Pressure points

Engineers at Lucite International’s Cassel Works in Billingham wanted a better way of measuring the level in a separator vessel.

The company, which designs, develops and manufactures acrylic-based products, is the world’s largest supplier of methyl methacrylate (MMA), the essential building block for all acrylics.

I chose electronic dp because of its accuracy in measurement and reliability, as well as the benefits of easy installation and maintenance

Tony Kydonakis, Lucite’s I/E asset engineer

The vessel keeps the chemical BPA under vacuum and at high temperature in order to separate off the volatile organics and reuse them, improving yield and plant efficiency.

It’s important that the BPA itself doesn’t enter the downstream process due to its viscosity and high sulfuric acid content, which carries a risk of corrosion.

The existing level monitoring system in the vessel was a high level alarm and trip, which had the desired effect of preventing overfill but gave no prewarning of any problems.

The obvious solution was to install a differential pressure device – mounting sensors at the top and bottom of the vessel in place of existing sight glasses – in order to monitor the level more closely.

Challenges

The company had concerns over accuracy and speed of response using a traditional differential pressure measurement system using oil-filled capillaries.

Due to the height of the vessel, capillaries of up to 10 metres in length would have been needed.

Changes in the ambient temperature would have affected the volume of the oil in the long capillaries, potentially reducing the measurement accuracy.

Installation of a traditional device would also have been difficult as it would have required feeding the capillaries through the floor of the plant in order to mount the transmitters at the top and bottom of the vessel, using the existing tapping points.

There were also maintenance considerations, says Tony Kydonakis, Lucite’s I/E asset engineer.

“If a diaphragm seal fails we would have to erect a very large scaffold to remove the complete assembly. The process would stop for the duration of the repair thereby incurring significant efficiency losses to the business.”

The solution

It was decided to install Endress+Hauser’s electronic differential pressure system, Deltabar FMD72.

“I chose electronic dp because of its accuracy in measurement and reliability, as well as the benefits of easy installation and maintenance,” says Kydonakis.

Unlike other electronic dp systems, Deltabar FMD72 consists of just one transmitter, instead of two, and two sensor modules.

Either sensor can be operated as the high or low pressure sensor. Due to the acidity of the BPA in the vessel, the device was supplied with tantalum diaphragm seals to avoid corrosion.

Endress+Hauser also supplied Lucite International with a HART splitter, enabling them to measure the vapour pressure at the top of the vessel as an additional parameter.

Additionally, Lucite purchased the Endress+Hauser Start-Up package, which included commissioning of the instrument by a qualified Service Engineer with 12 months’ on-site support.

“We saw it as free training for a new technology,” says Tony Kydonakis.

“We were able to sit with the engineer when he came to do the commissioning and ask any questions.”

The benefits

Ease of installation was a big benefit for Lucite International, says Kydonakis.

The system’s modular design meant that the transmitter, sensors and cable were mounted separately, which avoided the necessity to feed metres of cumbersome capillaries with attached flanges between floors or to modify the vessel.

It also meant that individual components could be replaced separately.

“Because the top and bottom sensors are the same, you can keep just one sensor in stores so if it goes wrong for whatever reason you can just quickly replace it,” says Kydonakis.

“It’s better than having to send the complete capillary system off site for repair or keeping an expensive capillary system in stores.”

Due to automatic sensor recognition there’s no need for recommissioning or recalibration once a sensor is replaced.

The FMD72 also uses a standard cable, reducing maintenance and repair costs.

The company has also benefited by being able to mount the single transmitter in a convenient location for the technician.

A second FMD72 is being considered for the site to replace an existing capillary system.