Case study - Totally Non Contact level on Chlorine

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Totally Non Contact level on Chlorine

“Full Marks” for VEGA Ohmart
VEGA have assisted A.H. Marks, the UK’s largest privately owned chemical manufacturer, to handle a difficult application without even touching it! VEGA are utilising the latest nucleonic technology to provide accurate and reliable measurement of a product, which requires the highest integrity control system.

Established by Mr. A.H. Marks in 1875, it has a history of over 125 years of supply. Sited on former coalmines, they started manufacturing “mordants” a chemical conditioner used for the preparation of wool for dying. This product improving on and replacing urine, which was used originally!

Early in the 1900’s the site moved into the manufacture of nitrates, the base for dyes as well as a propellant for the then fast expanding explosives market. At present, A.H. Marks make herbicidal compounds and intermediates for major manufacturers such as BASF, Avecia and Syngenta. They also produce a range of fine and performance chemicals, for example, polymerisation inhibitors for the plastics industry.

For most processes, chlorine is mixed with phenol and catalysts to provide the intermediate feedstock for the majority of the onsite manufacturing plants.

The storage of chlorine is taken extremely seriously at this site. This is a COMAH 1 category facility and constructed to the EUROCHLOR standard, which closely governs all aspects of safety, storage and handling of chlorine. The storage vessel is a horizontal cylinder, that is never filled above 50%, giving it a capacity of 60 tonnes. It has load cells for the primary contents monitoring and a second reliable alternative measurement was needed for safety, utilising a different technology.

Originally a capacitance level system was used, however the transmitter was unreliable, and needed filling and emptying to calibrate and test. Access to the vessel is on a 5 year cycle. With a product of this type minimising any entries is a priority too. 20mm thick vessel walls meant it was not possible to make any fixings to the outside of the vessel

A decision was taken to use a completely non-contact technology. Nucleonic systems are already used on site and technically proven for reliability and performance.

How they work
Nucleonic level gauges use low level gamma radiation sources with a narrow targeted beam, which will pass through the vessel walls and air space within, to ‘just reach’ an externaldetector on the on the opposite side of the vessel. The detector contains a special plastic scintillator crystal, that gives off light when exposed to gamma energy. The product rising or falling alters the low level radiation by blocking/unblocking it and the detector registers this “change of light” as a level measurement. With no moving parts, high stability and precise engineering, they are highly reliable needing almost no recalibration or maintenance. No contact with the product or the vessel offers an excellent solution for extremes of toxicity, temperature, abrasiveness and vibration. Nucleonic systems are very compatible for level switching, level measurement and density applications.

The site engineer was looking at a continuous level measurement system. However, he knew traditional straight detectors are limited in length due to the restrictions in length of rigid crystal, they would have heavy housings and at least two would be needed to profile a horizontal cylindrical vessel of this type.This would result in higher costs and a difficult installation in a confined, hazardous area.

The Flexible solution
After receiving a mailshot on VEGA Ohmart products, the site engineer spotted a solution. “It was the perfect answer for our application”, said the site instrument engineer, “There was a picture of a vessel similar to ours with a new, single flexible detector mounted around it, curving to suit the contour of the walls.”

A VEGA Ohmart product specialist was contacted and visited site within a few days, to carry out a site survey and advise the A.H. Marks. VEGA specified and supplied their FibreFlex detector. The detector element, normally rigid plastic, is made into a fibre optic bundle in the FibreFlex, because it transmits the light more efficiently, they can also be made twice the length of rigid systems! The engineer added, “VEGA offered not only the best technology but the best price too”.

The detector was lightweight, and has compact in-head SMART electronics. It is less than a fifth of the weight of what would have been used and can be supplied with clips to secure it easily to a frame or, as in this instance, it was placed inside a tube. The easy handling meant an installation without any heavy lifting gear and relatively quickly in an area around this hazardous product.

VEGA also went to site to carry out the critical survey, which gives the customer an official report on the system, installation and ensures signage and safety procedures are correct for their peace of mind.

VEGA successfully commissioned the system too, along with training on the easy use Ohmview software. This communicates via HART protocol over the signal cable, which in this high integrity application, was hard wired to the control room. Uniquely however, VEGA offer a Fieldbus Foundation option for all their systems.

“It does exactly what it is meant to do”, said the A.H. Marks engineer, “ We are happy with the performance and it provides ESD protection for the whole plant” He is now the Qualified RPS (Radiation Protection Supervisor) for the site, ensuring the company comply with the relevant legislation and procedures for operating nucleonic gauges.

VEGA Ohmart are still constantly developing new nucleonic systems, increasing the ease of use, performance and application areas. Along with innovative idea’s, they are the world leading company in this field with over 70,000 systems supplied in over 50 years of manufacture.