Deaerator Bursting Disc Noise Problems

[dugal]

Hello,
We have large deaerators in our generating station that have a rupture disc on them (approx 10&quot. Ocassionally one will blow up a pipe through the roof. It goes off like a jet engine and there is lots of steam. Problem is, this scares the nearby residents and there are always phone calls. The design is about 40 years old.
Would the code allow mufflers to be installed to quiet down the release and a way to reduce the steam going to atmosphere?

 

[saxon]

dugal, By installing mufflers on the discharge pipe you will be creating a back pressure on the discs and deaerators which could possiblly result in overpressuring the deaerators resulting in their failure. The sound problem is due primarily to high velocity flow at the discharge point. Reduce the velocity, reduce the sound. This can be accomplished by large dia. pipe, probably larger than you can install. Tough problem.

saxon

 

[dugal]

I know anything causing back pressure would be a major operational danger to the deaerator. As Saxon said probably just by increasing the vent pipe size several pipe sizes up before the vent goes through the roof would certainly tone down the noise some. It may change the sound from a jet engine to that of a frieght train Of course we would have to also strengthen the vent pipe hangers.
Would like to know what others may have done or would do in this situation. This is a problem that our plant has considered "normal operation" for years. I look at it as un-necessary noise polution that we constantly have to apologize for.

Dugal

 

[saxon]

dugal, I know of a similiar problem involving flare stacks; they could turn the night into day and sounded like freight trains . To solve the problem of neighbor complaints, the company bought out the property owners and created a 1.5 mile radius safety zone around the facility.

saxon

 

[jproj]

dugal

Why is your DA is over pressurizing? Popping a rupture disk is not normal operation and should not happen on a regular basis (even for a 40 year old DA). You should have control valves on your steam lines that sense the unit's operating pressure.... they should not be open when the unit is above a certain pressure (certainly below the design pressure). Could you post some more info on how the DA is operating so the rest of us can get a better idea of how to best help you?

Best regards,
jproj

 

[dugal]

Saxon,
We have about a 1/2 mile exclusion zone around the plant, then there is medium density residential, commercial and industrial area. Too much to buy out.
dugal

 

[dugal]

I will talk to the operators tomorrow and ask what is the most common reason for blowing a rupture disc. It does not happen often and could be an in-experienced field operator. I know maintenance do not change the discs out on any routine basis.
dugal

 

[CHD01]

Regarding the rupture disk, these disks fatigue with time with the length of service dependent upon: number of pressure cycles, how close to burst pressure the maximum operating pressure is and temperature of the disk.

Try to get a history of rupture disk failures to see if there is a consistent pattern; you may want to replace these disks on some interval - like evry 5 years or so.

Also, the type of disk and ists installation are critical as well and often merit investigation.

Last, you should consult with a silencer vendor and examine the potential for installing a silencer in-line; and also look at the diameter of the vent tip. This is a condition that you should be able to resolce with good engineering.


The more you learn, the less you are certain of.

 

[dugal]

Hi Folks,
I looked more into our rupture disc (20&quotfailure problems. It seems they are prone to fail (at 400kpa) on turbine start up and shut down. There are lot of pressure transients at these times. It has been noted that the CV's & MV's controlling the deaerator failed to do so on several ocassions causing overpressure. In 1999 the rupture discs were also put on a 4 year change out program to reduce the chance of failure from disc fatigue. The frequency of discs blowing seem to be once every 2 years on an average. I know there are a lot of variables why the discs blow and we need to look at the design arrangment for the RD's, pressure sensors, CV'& MV's and operational/maintenance practices.
Aside from premature rupture disc failure there is the BIG noise problem when they do blow. Am I right in believing that the code permits putting any silencing device on a RD steam line as long as it does not cause back pressure? Also that such devices exist for a 20" steam venting line ?
I have also noted several sideline problems while investigating this and one is that the panel operators have no alarm to say when a disc does blow. Also that there is no quick means of recovering from a blown RD. A blown disc usually means the units state is on hold for 16 hours. Is it practical to put a Y in a pipe downstream of a RD and put a valve on one Y and another RD on the other Y. These Y's would then join back into the common vent line. Then should the first RD blow it would exhaust out the open valve. When the operators regain deaerator pressure control the valve could be slowly closed leaving you with all the pressure against the second un-blown RD. A set up like this could buy you 16 hours of time back on line (unitl the second one blows).
dugal

 

[ibrahimdemir]

Hi Everyone,

I guess a silencer installation in accordance to ASME B31.1 Open System may solve the problem. Instead of safety valve you will have a rupture disc and a short pipe for the open system connection. This pipe will be inserted into the discharge pipe and silencer connected to this pipe. Just before the rupture disk failure, the back of the rupture disk will be at/around atmospheric pressure. There will not be any back pressure at the rupture. After the rupture, there will be some kind of pressure losses in the system of course due to silencer and pipe friction back pressure. Silencer back pressure ( I suggest 15 KPa or under ) may be reduced by the silencer manufacturer by compramising the size of it. This back pressure should be a lot less than the deaerator pressure.

