Dynaflow Lectures Surge analysis. Rotterdam, June 12 th, 2008

Transcription

1 Dynaflow Lectures Surge analysis Rotterdam, June 12 th, 2008

2 Agenda What are surges and what causes them How to reduce the impact of surges Case Conclusions 2

3 Surges are variations in pressures that are generated by a change in the operational status Several changes can lead to surges Foreseen operational changes Unforeseen operational changes Layout changes Some examples of changes Pump start-up Pump trip Variations in demand Valve opening/closing CONCLUSION Surges can lead to (un)anticipated effects such as extremely high or low pressures and unbalanced forces 3

4 Surges may lead to unacceptable conditions CONDITION POTENTIAL PROBLEM Pressures too high Pressures too low Reverse flow Pipeline movement and vibration Leading to permanent deformation or rupture of the pipeline and components May cause collapse of the pipeline; leakage into the line at joints and seals under subatmospheric conditions Causing damage to pump seals and brush gear on motors; draining of storage tanks and reservoirs Overstressing and failure of supports, leading to failure of the pipe Flow velocity too low Large unbalanced loads Mainly a problem in slurry lines, causing settlement of entrained solids and line blockage Pipe movement 4

5 Surges are basically pressure waves running thru the pipe system Pressure at a system node varies over time due to a pressure wave passing by and being reflected at the end of the system. Pressures can rise and fall to significant levels compared to steady state 5

6 Step 1: Valve closes Step 2: Pressure wave returns 6

7 Step 3: Pressure wave is reflected at the reservoir Step 4: Pressure wave returns at the closed valve 7

8 Step 5: Lower pressure wave travels back to the reservoir Step 6: Reflection again at the reservoir 8

9 Step 7: Pressure wave travels back to the closed valve Step 8: Initial situation is reached again (after t = 4L/c) 9

10 Combined a clear oscillation becomes visible 10

11 A basic transient pressure calculation can be made Instantaneous valve closure Fluid column length: L Change in flow velocity: Δv Fluid density: ρ Wave propagation speed: c CALCULATION Momentum change = mass x velocity change Momentum change balanced by pressure force impulse Fluid with density 1000 kg/m 3 (water), Δv = 1 m/s and c = 1000 m/s FORMULAS A* L* ρ * Δv A* L* ρ * Δ v = Δp * A* Δt L L* ρ *Δv = Δp c Δ p = c* ρ * Δv (Joukowsky) Δp = 10bar 11

12 Wave-speed in fluids varies for types of pipes K = fluid modulus of elasticity [Pa] ρ = fluid density [kg/m3] D = pipe diameter [mm] t = pipe wall thickness [mm] E = pipe modulus of elasticity [Pa] FORMULA TYPICAL VALUES FOR FLUIDS c = 1+ K ρ D t K E Steel: FRP: DI: m/s m/s m/s 12

13 Resulting in different surge behavior Pressure time history closing valve EXAMPLE valve closure time: 2 secs. FIBERGLASS STEEL 13

14 Transient pressures also differ per transported medium MEDIUM Fluids and gases Gases Fluids: TRANSIENT PRESSURES To a first approximation pressure pulses are proportional to the sound velocity, the fluid/gas velocity change and the fluid density. Velocities are high, velocity changes may be high, speed of sound is moderate, density is low Pressure pulsations are moderate Velocities are low, velocity changes are small however speed of sound is high and densities are high Pressure pulsation may be large 14

15 Pressure pulsations might result in unbalanced forces Pressure pulsation propagate thru the pipelines at the speed of sound Propagating pressure pulsation cause time dependent unbalanced forces between consecutive elbow pairs The unbalanced forces generate a dynamic response of the piping system Unbalanced forces may affect the integrity of the system. This ought to be a concern for the piping engineer Tool to predict magnitude and time history of unbalanced loads is required for the assessment of the fitness for purpose of the system. BOSFluids is such a tool 15

16 A brief discussion of some three effects related to surges 1. Column separation and vapor cavity formation 2. Air and gas entrainment 3. Resonance and auto-oscillation 16

17 1. Column separation and vapor cavity formation DESCRIPTION If the pressure falls below atmospheric it can reach vapor pressure The liquid column would part as the water boiled while the downstream section would slow down Eventually the downstream water column reverses its direction as well and recombines with the upstream column At this instant the pressure at the point of recombination will increase suddenly and create new pressure waves propagating in both upstream and downstream direction 17

18 2. Air and gas entrainment are highly unwanted Both dissolved and free air and gas Dissolved air and gases will come out of solution when the pressure drops, but the rate at which they can be reabsorbed is so slow that it can be ignored The system response depends on how the free air and gas are distributed In pockets the gas can behave like air cushions and become points of reflection Dispersed in bubbles the effect of even small quantities is to reduce the wave propagation speed to as little as 25% of the normal speed Extremely high shock loads can be generated when moving slugs of liquid following pockets of gas suddenly encounter valves, pipe bends or otherwise 18

