When I served as a corporate energy expert, requests for new permanent air compressors for plant air systems often caught my eye. Typically, a plant having problems keeping up with air demand would rent a compressor. Later, the rental's energy savings justified purchasing a new compressor. Department of Energy studies show that most inefficient plants could save 20% to 50% of compressed air. Good practices by maintenance crews save money. Blocking in utility air and capping is an essential part of post-maintenance clean up. Capping, rerouting or demolishing "unused" lines prevents unexpected losses. Have a crew walk around with an ultrasound detector to help find all valve leaks, including of process fluids, steam and other gases.
Flare, emergency and vapor recovery systems protect equipment from over-pressuring, but also can be used for unit evacuation, pressure regulation and waste gas disposal. Some systems use sweep gas to prevent oxygen accumulation in low pressure or vacuum lines. Most systems use 1,000 BTU/scf gas while the design can go as low as 300 BTU/scf. Replacing 50% of hydrocarbon gas with nitrogen lowers BTU content while still providing a margin of safety. Compare the cost of nitrogen versus sweep gas (whether natural fuel or other hydrocarbon) to see if this works for you.
Leaks into the vapor recovery or flare system are difficult to find. In most cases, you don't want to close isolation valves for fear they'll become unseated. If leaks cost you serious money, survey lines and take samples to determine what's leaking. Or add vibration monitors to system connections to help isolate leaks. Add them to flare relief valves and other valves that are normally closed.
In some regions you have to recover gas or register flare as a regulated emissions source. However, if fuel is constantly found in the flare, a gas recovery system can improve efficiency. If gas goes to a thermal oxidizer, and you're required to burn additional gas to keep the oxidizer functioning, investigate waste heat recovery. There's usually no regulation against adding coils to a thermal oxidizer to produce steam or heat water.
Cooling water systems have a tendency to foul; certain minerals may build up, which can increase scaling. An automatic blowdown system keeps the system running efficiently. But, in some places, use of brackish, salt or dirty water is a problem. There are a handful of exchangers where the process temperature is just a little too hot for the water. Using a closed-loop glycol water system automatically reduces scaling and fouling at the process exchanger where heat transfer is important. If there's a lot of maintenance or equipment replacement, or a need to reduce summertime rates because the water system gets too hot, investigate this option.
Condensate recovery systems save energy, water and money, but some plants have trouble with them. These systems are normally overloaded when flashing occurs in the line. Either live steam has leaked through a trap or high-pressure condensate was connected to the low-pressure line. If the steam is removed, the system would probably work fine. While it's best to find the problem and fix it, I've seen plants unsuccessfully search for years. So, try installing a pressure operated pump or flash pot system. The pressure operated pump is a brute force method that builds up backed-up condensate in a pot, which then is triggered to pump. The system is easy to set-up, but uses pressure to push condensate through the line. If your lines can take the pressure and no other systems start backing up, it will work fine.
The flash pot system is similar, but operates to flash steam at a pressure lower than the condensate return and then pumps remaining condensate. When designed correctly, this system eliminates all backpressure while providing more recovered condensate.
One of the biggest hidden gems is control system tuning. If the fuel system is swinging, start investigating how you control tank, drum and vessel levels. In systems that float on fuel or use a high and low pressure system control, control levels and spillback systems could be costly. I have seen whole plants swing because of one poorly tuned control on a badly designed vessel.
Are You an Energy Saver Too?
If you've got an energy saving idea you'd like to share with our readers, e-mail it along with your name, job title, company and location to: firstname.lastname@example.org. Submissions will be posted on www.ChemicalProcessing.com and will be referenced at the end of future Energy Saver columns.
Gary Faagau is Chemical Processing's Energy Columnist. You can e-mail him at GFaagau@putman.net.