CASE STUDY: High Cooling Water Supply Temperature

In this case study we have a refinery in the US Gulf Coast region, operating a three stage ejector system and an additional pair of ejectors for off gas compression. The ejector system consists of two 50% trains of ejectors, with each stage having a corresponding 100% condenser. The system has historically had poor vacuum and instability issues during the summer months, negatively affecting the process. The system is designed to be operating at a pressure of 10 mmHgA, however it was running closer to 25 mmHgA when the data for this case study was gathered.

Often cooling water issues are most prominent during summertime operation. This is the time of year where the ambient temperatures are the hottest. The specific refinery in this example, being on the US Gulf Coast, will be exposed to higher humidity as well. This has an impact on the wet bulb temperature and the ability of the cooling tower to provide cooler water temperatures. Often the supply temperature of the cooling water ends up limited by ambient temperature and humidity.

The cooling water supply temperature in this example was 92.5 ºF, while the design was 90 ºF. While a supply temperature of 2.5 ºF above design does not sound like much, to a condenser this can mean a noticeable increase in its operating pressure. Increase in operating pressures of a condenser will directly lead to higher back pressures to the upstream equipment. In most installations, the hotter cooling water impacts the first condenser in the system. This is because as you move to a deeper vacuum, you require colder temperatures to condense the steam and hydrocarbons. The first intercondenser in this system operates at the lowest vacuum when compared to the system's other condensers.

The design operating pressure of the first intercondenser is 78 mmHgA. The cooling water flow to the first intercondenser was near the design yet the operating pressure of the condenser was measured to be 82 mmHgA. The additional 4 mmHg backpressure were causing the first stage ejector to break operation. This poorer first intercondenser operating pressure was due to the hotter than design cooling water supply temperature. This was causing the first stage ejector to break operating during the daytime. At night, ambient temperatures dropped and the ejector would regain its internal shockwave and operate in a critical manner per its design. 

The remaining downstream condensers were operating at a better than design vacuum due to a lower than design noncondensable load and a good cooling water flow rate, even with worse than design cooling water supply temperatures. Those ejectors remained picked up and stable at all times. In this way, the poor cooling water supply temperatures were only affecting the first intercondenser and first stage ejector.

Ideally, every ejector system supplied would be designed for a cooling water supply temperature that would be adequate for year round operation. Unfortunately, this is not always the case and often systems are subjected to cooling water supply temperatures higher than the design as in this example.

This leads us to the question, “What can be done to help overcome the higher cooling water supply temperature?”

Here are some potential solutions:

In this specific example, the cooling tower was being limited by atmospheric conditions and all of the cooling tower cells and pumps were in service so no gains could be made there. The site contemplated renting a chiller to help cool down the supply temperature but decided not to pursue that option due to the proposed chiller's rental and operating costs. In the end, a water spray was added to the top of the first intercondenser. This provided an improvement in the vacuum stability. It helped relieve backpressure to the first stage ejectors and allowed them to remain picked up and operating at a vacuum that was close to the design. However, poor vacuum issues and instability continued to be an issue during the hottest parts of the day but were less frequent than before the water spray was added and were deemed tolerable until the summer months faded into fall.