Tank levels using Pressure

AshleyParr

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Join Date
Dec 2008
Location
Midlands, UK
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I have a lot of tanks that currently do not have any way of measuring the level, a few have sight glasses rigged up but these are a pain to CIP & ideally id like to have the level in the PLC.

The product is beer & the tanks are pressurized, around 1-2 bar head pressure applied. From looking around and speaking to people using pressure transducers at the top & bottom will be my best bet for this application.

Now onto my questions:

1. Can i install the pressure transducer in the pipe on the tank outlet rather than in the tank?

2. Will the reading be stable enough if i install a pressure transducer in the nitrogen/co2 supply pipe (the supply to provide the head pressure)?

My reasoning for this, it is going to be a nightmare to get to the top of each of the tanks to install each of the pressure transducers then inside to polish, clean & re-certificate the pressure vessel. If i can install in the pipe i dont need too. The costs for walkways, scaffolding, internal scaffolding all starts to mount up with around 100 tanks :S

Cheers,
Ash
 
Ash,

I suppose it depends on your pipes.

The one at the base of the tank, if you install the pressure transducer before the isolating valve (assume there is one?) then is it part of the tank, hence the certification? If you install it after the isolating valve, you lose the level when the valve is shut...

At the top, if you are installing in the N2/CO2 pipe, surely that will be pressurised and give a false reading?

here is a quote from ABB re DP Level : (http://new.abb.com/products/measurement-products/level/a-dozen-ways-to-measure-fluid-level)

A differential pressure (DP) level sensor is shown in Figure 4. The essential measurement is the difference between total pressure at the bottom of the tank (hydrostatic head pressure of the fluid plus static pressure in the vessel) and the static or head pressure in the vessel. As with the bubbler, the hydrostatic pressure difference equals the process fluid density multiplied by the height of fluid in the vessel. The unit in Figure 4 uses atmospheric pressure as a reference. A vent at the top keeps headspace pressure equal the atmospheric pressure.

In contrast to bubblers, DP sensors can be used in unvented (pressurized) vessels. All that is required is to connect the reference port (the low-pressure side) to a port in the vessel above the maximum fill level. Liquid purges or bubblers may still be required, depending on the process’s physical conditions and/or the transmitter’s location relative to the process connections.

Other than that, what about a sight glass with a magnetic follower?
 
You could get away with only a bottom level pressure sensor. Your Nitrogen Blanket Pressure differential with ambient air is small relative to the pressure of a certain depth of tank.

In terms of the outlet pipe, the pressure will change significantly with flow rate. If you measure the flow rate you can probably do some wicked maths to back calculate this. Might be worth doing the math if it is for 100 Tanks? Also John's problem with the isolation valve.
 
>1-2 bar head pressure applied
>Your Nitrogen Blanket Pressure differential with ambient air is small relative to the pressure of a certain depth of tank.

Consider the latter statement. 1 bar is about approximately 33 feet or 10m of water, so a 1 to 2 bar vapor pressure is most likely several times greater than the tank level.

But a relatively high vapor blanket pressure (compared to 1 to 2 inches water column vapor pressure in other types of storage tanks), would make it reasonably feasible to use two individual gage pressure transmitters and subtract their outputs to get level reading, acknowledging some error due to signal conversion and inherent (full scale) accuracy issues of two devices (rather than a single differential pressure transmitter).

I've witnessed level error indication when pressure transmitters are installed in liquid outlet lines when flow starts and runs. Flow only happens when there is a differential pressure and the transmitter sees the dynamic pressure drop from the hydrostatic tank level.

There would some commensurate dynamic flow error when measuring the pressure in the vapor supply line when there is flow. The error would stop and reflect only the static pressure when the flow stops.

Both DP and gage pressure transmitters are available with sanitary connections but you still need connection ports.

Your description of the effort needed to install connection taps puts me in the camp of suggesting load cells to avoid the cost of modifying sanitary tanks.
 
Thanks for the response guys!!

I thought i may have an issue with the sensor in the pipe at the bottom of the tank. This I do not mind as fitting the sensors at the bottom is relatively easy (access wise) and probably more stable/effective.

As for the one to measure the top pressure, putting the sensor in the supply line will show the pressure that is in the top of the tank wont it? We use that line to pressurize the tank, so theoretically the tank & pipe will be the same?

Load cells are probably out of the question, the install/modifications of pipes & access will make this a huge job.

I have looked at the sight glass with feedback but again some of that will require access tot he top of the tanks & if im going up there i may as well install a pt

Im just trying to see if theres a realistic way to record that top pressure without being at the top of the tanks :confused:
 
Bear in mind that the specific weight of beer is no equal to that of water.
Good point.

Hydrostatic pressure level measurement assumes a constant Specific Gravity (SG).

Does the SG of beer change while it is in a given tank?

If the SG changes as a result of processing, then the change is will be reflected as a proportional error from the SG value used for calculating level.

The correction for SG is made in the receiver so that's a minor PLC scaling task.

Is the need for level measurements
-for highly accurate inventory control
- filling/emptying control or
- for a less demanding "getting a handle on which tanks are about half, or 3/4 full or 1/3 full or almost empty"?
 
Use a differential pressure sensor, capable of withstanding the pressure. Take care that valves are open, obvious and there is no flow in the pipes, to be precise.
We use a airbubbling pipe in fishtanks, the air (or CO2) is blown in the bottom and this results in a pressure to show the level of water.
when the top pressure is equel in all tanks, and all tanks are same position, you can even use one sensor.
and some airvalves.
 

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