CENTRIFUGAL PUMPS - Modern Design and Practices

Softfoot

In technical term, soft foot is a condition where the mounting feet of an equipment (pump, compressor, blower, motor, engine, turbine, or similar machine) are not level or in proper contact with the mounting pads, or pedestals, on its baseplate; the feet are not on the same plane.

Consider a triangular table with three legs. The legs are identical, and the table stands on a level and flat surface. If someone applied a soft rocking force on its top, the table would remain steady and not move. If one of the legs were slightly shorter, or if the surface were uneven, and a soft rocking force were applied on top, the table would still hold steady and not move.

Now, imagine a rectangular table with four legs. If one of the legs were slightly shorter, or if the surface were uneven, and a soft rocking force were applied, the table would rock back and forth. The table would continue to rock, or ‘vibrate’, so long as the force is applied. This condition of the four-legged table is the simplest form of soft foot.

A pump soft foot condition arises if any of the following condition occurs:

· The feet are not on the same plane.

· The feet are on same plane, but the mounting pads are not.

· Both the feet and the pads are not on the same plane.

On the other hand, if a pump were mounted on only three touch points (instead of four), a soft foot condition would not exist – it is analogous to a table having three legs, as mentioned above. A good example is a small overhang process pump with a foot on each side of its casing, and a third foot supporting its bearing bracket, in standalone condition. But a soft foot condition can exist when taken in combination with its driver.

CHECKING FOR SOFT FOOT

Checking for the presence of soft foot can be done in two ways:

[1] Set the pump on its baseplate and place a dial indicator on one foot. Tighten the hold down bolt slowly and watch the dial indicator. The pump has soft foot if the pointer moved by 0.002” [0.05 mm] while tightening the bolt.

[2] Or, set the pump on its baseplate and bolt it down. Place a dial indicator on one foot, loosen its hold down bolt slowly, and watch the dial indicator. The pump has soft foot if the pointer moved by 0.002” [0.05 mm] while loosening the bolt.

The amount of movement measured by the instrument indicates how many shims are needed underneath its foot to level the pump. Repeat this procedure on each of the pump foot.

Soft foot should be corrected only by shimming using the right amount of shim thickness. Never try to correct it by over-tightening the foot hold-down bolts - it will induce harmful stress on the casing that can result in permanent casing distortion, or deformation, if left uncorrected for some time. Typical shims used are made of 300 series stainless steel, of different thickness, from 0.003” to 0.062”, as needed. If more than one shim is needed, a shim pack may be used but it is recommended that their total thickness not exceed 0.125” 

CAUSES OF SOFT FOOT

Despite careful efforts to machine the feet of an equipment and its baseplate mounting pads, so that they are flat and level on the same plane, a soft foot is likely to occur due to factors, such as:

· Expansion, or contraction, of materials due to changes in temperature.

· Eccentricity and tolerance build-up during machining and assembly.

· Induced or residual stresses from bolting, piping, and other connections.

· Using instruments that are out of calibration, or errors in measurement readings.

· Excessive external nozzle loads.

· Natural sag of baseplate.

· Improper grouting of foundation.

· Seismic activity.

EFFECTS OF SOFT FOOT

· Soft foot can cause rotor misalignment. 

· It can result in casing distortion, or deformation.

· It can induce elevated vibration level at 1X or 2X running speed frequency, or at impeller vane pass frequency.

Both pump and its driver should be checked separately for soft foot condition. Once the pump is verified to be free of soft foot, its alignment with the driver should be re-checked.

TEST YOUR KNOWLEDGE

Consider a situation wherein a pump was shimmed and aligned. If the bolts were loosened one at a time and the shims sprang back such as to open a gap in the four feet of 0.05”, 0.10”, 0.05”, and 0,08”, should all the four feet be considered ‘soft foot’, or only the ones with 0.10” and 0.08”, treating the 0.05” as a neutral datum this being the lowest value? 

What do you think?