Tests on Lempor Exhaust

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    John Hind

      On Saturday 4/2/17, Jamie Keyte, Mike Horne and John Hind went to the Keighley and Worth Valley Railway to log the smokebox vacuum on S160 5820, a 2-8-0 built by Lima in 1945 for the United States Army Transportation Corps. A Lempor exhaust for 5820 was designed by the Advanced Steam Traction Trust and fitted in 2013.
      The day was not specifically organised for our benefit. The KWVR, were running their normal public service with the Taff Valley tank No85 and were running 5820 on a test train to prove operational procedures for the visit of Flying Scotsman in April 2017.
      Under normal circumstances steam banked passenger services are not permitted. Flying Scotsman is a thoroughbred and the lifting of a heavy train on the steep curve out of Keighley and up Keighley Bank is not what the A3 Pacific was built for. Banking assistance is required to ensure that the services operate without hitches and delays, should the Pacific need assistance out of Keighley.
      The train was 5820 leading, 7 coaches and WD 90733 banking. The train was not open to the public and 3 runs were made during the afternoon.
      The preparation work that Mike Horne had done at the Kirklees Light Railway on commissioning our data logger paid off and it worked flawlessly, so we were able to log vacuum readings at 1000 samples per second. From Run 3 alone, we obtained 800,513 sets of data, which we are now starting to analyse.
      In addition to smokebox vacuum, we have the ability to record steam pipe and blast pipe pressures and temperatures, cylinder pressures and crosshead position, giving us the ability to generate super-accurate indicator diagrams. The test runs on 5820, were arranged at short notice, so we only had time to set up for measuring the smokebox vacuum.
      The trains ran non-stop from Keighley to Haworth, with a slack before Damens and slowing at Damens Signalbox to pick up the token. After re-starting at Haworth the trains continued to Oxenhope before returning downhill to Keighley. On Runs 1 and 3, 90733’s driver, was briefed to bank out of Keighley and take its own weight for the remainder of the run. On run 2, 90733 assisted throughout the run.
      What was apparent during the day was that 5820 had uneven exhaust beats which have developed over time since the locomotive was returned to traffic in 2013.
      The uneven beat shows up on the data. We had not expected that smokebox vacuum would show up problems with valves or cylinders to the extent that it can show a problem on one beat. This was an unexpected bonus. If we had been able to fit crosshead position sensors we could have said which valve head has the problem – as it is, we know that a valve head on one side has a problem.
      The data logger stores the data, which can be watched in real time on a laptop PC and downloaded for later analysis. The data clearly shows the pulsing of the exhaust (the chuff) and the logging speed means that every chuff can be seen. We can calculate train speed by measuring the time between pulses.
      We had wanted to measure smokebox vacuum for some time. The view is that the engine is performing better with new exhaust, and ‘to measure is to know’.
      It is too early to draw full conclusions, but some key data – on the climb out of Keighley on a 1 in 57 gradient at 14 mph an average vacuum of 8.16 inches of water with a peak of 16.6 inches was measured. Further on at Ingrow, when the engine had been notched up and at a speed of 24 mph, an average vacuum of 7.48 inches of water was measured with a peak of 12.23 inches.
      The only performance data for S160’s that we have seen, is that measured by the French Railways and published in Chapelon’s classic book ‘La Locomotive a Vapeur’. This gives an average vacuum of 8 inches of vacuum after a modification to reduce the blast cap area to 99 cm2- the key difference is that 5820 is operating with a blast nozzle choke area equivalent to 140cm2 – 41% larger. So 5820 is working at a lower backpressure therefore more economically and producing less CO2 emissions.
      More details will be published, once there has been a fuller analysis of the data.
      Pictures taken on the day can be found here – https://youtu.be/rVzQu16R3bw

      Martin Johnson

        Did those results ever get processed and posted somewhere? A variation of nearly 2:1 in vacuum, presumably in time with the “chuff”, sounds interesting. What implications might this have for design guides of smokebox volume, tube bank flow resistance and susceptibility to fire lifting / clinkering?


        Chris Corney

          I think I’m correct in saying that the smokebox pressure returned to “atmospheric” between each “chuff”.
          A while ago we had a discussion about pulsating smokebox pressure. My background is as an electrical engineer, and we are working with pulsating quantities all the time (i.e. ac). Adapting a method from this area, it occurred to me that there might be some advantage in having a double chimney with one being a Kylchap and the other being a Lempor. For each “chuff”, due to the difference in steam operating velocities, it would take slightly longer for the Lempor to reach “terminal velocity”, by probably a few tens of milliseconds, than the Kylchap and therefore the pulses of suction from the two chimneys would be slightly out of step. The overall magnitude of the oscillation would be reduced, compared with an equivalent double Kylchap or double Lempor. Following the discussion on the “Kylchap” thread I’m now thinking that under these circumstances the Lempor could be acting as a more efficient pump, if pulling against the vacuum which has just been created by the Kylchap.

          Obviously this would require a lot of work to optimize!

          Does it make any sense?

          Martin Johnson

            I see where your thinking is going, Chris. My own thinking was along the lines of changing the capacity of the smokebox, the resistance of the tube bank and the capacity of the firebox. I think in your electrical terms it would be a variable voltage feeding a capacitor connected to another capacitor via a resistor. Either way it is a differential equation of second or third order. I think that the frequencies involved are way below any possible resonance, so it would reduce to a mass oscillation problem.

            Oh how I wish my degree level maths was still up to standard!


            Chris Corney

              Hi Martin,

              There are some effects on the chart records from the tests that we don’t fully understand. My opinion is that it’s a resonance in the chimney of the Lempor, but other people have other opinions. I think there would be too much damping to get a resonance between the smokebox and firebox, but I could be wrong.


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