Steam Jet Air Ejector problems. 1

[ksprague]

We have a "Z" and "Y" stage steam jet air ejector on our 160 MW Unit. The design steam conditions for the SJAE are 265 psi @ 900 degf. At low loads we have been losing vacuum to the point of putting on the "hogger" and almost tripping the unit. I have noticed that during these low loads that when the source steam (Main Steam) temperature drops to ~930 degf (normal is 1005 degf) the SJAE will change pitch and the backpressure will start to climb. The SJAE rep says it isn't temperature related. I say it is. Any thoughts?

 

[eyec]

when was the last time you inspected the ejector tube bundle?

 

[rmw]

You didn't say what your steam pressure is for low load conditions.

Ditto with the cooling water for your inter and/or after condensers.

It could very well be temperature related, especially if the pressure is changing as well.

Is yours a controlled motive steam pressure, or is it from one of the extractions? If from the extraction stage, then the pressure is following the expansion line of the turbine steam, and I suspect that it is falling off at low load.

Just remember this. The nozzle in the jet is designed for a specific volume flow. If your steam conditions change with respect to pressure and temperature so that the specific volume stays reasonably the same, you should have no problems in that area. However, if it goes the other way, so that you are putting much more motive steam through the jet, then this just becomes load for the after condenser to have to handle, and it reduces the load that can be pulled through the jet. Remember that the load is a combination of N/C and water vapor.

Second, what is the design suction pressure of the first stage jet?

Your condenser pressure may be getting lower than the jet is designed for although that is normally more of a problem for liquid ring vacuum pumps that are married to the seal water temperature than jets. Still, if your jet curve is such that at lower pressures, it loses capacity, it just might not be able to keep up.

All vacuum equipment, some worse than others, but jets included have more capacity at higher suction pressures than at lower.

Plus, a jet, LRVP, etc is just a compressor with a design compression ratio. IF your condenser pressure gets too low, you may have exceeded the compression ratio range of the device.

Third, take a look at your turbine heat balances and look for piping such as steam seal regulator piping that while it is normally under pressure at higher loads, might go negative at low load conditions. If you find this condition, inspect this piping carefully as you might be sucking in more air than the jets can handle.

Saw this on a 800 MW unit once. Simply reparing a loose flange connection in the steam seal piping made the problem go away.

Graham at

http://www.graham-mfg.com

has a free jet sizing software that you can get over the net. If you can get one of these, you can model your jet conditions and see what happens when motive steam and/or load conditions change.f

Fourthly; Have you inspected the jet to see if there is any leakage at the nozzle to steam chest connection? Some jets tend to wire draw the threads and leak past this joint. This leakage steam just becomes load that the jet has to handle, reducing the capacity of the jet and inter/after condensers.

rmw

 

[drtimnjoe]

As rmw inquired, What is you steam operating pressure at low and high load? If the rated temperature is 900F and you operate at 1005F then the SJAE should operate better at the quoted 930F. Since this is not the observed condition please consider the following.

Air-in-leakage is common at low load. Do you have a means to measure air-in-leakage. There is overwhelming evidence that the formation of liquid water in the vent line is directly related (and quite common) to condenser configuration, operating load/pressure, and air-in-leakage. The pitch change can be the result of liquid water entrained in the vent line gases at the low load condition. As rmw stated, the SJAE is designed for fixed specific volume flow or in other words it is a "mass transfer" pump. Therefore, if you have liquid water in your vent line at low load you will use more of your SJAE pump capacity by pumping the liquid water as part of the mass transfer. This reduces your water vapor and noncondensible pumping capacity and can significantly increase back pressure in the condenser.

If this has been helpful, we can help you further to solve this problem.