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Where We Go Wrong In Pump Design Dick Hawrelak Presented to ES317Y in 1999 at UWO
Introduction • 5% of large property damage losses are caused by failures in pumps. • Average Trended Loss was $19.2MM, the lowest of all unit operations.
Poor Mass Balances • Normal mass balance. • Start-up, shut down or upset. • Recycle conditions on EBV closure. • Pump design for multiple services are tricky.
Multi-Discipline Design • Line sizing. • Check valve sizing. • Control valve sizing. • Orifice plate sizing. • Viscosity corrections. • Light density hydrocarbon corrections.
Control Valves • Poor CV selection - pump running on by-pass…may need two CVs. • If too large DP taken across control valve, it may be better to trim impeller, save CV wear & energy.
Blocked-in Operation. • Pumps can explode in a short period of time if left running while blocked-in. • Pump explodes, pieces rocket 275m, hits truck, kills driver. • Pump leaks, liquid catches fire and destroys plant.
Suction Conditions • Poor NPSH causes pump cavitation, high vibration & ultimately pump failure. • Pump fails to perform as designed without suitable NPSH.
Suction Specific Speed • SSS = rpm(Q)^0.5 / (NPSH@ BEP)^0.75 • Pumps operating at SSS greater than 11,000 had a high failure frequency. • Low capacity operation causes inlet recirculation, impeller erosion, shaft deflections, bearing failures and seal problems which lead to leakage.
Maintenance • Poor alignment causes bearing problems and ultimately pump failure. • If seal leakage rate too high, select better mechanical seal (single, double, triple). • Require a good preventative maintenance (PM) program.
Check Valves • If Check Valve designed properly, it will prevent back flow. • If Check Valve not designed properly, it will chatter continuously and destroy seal seat faces in a short period of time. • Without back flow protection, hazardous flooding conditions can occur on pump shut-down.
Phase 4 Design Checks • Preliminary Phase 2 & 3 pump calculations not confirmed in Detailed Design, Phase 4. • Pumps fail to perform as designed. • Corrections costly during start-up.
Pump Selection • Hundreds of pumps to select from. • Which selection is best? • Which RPM to use? • What HP size & type of motor to select, explosion proof, TEFC? • Download Durco PUMPSEL on internet (program is free).
Dissolved Gases • Absorbed gas follows Henry’s Law xa = (pp / Pt) / H. • Dissolved gases are like entrained bubbles. Residence time in suction vessel may be too short. • Dissolved gases causes problems similar to NPSH cavitation. • Prevent vapor entrainment with vortex breakers.
Material Transfer • Need multiple checks on quantity of material transferred to storage. • Weigh scales, level checks, mass = (flow rate)(time) on computer. • Time control EBVs to minimize Water Hammer problems.
Excess Flow Protection • Pumps cannot be allowed to run out on the impeller curve, may burnout motor if motor not selected for runout. • May need excess flow protection.
Possible Exam Questions • What is the problem of running a pump while it is blocked in? • What is the hazard created by poor pump alignment? • What is the hazard created by poor mechanical seal selection for a pump? • What is the hazard created by low capacity operation of the pump? • Why is it necessary to provide adequate NPSH for a pump?
Summary • This short list is indicative of some of the problems caused by poor engineering discipline in pump design. • Recommend you obtain a copy of the Chemical Plant Design programs and follow the procedures built into the pump design spreadsheets.