Lessons Learned from Vacuum System Troubleshooting

If not designed and operated correctly, vacuum systems can be prone to fouling, ice formation, and other issues that can affect product quality.

Vacuum systems are widely used in the chemical process industries (CPI). Distillation, separation, evaporation, reaction, and crystallization processes may all require vacuum equipment. Ejector systems and process precondensers are both used to create vacuum or sub-atmospheric conditions within unit operations.

Ejector systems create vacuum via isentropic expansion of high-pressure steam across a converging-diverging nozzle. High-pressure steam is more than twice the process operating pressure, thus steam passes through the nozzle at sonic velocity and exits the nozzle mouth at supersonic velocity. As the pressure energy of steam is converted to velocity, pressure exiting the nozzle mouth will be at vacuum conditions. This creates the sub-atmospheric pressure that pulls the process vapors into an ejector where they are entrained by and mixed with the supersonic steam. Another converging-diverging section, referred to as the diffuser, creates a shock wave that compresses the steam and process vapors to a higher pressure. Ejectors are staged in series to provide progressively greater levels of pressure.

A process precondenser creates vacuum by condensing process vapors at a high volumetric flowrate, which dramatically reduces the volumetric flowrate, thereby continually pulling process vapors into the precondenser. With operating pressure below atmospheric pressure, noncondensible gases must be continually evacuated from a precondenser. An ejector system is typically downstream of a precondenser for the purpose of evacuating noncondensible gases and vapors that do not condense.

Both of these vacuum systems are mainstays in the CPI. Reliable vacuum system performance is paramount to achieving the process throughput and product quality necessary to meet economic objectives. If designed and maintained correctly, they can provide reliable service for decades. However, performance shortfalls and maintenance issues do occur. A shortfall can be the result of the process conditions, fouling, poor utility supply, or mechanical damage to the vacuum system. The complexity and composition of chemical processes can present challenges in vacuum equipment; for example:

• the process constituents may not follow ideal vapor-liquid equilibrium (VLE) or may cause fouling/deposits
• the utility supply conditions may be off-specification (e.g., the inlet cooling water may be warmer than the design basis, or the steam supply to the ejectors may be wet)
• the actual plant operations may differ from the process simulations that formed the basis for vacuum system design. This is especially true years after the initial installation and/or when chemical producers adjust processes for greater throughput or purity.

This article presents five case studies and lessons learned from real-world troubleshooting of vacuum systems. The analysis and root cause of each case study is different, although each example is rather commonplace. These case studies emphasize the importance of ensuring good health and proper design and operation of a vacuum system through open discussion with the supplier before procurement and installation...