Where We Go Wrong In Pump Design Dick Hawrelak Presented to ES317Y in 1999 at UWO

Introduction n 5% of large property damage losses are caused by failures in pumps. n Average Trended Loss was $19.2MM, the lowest of all unit operations.

Typical Pump Sketch

Poor Mass Balances n Normal mass balance. n Start-up, shut down or upset. n Recycle conditions on EBV closure. n Pump design for multiple services are tricky.

Multi-Discipline Design n Line sizing. n Check valve sizing. n Control valve sizing. n Orifice plate sizing. n Viscosity corrections. n Light density hydrocarbon corrections.

Control Valves n Poor CV selection - pump running on by-pass…may need two CVs. n If too large DP taken across control valve, it may be better to trim impeller, save CV wear & energy.

Blocked-in Operation. n Pumps can explode in a short period of time if left running while blocked-in. n Pump explodes, pieces rocket 275m, hits truck, kills driver. n Pump leaks, liquid catches fire and destroys plant.

Suction Conditions n Poor NPSH causes pump cavitation, high vibration & ultimately pump failure. n Pump fails to perform as designed without suitable NPSH.

Suction Specific Speed n SSS = rpm(Q)^0.5 / BEP)^0.75 n Pumps operating at SSS greater than 11,000 had a high failure frequency. n Low capacity operation causes inlet recirculation, impeller erosion, shaft deflections, bearing failures and seal problems which lead to leakage.

Maintenance n Poor alignment causes bearing problems and ultimately pump failure. n If seal leakage rate too high, select better mechanical seal (single, double, triple). n Require a good preventative maintenance (PM) program.

Check Valves n If Check Valve designed properly, it will prevent back flow. n If Check Valve not designed properly, it will chatter continuously and destroy seal seat faces in a short period of time. n Without back flow protection, hazardous flooding conditions can occur on pump shut-down.

Phase 4 Design Checks n Preliminary Phase 2 & 3 pump calculations not confirmed in Detailed Design, Phase 4. n Pumps fail to perform as designed. n Corrections costly during start-up.

Pump Selection n Hundreds of pumps to select from. n Which selection is best? n Which RPM to use? n What HP size & type of motor to select, explosion proof, TEFC? n Download Durco PUMPSEL on internet (program is free).

Typical Pump

Selected Pump

Dissolved Gases n Absorbed gas follows Henry’s Law xa = (pp / Pt) / H. n Dissolved gases are like entrained bubbles. Residence time in suction vessel may be too short. n Dissolved gases causes problems similar to NPSH cavitation. n Prevent vapor entrainment with vortex breakers.

Material Transfer n Need multiple checks on quantity of material transferred to storage. n Weigh scales, level checks, mass = (flow rate)(time) on computer. n Time control EBVs to minimize Water Hammer problems.

Excess Flow Protection n Pumps cannot be allowed to run out on the impeller curve, may burnout motor if motor not selected for runout. n May need excess flow protection.

Possible Exam Questions n What is the problem of running a pump while it is blocked in? n What is the hazard created by poor pump alignment? n What is the hazard created by poor mechanical seal selection for a pump? n What is the hazard created by low capacity operation of the pump? n Why is it necessary to provide adequate NPSH for a pump?

Summary n This short list is indicative of some of the problems caused by poor engineering discipline in pump design. n Recommend you obtain a copy of the Chemical Plant Design programs and follow the procedures built into the pump design spreadsheets.