flow metering


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I have a project where I have to measure the mass flow of liquid chlorine (100% chlorine not sodium hypochlorite) and need to know what my options are?

The chlorine is from rail tankers (through a 2 inch line) to 7 evaporators then to 7 chlorine gas feeders operating under vacuum to the injection point. The flow rate is 2000 pounds per day (approx 25 L/hr)

We want to measure the amount of chlorine used to determine when the reported mass of chlorine in the rail tanker is near zero (this minimizes the return of the railcar with chlorine residual and notifies the plant that the second railcar has to be put on line).

I appreciate any help even though the flow is very low.

Could you give an example of what's in the Startup routine.
I work with many different PLCs and DCS systems. I don't have time to learn them all in great detail.

My interpretation of good programming is code with lots of documentation, not fancy routines with none.

This is not an easy measurement to make. Industrial grade Cl2 is nasty, impure, and full of water vapor.

Many years ago, I wrote the engineering section of the then Manual of
Practice for the Disinfection of Wastewater for the then Water Pollution Control Federation, now WEF. So I have some relative expertise.

The problem is that you are extracting chlorine from the railcars under a nitrogen pad (I hope) and pushing it to evaporators. If you use anything that creates a significant differential in pressure across the flow element, you will cause the chlorine to flash into gas, and your measurement will be bad, and the evaporators, which are expecting chlorine liquid, will also have problems. Most differential pressure measurement techniques are not going to work well.

You can try to use coriolis meters, but remember that this chlorine is not, by definition, 100% and it is chock full of impurities, waxes, and other crud, which will plate out on the walls of the piping...especially ferric chloride. This means that you cannot use a standard stainless steel body for the coriolis meter, or its measurement tube. Metals like tantaloy, Hastelloy C276, or sterling silver work. Most others don't. Tank car chlorine is supposed to be anhydrous, and not corrosive, but that NEVER happens.

I have used the Ramapo target meter, now manufactured by Venture
Measurement, with all Hastelloy C276 body for this measurement. It used to be a standard for this service at many chlorine manufacturing plants. I don't know what people like Dow and PPG use now.

You also have the problem that you are measuring a low velocity flow.

If I can be permitted a question, why are you evaporating such a low flow rate from a tank car?

Using a standard vacuum regulator (Advance or similar) you ought to be able to go directly to the chlorinator from the gas side of the railcar (removing the N2 pad) and pull off that much gas without evaporating...

Or is it 2000 PPD times 7 evaporators? Or 14,000 pounds per day? If so, the flow rate isn't as low as indicated...

Walt Boyes
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I don't know how my previous reply ended up on this thread. I agree with Walt about the flashing problem with a coriolis meter. Even if the pressure and temperature are well outside flashing conditions I think the mechanical energy of the vibrating tube causes flashing. I have also tried a vortex, it also flashed. I suggest you have the vendor garantee whatever you buy. Hastelloy C would be my choice.



Our chemical engineer has some questions and answers as follows.

What do DOW or PPG use to measure chlorine liquid flows - good question indeed. Can we find out?

I might have misled you on the flow, but my info is that a 90-tonne load lasts for up to 7 weeks, so around two tons per day is what gets used on average.

The railcars are self-padded since the gas evaporates... and the cars can have padding augmented on site using dry compressed air (rather than nitrogen - I think - if it's important we can check).

We use evaporators probably because there are/might be transient needs for much higher flowrates than the average.

Dave Ferguson

My interpretation of good code is properly written code for the controller used with both process and human safety in mind, with lots of documentation and a proper HMI interface. If the HMI is designed correctly, then there is no need for a Human to ever go into the Machine (other than to change the process). I have seen plenty if HIM's Humans Into Machine due to improper programming and design engineering.

As to the question:

As has been said on here by a couple of people. On a startup routine you would clear out set points, load constants, clear latched bits, reset sequences, etc.

This could also be done on a shutdown routine and there are also programs that will run a fault routine if there is a major fault where you could reset bits and return equipment to a "safe" state.

Of course as one poster said, be doing some resets if your code is good, should put the rest of the machine into a safe state.

And as always, this takes a knowledge of the process, the controller used and the program written. In other words it is a software engineering project like most others.

Dave Ferguson
I have no experience with this medium, but what about thermal dispersion meters (relatives of the old hot wire anemometer)?

They generally have minimal pressure drop (an open tube with a small protrusion for the heater and RTD), and work fairly well at low flows.

I have a Thermal Instrument brand on a cold bleach line and it works very well.