However, a design engineer's and a silencer manufacturer's involvments are essential. They may require the whole process information, pressures, temperatures, flow rates etc... Silencer and the exhaust pipe have to be supported independantly to take ultimate pressure forces due to losses in the system.

Don't you think this system work? Any comment.

Ibrahim Demir
Design Engineer

 

[jproj]

dugal:

I really don't know if this is allowed (by code) or even possible (not sure how rupture disks are sold.... in terms of rupture pressure), but you may even look into buying a silencer and then purchasing rupture disks such that the pressure drop across the silencer and the RD bursting pressure add up to the deaerator's design pressure.

jproj

 

[abcmex]

my first comment is that the best way of avoiding the noise is to avoid that a disk blows. Try to find out why this is happening and improve your operation, controls and procedures. A big help can be if you have a reliable information coming from an automatic system and not from the probable culprits of the event.

the second comment is try to control the pressure and if this is not possible: you may install an additional pressure relief valve with a silencer with a lower set point, in this way you will not endanger the installation

 

[CHD01]

1. You need to refer to API standard 611 and 612 for steam turbines which may have specific requirements you must meet.

If there are no exceptions per API611 and 612 then the following general statements should apply:

2. You need to calculate the total system pressure at the design (required relief rate) flow for the rupture disk with what ever silencing equipemt you install. Rupture disks have no back pressure requirement like a relief valve does; the only requirement is that the equipement protected not exceed its design pressure (MAWP) plus allowable overpressure (usually 10%). TO do this you must determine the system resistance K factor. Get a pro if you do not know how.

3. I doubt you will easily find a 3-way valve 20=in in diameter. If your required vent line really is 20-in; you will probably need at least a 24-in 3-way valve to get the dp down. You can do this but it will be expensive; the best solution may be key-controlled butterfly valve to isolate a disk for service. The concept would be tohave a "Y" with each branch of the Y going to a rupture disk with a valve istalled on the inlet and outlet of each rupture disk. The valves would have to be controlled so that when one disk is in service the other is out of service. All of this would have to meet dp requirements relative to the equipment protected for both branches.

4. Last, make sure the disk is your best application per the API standrads I listed; I have not had the occasion to use those standrads - so you need to check them out.

5. Regarding disks, also remember that they are sensitive to pressure surges caused by velocity of changes in direction; if the disk is at the end of a line at an elbow it may be possible to create a pressure spike perhapsa 1.5X above normal operating pressure on sudden changes is flow.



The more you learn, the less you are certain of.

 

[dugal]

Thanks Folks,
Some good ideas tossed around here.
I think it would take a team of one of our mechanical piping design engineers, turbine system engineer, a rupture disc and steam silencer company to resolve the problem. I am not at all convinced our configuration is proper. These rupture discs do not help our annual generating capacities and does less for our public relations with the noise problem.
I shall now endeavor to find out if similar generating stations have the same problems. If not, then I shall go after our upper stratums to have a design review done of our configuration. This is always the hard part.
Thanks
dugal

 

[CHD01]

I want to clarify one point, rupture disks are sensitive to constant superimposed backpressure since this will increase the burst pressure; but they are not limited to variable backpressure except as I noted in my earlier memo.

The more you learn, the less you are certain of.

 

[mortos]

Hi I am trying to research noise reducing solutions for an existing geothermal steam rupture disk configuration.

I saw that rupture disk silencers where mentioned but have been unable to find anyone with experience in rupture disk - silencer configuration or suppliers.

Could anyone point me to a possible supplier with experience in this type of configuration?

 

[EdStainless]

I have seen one system where they went to a larger diameter vent. This lowered the frequency of the sound. They then installed active noise suppression units on the duct. They were able to lower the sound to a reasonable level.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[hacksaw]

Vent silencers are not really that un-common, but they are a bit expensive.

They are not a simple add on device. You'll have to come of with an engineered assessment of the D/A system and the various relief conditions, too. Sounds like piping changes will be significant.