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Solving Liquid Hammer Problems, Using PIPENET VISION and CAESAR II نرم افزار PIPENET VISION >

 

INTRODUCTION:

Liquid hammer is the destructive force, pounding noises and vibration in a piping system when liquid flowing through a pipeline is stopped abruptly. When sudden changes in flow occur, the energy associated with the flowing liquid is suddenly transformed into pressure at that location. This excess pressure is known as surge pressure and is greater with large changes in velocity. Characteristics of the pipe such as the materials used in construction, the wall thickness, and the temperature of the pipe all affect the elastic properties of the pipe and how it will respond to surge pressures.

The thicker pipe wall and stronger pipes and fittings the faster shock wave. Thin walled plastic pipe will only bounce a shock wave back at 914 m/s while heavy wall steel pipe will bounce a shock wave back at 2438 m/s. Liquid does not actually travel down a pipe line at these speeds. For example 1.5 to 2 meters per second is very fast for water to flow in a pipeline. A pressure wave or shock wave in liquid happens when one liquid molecule pushes on another liquid molecule and the second molecule pushes on a third and so on.

If you have a pipeline 1,000 meters long full of water, injecting a thimble full of water in one end of the pipe, will cause another thimble of water to almost instantaneously come out of the other end of the pipe. The non compressible nature of most liquids is what transmits a shock wave through pipelines at such unimaginable speeds. Stronger or thicker walled pipe and fittings are better able to withstand the repeated impacts of liquid hammer but, as the strength of the pipe and fittings increases, the velocity of the shock wave increases causing more damage.

CAUSES OF LIQUID HAMMER:

The causes of liquid hammer are varied. There are, however, four common events that typically induce large changes in pressure:

  1. Pump startup can induce the rapid collapse of a void space that exists downstream from a starting pump. This generates high pressures.

  2. Pump power failure can create a rapid change in flow, which causes a pressure up-surge on the suction side and a pressure down-surge on the discharge side. The down-surge is usually the major problem. The pressure on the discharge side reaches vapor pressure, resulting in vapor column separation.

  3. Valve opening and closing is fundamental to safe pipeline operation. Closing a valve at the downstream end of a pipeline creates a pressure wave that moves toward the reservoir. Closing a valve in less time than it takes for the pressure surge to travel to the end of the pipeline and back is called “sudden valve closure.” Sudden valve closure will change velocity quickly and can result in a pressure surge. The pressure surge resulting from a sudden valve opening is usually not as excessive.

  4. Improper operation or incorporation of surge protection devices can do more harm than good. An example is over sizing the surge relief valve or improperly selecting the vacuum breaker-air relief valve. Another example is to try to incorporate some means of preventing liquid hammer when it may not be a problem.

 

MANUAL LOAD CALCULATION:

dp = ρ.c.dv

Where:

dp - The pressure rise due to the fluid’s “instantaneous” stopping

ρ - The fluid density

c - The speed of sound in the fluid

dv - the change in velocity of the fluid

The speed of sound in the fluid can be estimated from

c = [Ef / (ρ + ρ(Ef / E) (d/t) )] 0.5

Where:

Ef - The bulk modulus of the fluid

E - The modulus of elasticity of the pipe

d - The pipe mean diameter

t - The pipe wall thickness

ρ - The fluid density

The magnitude of unbalanced load can be computed from:

F = dp . Area

PIPENET MODEL:

In the following piping system, Water Hammer phenomenon happens when stop valve closes in 4 seconds. Instead of the manual calculation, it’s far better to use fluid transient analysis software like “PIPENET Transient” to simulate pressure surge in piping systems. The part of the piping system shown in figure 1, is our interest for analysis.

Figure 1
This is a 10”, Std. WT, A106-B pipe. Water is flowing through the pipe with 11.2 bar pressure and 20°C temperature.
Suddenly, valve closes in 4 seconds. Figure 2 illustrates the specification applied to valve closure with power-ramp time function.
 

Figure 2

The best candidate for water hammer load is segment 200-300 (force 2), shown in figure 1. The magnitude of applied dynamic force due to pressure surge in elbow-elbow pair 200-300 illustrated in figure 3 below. This load spectrum is uni-axial force along pipe which its direction must be specified in CAESAR II dynamic model correctly, later.
 


