Frequency Detuning in Vertical Pumps _ Pumps & Systems Magazine

8/12/2019 Frequency Detuning in Vertical Pumps _ Pumps & Systems Magazine

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11/3/2014 Frequency Detuning in Vertical Pumps | Pumps & Systems Magazine

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Written by: Carlos Moreno, Flowserve

Detuning devices solve high vibration problems in power station water circulating pumps.Mechanical res onancethe buildup of a large vibration amplitudewhen som ethingis excited at its natural frequencycomes in many for ms . In its most desirablestate, it occurs in m usic. Instruments and acoustics are finely tuned to producedramatic and beautiful resonant effects. In its mos t undesirable s tate, mechanicalresonance has extremely dramatic effects.They can cause bridges , structures andmachinery to break or malfunction, resulting in dange rous and e xpensive failures.

Engineers w ork diligently to fine-tune mechanical s tructures an d machine ry toreduce or eliminate the effects of mechani cal resonance b efore construction or installation. Engineers m ust account for the following factors when des igningpumping systems to avoid mechanical resonance:

Operating speedFoundation stiffness characteristicsDrive reed critical frequencyBearing spanBearing dampin g coefficientsBearing stiffness coefficientsDrive mass and distribution

Pump mas s and distributionPump fluidGeometry of the rotor and structural components

When ass umptions are wrong or the actual installation does not mirror the engineering models , the potential for mechanical res onance could exist if the systems actual natural frequency is too clos e to the operating speed. Issuesdo aris e, often from unexpected foundation conditions or in accurate drive reed critical frequency data.

Natural Frequency in Pumps A mechanical systems natural frequ ency is a function of its weight and s tiffness , most commonly express ed as :

n = (k/m)

Where :

n = natural frequency

k = stiffness

m = mass

For horizontal pumping s ystems, the solution to resonance iss ues has generally involved simple m odifications thatincrease s ystem weight, reducing natural frequency as a result and creating an adeq uate margin from the operatingspeed.

Resona nce iss ues can be much m ore difficult to address with large vertical circulating water pumps. Typically found inpower generation, steel manufacturing, water intake and flood control applications, these ma ss ive, expensive pumpinstallations can be difficult and costly to mod ify once ins talled. Because of these difficulties, engineers are forced toemploy more soph isticated techniques that alter system stiffness as a means of changing natural frequency oncepumps are installed.

Often, this can be accomplis hed by adding frequency detuning devices to the pump s ystem structure. In principle, theaddition of these devices reduces s ystem stiffness, lowering its natural frequency away from potential points of excitation. While this practice has been around for decades , advances in tes ting and modelin g techniques have madedesign and application of detuning devices e xtremely reliable, avoiding the unnecess ary and costly trial and error previously associated with this technique.

Frequency Detuners for Vertical Pumps Axial and m ixed-flow vertical circulating water pumps (see Figure 1) are we t-pit instal lations with thes e main

Frequency Detuning in Vertical Pumps

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components:

Suction bellRotor ass embly (impeller, shaft, bearings, couplings and drive rotor)ColumnDischarge headDrive (not shown )

In this type ins tallation, one of the eas iest and m ost inexpensive places to affect a change in na tural frequency is a t theinterface between the dis charge head a nd the driver. A detuning device for this type pump system cons ists of twocarbon steel plates (so metimes called spring plates ) mounted between the driver flange and the pump driver supportflange of the discharge he ad. A gap between the cantilevered portions o f these plates a dds flexibility to the systemlike a pair of springs acting in series reducing system stiffness. To illustrate the engineering and appl ication of

detuning devices, consider the following circulating water pump appl ication at a Midwest U.S. power s tation.

Figure 1. Typical vertical circulation pump

Frequency Detuning in PracticeDespite careful en gineering, two vertical circulating water pum ps exhibited higher than expected vibration levels s hortlyafter installation (see Figure 2). Engineers performed thorough vibration analysis on both units, revealing highamplitudes in both systems , the most severe being in Unit 1B. Measurem ents for Unit 1B are shown in Table 1. Toidentify and confirm a resonance is sue, consulting engine ers engaged in the following field testing and analysis.

Figure 2. Original pump installation at the powe r gener ation plant

Impact Testing with a Calibr ated Hammer In this method, engineers strike the pump system at multiple locations using an instrumented hammer and use avibration sen sor and a vibration analyzer for measurem ent. This m ethod of striking a system with a mas s andmeasuring its response is common for determining natural frequencies in mechanical structures. Best practice


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requires that the impact testing take place at m ultiple locations on the structure to identify several resonantfrequencies. This test mus t be performed with the pumping equipm ent off. The recorded first and secon d naturalfrequencies are show n in Table 2.

1 2 next last

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