Flow dynamic is affecting stillwell, and level reading, for hydrostatic level transmitter

Cooling tower level probes have stillwell with 1/2 inch holes every 6 inches and 90 degrees apart, from top to bottom of the 18 foot pit. A strange dynamic has occurred causing the level in the stillwell to change by as much as a foot.

It looks like the flow across one of these stillwells is causing the water to draw down within the stillwell, giving a false level reading. At first I thought maybe the holes were plugged but they are not. There can be changes in the flow across the stillwell due to plugging of inlet screens so there may be an increase in velocity at various regions of the 18 foot probe. This is normal and the probe needs to be able to work with that. One question I had was what would be an appropriate quantity and size and location of the holes? I original wanted many holes to prevent such a dynamic. Hydrostatic level measurement has historically been good for this application and I do not want to change that. Maybe I need larger holes or more holes, but someone else has suggested fewer holes. Any comment or suggestion would be appreciated.
 
C

Curt Wuollet

The holes defeat the purpose of a stilling well allowing flow past the holes to affect the level. It should do fine without provided conditions at the base are reasonably still. The water will seek it's own level. If it's too dynamic, a restriction at the bottom will provide rate of change limiting and an averaging by that rate.

Regards
cww
 
R
If there is only one liquid in the well all you need is a hole in the bottom and another at the top above maximum level.

Are you sure you have your well on the right side of the screen, if it's downstream and the screen becomes partially blocked it will read lower for sure, not that that's a bad thing.
 
I'm not convinced it's fluid dynamics, but who knows?

1.>A strange dynamic has occurred causing the level in the stillwell to change by as much as a foot.

>is causing the water to draw down within the stillwell, giving a false level reading.

If the actual level is truly changing then a changing level indication is not false, it is true.

I'm confused, so can you clarify, is the actual level really changing by as much as a foot and you're trying to figure out how to stablize that change or is the actual level stable but the level indication changing and you're trying to troubleshoot false changing level indication?

2. >Cooling tower level probes
'Probes' is plural. What are there multiples of? More than one stilling well with level transmitter per tower? Or are multiple towers exhibit this same error?

3. How long do the false readings last? a minute? an hour? a day? 3 days?

Do they ever cyclically repeat, up/down/up/down at some periodic interval? That interval being?

4. Do the false readings self correct all on their own?

5. 'Hydrostatic' is implemented with what instruments? Flange mounted DP level transmitter or dual port DP piped into the process?

A flange mount vents the low side to the atmosphere somewhere internal to the transmitter. If the xmtr is a dual port DP, how is the low side vented? Left open to atmosphere? Piped to a vertical drip leg that somehow got a vertical extension that could fill with rain or condensed water which would create a low reading?

How does the high side connect to the stilling well?n hose? pipe/ SS tubing?

Is the transmitter itself mounted securely on something that can't change elevation with the wind?

What climate is this installation located in?
 
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