Sep-2003

Crude unit start-ups: the results of a 
high liquid level

Pressure-measurement techniques can avoid the dangers of rapidly changing flow rates and levels during start-up

N M “Tinus” Erasmus, Sasol South Africa
Tony Barletta, Process Consulting Services

Article Summary

National Petroleum Refiners of South Africa Ltd (Natref) revamped its crude unit in 2002 to increase capacity. The start-up and operating procedures were revised to reflect extensive flow scheme changes, which included many new pieces of equipment. Nonetheless, even after careful planning, abnormal events can take place during start-up, such as a high liquid level. Level instrumentation alone is not enough to avoid this problem and its consequences. However, pressure is a simple, yet often overlooked, measurement that operators and start-up personnel can rely on instead.

Start-up can be a dangerous time, as flow rates and levels are changing rapidly and several things are occurring at the same time. Many crude units experience internal damage during start-up, which most engineers associate with wet steam; for example, pressure surges from the rapid expansion of water into superheated steam. However, a high liquid level occurs so often it is the most common cause of internal damage during start-up. It occurs when high-velocity steam in the bottoms or two-phase feeds from the transfer line generates sufficient forces to dislodge equipment from the stripping, flash or wash sections.

When crude unit stripping sections are damaged during start-up, atmospheric crude and vacuum column distillate yield losses can be as high as 4 and 3% of whole crude, respectively. Furthermore, when flash section internals do not function properly, the distillate product can be contaminated with metals and carbon residue. With such large potential economic consequences, operating procedures need to be reviewed to find practical ways to ensure the liquid level is monitored. Also, column internals directly above and below the flash zones must be designed for higher uplift forces that inevitably result from a high level or wet steam.

Measuring level with pressure
The liquid level generates differential pressure between any two points. Figure 1 shows the calculated (expected) and measured pressures for a vacuum crude column designed with a stripping section. In this example, a refiner was having problems maintaining a low operating pressure and the heavy vacuum gas oil (HVGO) product yield was low.

Even though the vacuum column bottoms level controller showed the height of the liquid to be 60% of the instrument range, which would have maintained level in the boot section, the level was checked. The exact height of the liquid in a column can be determined by measuring the differential pressure between elevations. Accurate vacuum column measurements require absolute pressure mercury manometers or appropriately ranged electronic gauges.

In this case, two electronic absolute pressure-measuring devices were used. Before taking field measurements, exact elevations were established for all pressure-measuring points. Typical locations are level instruments, pressure instruments or other taps. Pump suction pressure can be used as the reference point (elevation zero) for the other elevations. The pressure difference between any points and the fluid-specific gravity is all that is needed to calculate the level. When a system is  liquid full, the pressure difference between any two points will correspond to the pressure exerted by the static head. Hence, measuring three locations always provides a consistency check.

In Figure 1, if the instrument reading of 60% level had been correct, the absolute pressure at the pump-suction would have been approximately 589mmH. Yet, when the pump suction pressure was measured, it was 1263mmHg – nearly double the level instrument indication. In this case, the liquid level was nearly as high as the bottom of the transfer line. Since vacuum column transfer line velocity is 91m/s or higher, a high-energy wave of liquid is created in the flash zone when liquid covers the transfer line. In many columns, internals above the flash zone have been severely damaged by this high level.  

Over time, the gas flow from the ejector hotwell had increased and the liquid level was being checked because there were no apparent changes in the heater operation. The vacuum heater is the source of 90% or more of the non-condensable gases in most vacuum units.  Therefore, once the pump suction-pressure reading confirmed a high liquid level, the true level was pulled down until the measured pump suction was approximately 589mmHg. At this point, the level instrument reading was 0% level. However, once the true level reached the boot, cracked gas production decreased by almost 25%.

Cracked gas is generated by high temperature and oil residence time. If either of these two increase, cracked gas product will increase as well. When the level instrument malfunctioned, the liquid level increased until it reached the flash zone, which significantly increased the oil residence time. This is why vacuum column boots are typically small, with only 30–45 seconds of residence time. The boot temperature is often reduced by recycling cold vacuum tower bottoms (VTB) quench from the heat exchanger network to further suppress thermal cracking and gas formation. When the liquid level reached the larger diameter stripping or flash zone sections, the liquid above the quench inlet was at the same temperatures as the flash zone. Flash zone temperatures on deep-cut units can be as high as 410°C, thus severe thermal cracking results if there is a high level. 

Since high column-operating pressure alone could not explain the low HVGO product yield, the stripping section tray pressure drop was also checked. Thus, a consequence of the high level was probable stripping tray damage. Once the level was reduced, the measured pressure drop across the stripping section was nearly zero. A well-designed tray in this service will generate between 3.5–4.5mmHg pressure drop. All the stripping section trays had been dislodged by the high liquid level.

Avoiding wet stripping steam
Wet steam can also cause internal column damage. When the column operating temperature is high, the stripping steam needs to be dry before it is put in service, because water will rapidly vapourise, creating a pressure surge. The stripping steam system needs to be designed with the provision to blow the line to atmosphere to ensure no significant amounts of water enter the column. In vacuum service, the isolation block valve is normally installed just downstream of the stripping steam control valve, because the steam line size increases dramatically after the control valve, and most refiners do not install an isolation valve at the column. If water accumulates in this line, it should not be a concern, as water boils at a low temperature under vacuum. Water will vapourise at a low temperature when the ejectors are put in service.