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23-Chem-A5 Chemical Plant Design and Economics · May 2017

Question 4 of 6: Pumps — Types, Materials, and Selection Data

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Closed-book exam, 3 hours; one aid sheet permitted. Six questions of equal value (20 marks each); five constitute a complete paper — full solutions to all six are given here. Questions 1 and 2 are quantitative (plant material balance and discounted-cash-flow return); Questions 3–6 are design-practice list/essay questions.

Reference texts: M.S. Peters, K.D. Timmerhaus & R.E. West, Plant Design and Economics for Chemical Engineers (5th ed., McGraw-Hill) — the exam's named primary text (process design development Ch. 2, general design considerations: plant location, safety, materials Ch. 3, interest and profitability Ch. 7–10, materials-transfer/pumps Ch. 14); R.H. Perry & D.W. Green, Perry's Chemical Engineers' Handbook (9th ed.) — pump types and selection (Sec. 10), pyrolysis kinetics data; O. Levenspiel, Chemical Reaction Engineering (3rd ed.) — first-order plug-flow space-time behind the reactor sizing in Question 1; supporting Canadian practice from CCOHS and the CSA Z767 / provincial OH&S process-safety-management framework for Question 5.

Question 4: Pumps — Types, Materials, and Selection Data (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Four important types of pumps

  1. Centrifugal pump — a rotating impeller adds kinetic energy to the liquid, which is converted to head in the volute/diffuser. The workhorse of the chemical industry: simple, low-maintenance, high flow at moderate head, smooth (non-pulsating) delivery, but head falls off as flow rises and it must usually be primed. Handles clean to mildly dirty liquids.
  2. Reciprocating (positive-displacement) pump — a piston or plunger displaces a fixed volume each stroke, delivering nearly constant flow against very high, variable discharge pressure. Used for high-pressure, low-flow, metering, and high-viscosity duties; flow is pulsating and a relief valve is mandatory.
  3. Rotary positive-displacement pump (gear, lobe, screw, vane) — meshing rotating elements trap and carry liquid from suction to discharge. Self-priming, gives steady flow of viscous or lubricating liquids (oils, polymers) at moderate pressure; not suited to abrasive slurries.
  4. Diaphragm / air-operated pump — a flexing diaphragm displaces liquid with no seal to the process, ideal for corrosive, toxic, abrasive, or shear-sensitive fluids and slurries. (Alternative fourth type: axial-flow / propeller pumps for very high flow at low head, e.g. cooling-water circulation.)

(b) Materials of pump construction

Pump wetted parts are matched to the fluid's corrosivity, abrasiveness, temperature, and pressure. Common choices: cast iron and carbon/cast steel for water, oils, and non-corrosive duty; bronze for water and mildly corrosive service; stainless steel (304/316) for oxidising acids, food/pharma, and general corrosion resistance; high-nickel alloys (Monel, Hastelloy) for reducing acids and chlorides; titanium for hot oxidising acids and seawater; hard/high-silicon irons for abrasive and acid slurries; and non-metallic linings and bodies (rubber, PTFE, ceramic, glass, or engineered plastics) for highly corrosive or ultrapure service. Elastomers/PTFE are selected for seals, diaphragms, and gaskets to suit the fluid.

(c) Ten information items required to select a pump

  1. Liquid to be pumped (chemical identity, corrosivity, toxicity).
  2. Required capacity / flow rate (and turndown range).
  3. Total dynamic head (discharge minus suction, including friction).
  4. Suction conditions and available NPSH (to avoid cavitation).
  5. Discharge pressure required at the destination.
  6. Operating temperature of the liquid.
  7. Density / specific gravity and vapour pressure of the liquid.
  8. Viscosity of the liquid (Newtonian or not).
  9. Solids content / abrasiveness / particle size (slurry vs. clean).
  10. Nature of service — continuous or intermittent, and control turndown.

Additional selection data often specified: available driver/power and utilities, materials-of-construction limits, and any special features (sealless/canned for hazardous fluids, sanitary finish, self-priming).