resonation in pump station

[mae1133]

We have just commissioned a triplex pump station consisting of a 30 ft deep wetwell and a 10 ft deep drywell. The drywell houses a 12" rubber swing flex check valve and a 12" plug valve on each discharge pipe prior to coming together at a common 12" header. The header is routed underground to another pump station located approximately 1 mile away. The pumps are rated for 1800 gpm and are on soft starts/stops. On the common discharge line, there is a sewage air release valve. The pump station is fully constructed of concrete with walls approximately 32" thick. Now that I have the physicals out of the way, here is our problem:

When any one of the pumps kick on, a resonation exists at various locations in the room that become unbearable, however, only at particular locations. If you step one step one way or the other, it virtually disappears and there is never appears to be any noticable vibration occuring with the piping or with anything else in the room. It seems to be only occurring withing the air space. On the wetwell side, there is never any indication of this noise. Everything appears to run very smoothly and quietly.

The resonation is strongest in the two corners separating the wetwell from the drywell and at the mid wall opposite to these two corners in the drywell. You can almost trace the resonation in a sinusoidal or triangular pattern across the room. Like I say, the piping appears to have no vibration when this is occurring and there is no sign of it in the wetwell.

I thought at first that there may be an air locking situtaion causing this, but we have ruled that out.

Does anyone have any hints or clues as to what the source of this may be and what a possible remedy is? I am not too concerned with this causing any physical damage, especially where there appears to be no vibration, however, it certainly is something we would like to get resolved.

Thanks to anyone who has suggestions.

 

[GregLocock]

It is called a room mode, or cavity resonance. The architectural acoustics people have more experience with these, you will find a good article on this in any acoustics textbook.

Cheers

Greg Locock

 

[MikeyP]

Can you say at what frequency the resonance is occuring (if you know - if not, tell us the running speed of the machinery. Can you also give us the room shape and dimensions?

M

 

[mae1133]

I first will offer the following correction to the flowrate provided above. There are three pumps at 910 gpm. They are 30HP, 1750 rpm pumps with 65 ft TDH. I do not know the frequency that the resonance is occurring at. The room has a footprint of 2.8m x 5.5m. There is a stairway separated by the 5.5m long wall that is 1.5m x 4.15m. Access to the stairway from the drywell is through a 2.2m high x 1.23m wide opening in the 5.5 m long concrete wall that separates the drywell and stairwell. Access to this stairwell from the exterior is through a Bilco hatch. Both drywell and stairwell is 2.4 m from floor to ceiling. The pipes from the three pumps extend out of the 5.5 m long wall that separates the wetwell and drywell at approximately equal spacing. The header runs parallel to the 5.5 m long wall at a distance of 1.65 m from it. It disharges downward through the floor at 1/3 the header length. The piping centerline is 0.89 m above the drywell floor. As indicated previously, the complete chamber is constructed of concrete. Hope this offers some additional information into this issue.

 

[MikeyP]

I'm afraid I don't understand all the pump and piping terminology, but as I understand it, you have a rectangular concrete box which is 2.8 x 5.5 x ?.?m

From the description you give of where the sound is strongest and weakest, it sounds like it is a half wave resonance along the length of the room.

The frequency of the pump and the frequency where one half-wavelength fits into the length of the room are both very close to 30 Hz. This would sound like a very low rumble in pitch. Is that whoat you are hearing? Alternatively it could be at a multiple of 30 Hz (i.e. 60 or 90)

So how can you deal with it? Well as it appears to be the coincidence of the pump running speed and the acoustic modes of the room, you could either change the size of the room (not very practical!) or change the running speed of the pump. Even if changing the pump speed is not going to be an option once the station is commissioned you could still try test runs at say 1500 or 1000 rpm to see if that really is the cause of the problem. Depending on circumstances, you may find that a change of +/- 10 rpm may be enough to significantly reduce the resonance it may be +/- 100. Play around and see what works.

If the pump speed cannot be changed, then unfortunately there is very little you can do. Damping out room resonances as low as 30 Hz is a difficult (and expensive) business.

