TECHNOLOGY Fiber optic sensing detects hot spots in methane reformers

Sasol Ltd.'s Mossgas plant, in Mossel Bay, South Africa, is using fiber optic sensing technology to detect potentially hazardous hot spots in its secondary methane reformers.

The system is part of a distributed temperature system (DTS) that warns operators when hot spots begin forming. Without such warning, the localized high temperatures could lead to vessel damage or even process shutdown.

The Mossgas DTS installation was described in an unpublished report by its manufacturer, York Sensors Ltd., Southampton, U.K.

Syngas plant

In 1992, Mossgas began producing transportation fuels from natural gas produced 85 km offshore in South Africa's Mossel Bay.

The natural gas is reformed to produce synthesis gas, or syngas. The Fischer Tropsch process converts the syngas to synthetic oil, which is then refined to produce gasoline, diesel, and kerosine.

Temperatures in the secondary reformers that produce the syngas can reach as high as 1,200 C. Small defects in the refractory material covering the vessel shell can create hot spots.

Because it is nearest the burner, the vessel cone is particularly susceptible to rapidly developing hot spots.

Monitoring

Because of the reformer vessels' propensity for forming of hot spots, it was necessary that Mossgas be able to measure the temperature over the entire surface of its three secondary reformers.

Each vessel is wrapped with 2 km of fiber optic cable encapsulated in stainless steel (Fig. 1 [104741 bytes]). The cable is placed at 100-mm spacings, with 800 m on the cone and 1,200 m on the barrel.

The system uses a technology called optical time domain reflectometry (OTDR) to monitor temperature. In OTDR, a train of laser pulses is injected into the optical fiber sensor.

"Variations in the reflected optical-power signal convert to temperature readings, while the time delay between injecting and sampling the reflected signal equates to a distance along the sensor," says Peter Orrell, sales and marketing manager for York Sensors.

The cables are linked to three DTS units in the plant's central control room. The DTSs produce and continually update a temperature analysis along the entire length of the cable. A fourth DTS double-checks the cone sections of the reformers and provides backup for the main DTSs.

Temperature profiles of the three reformers are displayed continuously. An alarm sounds immediately if the temperature in any location reaches a preset limit.

In the event of an alarm condition, the process controller uses a thermal imager to check whether a hot spot is indeed forming. The operator can then reduce reformer loads, shut down the plant, or cool the hot spot with air.

"Before we introduced the DTS, the only prior warning of a hot spot came from monitoring a heat-sensitive paint on the reformer," said Marc Middendorp, control systems engineer for Mossgas. "Traditional fixed-point technology was not feasible, because we could not predict where small hot spots might develop."

Orrell says the DTS is particularly suitable for hazardous environments such as this because the fiber optic cable is unaffected by electromagnetic interference. In addition, the system uses no electrical currents to transmit data.