Last month’s column discussed how reviews of process flow diagrams (PFDs) could identify opportunities to save energy in process plants (see: “Make the Case for PFD Reviews”). The opportunities then need screening. This leads to a shortlist of the most attractive projects, usually ranked by simple payback (investment/annual savings). The plant operations group can then implement practical options that don’t require new facilities. Investment projects with acceptable paybacks should receive a full engineering evaluation, and implemented if they prove attractive.
Screening can take various forms. Some opportunities are rejected during the PFD review itself, based either on quick calculations or prior experience with similar ideas. Opportunities that survive garner more rigorous screening to validate technical credibility, quantify energy savings, and, if new facilities are needed, provide an estimate of investment costs.
Technical credibility. PFD reviews yield a wide range of ideas. While many are valid, others are manifestly infeasible — for example, they may violate the laws of thermodynamics. The technical credibility assessment helps weed out the infeasible ideas. Some ideas are conceptually valid but demand operating conditions beyond the limits of the existing equipment and systems. In such cases, consider the cost of equipment upgrades; this usually makes the ideas uneconomic. Another cluster of ideas could potentially lead to significant energy savings and other benefits but pose many technical risks and unknowns. Such items may be good candidates for research and development projects.
Energy savings. For projects that survive the technical credibility assessment, ask three main questions when estimating potential energy savings:
• How much energy is saved? Sometimes the answer to this question is so simple it requires no calculation. For example, if the only energy impact of a particular idea is to shut off an electric pump, the energy saving is — most likely — the elimination of the electric power used in the pump motor. Always exercise care, however, because closer scrutiny often highlights hidden energy savings or penalties. For example, shutting down one pump may increase the energy consumption in another pump. Another example: Heat recovery projects that increase feed preheat ahead of a charge heater usually raise the stack gas temperature, which in turn reduces the heater efficiency. Be sure to include the change in efficiency in the energy savings evaluation.
• What type(s) of energy is saved? The two main classes of energy in process plants are thermal energy (heat) and electric power. A fired heater may provide heat to the process directly, or indirectly by way of steam or some other heat transfer medium. Many sites have steam at several different pressure levels. It is important to identify which level or levels are impacted, as this can affect the steam cost (see below).
• What is energy worth? This can be difficult to answer. In the case of electric power, a saving usually translates into a reduction of power imported from the grid, so we need to know how much the power bill changes as the electric load changes. However, electricity contracts can be complex; we must understand these complexities to calculate the value of electrical savings (see, “Learn from Your Electric Bills, Part 1,” and “Part 2”).
The most important cost item for thermal energy is the cost of fuel. In fired heaters we can usually calculate the value of energy savings directly from fuel cost and the reduction in fuel usage. For plants with multi-level steam systems, the credit per pound of steam saved is usually different at each pressure level; it may also depend on the path the steam takes through the steam system. For example, does the steam pass through steam turbines or through letdown valves? Does excess steam go to a vent? Steam system models are typically used for marginal steam costing for each steam header (See August 2019’s “Homemade Spreadsheets Aren’t Enough”).
Cost estimates. In screening studies, these must be realistic. If the cost of the main equipment items is known, we can multiply it by an appropriate installation factor, which accounts for additional elements, including foundations, piping and control, as well as engineering and various labor disciplines, and overheads. You can also use various software tools and literature sources.