Centrifugal Pump Troubleshooting |
When sizing and selecting centrifugal pumps for a given application the pump efficiency at design should be taken into consideration. The efficiency of centrifugal pumps is stated as a percentage and represents a unit of measure describing the change of centrifugal force; velocity of the fluid, into pressure energy; Head In Feet, at a constant flow; gallons per minute.
Tested and published performance curves; generally based on water, illustrate the changes in efficiency throughout the entire capacity range of the pump. The B.E.P.(best efficiency point), is the area on the curve where the change of velocity energy into pressure energy at a given gallon per minute is optimum; in essence, the point where the pump is most efficient.
To avoid excessive hydraulic thrust, temperature rise, erosion and separation cavitation, operation of a centrifugal pump should not be outside the furthest left or right efficiency curves published by the manufacturer. Performance in these areas induces premature bearing and mechanical seal failures due to shaft deflection, and an increase in temperature of the process fluid in the pump casing causing seizure of close tolerance parts and cavitation.
In reality a centrifugal pump rarely sees the system requirement it was originally selected and due to its ability to fluctuate within a broad head capacity range will meet the expectations of the application. However; when the process changes, system differential pressures increase and operating conditions vary dramatically, consideration must be given to thermal and mechanical minimum flow to decrease the possibility of personal injury and downtime.
In a centrifugal pump when operating toward shut-off; far left on the performance curve,a percentage of the process fluid recirculates in the eye of the impeller and between the impeller shroud and backplate. Evidence of minimum flow problems are more dramatic on applications where NPSHa exceeds the NPSHr by a given pump two, three and four fold.
To avoid thermal problems during low flow
operation and prevent a potentially hazardous temperature rise
within the pump, the temperature rise at shut-off and the minimum
flow required for thermal protection must be calculated and the
required volume of the fluid to be by-passed to dissipate this
heat established.
Prior to calculating the minimum flow required for a given application the maximum allowable temperature rise must be established. This defines the temperature that exceeds the corresponding saturation temperature the impeller eye.
While most maximum allowable temperature
increases are based on the temperature where flashing;
vaporizing, of the process fluid occurs, it's important to
realize other pump components may dictate a lower temperature to
ensure long trouble-free service. For example; while the maximum
allowable temperature to avoid cavitation may be 210oF
the upper temperature limitations for a polypropylene pump may be
180oF Other pump
components which may require consideration will include
mechanical seals, packing, bearings and wear ring tolerances.
Specific heat (SH) is a relative unit of
measure which describes the heat required to elevate one pound of
a fluid one degree Fahrenheit. Water has a specific heat of 1;
one BTU is required to elevate one pound of water one degree
Fahrenheit. The specific heat of a given fluid will be dictated
by the fluids properties.
When handling non-newtonian, viscous and shear sensitive fluids where temperature may affect product integrity the maximum allowable temperature introduced by the pump at the design capacity should be considered. The temperature rise at various on a centrifugal pump head capacity curve should be identified and the required by-pass volume established.
During the centrifugal pump selection
process consideration must also be given to minimum flow for
mechanical protection. With cool clean liquids the required
minimum flow for thermal protection may be minimal. However;
excessive shaft deflection due to unbalanced radial loads,
vibration and rotating element instability will result should the
mechanical minimum flow requirements not be met. These scenarios
become more evident as suction pressures increase further beyond
the NPSHr by the pump.
While intricate formulas and calculations exist which determine the amount of minimum flow for mechanical protection it's recommended the pump supplier be contacted during the selection process to supply this information. Generally; most manufacturers will supply a by-pass capacity figure based on the percentage of B.E.P. at the design impeller diameter and speed.
When sizing the pump it must be determined whether the minimum flow by-pass will remain open at all times; as with a fixed orifice, or will the by-pass be closed when the pump is operating at design capacity. This decision will affect the impeller diameter and in some cases a change in pump size.
One of the most practical, cost efficient methods of maintaining the by-pass capacity would be an automatically controlled device which senses changes in the process pressure. As this pressure increases to a predetermined setting the device will open diverting the by-pass flow, as the pressure decreases the device will modulate limiting the by-pass flow.
There are numerous methods available for controlling centrifugal pump performance fluctuations. Variable speed pump systems, and operating pumps in parallel have become popular methods when applied properly.
Acknowledged and incorporated into the process system; a minimum flow by-pass will have a direct impact resulting in extended service life and a dramatic reduction in down time and the affiliated expense of repair parts.
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