Tower Pressure Survey

Make pressure measurement at flash zone, tower top and points in between. 

Determine Delta P across the strippers, both with steam on and with steam off. 

Measure ejector inter-stage pressures and condenser Delta P, noting temperature differences between condenser liquid and vapor outlet as well as Delta T across the condensers. 

Determine pressure drop across spray nozzles. 

If overall tower pressure drop is too low from the flash zone to the top then there may be damage to the tower internals or hydraulic problems. If the pressure drop is too high then flooding, plugging or internal damage may be indicated. 

---

The key tool in troubleshooting flash-zone pressure problems is a vacuum-tower pressure survey. The time to initiate this survey is just after start-up when the trays, demister, and ejector system are clean and in good condition. Pressures are best measured with a portable mercury-filled vacuum manometer. Using a vacuum pressure gauge will reduce the accuracy of observed pressure drops. Relying on permanently installed gauges for pressure drop data will not give reliable results. 

Figure 13-3 summarizes two vacuum-tower pressure surveys one just after unit start-up and the other a year into the run. The data clearly show that the demister is partially plugged with coke. 

Tower pressure survey Normalized pressure survey Coke buildup on demister Flooding of PAR trays Ejector deficient Excess thermal cracking... 

While X-ray pictures of tower internals easily detect most types of tray damage, this can be a cumbersome and expensive troubleshooting procedure. A simpler way to obtain almost the same information is by a pressure survey. Tower pressure drops of less than 1 in. of water per tray typically indicate tray damage, assuming a tray spacing of 24 in. 

Thermal cracking will eventually produce coke in the vacuum tower. The coke will wash down into the pump suction and plug the suction screen. A simple pressure survey will identify this problem. 

Other causes of excessive coke drum back pressure are badly fouled combination tower overhead condensers, partially plugged trays, or insufficient tower pumparound heat removal. Fouled condensers and lack of pumparound heat removal overload the wet gas compressor. Plugged trays are best identified with a pressure drop survey. 

A careful pressure drop survey indicated that there was an inexplicably high pressure drop in the liquid inlet line. When the tower was opened for inspection, the carcass of a dead rat was discovered lodged in the reboiler liquid inlet nozzle. Inadvertently, this rat caused the tower to flood as the reboiler rate was increased. If the rat had expired in the vapor outlet line, the effect would have been the same. 

---