Is it wise or dangerous to back up differential pressure with nuclear instrumentation?

Is the addition of a radiation-type level detector advisable, and what are the advantages and disadvantages?

Q: Our project involves a reactor 5 m in diameter and about 25 m in height. Presently, its level is being measured by a differential pressure (DP) gauge, capillary type, and I was told to install, in addition, a radiation level detector. Is the addition of a radiation-type unit advisable, and what are the advantages and disadvantages of these two options? Is the radiation-type a better choice? Can radiation sensors be dangerous if the reactor walls are thick and hot?

Rahim Salamat
rahim1356@gmail.com

A: For a couple of decades, I was the chief instrument engineer at C&R, and during that period, we must have designed nearly a hundred polymer reactor control systems. So your question is familiar, but it is lacking the key information: Is this a batch or a continuous reactor?

If it is a batch reactor and if you have good flowmeters on the charging side, you might not measure the level at all, but just depend on the batch flowmeters for recipe formulation and add a high-level interlock for safety. If the accuracy or reliability of the flowmeters is insufficient, and if the full weight of the reactor is more than four times its empty weight, you might put the reactor on load cells.

To consider nuclear sensors, you need an NCR license, an on-site certified radiation officer, and if you have heavy coating, it will still affect accuracy. In addition, you must also arrange for source disposal. For DP, extended diaphragms (Figure 1) with equal-length capillaries and temperature compensation for ambient temperature and sun exposure variations can also give reasonable performance.

While in batch reactors the residence time is measured by a timer, in continuous reactors, residence time is a ratio of reacting volume divided by the outflow (V/F), where V is a function of level. Therefore, in controlling continuous polymer reactors, level measurement is not optional, but a must.

Figure 2 illustrates such a control system. The selection of the level sensor should consider the comments I made in connection with the batch reactors, and some people might also consider the use of self-diagnosing laser (up to 300 °F, if the transmittance in the vapor space and the reflectance of the polymer surface is acceptable) or noncontacting and self-diagnosing radar (up to 500 °F, if there is no coating, condensation or crystallization on the antenna).

Béla Lipták
liptakbela@aol.com 

A: The biggest reasons not to use radioactive measurement are:

1. The instrument rays have to shine through the walls of the reactor so the receiver can absorb them, but the radioactive beam is not concentrated in one point like a laser, so sometimes you will have scattering of the radioactive beam, which could harm personnel.

2. Generally, reactors have extremely thick walls requiring a very high-energy source, which, over time, may make the reactor walls radioactive around the the beam area.

3. Most radioactive systems need to be close if not in contact with the surface of the walls of a hot reactor. This could damage the source, which could cause radiation to leak.

Alex (Alejandro) Varga
vargaalex@yahoo.com

A: I have not come across DP level used in polymer reactors. Even with designs from 40 years ago, we were using nucleonic/radiation level, however, that is a small sample of the total number of polymer reactors in the world.

Most of the reactors I worked with did not use level. They just batched in a certain quantity and called it good. In the cases I am familiar with, there is no real benefit to knowing level in a batch reactor. Perhaps most of the modern polymer reactors are now semi-continuous.

There are regulatory difficulties involved with nucleonic installations. In the jurisdictions I know about, you have to have a trained and certified radiation officer on-site. There is also a perception that such devices are very dangerous and difficult to monitor/maintain/control.

Simon Lucchini, CFSE, MIEAust CPEng (Australia)
Chief controls specialist, Fluor Fellow in Safety Systems
Simon.Lucchini@fluor.com

A: I lived with this question for 15 years. The only successful approach was a blow-back dip tube. This did plug now and then, and we used a long rod with a drill bit welded to the end to clear it. It plugged because the blow-back air was supplied at too low a pressure. The reactor normally ran at a high vacuum but we discovered (by watching the operators late at night) that when the valves became plugged, the operators would turn off the vacuum and pressurize the vessel until the valve plugging cleared. And the blow-back tube was filled with polymer.

The proper installation sequence would be: air supply header with filter (not regulator) > check valve > needle valve > rotameter > dip tube.

