Resources
December 3, 2018

Steam System Engineering – Flash Steam Recovery with a Cascade System or Thermocompressor Unit

Steam System Engineering and Steam System Optimization

My name is Kelly Paffel. I’m a technical manager for Inveno Engineering LLC, located in Tampa, Florida. We are an international engineering firm focused on steam and condensate systems.

Today, I would like to discuss steam system engineering, but specifically flash steam recovery from steam process and how we recover flash steam in a cascade or thermocompressing system.

Why recover flash steam? Industrial plants are focused on steam system optimization to improve safety, increase production, enhance reliability, and reduce energy and omissions. Unnecessary steam venting is, today the number one priority to improve steam system optimization and increase your steam system thermal cycle efficiency.

Flash steam generation. This is a proper operating steam trap.  Why recover flash steam? Steam processes, high percentage, will allow condensate to discharge into a pressurized condensate, flash recovery. Plants need to take advantage of this as you saw in the previous slide, the large volume of flash steam. This is a tremendous opportunity for enhancing the steam system thermal cycle efficiency.

We talk about the two systems that we have in the plant operation. We called non-modulating steam applications, the other one, high temperature process applications where we can utilize cascade systems and thermal compressing systems.

Non-modulating steam processes, these processes exists into our steam system today. The things with this type of a system, there is no modulating valve here. The process modulates. When there is no modulating valve then there is a constant differential pressure across the drain devices right here. Therefore, we can take elevation but also we can go into a pressurized return system.

Two-phase flow, flash and condensate from the steam process discharge can be directed into a pressurized flash tank for separation of the steam and the condensate. Here we have high pressure or non-modulating loads from the process directed into this flash tank. The flash steam is separated from the condensate and delivered into a lower pressure system.

The separation of flash and condensate, we allow no entrainment of condensate and flash or we get 100% steam quality coming out of the flash tank.

Flash steam is described in the previous slide, this is what we refer to cascading flash steam. Common in industrial plants to cascade flash steam several times from the very high pressure down to the lower pressure.

Some of the examples of non-modulating steam processes or steam applications; steam tracing systems, drip leg steam traps, process heaters, reboilers, corrugator, and et cetera.

Anytime we have a non-modulating load we can take it into a flash tank and cascade down to a lower pressure or recover all the flash steam.

The next one is steam processes with a high temperature process outlet. On this example here the process outlets 310 degrees Fahrenheit. 310 degrees Fahrenheit, if I look at the steam tables 90 psi so the lowest pressure is going to be here on my drain device at P4 is going to be 90 psi. Therefore, I have enough differential here to go discharge into a pressurized return system and go into a flash tank down here. This is regarded to a high temperature process outlet. Another example that I can recover flash steam and send it down into a lower pressure system or a cascade.

Steam process with a high temperature process outlet; wood product presses, tire presses, paper machine, metal processing, process heaters, just any textile presses, just to name a few of the applications.

Now flash tanks, the benefit of using a flash tank is for the separation of the condensate and the steam. Delivery of the flash steam at low velocities into a low pressure system to ensure condensate recovery. We calculate up, there’s enough space to store the amount of flash and the condensate in the system. Please note, the release of energy or flash steam occurs in a condensate that is properly sized for the flow rates. The flashing does not really occur in the tank it occurs in the condensate line prior to the tank. Flash tanks can be mounted vertically or horizontally. Vertical arrangement is the preferred method that we use in industrial plants.

The most important dimension and the design of a vertical flash tank is an internal diameter large enough to ensure low steam discharge velocity through the outlet nozzle to eliminate any possibility of carry-over, again, getting us a 100% steam quality. Outlet velocity from the flash tank should never exceed 3000 feet per minute. The condensate line must be properly sized to ensure flashes occur in the condensate system.

The next application I’ll talk about is thermocompressing. This is where we do not have a use for low pressure flash. Therefore, we use a thermocompressor to take the low pressure flash and but it up to a higher pressure. This is here is the thermo-compressor system here. This is the lower pressure steam coming off of the flash tank and of course we have to have a mode of steam there, which is a higher steam pressure. Example of the tank was at 30 psi, our motor steam could be 150 psi or higher and we would produce 60 psi intermedia pressure to go out into the system. Basically, thermo-compressing takes low pressure steam that’s normally not able to be reused and transform it into higher usable steam pressure we can use in the process.

Thermocompressor is a simple device, it’s exist for many years. A nozzle where high pressure steam at the plants are accelerated at a high velocity fluid to high velocity entrains low pressure steam from the flash tank by momentum transfer then recompresses in the divergent venture resulting in intermediate, usable pressure that is useful for the plant operation.

Again, just another way of taking low pressure steam instead of venting to an atmosphere which is the last thing we want to do today and reuse it back into a steam system, thermo-compressing.

Inveno Engineering LLC, our approach is short-term impact steam. We do steam system assessments, steam balance, steam engineering, training. Long term upgrades, risk assessments, project engineering, project management, water-hammer elimination, which is a big item for us. No one should have water-hammer. And process changing.

Here is our information, have any questions regarding steam system please contact us. Have a great day.

Thank you.