Sep-2003

Vacuum unit troubleshooting

Case studies show how accurate field-measured differential pressure can be interpreted incorrectly when troubleshooting refinery vacuum columns

Norman P Lieberman, Process Improvement Engineering
Daryl W Hanson, Process Consulting Services

Article Summary

The troubleshooting of refinery vacuum columns must begin with accurate differential pressure measurements. Specific pressure measurement elevations and detailed drawings of the column internals are essential to interpret the data properly. Without complete information, the wrong conclusions can be drawn from the measurements. The three case studies in this article present only some of the ways the data can be misinterpreted.

Measuring low pressure
The ability to accurately measure pressures under deep vacuum requires proper equipment. Acceptable measuring devices are absolute pressure mercury manometers or absolute pressure electronic devices. Moreover, tubing from the instrument to the process connection must not leak or the measured pressure will be higher than the true process pressure. To test for leaks, the process isolation valve should be first opened so that the measuring device reads the process pressure, then the valve must be blocked in. If the measured pressure gradually increases, the tubing is leaking. If it remains constant, there is no leak.

If the pressure does not fall to a steady value within a few seconds, the connection is partially plugged. The steady state reading will then read high, because a very minor air leak cannot be completely offset by a relatively larger opening into the connection.

Column internals generate a pressure drop if they are functioning properly, but so will liquid level. Since a pressure drop generated by packed column internals is very low, two gauges must be used, with the readings taken simultaneously. To ensure there is no offset between the two instruments, they must both be connected to the same process point and each have the same reading. By measuring differential pressure simultaneously, the readings are not affected by normal pressure variations.

Most digital gauges read a few mmHg low when measuring vacuums less than 10mmHg. Special high-precision gauges are required if accurate pressure readings are necessary. This is not a problem when measuring pressure drops.

Rules of thumb
Column internals generate a pressure drop and so does liquid head. Properly functioning stripping section trays will generate between 3.5–5mmHg per tray. Liquid level also produces dP, so 10” of 0.75 specific gravity liquid generates approximately 13–14mmHg. Since packing generates only a 0.375mmHg pressure drop per foot of packing, pressure must be measured simultaneously to determine the differential pressure. Five feet of packing generates only 2mmHg when operating properly, and 5mmHg when coke is being formed. Small variations in the operating pressure can materially change the conclusions about the condition of a packed bed when a single-gauge survey is performed. 

Case 1: “Low” wash section pressure drop
Cat feed hydrotreater (CFHT) run lengths were being reduced dramatically by rapid metals deposition on the catalyst. Recent refinery configuration changes allowed heavy Mexican and Venezuelan crudes containing very high vanadium to be processed. While the vacuum unit had a history of coking after a three-year run, switching to heavy crude caused downstream unit contamination problems in only 18 months. CFHT catalyst had reached end-of-run (EOR) and an unscheduled outage was planned. A decision needed to be made either to go into the vacuum column and replace the packing or to continue running. 

A pressure survey was conducted, because coke reduces the packing open area and generates higher dP as the coke builds up in the packing. Once the  pressure drop reaches approximately 1mmHg per foot of packing, heavy vacuum gas oil (HVGO) product will begin to turn black due to vacuum tower bottoms (VTB) entrainment. Pressures were measured simultaneously in the flash zone and above the wash section because of convenient platform access. Figure 1 shows the measured pressures and the locations. Measured dP across the slop wax (dirty gas oil or overflash) collector tray and 5ft of wash section packing was only 3mmHg.

Interpreting pressure drop correctly requires full knowledge of the column internals and how they influence pressure. Pressures were taken above the wash bed and at the same location as the flash zone pressure instrument. A detailed review of vendor drawings showed a vapour horn had been installed, yet it did not appear on the vessel elevation drawing. The flash zone pressure measurement was taken inside the vapour horn.

Pressure should never be measured inside vapour horns, because vapour and liquid velocities are very high. Kinetic energy can make the pressure reading lower, or momentum from liquid impinging on the pressure point can produce readings higher than the flash zone pressure. Pressure in the flash zone was 8mmHg higher than inside the horn. Therefore, the kinetic energy component of system pressure was very high, causing the measured pressure to be lower than the flash zone. Since the true pressure drop across the packing was very high, it confirmed the bed was badly coked and a shutdown was needed to replace the packing. Furthermore, a high column pressure drop reduced HVGO product yield.

Pressures outside the vapour horn and below the overflash pan are often 3–6mmHg higher than actual due to vapour and liquid impingement on the pressure tap. Taking this reading alone will lead the troubleshooter to a conclusion that the wash zone packing and overflash tray are coked, but this is often not so when the refiner opens the tower. The lesson is clear: a pressure tap immediately below the wash zone packing is needed to measure critical wash zone differential pressure.