Operations and inspection personnel should perform sperate TMT measurements using different devices and keep these in separate monitoring records (sheets). Anomalies must be reported immediately and monitoring sheets periodically checked for any abnormalities.
(a) Gold cup pyrometers
The gold cup pyrometer is a trusted method of TMT measurement (Figure 6). A gold plated hemisphere (gold cup) surrounds the target object, effectively acting as a blackbody. Errors due to background contribution and inaccurate emissivity assumptions are eliminated, and the radiation measured can be directly converted to the true temperature of the object. Due to its accuracy, the gold cup pyrometer is often used as a reference standard.
A disadvantage of the gold cup pyrometer is its heavy and cumbersome weight. It is also limited by the distance it can be inserted into the furnace. Use is otherwise relatively simple, with minimal training reauired.
(b) Online measurement
Online measurement includes:
• An internal array of image collector or direct contact welded thermocouples. However, due to the harsh environment in the furnace, the lifetime of these instruments is limited and relatively costly.
• In-tube process gas temperature monitoring; an instrument is inserted in the tube, and the readings used to calculate the tube wall temperature.
Catalyst tube inspection
Proper determination of tube conditions and ultimate lifespan requires specific in-situ examinations. Reformer tube condition is assessed in-situ using various NDE techniques: visual inspection, eddy current testing, ultrasonic attenuation, laser profilometry, radiographic inspection, replication, wall thickness measure, dyepenetrant test, combined NDE technique ‘H Scan.’ Each technique has its limitations which plant personnel should be aware of, and tube conditions cannot be determined conclusively by one stand-alone technique.
Tube expansion monitoring
The effect that high temperatures have on the tube is reflected in the overall thermal expansion, which is detectable on the suspension systems. It is essential to maintain the tube suspension systems (spring hangers or balanced weight) in good condition to ensure the proper support of the tubes without hindered expansions and excessive loads. The cold and hot positions of the suspension systems must be marked clearly, with safe access for inspecting, to indicate axial tube expansion.
Tube growth monitor (TGM)
TGM measures the thermal expansion of reformer tubes, giving accurate information regarding operating conditions relative to tube metal temperatures and providing feedback to the DCS. An alarm is triggered when expansion correlates to a tube temperature approaching the design temperature. TGM is effective for cases of hot tubes (temperature increases for the full tube length or the majority of the tube length). However, for cases of temperature increase in portions of the tubes (hot band, tiger tailing, giraffe necking) the reflection in the tube expansion is relatively small and may not alert the TGM alarm values.
Tube life assessment
The two most common methods for calculating the remaining creep life of a component are the Larsen-Miller method and the Omega method. The Larsen-Miller method has been around for decades and is used in API 530, Calculation of Heater-Tube Thickness in Petroleum Refineries. The Omega method is used by API 579-1/ASME FFS-1, Fitness-For-Service.
In both methods, the inputs are material-specific constants, stress, and temperature. The result is time-to-failure or end of design life. Accurate material properties and operating history must be used.
Process monitoring & control
Monitoring the operating parameters of the reformer within the acceptable safe limits and the developed Integrity Operating Windows (IOW) is essential to maintain safe reformer operation. Continuous monitoring and trending of the data are also necessary to show early indications of changes in parameters values before they become critical. While monitoring and control of the operating parameters is the role of the operator, the involvement of the process engineer for trends and periodical reports on the reformer performance is also required.
Typical parameters to be controlled include:
• Pressure Drop, DP
• Carbon formation: S/C (steam /carbon) control and optimization
• Over-firing protection
• Operating temperature, pressure and flow rate
• Approach to equilibrium, methane slip
• Monitoring of the firebox vacuum, oxygen and temperature
• Firing parameters