Vortex Induced Vibration in Centrifugal pump ( case study)

Contents: 1- Definitions. 2- The problem. 3-problem analysis and findings. 4-solutions and conclusion.

Definitions : 1- centrifugal pump: A centrifugal pump is known to be a “pressure generator,” vs. a “flow generator,” which a rotary pump is. Essentially, a centrifugal pump has a rotating element, or several of them, which “impel” (hence the name impeller) the energy to the fluid. A collector (volute or a diffusor) guides the fluid to discharge.

Pump characteristic curve:

Best efficiency point (BEP): This is the point where the brake horsepower going into the pump is the closest to the water horsepower coming out of the pump. It is at this point we experience the least amount of shaft vibration and deflection.brakewater

Vibration in centrifugal pumps: Vibration due to low flow from (BEP), occurs due to circulations in pump (vortex induced vibration). Vibration due to higher flow from (BEP), because of cavitation.

Circulations in impeller: Pump generated dynamic pressure sources include turbulence (vortices or wakes) produced in the clearance space between impeller vane tips and the stationary diffuser or volute lips.

What is VIV (Vortex induced vibration) When a fluid passes a structure sufficiently rapidly (e.g.>60), it generates vortices in the wake region behind the body. This phenomenon (also known as Kármán vortex shedding) imposes aperiodic surface pressure on the body causing flexible structures to oscillate. The vortex street formed behind the structure and parallel to the flow direction, regardless the geometry of the structure, Are usually similar. The vortex shedding frequency is characterized by anon-dimensional parameter called the Strouhal number

Vortex shedding frequency Pulsation induced by vortex shedding a simplified analysis can be performed using the concept of the Strouhal number. The frequency of the vortex shedding can be described by the following Equation : where: f = Vortex shedding frequency, Hz V = Flow velocity, m/sec S = Strouhal number D = Effective diameter of the obstruction (m)

How it occurs?

Reynold number The Reynolds number is the ratio of inertial forces to viscous forces within a fluid which is subjected to relative internal movement due to different fluid velocities Reynold number at pump impeller: where: N is the rotational speed of the impeller D is the impeller diameter

Strohal number it is used to define the flow forcing: (St excit = f excit D/U), where f excit (vortex shedding frequency) is the underlying frequency of the oscillatory motion (flow excitation), D is the characteristic dimension (for example cylinder diameter or channel width) and U is a characteristic velocity, typically the amplitude of the velocity oscillation

Turbulence intensity Excitation level depends on turbulence intensity (TI) and the random behavior of the flow due to the upstream flow turbulence or the turbulence generated. The turbulence intensity: the ratio of RMS velocity to mean velocity.

Locked in phenomenon:

The problem: Observation: a process condensate centrifugal pump while operating at a discharge pressure of 24 bar, a high vibration was recorded with FFT analyzer in one direction. When increasing the flow a vibration reduction dramatically occurs after a certain condition of discharge pressure and flow.

FFT recorded spectrum:

Vibration change along with flow trend: In horizontal direction,

In vertical direction (exhibits no change in vibration along with flow change)

Pump physical layout :

Pump with motor driver,

Pump characteristic curve, Best efficiency point Current work point

Pump technical specifications,

Problem analysis and Findings: Analysis of FFT Spectrum, 1- the dominant component is 70 HZ, A non synchronous frequency that 1X= 50 HZ. 2- Directional vibration which only strong in horizontal direction. 3- existence of small harmonics of the 70 HZ component. 4- hay stack with the 70 HZ component.

Calculations: Given data, - shaft RPM= 2983 Rev./min. - number of impeller blades K= 4 blades. -impeller diameter= 402 mm. -Liquid density=926 kg/m3. -viscosity=0.2 cp. -

Reynold number at impeller tips, Re. no=(2983) (0.402*2)(926)/(0.2)=2,231,959 Characteristic length of impeller= 2* π*impeller rad./k

Strohal no at this given Reynold number,

Vortex shedding frequency at the given condition, U= ω*r = (2*π*N/60)*r= m/sec. (f excit = st excit u/d), st=0.35 Characteristic length of the impeller = 2*3.14*0.201/4=0.315 F shedding vortex = 0.35*62.788*/0.315 = Which is equal to the observed frequency.

Experimental data, vibration change with flow change (waterfall data),

- low vibration 70 HZ component at vertical direction= mm/sec.

Bump test for the discharge pipeline,

Interpretation of this directional vibration, With the previous given information, it is obvious to be in a lock-in vibration. Vortex shedding frequency = discharge pipe line natural frequency= 70 HZ. Noticing that When increasing the flow the vortex shedding frequency will not change however, amplitude will greatly diminished due to the decreasing turbulence intensity which represents the vibration energy in the flow. Note: This type of instability is more probable at low flows because acoustic damping that is generated by flow friction effects is greater at higher flow rates.

Conclusion, With all information and analysis shown above it is evidenced that it is vortex induced vibration with lock-in phenomenon. Solution: There are many proposed solutions may work which are, 1- removing vibration source (reducing turbulence intensity) 2-separate natural frequency of the system from the existing vortex frequencies 3- add a vibration absorber.

However, the most convenient solution is to minimize the vibration force accompanied with the 70 Hz frequency to avoid the lock-in phenomenon. So, the recommendation is to install the minimum flow line to accommodate the surplus flow of the process at the same time meet the BEP (best efficiency point ) at pump characteristic curve which accordingly mitigate the TI( turbulence intensity ).