19 Example of incident as a result of trapped gas EXAMPLE DESIGN WITHOUT ANALYSIS Injection startup failure 19

20 3. Resonance and auto-oscillation Result of cyclic variations Resonance MODEL OF RESONATING SYSTEM Oscillatory behavior may be generated for example by a component or by unsteady flow The oscillations, or higher harmonics, can be at the resonance frequency of the system System resonance can lead to very high pressures and can be very violent 20

21 Examples of incidents as a result of fluid transients (III) EXAMPLE DESIGN WITHOUT ANALYSIS Supports not designed for its axial loads Deflected beam 21

22 Examples of incidents as a result of fluid transients (II) EXAMPLE DESIGN WITHOUT ANALYSIS Unforeseen unbalanced forces 22

23 Examples of incidents as a result of fluid transients (I) EXAMPLE DESIGN WITHOUT ANALYSIS Pipe lifted from its support after the support is deformed Deflected beam 23

24 Examples of incidents as a result of fluid transients (V) EXAMPLE DESIGN WITHOUT ANALYSIS Pipe not designed for surge effects (minimum pressures) Unforeseen vacuum 24

25 Agenda What are surges and what causes them How to reduce the impact of surges Case Conclusions 25

26 There are a number of ways to reduce the impact of surges Stronger pipes Rerouting Changing valve movements Avoiding check valve slam Increasing the inertia of pumps and their motors Minimizing changes for resonance Surge vessels Vacuum breakers and air release valves Pressure relief valves and bursting discs Bypass lines 26

27 Stronger pipes, rerouting and bypasses MEASURE Stronger pipes Rerouting Bypasses DESCRIPTION Increase the pressure rating of the pipe. Expensive and often used as last resort. Changes such as lowering high-points, less or different bends and different rising profiles. Expensive measure and often not an option Installing a bypass line around a pump to minimize the effect of a pump-trip When the pump trips the pressure at the discharge will be low enough to open the bypass to continue feeding the system bypassing the pump 27

28 Changes of valve movements ADJUSTING THE VALVE CLOSING CHARACTERISTIC Critical opening/closing time: T = 2L c 28

29 Avoiding check valve slam ADJUSTING THE CHECK VALVE CHARACTERISTIC 29

30 Installation of surge vessels Surge vessels are used to maintain water supply after for example a pump trip. As the trapped air expands and the driving pressure falls, the flow is allowed to decelerate in a controlled manner 30

31 Installation of vacuum breakers and air release valves Valve chamber Vacuum breaker (air valve) Pipe line 31

32 Pressure relief valves and bursting discs REDUCE HIGH PRESSURES RELIEF VALVE Aimed at avoiding too high pressures in the system Typical situations include the rapid closure of oil tanker loading valves and control valves in process plants Often the relief valve is connected to a blowdown vessel to avoid the liquid/gas to escape 32

33 Agenda What are surges and what causes them How to reduce the impact of surges Case Conclusions 33

34 A surge analysis of a system using BOSFluids in 5 steps ESD valve shutdown in NH 3 network 1. Make a surge model of the system and check steady state Separate model for each system maximum and minimum pressures 2. Define upset conditions and analyze 3. Evaluate transient results Pressures, velocities and unbalanced forces 4. Generate unbalanced force time histories and insert in mechanical model Time histories per relevant node 5. Introduce (support) modifications where required Additional supports, axial stops, reinforcements, etc. 34

35 1. Model piping system in BOSFluids and analyze Steady State EXAMPLE PRESSURE FLOWRATE Steady state pressure distribution over the system Steady state flow rate distribution in the system 35

36 2. Define the upset conditions and analyze Transient analysis EXAMPLE VALVE CLOSURE TIME HISTORY Pressure time-history in one of the system nodes ESD valve closure in 5 seconds 36

37 3. Evaluate the transient results (I) Pressures, velocities and unbalanced forces EXAMPLE MAXIMUM PRESSURES [BARG.] MINIMUM PRESSURES [BARG.] The maximum pressures per node are graphically represented. The pressures however don t arise simultaneously. Also the minimum pressures are graphically represented. Minimum pressures can fall to a minimum of -1 barg., i.e. full vacuum. 37

38 3. Evaluate the transient results (II) Pressures, velocities and unbalanced forces EXAMPLE UNBALANCED FORCES [N] 38

39 4. Generate unbalanced force time histories and insert in mechanical model EXAMPLE UNBALANCED FORCES PRE RELEVANT PIPE SECTION [N] 39

40 5. Introduce (support) modifications where required EXAMPLE MODIFIED ARRANGEMENT Axial stop & Lateral Guide Valve anchored Lateral Guide 40

41 Agenda What are surges and what causes them How to reduce the impact of surges Case Conclusions 41

42 Conclusion from Case Study & Presentation Surge effects can be of great importance Just static design only often proves to be insufficient Several effects can lead to surge and should be checked for Unacceptable surge effects can often be avoided using suppression tools Surge vessel, air valves, different system operation, etc. Only thru determination of the dynamic loads an assessment of the integrity by the piping engineer is feasible Determination of surge effects is only possible by means of a transient flow analysis (such as BOSFluids) 42