Vacuum low (Condenser exhaust steam pressure high)

[instrumentTH]

I have issue about exhaust steam pressure high, cannot build up vacuum condenser.
For detail:
1. Combine cycle power plant 2 unit of GT(I will call GT-A 40MW and GT-B 40MW) 1 unit of ST (40 MW)
2. Found this issue after minor inspection GT and ST (first minor inspection)
3. Now, operated with open cycle due to some problem about Steam turbine
4. For start with GT-A can build up condenser vacuum normally (-0.95 barg) at 5 MW
5. But for start with GT-B cannot build up condenser vacuum (vacuum -0.80 barg) higher than permissive setting to control load (-0.85 barg) at 5 MW
6. For HP steam can control by De-super-heat but IP not have.

For more detail,
1. I found that Exhaust steam temperature of GT-B (60 C) higher than GT-A (40 C)
and then I found temperature after IP bypass of condition start with GT-B higher than GT-A around 10C
2. During Start GT-B found that between pipe line of IP main steam bypass valve of unit A (IP stop valve bypass and IP control valve bypass of unit A are close) are vacuum (-0.5 barg) and we sure that IP bypass valve passing.
3. I found that IP main steam pressure of GT-B pressure below than GT-A (due to bypass control valve of GT-B open more than GT-A) but temperature are nearly.
4. Exhaust hood spray normally sequence and normally operated

So, for my opinion I think may be cause below
1. Exhaust steam temperature high due to spray de-super-heat after valve bypass abnormal (spray pattern not good)
2. IP bypass control valve passing of unit GT-A effect to cannot build up vacuum of GT-B. (possible or not?)
3. Main steam condition (Pressure, Temperature, Flow)
4. Exhaust temperature of GT (I think it's effect to steam super heat condition for control entralpy)

I like to know other opinion please advise me.
1. What is the cause of this problem and how to solution?
2. If I try to manual mode for control spray water after bypass, what is effect?
3. and If I try to manual open exhaust hood spray what is effect, and it can be reduce exhaust steam temperature or not?
4. If IP bypass valve of other unit passing it's possible cause of vacuum low?
5. How about the relation between vacuum and exhaust temperature (from bypass directly)? Vacuum is lead effect to steam temperature or exhaust temperature is lead effect to vacuum?

 

[EdStainless]

Is your vacuum supported with pumps or eductors?
You are talking about running in bypass, right?
If you have different steam temps then your spray systems are not responding the same, or one of your HRSGs is not making steam correctly.
The steam in bypass mode should be saturated, but with no entrained water in it, at the condenser.

Has this system worked correctly in the past?

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P.E. Metallurgy, Plymouth Tube

 

[instrumentTH]

To EdStainless, Thank you for your attention.

- Condenser system or Vacuum it's supported by steam ejectors system.
- Yes, running in bypass mode

For bypass mode, detail from instruction manual given that Spray water for control temperature after bypass valve will be control minimum 15C above saturated of condenser pressure, which condenser pressure of unit GT-B (problem unit) = 0.79 barg So, temperature after bypass valve should be around 75-80 C (right or not?). But in situation temperature after bypass valve are around 110C.

In the past this system worked correctly, not problem.

 

[EdStainless]

I would look at both parts.
First (since it is easy) make sure that the eductors are working correctly. You should have the blank off pressure in the specs, so you can seal their inlet and make sure that they reach the correct lower pressure.
As for the DSH system you have a sensor, the control loop, the valve and the spray. My hunch is that this isn't a closed loop system or it would be adding more water. So is it sensing correctly? Is the program calling for the correct amount of water? Though it is more likely that the valve is not working correctly or that the water is not being sprayed correctly.

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P.E. Metallurgy, Plymouth Tube

 

[instrumentTH]

Thank you

First, We sure that ejector working correctly because parameter control and monitoring are normal and it's common unit, ejector system can operated normally in other unit.
For DSH system is close loop control sensor and nozzle of spray are normal (check complete - disassembly and clean) and it has program to correct amount of water by enthalpy control.


Now, We solve this problem already.
We are sure and proof that root cause of this case is bypass valve in the other unit passing.
After we check and overhaul valve, system are normal operated - can build up vacuum to normal condition.