Where We Go Wrong In Pump Design

1
Where We Go Wrong In Pump Design

• Dick Hawrelak
• Presented to ES317Y in 1999 at UWO

2
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.

3
Typical Pump Sketch
4
Poor Mass Balances

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

5
Multi-Discipline Design

• Line sizing.
• Check valve sizing.
• Control valve sizing.
• Orifice plate sizing.
• Viscosity corrections.
• Light density hydrocarbon corrections.

6
Control Valves

• Poor CV selection - pump running on by-passmay
  need two CVs.
• If too large DP taken across control valve, it
  may be better to trim impeller, save CV wear
  energy.

7
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.

8
Suction Conditions

• Poor NPSH causes pump cavitation, high vibration
  ultimately pump failure.
• Pump fails to perform as designed without
  suitable NPSH.

9
Suction Specific Speed

• SSS rpm(Q)0.5 / (NPSH_at_ 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.

10
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.

11
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.

12
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.

13
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).

14
Typical Pump
15
Selected Pump
16
Dissolved Gases

• Absorbed gas follows Henrys 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.

17
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.

18
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.

19
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?

20
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.