Chicago Bridge & Iron LNG Cooling System Surge Analysis
- Industry: Liquified Natural Gas Industry
- Region: Illinois, USA
- Focus: Gasification System
Chicago Bridge & Iron’s (CB&I) Process Engineering Division Gasification System was tasked with developing the design for a gasification system serving an LNG off-loading terminal. An innovative and energy efficient approach was taken utilizing waste heat from a nearby power plant. A key criterion in the design was to insure the new, interconnecting system did not compromise the mechanical integrity or operation of the plant cooling system.
A previous analysis resulted in a design incorporating check valves, flow control valves, booster pump bypass valves and full voltage booster pumps. While meeting some of the design criteria, an analysis of the system using Datacor Impulse revealed vacuum conditions would exist in the piping under various operational scenarios, including pump trip. As information was not available on the vacuum capabilities of the existing piping, a new system design was developed to eliminate these possible problem areas.
The Datacor Impulse model consisted of both the existing cooling water system and the new, interconnecting system flowing in excess of 108,000 gpm. The existing system included:
- Six cell cooling tower with two vortex breakers
- Two circulating pumps
- Two condensers
- All interconnecting piping, valves, fittings, etc.
Adding to this the new, interconnecting system included;
- Four booster pumps
- Seven plate-frame heat exchangers
- Tower connecting valves
- All interconnecting piping, valves, fittings, etc.
One of the first steps was to benchmark the existing system. Running steady-state cases in Datacor Impulse, Doug DeGraaf, CB&I Senior Engineer, determined discrepancies in predicted and measured parameters was due to differences in the theoretical and actual losses in the condensers due to plugged tubes and fouling. Condenser loss values were adjusted to calibrate the model. As Doug explains; “Datacor Impulse allowed the separation of ‘reality’ from ‘theoretical’ to arrive at a true model of the existing system.”
Several transient cases were studied including tripping combinations of the existing circulating pumps and addedbooster pumps and closing of the new connecting valves. Dynamic modeling revealed considerable flow would continue after pump trip due to the momentum of the water in the long pipelines. This continued flow after pump trip caused low pressure in the existing cooling tower return lines resulting in reverse flow and air being drawn into these lines.
Initially check valves and other components had been included to avoid such conditions, but Datacor Impulse revealed this would result in transient pressures below atmospheric, a situation that needed to be avoided.