Liquid level bridle tapping on liquid outlet line

[lewismg]

This question relates to the position of the low tapping point of a liquid level bridle controlling level in a vessel filled with either water or condensate.

Unfortunately, although it is not standard practice, the only option we have been left with is to connect the lower bridle tapping to the liquid outlet of the vessel (instead of a tapping point off the side or bottom of the vessel itself). Given that this is the only way of level coordination for this vessel;

1. what are the implications of doing this?
2. I would imagine that a vacuum is created in the level bridle that will create an offset in the level reading. Does anyone know to what extent this will be or how to calculate this. The vessel operates under on/off control discharging liquid at 5.8m3/hr.

Any help is much appreciated

 

[SooCS]

I made the mistake of allowing a level tapping from the liquid outlet many, many years ago. The result was out of control liquid level causing frequent shutin and had to cut a hole in the vessel to make proper liquid bridle.

So do a proper job now or regret later.

 

[hacksaw]

caution is always advised, yet it has been done from time to time.

you have to identify and estimate all flow dependent pressure losses. if they are less than 1% of your span, it might work.

the recommended practice for bridles is to locate the tapping point ~ 6" from the tangent line or so, and call that you datuum (0 mm/0 in point). The operators have less proble with that than a flow dependent level reading.

 

[scanguy]

If monitoring this level is of great enough urgence, you may want to consider external gauges such as nucleonics. However this may be cost prohibitive based on the level of importance.

 

[25362]

Since the operation is on/off, apparently, at the "off" status one could, at least, detect the correct level. By the way, how do you get constant water flow rate at the "on" stage ?

If it is an open vessel, another possibility is to bubble a constant flow rate of air or whatever gas compatible with the system through the upper tap, measuring the gas pressure after the reducing valve. See, please, Perry VI Chapter 22.

 

[jsummerfield]

You correctly identified the problem. When you open a valve or start a pump a head or displacement level instrument will be affected. You need to identify whether this is point or continuous level application and some further design details. However, there are several solutions to consider depending upon the specific application.

If this is a vessel with multiple compartments, perhaps a water boot on a hydrocarbon vessel the water should be less significant than the oil phase. On-off control is often used to dump such accumulations. Also, perhaps you know the rate at which the level fills. In this case you could the measurement instrument to initiate opening the valve or starting the pump and use time to determine how long to drain.

I think that Valvek offer a smart control valve positioner that includes internal flow rate data. Perhaps this could be useful if you have some flow inflow indication.

However, if your objective is to maintain a constant level, not the normal case for vessels except steam drums, you will likely need another vessel nozzle or some other measurement technology. Note that many new technologies apply to level measurement. Some may be suitable. Pressure, temperature, physical properties or other details are significant in determining options available. You may want to share your situation with some instrument vendors such as Rosemount (Emerson Process, Fisher Rosemount or whatever name they use this year), Endress + Houser, Pepperl-Fuchs (

http://www.pepperl-fuchs.com/pa/levcon/cont-lev-con/main_e.html),

or others at

http://www.isadirectory.org/online/master.cfm?Letter=L

like for level.

Good Luck.
John Summerfield