the pipe is 4" and you do place a 3" or even 2 1/2" valve in line, I know the Cv must be corrected for the piping reduction. It basically nets less Cv than listed flow for equal pipe sizing. Also a good point about the ball and characterizing discs. The bypass port on a Belimo (B) port is rated at only 70% of the A to AB port. Just a little reminder.