Techy query - bad vibrations!

[chrishutt]

I've recently installed a Telford Tempest unvented cylinder with a Keston C25 boiler. All fine, but the corrugated stainless steel coil in the Tempest vibrates when water is pumped through. At certain slow pump speeds this seems to resonate and cause a loud buzzing noise.

I'm thinking of introducing something to absorb the vibrations (plastic pipe and/or mini expansion vessel), but I'd rather deal with the source of the problem. I'm sure it's not down to the fixings of the flow and return pipes, and it doesn't arise when the boiler's doing CH, so I'm pretty sure it's the coil that generates the noise. Anyone come across anything similar or have any ideas?

 

[ChrisR]

ONly simple suggestion I have would be a gate valve in the primary

 

[chrishutt]

I briefly thought about that, but since the vibrations are noisiest at the lowest pump speed (the C25 varies pump speed with boiler output) the gate valve would have to be almost shut to further restrict flow at that low flow rate, causing very high resistance at higher flows. I've already got ball valves in the circuit so I could play around with those to see what happens in practice (which I hadn't thought of doing before, so your comment was helpful, ChrisR).

 

[gas4you]

I don't know these cylinders, but could it be the coil is faulty somehow?

 

[chrishutt]

I had a look through the immersion heater boss with a torch and couldn't see anything amiss, although I couldn't see all the coil. Apart from the coil inlet and outlet connections the only fixings are a couple of metal clips that hold the different rings of the coil apart from each other at the correct spacings. Maybe there should be three?

With the cylinder empty of water you can observe the coil vibrating as water is pumped through. I guess when the cylinder is full the vibration is damped down, but at very slow flow rates it seems to set up resonance which is when it gets noisy. Thanks for the comments so far.

 

[Agile]

It certainly sounds like a manufacturing/design problem. I doubt that you would have done anything wrong to cause it.

I would suggest you call the manufacturer and ask them to come and see it.

Please keep us informed !

Tony

 

[TicklyT]

Resonant vibrations can be odd things to try and cure. It may be that you've got just the right (or wrong, depending on your point of view) length of pipe between the pump and cylinder, so that the whole lot resonates in sympathy with the pump impeller running at low speed, just like engine tuners do to inlet and exhaust systems to maximise power outputs.

Maybe adding a couple of feet or so to the length of the pipe runs would be enough to stop any sympathetic resonance.

 

[Agile]

How about adding a quarter wave stub to the inlet or outlet ?

The topic should remind you of the beach Boys hit !

Tony

 

[chrishutt]

How about adding a quarter wave stub to the inlet or outlet ?

Tony, can you explain what a quarter wave stub is, and what a hydraulic equivalent might be? I've had a Google on it but the links look extremely technical.

 

[Agile]

Its basically a short circuit length of wire or tube with a length of 1/4 wavelength at the frequency you want to filter out.

In this case it would be a length of tube of the same size, say 22 mm, connected in parallel to form a dead leg as its remote end would be capped off.

In effect it forms a resonant length like an organ pipe and the objective is to create a "short circuit" at the offending frequency where it joins the flow pipe in the "T".

Paul Barker may be able to explain it more simply!

Chris R will provide the formulae to calculate the required length using the speed of transmission of sound in water, 1500 m/s.

Tony

 

[ChrisR]

Length is 6000/f, obviously
So if the frequency is middle C, yer stub would be 23 metres long. Very useful

Maybe best not to go that way then, though I suppose a coil of microbore could do it... Now would you need an air gap at the end of the pipe - in which case wouldn't a shock arrestor do anyway?

You could try knobbling the Q of the resonant circuit.
Springy systems resonate when the kinetic energy of the movement is efficiently converted to potential energy of the strain of the elastic thing, whatever that is, and back again. The electrical parallel is tempting to use but only if you've seen it, otherwse too abstract to visualise.
SO think of a tuning fork, bending , storing the energy, then whizzing to the other extreme of bend where it stops again, and so on. If you want to knobble it, put a blob of treacle on it.

Assuming you don't want to dribble any goo into the cylinder, you could try the electrical solution, which is in effect to leak some current away from the system. So put a bypass, with a valve in it, between the flow and return. Then any peaks in vibrational movement in the water will not cause the pipe to spring (hopefully) because the movement in the water can be accommodated in the bypass.

Well it sounds good, dunnit?

 

[Agile]

Taking that last point a little further, if the resonance is severe then a diode damping circuit would seriously lower the "Q" of the resonant circuit.

The hydraulic equivalent is a non return valve. This should be a low forward voltage type or low spring pressure ! Connect it across the heating coil so that its normally closed in the flow direction. Any resonance will cause it to open and dissipate the resonant energy by short circuiting it.

In spite of all this, I still expect that it results from an inadaquate manufacturing aspect.

Tony

 

[ChrisR]

That did occur to me, as did the noise the darned things make!

How close is the immersion heater hole? If you know anyone good at ships in bottles, or a certain type of ladies' doctor, perhaps you could get a plastic ty-rap round some of the heating coils.

 

[chrishutt]

Interesting input, thanks. The immersion boss is smaller than normal (1.5" thread I think) so not much chance of getting a hand through, even less of getting it out again! As stated above the coil material is corrugated stainless tube which is naturally springy, hence it's tendency to vibrate, hence resonance which I think can occur at different frequencies.

Currently the resonance starts when the C25 pump is on it's lowest speed stage (one light on temp LEDs, next stage burner off and higher pump speed to dissipate heat). However I have also had resonance with higher pump speeds with different pipework connections (a long story!).

I've already found one way of damping the vibrations and stopping the resonance. I put tees in the flow and return pipes and attached a length of 15mm plastic pipe between the two (stressed by being sharply bent), intending to test the affect of a by-pass. To my surprise the vibration was damped as soon as I opened just one of the two isolating valves at either end of this by-pass.

I'm not sure exactly why this dead-end branch works as a damper, but it may be helped by trapped air in this pipe and/or the resilience of the pipe (if I hold the plastic pipe rigidly the vibration returns). I expect an anti-shock mini expansion vessel will be the simplest way of dealing with the problem. I intend to do a few more experiments and will report back.

Incidentally, fully opening the by-pass worked too, but of course robs the coil of hot water due to comparatively low resistance of the short 15mm by-pass. Perhaps a 6mm by-pass tube might be an answer. I've got an ABV between the flow and return pipes (for pump overrun purposes) but that is intended only to open on higher differential pressures.

 

[BenCross]

I too have just had a Telford Tempest installed. It works fine when heated by the immersion heater but sounds like a Tuba when the heating coil is being fed. I have tried all three speeds on my heating pump but it still sounds like a Tuba.

I have yet to contact Telford but the fact that I have found someone else with the same problem on this forum seems to indicate a problem with the design of the heating coil. My plumber has never come across a tank with such a problem (but has never plumbed in a Telford before either).

I would be keen to know if someone has a working solution to stop such resonant vibrations in the coil or if anybody has further experience of Telford as a manufacturer and what the likelihood of gaining resolution through Telford themselves.