 Figure 3


CEASAR II MODEL:

After creating the same model in CAESAR II and performing static analysis shown in figure 4, we see that the support loads and the maximum stress are very low and acceptable. (Code Stress Ratio is 13.0 at Node 410 LOADCASE: 2 (SUS) W+P1)
 

Figure 4


PERFORMING DYNAMIC CALCULATION:

PIPENET creates .FRC file for each defined force in the model. CAESAR II can read .FRC files and create dynamic input file automatically. We can import .FRC file in CAESAR II by using Tools>External Interfaces>PIPENET from CAESAR II main menu. After performing dynamic analysis, we'll see stress failure and excessive forces on some of pipe supports shown below.
 

RESTRAINT REPORT, Loads on Restraints

  (OCC)COMBINATION #  1

    ------Forces( N.)-------     -----Moments( N.m. )-----

  NODE      FX       FY       FZ         MX       MY       MZ

   100    4140     2063     1451       2063    21443       23   Rigid ANC      

   250       0    14733        0          0        0        0   Rigid +Y       

   410       0    10935        0          0        0        0   Rigid +Y       

   600   13618     7591     1164       1308     8335     7723   Rigid ANC    

****  B31.3 -2006, May 31, 2007 

****  CODE STRESS CHECK FAILED                     

 HIGHEST STRESSES: (   KPa    )

    CODE STRESS %:  106.1  @NODE  298

              STRESS:  194578.8  ALLOWABLE:   183401.

BENDING STRESS:        183613.0  @NODE  298 

TORSIONAL STRESS:        5011.1  @NODE  300

AXIAL STRESS:           11947.4  @NODE  298 

3D MAX INTENSITY:      201337.9  @NODE  299

THE FIX:

  1. In long pipelines, surge can be relieved with a tank of water directly connected to the pipeline called a “surge tank.” When surge is encountered, the tank will act to relieve the pressure, and can store excess liquid, giving the flow alternative storage better than that provided by expansion of the pipe wall and compression of the fluid. Surge tanks can serve for both positive and negative pressure fluctuations.

  2. Air chambers are installed in areas where water hammer is encountered frequently, and are typically seen behind sink and tub fixtures. Shaped like thin, upside-down bottles with a small orifice connection to the pipe, they are air-filled. The air compresses to absorb the shock, protecting the fixture and piping.

  3. In this situation, the best form of water hammer prevention is to have automatically-controlled valves, which close slowly. Closing the valve slowly can moderate the rise in the pressure when the downsurge wave - resulting from the valve closing - returns from the source or reservoir.
     

Going back to the PIPENET model and changing the valve closure time to 6 sec. (figure 5) will cause less water hammer loads, as shown in figure 6.
 

Figure 5


NOTE: Lower values of the exponent cause the majority of the flow cutoff to occur at the end of closure, while larger values of the closure exponent cause the majority of flow cutoff to occur at the beginning of closure.
 

Figure 6

Now, dynamic analysis is performed for this model again with new .FRC file. At this time, supports loads are far better than previous situation and code stress also passed as shown below.
 

RESTRAINT REPORT, Loads on Restraints

  (OCC)COMBINATION #  1

 

         ------Forces( N.)-------     -----Moments( N.m. )-----

  NODE      FX       FY       FZ         MX       MY       MZ

   100    1991     2063      701       2063    10648       18   Rigid ANC      

   250       0     9607        0          0        0        0   Rigid +Y       

   410       0     8885        0          0        0        0   Rigid +Y       

   600    6844     6824      584        680     4230     5943   Rigid ANC      

****  B31.3 -2006, May 31, 2007                                                

****  CODE STRESS CHECK PASSED                                                                                                                          

 HIGHEST STRESSES: (   KPa    )                                                

    CODE STRESS %:             57.0  @NODE  298                                

           STRESS:         104517.9  ALLOWABLE:   183401.                      

    BENDING STRESS:         94708.5  @NODE  298                                

    TORSIONAL STRESS:        2521.1  @NODE  300                                

    AXIAL STRESS:           10331.8  @NODE  399                                

    3D MAX INTENSITY:      110781.1  @NODE  299 

CONCLUSION :

Water hammer will continue to challenge engineers, operators, and managers of water systems because it is associated with systems that cannot be exactly defined due to the size and length of the water distribution system with undulating profile or the lack of definition of the system components such as valves or pumps. By knowing how to avoid situations that will create water hammer or pulsations during the process, or while trouble shooting, you can eliminate a lot of problems, failed valves and equipment, and costly downtime.
 

 

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