Conflict Resolution: Pump & System Interaction. April 13, 2017

Transcription

1 Conflict Resolution:

2 Agenda What is a system curve and what is it good for? Friction vs. static head dominated systems Effects of manual vs. control valves Affinity laws Parallel composite curves When will parallel pumps give more flow vs. when they won t - and why Effects of dissimilar pumps Series composite curves Complex curves Multiple branch points Different liquid level elevations System curves that change over time Effect of control features Multiple system curves for a single system Problems creating system curves API 610 and ANSI/HI best practices 2

3 Pumps and Systems A pump must overcome two fundamental system-related aspects Friction Static Liquid elevation differences between supply and discharge Pressure differences between supply and discharge*» E.g., when tanks are pressurized Elevation Changes, Same Pressure in Tanks Elevation Same, Different Pressure in Tanks P 1 P 2 Supply Discharge Supply Discharge * Since the effects of supply and discharge pressure differences are the same as liquid elevation differences, we will simplify things and only discuss elevation differences in this course, with the understanding that pressure differences can cause the same effects 3

4 What Is a System Curve? A system curve represents the head required to move fluid through a system at various flowrates In the absence of control features, the system will operate where the pump and system intersect 4

5 What Is a System Curve? (2) Pump Curve System Curve Friction H f Total Dynamic (TDH) Static H s Operating 5

6 What Are System Curves Good For? System curves help demonstrate pumping system behavior in a graphical manner If a system curve can be determined, it can help identify the effects of pump and/or system modifications As systems get more complex, system curves lose usefulness and in fact it is not possible in some cases to determine a unique system curve 6

7 Friction in Pump Systems Friction occurs in pump systems due to irrecoverable hydraulic losses in: Piping Valving Fittings (e.g., elbows, tees) Equipment (e.g., heat exchangers) Friction is also used to control flow or pressure Automated flow and pressure control valves Orifices Manual throttling valves 7

8 Friction Characteristics It is often convenient to think of pump systems in terms of head rather than pressure loss and pressure loss are related H P = ρg Frictional head loss typically depends on the square of velocity and flow rate H H H 2 fl V = D 2g 2 fl Q = D 2 2gA 2 = RQ 8

9 Pure Friction System Curve For systems with pure friction the system curve head goes to zero at zero flow Closed systems are always purely frictional Any pump can produce flow (no elevation to overcome) Pump Curve System Curve H f Example: No Elevation Changes Supply Discharge 9

10 Effect of Elevation Differences When the supply or discharge liquid elevation is changed, the system curve shifts up and down When there is a liquid elevation increase, no flow can occur unless the pump generates at least enough head to over come the elevation increase Example: Pump Curve System Curve Elevation Changes Supply Discharge 10

11 Effects of Elevation Differences (2) New System Curve (With Increased Static ) Pump Curve Old System Curve New TDH Old TDH H f * H f H s Increased Static H s Old Flow New Flow * In this case, friction head H f decreases because the flow rate is reduced Old New 11

12 Static Dominated Systems A system that is static head dominated is one where the primary effect of the pump is to overcome static head (i.e., gravity, or liquid elevation) Pump Curve Large Elevation Change System Curve H f H s Supply Discharge 12

13 Effect of Control Valves Control valves (CV) are a form of frictional head loss Pump Curve System Curve Loss Across CV H cv H f H f H s H s Flow with CV Flow w/o CV w/o CV 13 with CV

14 Effect of Manual Throttling Valves Manual valve throttling increases the friction head loss Pump Curve System Curve Valve Throttled System Curve Valve Open H f H f H s H s Flow Valve Throttled Flow Valve Open Valve Open Valve Throttled 14

15 Pump Affinity Laws The pump affinity laws (also known as homologous pump laws) are based on dimensional analysis and allow prediction of pump performance for other impeller sizes and speeds Q Q 1 2 H H P P D = 1 D2 2 D1 = D 2 D1 = D 2 3 N = 1 N2 2 N1 = N 2 N1 = N 2 3 Where: Q = D = (Impeller) Diameter N = Speed H = P = Power 15

16 Impeller Size Changes Using the affinity laws the pump head curve can be adjusted for a different diameter impeller Pump Curve 100% Impeller Diameter System Curve Pump Curve 90% Impeller Diameter 16

17 Pump Speed Changes Similar to impeller diameter, using the affinity laws the pump head curve can be adjusted for a different speed Pump Curve 100% Speed System Curve Pump Curve 90% Speed 17

18 Pump Efficiency Effects Pump Curve Pump Efficiency Curve System Curve Efficiency Operating Best Efficiency Point 18

19 The efficiency of a pump does not change significantly with speed Similarly, but to a lesser degree, the same is true for impeller changes Pump Efficiency Effects (2) = = = N N N N N N P P H H Q Q η η 19

