Vanquish Vertical Pump Vexations

Address common problems including undersized motor drivers

By Amin Almasi, rotating equipment consultant

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Vertical pumps suit a diverse range of industrial applications for services ranging from low to high pressure, say, from 0.3 barg to 700 barg, and cryogenic to high temperature, e.g., -190°C to +500°C. Moreover, they cover a wide range of flow rates, roughly from 6 to 60,000 m3/h. The pumps contain from 1 to 50 stages and run at speeds from 180 to 20,000 rpm. They come in numerous varieties.

A vertical wet-pit pump can handle applications involving clean or lightly contaminated liquids. The drive motor is mounted directly above the pumping head, which is immersed in the liquid. Such a pump has a common discharge and support column. It has a vertical line shaft with the slide bearings typically lubricated by the pumped liquid.

A vertical sump pump is used primarily in services with lightly contaminated liquids, slurries and liquids containing solids. It also has a motor mounted directly on top of the pumping head, which is suspended in the pumped liquid. Such a pump has a separate discharge pipe and support pipe column. It has a vertical line shaft with slide bearings or cantilever design without slide bearings.

Submerged pumps are the other major category of vertical pumps but there are other types and categories as well.

Bearings and the lubrication system significantly impact the pump’s reliability, maintenance and long-term operation. Indeed, bearings and their lubrication usually account for most problems. So, specify all bearings according to API-610 whether the pump is an API type or not. Likewise, insist upon a sophisticated lubrication oil system as per API-610 or via a robust specification (preferably with two lubrication pumps, dual filters, etc.) for any large vertical pump.

Thrust bearings are critical components. They should be designed to carry the weight of all rotating parts plus the hydraulic thrust (both steady state and dynamic forces) and should be readily accessible for inspection. The hydraulic design of the pump should ensure that residual loads on the thrust bearings are always downwards.

Because vertical pumps commonly are direct driven, couplings should have provisions to secure the hub to the shaft with minimal dismantling.

Vertical Pump Dynamics

A vertical pump, particularly a suspended unit, usually is a flexible system that runs at a speed located between or around natural frequencies. This makes it susceptible to resonant vibration if its separation margins and dynamic situation aren’t verified during design and installation. Preferably maintain a 10–15% margin of separation between any natural frequency of the support structure and operating speed (and its first few harmonics).

In case of high vibration, it may be possible to rebalance the rotor assembly on site. However, in the case of resonance, this usually won’t markedly reduce the vibration level. Furthermore, the balancing planes probably aren’t easily accessible on site and rebalancing has some limits. The unbalance condition somewhat depends upon temperature and other operating conditions. So, expect increased vibration levels during transient conditions, due to fouling of a pump impeller or for other reasons. Also, any changes to the pump, such as maintenance work on the electric motor, coupling, pump rotor, etc., can affect vibration level.

Basic structural elements typically include the foundation, pump structure and electric motor frame. Strive to use accurate foundation data in a dynamic analysis. However, the deflection of the foundation typically represents less than 5–8% of the total deflection of the structural elements. Therefore, if accurate foundation data aren’t available, use estimated or rough data.

Because of its flexible structure, a large vertical pump can get excited by random broadband turbulences. However, vibration levels due to turbulent excitation usually pose no major concerns.

Large vertical pumps located side by side can be excited by the foundation structure to some extent. Some cross-couplings of vibrations between pumps can occur via the foundation or soil; measurements can detect these. If manufacturing and construction tolerances are controlled, identical pumps will exhibit only minor difference in vibration characteristics. In other words, natural frequencies usually differ by approximately 0.5–1.5 Hz. Slightly different speeds and natural frequencies of identical pumps occasionally result in moderate beat phenomena.

Operation may change the natural frequency of a vertical pump system. For instance, liquid level can alter the natural frequency slightly. However, such a variation usually is small. As a very rough indication, you can expect 0.05–0.4-Hz higher bending frequency at low liquid level relative to high liquid level. Also, the natural frequency often increases when a vertical pump is stopped (because, for example, the pump column isn’t full of liquid). Such a change in the bending natural frequency might be on the order of 0.1–0.6 Hz.

A vertical pump, particularly one with a long shaft, has relatively large inertia in the driver and pump stages and is susceptible to torsional excitations. Therefore, it requires a complete torsional study.

Any critical vertical pump package demands careful condition monitoring. So, specify electric motor winding temperature sensors, an X-Y vibration probe for each bearing as well as an axial probe and key phasor for each shaft. In addition, insist upon an accelerometer at the pump casing to monitor the casing vibration — particularly for a large vertical pump with flexible structure. Ensure the number and location of vibration sensors will suffice for machinery diagnostics.

Parallel Operation

Plants often operate vertical pumps in parallel. For such operation, the “head versus flow” curves of the pumps ideally should match within 2–3% between the minimum continuous flow and the end of curve flow. Always check that all combinations of parallel running are efficient for long-term operation without any performance or reliability issues.

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