Radar-based vessel gauging system at Shell refinery

Shell Geelong refinery, which sits on 120 ha on the shores of Corio Bay, produces vehicle and aviation fuels, solvents, bitumen and other products. With up to 500 employees and some 150 contractors, the refinery is one of the largest in Australia, with a replace ment cost of around $1.5 billion.

LPG for vehicle use is produced and stored both onsite and at the Lara LPG storage facility about six km away. An underground pipeline transfers the LPG to Lara where it is then distributed by truck.

The new radar technology replaces two old electro-mechanical gauges that were installed in 1978 and due for inspection. That equipment consisted of a mechanical servo-driver with balance circuit, drum, and fine wires to hold floats. These wires could break and snag the floats or cause the floats to drop off.

Monitoring equipment
The LPG vessel only comes out of service every 10 to15 years, which pres ents the only opportunity to do mainte nance on the monitoring equipment. So, as Shell Refining’s Pat Dinaro points out, critical requirements of the new technology include high reliability and low maintenance.

“High pressure is required to keep the gas liquefied, so opening the vessel for inspection can be complicated and expensive. With a radar gauge there are no components inside the vessel to measure the liquid level that may require serv icing, and measurement verification can be carried out with the tank in operation.

“While the old system was electro- mechanical, the new Rosemount Rex RTG 3960 gauge operates by sending a radar signal, so there are no moving parts. As well as measuring the LPG level, the system can also measure temperature and provide an alarm func tion,” Dinaro said.

“We can talk to the Rex RTG 3960 online, monitor the signal strength and diagnostics, and observe any error codes. In addition, one of the two old gauges has been retained as a backup to correlate with the radar and verify the LPG level.

Radar signal
Installation of the new system proved to be a challenging operation. The Rex RTG 3960 is sited on top of the LPG vessel and connected to 34 metres of 4 inch still-pipe to condition the microwave radar signal.

The pipe had to be lifted up 70 metres by two cranes and lowered into the vessel. This occurred in the midst of a live plant and required safety issues to be addressed to enable a crew to work in a confined space inside the vessel to secure the bottom of the pipe, which needed to be plumb and within one degree of vertical. Brackets in a spider formation were used to secure the bottom of the pipe, and no welding was permitted.

Verification pins
The Rex RTG 3960 software includes a module that calculates a correction. Verification is provided by measuring against verification pins installed at known distances in the still-pipe, which allows verification on different distances from the level gauge spread over the whole measuring range. This informa tion can then be read by the operator.

According to Magnus Petersson, product manager at Emerson Process Management Australia, which supplied the Rosemount Rex RTG 3960, several factors need to be considered to ensure successful radar gauging in LPG vessels.

This includes accurate temperature and pressure measurement, correction for reduced radar wave velocity, and verification.

“Ensuring a reliable overfill alarm for a LPG storage vessel is particularly important to avoid gas escaping to the atmosphere and resultant safety issues. Conventional overfill alarm sensors have a major weakness because the only time that you really know they are func tioning effectively is when a test is run. On the other hand, a Rosemount radar gauge will give the operator continuous information about the status and performance of the system,” Petersson explained.

Bulk liquids
The ability to accurately determine the quantity of bulk liquids in storage is of critical importance to effective refinery operations, including in relation to factors such as inventory value, loss control, storage capacity utilisation, leak detection and overfill protection.

Petersson believes that the state-of- the-art technology provided in the Rex RTG 3960 brings a number of significant benefits including improved productivity, increased safety and higher profitability.

“Rosemount radar gauges are virtually maintenance free because there are no moving parts. The only maintenance needed is to clean the antenna, and the signal strength can be monitored by the oper ator via computer,” says Petersson.

Overfill situations
“There are big safety and productivity benefits in not having to climb the vessel so often, whereas the old system needed more physical inspections,” explains Petersson. “Safety considerations are a high priority in refinery operations, so an automatic vessel gauging system with alarm functions can help to avoid over fill situations and protect both people and the environment.

“In addition, improving measure ment accuracy in the vessel gauging system reduces uncertainty during both transfer control/verification and inven tory management, which can have significant benefits for a company’s bottom line.”

Dinaro says the Rex RTG 3960 has now been operating effectively at the Shell Geelong Refinery for about 10 months without any problems. In addition, there are now plans to replace several other gauges on LPG storage vessels at the refinery, so the opportunity will be taken to also install Rosemount radar gauging technology on the new vessels.