20 Pump Efficiency Effects (3) Iso-efficiency lines on a head/flow diagram follow the behavior of head (quadratic) and flow (linear) 40% 50% 60% 70% Efficiency 80% 82% 80% 100% Speed 90% Speed 80% Speed 70% Speed Pump Curves 60% Speed 20

21 Variable Speed Pumping Efficiency Friction dominated system curve parallels iso-efficiency lines System Curve (Friction Dominated) 40% 50% 60% 70% Efficiency 80% 82% 80% 100% Speed 90% Speed 80% Speed 70% Speed Pump Curves 60% Speed 21

22 Variable Speed Pumping Efficiency (2) Static head dominated system curve doesn t parallel isoefficiency lines 40% 50% 60% 70% Efficiency 80% 82% System Curve (Static Dominated) 80% 100% Speed 90% Speed 80% Speed 70% Speed Pump Curves 60% Speed 22

23 Parallel Composite Pump Curves All parallel pumps are the same size One Pump Two Pumps Three Pumps 23

24 Steep System Curve With Parallel Pumps Flow increase very small with additional pumps (in this case) System Curve One Pump Two Pumps Three Pumps 24

25 Flat System Curve With Parallel Pumps Flow increase much larger with additional pumps (in this case) System Curve One Pump Two Pumps Three Pumps 25

26 Dissimilar Parallel Pumps Pump 1 Pump 2 Pump

27 Series Composite Pump Curves All series pumps are the same size Three Pumps Two Pumps One Pump 27

28 Complex System Curves Supply Discharge * From the Pump Handbook, Karassik et al. 28

29 Complex System Curves (2) The flow rate for a system curve is typically referenced to a pump System curves can be generated with reference to other locations in the system Some systems (especially those with multiple pumps) may not have a unique system curve In such cases, the concept of a system curve breaks down, and system curves have no real value 29

30 System Curves Over Time: Tank Filling Pump Curve Example: Supply Old System Curve Elevation Changes New System Curve (With Increased Static ) Discharge New TDH Old TDH H f * H f H s Increased Static H s Old Flow New Flow Old 30 New

31 System Curves Over Long Time Periods System and Pump Degradation New System Curve (With Increased Friction) Original Pump Curve Original System Curve New Pump Curve (Degraded ) New TDH Original TDH H f H f * H s H s Original Flow New Flow Original New 31

32 System Curves Over Long Time Periods: System Degradation with Control Valves New System Curve (With Increased Friction) Pump Curve Original System Curve Loss Across CV Originally Loss Across CV New H cv H cv H f H f H s H s Flow with CV Flow w/o CV with CV Original with CV New 32

33 Multiple System Curves Different System Operations * Tank A Tank B Tank C Supply Discharge * From * From the Pump the Pump Handbook, Handbook, Karassik Karassik et al. et al. 33

34 AFT Fathom Detailed Data on Pump vs. System Curves * * From the Pump Handbook, Karassik et al. * New feature in release

35 Problems Creating System Curves and Composite Pump Curves Heat transfer occurs or anything that changes density System curves are based on head and the concept of head breaks down when the density changes Multiple pumps in parallel when the pump piping is not symmetrical If the flows through the pumps are not equal then: a unique system curve cannot be created the heads through the pumps are usually different and it is not possible to say what the composite head actually is Often the composite system curve and composite pump curve do not cross at the operating point The operating point itself is not well defined 35

36 Problems Creating System Curves and Composite Pump Curves (2) Total = 2540 Avg =

37 Problems Creating System Curves and Composite Pump Curves (3) Total Flow = 2540 gpm Avg = 56.5 ft? 37

38 Pump Efficiency Effects Pump Curve Pump Efficiency Curve System Curve Efficiency Operating Best Efficiency Point 38

39 API 610 and BEP Pumps shall have a preferred operating region of 70% to 120% of best efficiency flowrate of the pump as furnished. Rated flow shall be within the region of 80% to 110% of best efficiency flowrate of the pump as furnished. 39

40 ANSI/HI

41 Reliability vs. Distance from BEP Source: Barringer, H.P., 1997 Reliability Engineering Principles training course, slide 45, Barringer & Assoc. Humble, TX 41

42 Vibration vs. Distance from BEP Thrust Bearing / Horizontal Overall Vibration Vs. Flow Fixed Speed w/ Control Valve vs Variable Speed Model X6-18 Vibration (IPS) Flow (GPM) BEP Test 11 Variable Speed Test 17.5" Dia Test Rpm 17.5" Dia Magenta Fixed Speed Blue Variable Speed Source: ITT IBG Goulds Pumps 42

43 Questions? 43