I certainly agree that this type of noise is pretty unbearable. I did a test in a section of an aircraft fuselage with a half wave resonance of around 50 Hz. Standing in the centre of the fuselage facing the wall meant that each ear experienced opposite pressures. A very uncomfortable feeling indeed.

M

 

[mae1133]

MikeyP

Thanks for your input. My first thoughts when I was out at the pump station was to change the speed of the pumps, however, since the pumps are fixed speed, the only alternative I had was trying two pumps at once as opposed to one. By doing so, there was really no improvement. I should mention that the pump motors are 60 Hz motors so really, by running two pumps at the same time, I am not changing the speed or the frequency of the motor, just the volume of liquid being pumped. As mentioned earlier, the piping system does not appear to have any vibration to it and to reinforce that fact, I noticed an electrical cover plate with a few screws sitting on top of one of the valves. When the pumps run, there is absolutely no movement of the cap or screws. However, from the same valve, there is a paper tag hanging from a string. The paper waves around in the air as the pumps run. Very strange to see. Is there any fear of this type of thing causing any damage or is it simply a cosmetic/annoyance issue? If so, we will have to try to convince the owner that this is just "fluke" due to room size and shape and that there is no reason to be concerned for structural or mechanical damage. Thanks again!

 

[MikeyP]

Not knowing enough a) about pumps and their ancilliary equipment and b) about your particular situation, it is impossible to comment on potential fatigue or damage due to this resonance. You should also bear in mind that there amy be hearing protection issues should personnel be in the room for long periods of time.

Is the problem also present during your soft start procedure or only at running speed?

M

 

[mae1133]

MikeyP

There should never be personnel in the station for long periods of time and the duration of the resonance while the pumps are on is probably about 2 to 3 minutes, followed by 15 to 20 mins of downtime. Of course this will vary with flow into the wetwell.

When the pumps ramp up to speed, the resonance does not seem to be apparent until the pumps are at full speed. We are going to try to confirm this again on our next visit.

Even though there is no vibration in the piping, is it possible to have these resonances cause damage?

 

[hacksaw]

Following up on greg's comment. You have an accoustic resonance. These are solved by changing the boundary conditions on your walls (sound abs. material) or through the use of partitions that modify the accoustic size of your chamber.

You motor supplier can help you sort out the solution. It is hardly a new problem.

 

[MikeyP]

mae1143

The fact that there are no other resonances during ramp-up suggests that it is indeed the first half-wave resonance that is the problem.

There is always the potential for damage to occur in these situations but you would need to hire an acoustic consultant to investigate further. It is impossible in these forums to provide definite answers on questions like that. We have not seen the situation or taken measurements and the legal implications of making a statement on potential damage in that situation are decidedly dodgy!

Hacksaw,
I think the frequencies involved are too low for absorption to be a viable solution.

M

 

[mae1133]

MikyP:

I wasn't asking if we could expect damage caused by this for our specific situation. Rather, I was wondering in general if resonances of this type have been known to cause damage even without the presence of vibration.

We will be trying to confirm today whether or not the resonances exist during ramp up and ramp down.

I tend to agree with you that the frequencies are too low to expect absorbing panels or baffles to reduce or eliminate the problem.

Is this something that is simply a "fluke" that it has happened, having the right size and shape of room combined with the operating frequency of the equipment? ie. If the room was 1 ft larger or smaller in any direction, this may not have occurred? It is the first time that we have experienced it after many years and several similar installations. Are there methods of determining this phenomenon during the design stage?

 

[MikeyP]

Yes it is coincidence.

1 ft either way may make a difference, it may need to be 2 it may be 5 (similarly to the way that pump speed would effect the resonance). It rather depends on the level of acoustic damping (ie absorption) at the frequency of interest. As you are dealing with a simple concrete box, the reverberation time is probably quite high and hence the absorption quite low. This would meen that quite small changes in dimensions would have a marked effect. It's difficult to quantify without doing some acoustic tests. For a cuboid acoustic space, the natural frequencies of the lowest modes are easy to calculate. A building acoustics text should give you the required equations.

My estimate of 30 Hz for the first resonance was calculated simply by taking the longest dimension of the room as one half of the wavelength of the first natural frequency.

M