Control valve noise /vibration 1

[fatlenny]

We have developed a strange noise in the area of our control valves over the last month..would only come in for a minute or so up until the past few weeks.Along with the noise we see a very small change in our first stage psi..we have had the noise stop while stroking all 4 cv's but it stops at a different position depending on the valve.Any ideas would be welcomed.Thanks

 

[byrdj]

what type unit might help
Are the disk solid to the stem or loose?
are the disk balanced (or maybe samuelson design)?
seperate or shell mounted?

It is possible for flow around a disk to generate a lifting force that causes the disk to flutter. this can happen with wear, if the design has been proven.
any recent changes to valve design?

what type of controls?
do you have any actuator movement?

 

[fatlenny]

We are a 720mw unit 2460psi rated flow and 1050 degrees @ inlet of machine..We have a Toshiba turbine Tandem Compound double flow type..no changes..new plant..been commercial for a year.I'm gathering other info from our Engineers...i'm the Shift supervisor.Thanks

 

[byrdj]

From the size, I'll assume the CVs are mounted in a chest seperate from the HP shell. I would also assume at least 3 of the 4 are balanced. The several valve flutters for this configuration I worked on were with CV#3 or CV#4 as it opened sequentialy. the first stage pressure that would be under the CV reduced the downward force of mainsteam acting on the disk. In the case of CV#4, that force under the disk is great enough that this valve may not need balancing. If CV#4 is fluttering near its crack point, it will be hammering the disk and seat

Can you change the admission mode, ie go from sequential CV opening to all 4 opening together, however if the design is marginal, all 4 may start to flutter. If you are operating at CV#4 crack point, going single admidssion will at least get it of its seat

 

[fatlenny]

We are in gov control mode ..and sliding pressure..i am not used to not having valves backseated when sliding pressure...the noise has become more pronounced when valve position @ 87% or higher...when we stroked #4 the noise became even louder..in gov mode the valves do indeed move together..we also have load limiting capability...which i believe would be the sequential order you mentioned...thanks ..this is good information

 

[fatlenny]

Oh and yes the control valves are mounted right before the chest and immediately after the MS Stops

 

[fatlenny]

One more bit of information might be helpful..we have UCC (unit coordinated control) we can use sliding psi or fixed..we have options of turbine follow,trb follow + mw,boiler follow ,blr follow +mw and base...thanks again Byrdj

 

[byrdj]

Terminology between OEMs might add to confussion in discussions here. I have not dealt with Toshiba, I'm familair with GE.

85% of total CV would be the expected range where I would expect CV#4 to be opening (ie at crack point)for any sequential admission scheme. I do not think CVs have a backseat, the actuator usally has a top stop, I was saying operating just above crack point the disk could be hammering the seat. If a valve is fluttering above crack point, the disk is rattling on the end of the steam. A balanced valve can have considerable freedom between the disk and stem for its equalizer valve, which would then be hammered.

being on govenor usually means the CVs are constanly being adjusted to compensate for grid frequecny variations. this is usually very small movements (<1% total) ever couple seconds. But being on the govenor opens up the possibilty for noise in the control system to cause the valves to be cycled excessively. for example, I have seen CVs shaken 5% stroke at 10hertz due to a bad govenor.

being on load limit usually means that the opening potential of the govenor has been limited and thus the CVs no longer respond to the grid variations, unless the frequency increases significantly and then the CVs would close.

for whether or not you are in seqenced admission, if you have position meters for each of the CVs, you would notice at rated pressure, rated load, the CV 1, 2 & 3 would be near 100% but CV4 only 50%. Full load rated ususlly occurs at 95% total CV


when you slide pressure, you could adjust the govenor to allow all 4 CVs to go to 100% and stay there with load reduction with pressure drop. If you operate CVWO, does the noise go away

 

[fatlenny]

I guess 100% is what Toshiba refers to as back seating. We have some Toshiba Engineers scheduled to come out this week.I'll let you know what they determine it to be.Thanks so much for your input and time.

 

[byrdj]

Hope you find something minor and external to fix.

I'm not familair with Toshiba and I find CVs with back seat interesting. I guess it would reduce stem leak off when at CVWO and thus some effeciancy gain. IF the actuator does not have open end over travel and the actuator tops out just before the back seat, I would think that would cause a whistle

Is the unit in the US?

 

[fatlenny]

Yes sir, we are in arkansas.

 

[rmw]

Anything else unusual in your operation from the time when you ran without any noise until the symptoms began?

Any unusual trips or outages, etc?

rmw

 

[byrdj]

BTW congradulations to your site, I saw PPES featured in POWER this month.