Vacuum problem during start-up

Question

Vacuum problem during start-up

powerzizo (Mechanical)

(OP)

29 Jul 15 14:49

We are running a power station with two steam turbine units 360MW each. in which SEIMENS EMLO-F 2BE1 liquid ring vacuum pump installed two per each steam turbine unit. Normally with 2 pumps in service we are able to pull condenser vacuum from atm to 130mbara in 70 minutes. We had a problem last week during one unit start up where vacuum pulling process went very slow and it took almost 30 hours to reach 130 mbara. Finally the unit is back in operation and we are investigating the root cause. The main concern is that the issue is not resolved yet and we will face the same situation during any startup . we doubt the performance of the vacuum pumps or air ingress source to the condenser, based on the attached trend that shows the behavior of turbine exhaust pressure/vacuum pumps inlet and liquid ring temperature/turbine exhaust temperature.we did not make Helium leak detection test to check if we have air ingress. Appreciate your thoughts on which is more probable root cause (i.e issues with the vacuum pumps or air ingress).
Thank You

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crshears (Electrical) · Answer 1 · 2015-07-29 16:50:51

In the absence of any mechanical work having been performed on turbines that would disturb correct function of seals etc., I'd check for under- or over-priming of the liquid ring.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

ScottyUK (Electrical) · Answer 2 · 2015-07-30 05:58:49

Is the gland sealing system working correctly? Are you sure it is working correctly?

powerzizo (Mechanical) · Answer 3 · 2015-07-30 07:33:10

The steam turbine had major overhaul in April 2015, where all seals were reviewed and tolerances checked-we had few start ups without vacuum problems.
we are preparing for vacuum pumps capacity test this week.

powerzizo (Mechanical) · Answer 4 · 2015-07-30 08:05:44

regarding the sealing steam- higher steam flow than normal noticed during start up. it was strange to see the LP turbine exhaust temperature increasing to 60-70C just after charging the sealing steam header and starting the vacuum pumps. normally the LP exhaust temperature decreases with vacuum pulling.
also noticed the inlet temperature to vacuum pumps is 35 to 37 C while the LP exhaust temperature is 60-70C, the liquid ring temperature is 37 to 38C almost equal to or higher than the vac pump inlet temperature which is strange.
Thank you

ScottyUK (Electrical) · Answer 5 · 2015-07-30 16:46:56

I recall one nightmare return-to-service which lasted days because we couldn't pull vacuum. We dismantled and recalibrated / rebuilt most of the gland steam control system before the mechanical group finally conceded that there was a problem elsewhere. They opened up the gland steam condenser, virtually at gunpoint, and discovered about 200lb of silt in the water side of the condenser. The machine behaved normally once that was sorted out.

EdStainless (Materials) · Answer 6 · 2015-07-31 01:42:45

If your pumps were running warm you need to check that cooling system very aggressively.
These are liquid ring pumps, water forms the seal, you can't pump below the vapor pressure of the water in the pump.

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Vacuum problem during start-up