Effective Maintenance Management. V. Narayan

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СКАЧАТЬ cost-drivers; risks associated with unstructured cost reductions; ways to reduce costs without losing control on safety and profitability;

      •Process plant end-of-life activities and associated risks.

      Commissioning a new plant can be an exhilarating or frustrating experience, depending on how well the designer has anticipated start-up problems, and whether the plant functions as required. It is not unusual to find a number of change requests being initiated during and shortly after commissioning the plant. If the change requests relate to the original functional requirements, operability, or maintainability, they indicate deficiencies in the design. The number and scope of such requests are measures of the level of dissatisfaction.

      Other change requests relate to the desire to increase plant capacity. By operating new plants at design and higher-than-design throughputs,we can test them. Some equipment, piping, or logistics will stand out as bottle-necks. Lack of balance between the different parts of the plant is the cause of these bottle-necks.Change requests that relate to the removal of these bottle-necks are capacity-increase projects. De-bottle-necking projects can potentially lead to reliability problems. Hence,these risks need careful evaluation.

      We cannot avoid some of this imbalance, for which there are several contributing factors. First, when the designer needs items such as a length of pipe, a centrifugal pump or a gas turbine, the vendors would offer it in a standard range of sizes. The designer does not have the choice of trimming the sizes. As long as the item on offer is close to the specifications and budget, it is acceptable. Hence, the selected items are usually larger or stronger than required. Second, there is always a residual amount of uncertainty in any new design, in spite of all the analysis and expert inputs. The designer will build in some ‘fat’ to take care of these uncertainties.

      Third, there may be bonus or penalty clauses in the contracts to ensure that the plant design meets its functional requirements. Turnkey contracts often have such provisions. The cost of building in a little extra capacity is usually quite small in comparison to these bonuses and penalties. The designer avoids the penalties and adverse publicity by building in some over-capacity.

      Last,the designer uses redundancy to guarantee the reliability of the plant. Sometimes installed spare equipment is necessary for safe and reliable operation of the plant. However, in many cases, custom and practice dictate the decision-making process. The correct method is to carry out a risk analysis before choosing installed spare capacity. However company standards and codes of practice often mandate such practices. All of these factors contribute to over-capacity or fat in some parts of the plant.

      We often purchase oversized equipment without realizing that this is happening. As an illustration, consider the selection of a centrifugal pump. The sequence of events is often along the following lines:

      •The process designer calculates the discharge pressure required to overcome the back pressure at the rated flow,the available suction head, and the drop in the piping,valves, and fittings. These include an allowance for uncertainty.

      •The instrument designer adds the pressure drop across orifice plates and control valves, again including an element for uncertainty.

      •The project engineer writes the requisition for the pump,and invites bids from vendors.

      •The buyer’s equipment specialist looks at the pump selection charts among the offers received, and selects a suitable pump, usually the next size above the required capacity. The selection charts show the flow and pressure combinations that a given model can provide.

      •In producing the selection charts, the vendor has allowed for some manufacturing deviations and de-rated the equipment slightly. This gives the vendor a comfort cushion to cater for uncertainty.

      •As a result of all these allowances, the pump discharge pressure can be, say, 20-40% higher than required at rated flow. This additional pressure energy will be dissipated as heat, vibration, and noise in the control valve.

      Admittedly, there is some exaggeration in this example, but it is not far off the mark. If you take a walk in a chemical plant or petroleum refinery, you are likely to find some noisy control valves on pump discharge lines. The valve body can be quite hot, and may even have blistered paint-work. Further examination will reveal that the pump’s discharge pressure is excessive, and that this additional energy is being dissipated in the control valve. Apart from the fact that energy is being wasted, the pump is also operating with a throttled discharge. This causes excessive wear—the internal leakage past the wear rings will increase. The local flow rate inside the control valve can be very high, resulting in erosion of the trim. The probability of failure increases, of both the pump and the control valve. Due to the additional erosion inside the bodies, the physical damage to the internals is larger than otherwise. Thus, the consequence of failure is also higher, and repair costs will rise. Depending on the level of redundancy built-in, the loss of the pump could result in an immediate operational consequence.

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