True Professional Engineer
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Thursday, January 28, 2016

Think "dynamic" in Engineering

Production executive who has basic engineering knowledge asked me about a recent problem faced with regards to the LPG supply line.

In the plant, we have a few LPG storage tanks, passing through two units of evaporators into a common main supply line. This main line is then branches into four sub-lines, supplying to four production lines.

It is Malaysia's industrial practice that the main petroleum gas (PG) supply line into any building must be set at a pressure not more than 18 psi. This 18 psi line pressure will then be a standard to work on for normal equipment operation.

Our four PG lines comprise of three oven with ribon burners and a thermo oil heater unit with industrial burner. Recent problem faced is the industrial burner did not manage to switch into high fire during operation. This was traced and I noted the minimum gas pressure switch was activated on low pressure when the system changed from low fire to high fire mode.

My explaination to the team is simple. The gas supply rate is not sufficient to maintain industrial burner at high firing rate. Thus I arranged for the team to check and clean the supply line filters for any possibility of blockage. Meanwhile,  a detail trace was then carried out and noted that it was a cool day and one of the LPG evaporator unit was malfunction because it is electrically operated and the heater unit was out of order.

Below are questions by the production executive and my answers:

Why we have four lines but only the thermo oil heater is affected?
Answer: The firing rates are different. Ribon fire burners are having low firing rate, thus minor supply line pressure fluctuation is not an issue on individual burner. As for industrial burner, the firing rate is higher and thus gas supply rate is important to maintain efficient firing. Thus a minimum gas pressure switch was there to ensure gas pressure prior to combustion is above the minimum requirement. It was this minimum gas pressure switch being activated that leading to shutting down of burner.

We do have a pressure regulator supplying to thermal oil unit industrial burner, the upstream pressure was still higher than down stream and the down stream was seemed to maimtain at its normal reading, why there was still tripping?
Answer: The normal line pressure was 18PSI. With only one evaporator, especially in a cool day, the line pressure drop to 16PSI when the consumption is high. Even the down stream pressure was tegulated to only 3 PSI, the actual flow rate of gas across the regulator opening was lower due to lower upstream pressure. Thus eventhough the regulator still manage to regulate to set pressure finally, there was a minor time lag. Too bad that this time lag was long enough to activate the minimum pressure switch which finally shut down the burner.

On the second query, I took opportunity to highlight to the team on what is so called "theory and practical are different". Theoretically we normally apply some assumptions, which sometumes are so obvious and yet negligible, thus we do not highlight them specifically.  Nevertheless, in practical world, this assumption is normally what causing an issue. Hence it is important as an engineer to observe this and remember, engineering is working with dynamic condition and we should not apply theory learnt statically.

For this incident, I was glad that the productuin executives are now paying more attention on understanding the root causes of "equipment breakdown". With extra knowledge or experience gained, they would be able to understand that it is not an equipment issue but a system issue. With this understanding, they will then be more appreciating on why a certain preventive task was carry out not directly on an equipment.

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