SIMPLE EQUATIONS TO SOLVE THE PROBLEM WITH FUNDAMENTALS

While there have been several articles about chemical engineering academic training not being very applicable to jobs many of the graduates have in industry, I think that there is another facet of the failure for new graduates to be industrial problem solvers. I believe that this facet is actually two pronged. The current management mantras such as” work smarter not harder”, “just try something even if it is wrong” “lets brainstorm that” or “don’t leave the plant until you have a recommendation” provide the wrong tactical approaches to problem solving. These cliches can be contrasted to my experience that most engineering problems are solved by an introspective application of engineering fundamentals. The second prong is that many engineering graduates do not know how to apply first principles to actual real-world problems. There are multiple sub-sets of this prong. Two of them are:

·        The new engineer often does not know how to isolate the problem. Industrial problems almost always involve multiple interacting components with only a single component causing the problem.

·        The engineer (new or experienced) often will not use first principles to isolate and develop a correct hypothesis for the problem

The following problem is an illustration of these shortcomings.

               A newly graduated chemical engineer was assigned to look at the failure of a centrifugal compressor to handle the volume of gas being generated at a steam cracker. The cracker rates had been reduced to compensate for this lack of capacity. It was not obvious whether the problem originated in the steam turbine or the compressor. In addition, operating personnel believed that the problem originated when the pressure of the steam driving the turbine was reduced from 200 psig to 190 psig. This change was made as part of an overall steam pressure optimization which could not be easily changed back.

               After looking at operating data, he/she estimated based on first principles using suction and discharge temperatures that the compressor seemed to be operating at the design efficiency. These simple fundamentally correct first principles are given in a previous article and in my book Process Engineering Problem Solving. Based on the compressor efficiency calculations, it appeared that the problem originated with the steam turbine. Thus, the hypothesis that the lack of capacity was due to the reduced steam pressure seemed reasonable.

               However, the engineer elected to investigate the efficiency of the steam turbine before making a recommendation to return the steam pressure to the previous level of 200 psig. He/she obtained actual plant and design data. From the actual and design temperatures and pressures, the enthalpy and hence work could be determined. From this the turbine efficiency could be determined. In developing the turbine efficiency, the concept that for a 100% efficiency turbine the entropy change is zero was used. Knowing the inlet steam conditions (190 psig and 485oF), the Entropy could be determined by the Mollier diagram or steam tables. This allowed determining the enthalpy change for a turbine operating at 100% efficiency (no change in Entropy).

The actual enthalpy change was determined by actual inlet and outlet steam conditions. The equations below show these calculations:

                               Enthalpy Change at 100% efficiency = (Hi -Ho’)

                               Actual Enthalpy Change                       =(Hi-Ho)

                               Actual Efficiency = 100*(Hi-Ho)/(Hi-Ho’)

                                               Where:

                                               Hi = Inlet Enthalpy based on actual inlet temperature and pressure

                                               Ho = Outlet Enthalpy based on actual outlet temperature and pressure

                                               Ho’ = Theoretical outlet Enthalpy based on no change in Entropy

The results of these calculations are shown below:

Data

                                                            Plant     Design

Inlet Enthalpy, BTU/lb         1260                                                        1267

Outlet Enthalpy, BTU/lb                                               1191                                                      1176

Enthalpy Change, BTU/lb                                             69                                                          91

Enthalpy Change at 100% Efficiency,

BTU/lb         138                                                        140

Steam Turbine Efficiency, %      50    65                                                                                       

               From these simple calculations, it appeared obvious that the problem was associated with the steam turbine and further more it is likely to be some sort of mechanical damage rather than just a small decrease in the inlet steam pressure.

               Two alternatives were evaluated to avoid shutting down the steam turbine/compressor which would shut down the entire plant. These alternatives were:

·        Increasing the steam pressure back to 200 psig – This would increase the enthalpy change and amount of work from the turbine by about 1.5% assuming that the efficiency did not change.

·        Decreasing the outlet steam pressure to 20 psig - This would increase the amount of work from the turbine by about 5.8%. However, it would decrease the condensing steam temperature by 8oF.

After consideration of these alternatives it was decided to shut the steam turbine down for repairs. This would allow other plant repairs to be made.

               While this may seem like a purely fictitious problem, a similar problem really did occur in a plant that I am familiar with. Identical calculations were used to isolate the problem to mechanical damage of the turbine. The turbine was damaged. Fortunately, there was a spare compressor that allowed for partial operation of the plant rather than a complete shutdown.