Chemical plant weighs in

Croda Application Chemicals contracted Negretti Automation to provide a solution for weighing bulk liquids into 25 tonne blending vessels with minimal plant disruption. David Edwards from Negretti reports

As part of the plant upgrade at Croda Application Chemicals, Doncaster, Negretti Weighing were asked to provide a solution to accurately weigh bulk liquids into 25 tonne blending vessels used in the manufacture of specialised chemical products.

The 10 blending vessels, varying in capacity from 15t to 25t are continually in use as part of their batch production facility. Faced with the possibility of unacceptable down time to modify the support structure, Negretti were tasked in finding the best weighing solution to meet the required system performance and accuracy of 0.1% full scale deflection (fsd), yet minimise plant disruption. To add to the problems the production area was classified as a Hazardous Area Zone 1 with several of the vessels also fitted with heating and cooling jackets.

Firstly, Negretti engineers carried out a full site survey, which is a pre-requisite for any installation of this type where the original vessels are installed without weighing in mind. Hazards was encountered that could impede the weighing system, such as handrails attached to the vessels, walkways fouling, cable tray suspended from the vessel sides, etc. These were the first to be rectified prior to the main installation of the load cells.

As a minimum, each of the four vessel legs would have to be modified to strengthen them and accommodate the load cells. The modification was to be made without altering the overall working height of the vessel because of the existing process connections.

LOAD CELL INSTALLATION

Taking into consideration the time constraint to install and the environmental conditions, Negretti selected their VC3500 I.S. load cell and mounting kit. The cell is a stainless steel double-ended shear beam, a fully welded construction, sealed to IP68. The cell, coupled with the tank mounting kit with its integral jacking facility and built in self alignment, would give the flexibility needed during the installation. To simplify the design further the 10t capacity cell could be used throughout without any degradation to system accuracy.

The vessels were modified one at a time to minimise disruption to production, although this gave particular problems within the area classified Zone 1. Each leg was modified in turn by attaching a temporary brace to support the vessel weight whilst a section was cut away. Load bearing plates were inserted with additional webbing to give greater strength. The advantage of using the VC3500 tank mounting kit enabled installation in one operation, before final installation of the cells. Once the kits were in position the cells were simply inserted and the vessel lowered using the mounting kit integral jacking bolts.

THE WEIGHING SYSTEM

The innovative self-aligning and self-locating features of the mounting kit enabled speedy installation, each vessel taking about four hours from start to finish. With the built in anti lift mechanism there was no need for additional tie rods or restraints.

The Negretti instrumentation consists of the GS350HR high-resolution amplifiers interfaced to VC3500 intrinsically safe load cells via zener barriers. The amplifier, with its dual isolated analogue outputs, provides re-transmission to local display units adjacent to each blending vessel and batch data input to the distributed control system.

The order to upgrade the batch facility at Croda Application Chemicals is the latest weighing system installed on this site by Negretti Weighing. The system forms part of their continuous production process with Negretti Weighing providing service, calibration and support.