The polymer would not be pushed into the dip tube because the pressure downstream of the needle valve would rise to block backflow. The standard air supply regulator has downstream pressure protection, and when the downstream rises, the regulator vents and allows backflow.

It took a long time to discover this. Our installation did have a radioactive source in the agitator shaft, and the detector outside. A "micro-micro-ammeter" amplified the signal. Most of the time it worked well. The problem was the way the various parts were electrically grounded, so it appeared to be unreliable. Any welding in the building caused a panic.

Cullen Langford
cullenl@aol.com

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  • Mr. Salamat, we have a very similar situation in our refining plant. Our Delayed Coking unit has a fractionator tower with main botton level measurement with DP backed by nuclear. As a matter of fact we use the DP to calibrate the nuclear. This system has been working since 2012. The nuclear system is comprised of a nuclear source and the transmitter localized diametrically on the opposite side of the tower. We had some issues with temperature at the transmitter which were solved with proper thermal insulation. Some things to consider if nuclear is to be installed: - legal issues concerning the regulating agency in your country; - people tend to be afraid of nuclear radiation so proper training has to be considered regarding radioprotection and technical matters. Also the maintenance crew will have to use portable dosemeter whenever servicing close to the nuclear source; - HSE people usually are the ones responsible for monitoring safe levels of radiation and usually are involved whenever maintenance has to be executed near the nuclear sources; - the sources have to be tested anually to garantee they can close if needed, and if not they have to be lubricated if stuck. Also every two or three years a leak test has to be performed to make sure no leak is going on. - procedures have to be created and followed regarding access, isolation, monitoring maintenance and emergency situations . So after these considerations, I would recommend nuclear if no other possible technology is available, even using the same technology, DP in our case, would have been ok. We have an application here where nuclear is the only possible technology, as far as I know, which is in our Coking Drums. In these Drums, we have hot heavy oil entering and as it fills the drum the heavy oil temperature lowers and it starts to solidify what is called coking. So due to the solidification and high temperature inside the drum other level Technologies have not had success.

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  • From my experience in polymer plants around the world, operating plants like have a redundant level detector of a different technology. The main advantage of the radiometric level measurement is that it isn't exposed to process conditions. In polymer plants, the inside of the vessel has a tendency to coat up with polymer, this includes the pressure ports. If the pressure ports plug then that can lead to an error in the level reading of the DP. A deviation alarm can be setup in the DCS to alarm when the two different technologies differ by a certain percentage so that maintenance can be notified to address the issue. The radiometric level device can be added to the outside of the vessel with out any modifications to the vessel and provide a safe and reliable level measurement. The reliability of the radiometric level device is its greatest asset. As mentioned there are some negatives to using a radiometric level device but they can be easily mitigated especially if the plant all ready has other radiometric device, such is the case for most polymer plants.

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  • Having spent the better part of my career in the polymer industry around reactors, I read your article with interest. I would like to add a few points to the article in the May Control magazine: 1. Residence time in a polymer reactor is really measured on a mass basis rather than volumetric basis (i.e. reactor mass divided by mass flow). This may not be important in reactor systems that maintain constant density, but is significant in boiling reactor systems where density varies due to vapor bubbles in the mixture. In reactor systems such as these, DP level detection is preferred because it related to mass above the lower detector and therefore compensates for density differences by its nature. Nuclear detectors on the other hand show actual level and will vary as the reactor boiling varies even though residence time is not changing. This leads to errors in level control. 2. Some polymer reactors use load cells for maintaining level since mass is the critical variable. 3. An effective way to keep DP level instruments from plugging up is to take a small slip stream of the reactor feed and use it to continuously purge the port for the lower DP instrument where it goes into the reactor. This can be accomplished by installing a slim ring with a port between diaphragm and the reactor nozzle. As long as the purge is maintained at a low rate (2 GPH), the back pressure is minimal and the level measurement will not be affected. 4. Nuclear detectors are very effective and are used routinely in high temperature/high viscosity situations such as polymer devolatilizer vessels. Other level detectors have issues with this environment. Mark Cicerchi

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