There is a portion of scripture referring to little foxes ruining the vineyard. Along the same vein I have spotted a number of little foxes ruining boiler operations as well, often to the utter confusion of operating staff and the dismay of Production staff. One of them in particular is simple to avoid, but seemingly difficult to detect. I am referring to the draught sensing pipe running from the ignition arch through to the control panel. It is normally a ⅜ inch (10mm) copper pipe which is run along a cable tray, with a number of pipe joints and connections along the way. Quite often the copper pipe is joined to a plastic or rubber pipe on its final run to the control panel, where it is connected to a furnace pressure controller, such as a photohelic or other electronic pressure differential sensing device.

One thing is for sure: if draught (furnace pressure) sensing is lost, boiler operations tend to end up in chaos. Either the boiler house rapidly fills up with smoke and flames, or the FD fan and stoker come to a standstill. Both are of cause equally disruptive; on the one hand choking operators trying to engage manual operation to restore steam production, on the other hand dropping steam pressure and irritated production supervisors swarming and/or calling the boiler house to find out why there is no steam.

I walked into our boiler house the other day after a 36 monthly statutory inspection. I noted that the draught sensing pipe has been replaced, but I also noted that the pipe sloped slightly upward where it took the first turn. Not good at all! With a freshly cast ignition arch I knew there was a significant amount of water evaporating from the wet refractory and condensing in the sensing pipe. Very soon the condensate would fill the pipe at the first bend and block the furnace pressure signal to the control panel. My suspicions were confirmed when I disconnected the pipe at the first joint and pressed down on the loose end – some 5 ml of condensate drained out on to the floor.

So here are a few practical tips to consider regarding draught sensing pipes and eliminating the little furnace pressure fox:

  1. Make sure the sensing pipe slopes downward all the way to the panel, but install a deliberate loop where condensate may gather and which is easy to disconnect and drain. I have even seen automatic condensate drains to be installed on these loops.
  2. Condensate only really becomes a problem when a new ignition arch is cast. Regularly check for the presence of condensate in the sensing pipe during the first two weeks after the arch was cast.
  3. Reduce the number of joints and connections to the minimum. Each one of them is a potential source of leakage which can disrupt the quality of the furnace pressure control signal. Ensure joints and connections are tight, well supported and generally in good condition.
  4. Avoid long lengths of sensing pipe. We once had to do a sensing pipe run of 60 metres long, where the control panel was located in a remote room. We could spot a definite delay in the response of the system to changes in furnace pressure. Eventually we could improve the situation by increasing the diameter of the sensing pipe, but it was everything but ideal. It is probably better to run long lengths of control wiring than sensing pipes.
  5. Be careful when using plastic sensing pipes which may be exposed to high temperatures. We once encountered a situation where someone welded on a steel structure in the boiler house and a droplet of weld spatter burned a small hole through the plastic pipe somewhere on a cable tray. It took us almost a day to find the cause of the erratic draught signal.
  6. Furnace pressure control going wrong can cause significant damage, especially if it turns positive in the absence of a boiler operator. Consider installing protection that will trip the boiler when furnace pressure turns positive for too long – a feature that may one day spare one the cost of expensive repairs.

This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about efficiency of combustion or combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.

Kindly note that our posts do not constitute professional advice and the comments, opinions and conclusions drawn from this post must be evaluated and implemented with discretion by our readers at their own risk.

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