To find the actual operating point for a pump discharging to a pipe distribution system, a comparison is made between the pump’s performance curve and the system’s resistance curve. Both are parabolic curves that compare flow rate to pressure head. The system curve shows high head with high flow, and low head with low flow. Conversely, pump performance curves show high head with low flow rate and low head rate with high flow rate. Where the two curves intersect is the overall pump system operating point where pump and system characteristics match up.
PUSHING THE ENVELOPE: THE EFFECTS OF EXTREME OPERATING REGIMES
How long should a pump last? Its operational lifetime is a function of many factors, but certain types of pumps are designed to last for different durations. The table above provides a list of different pump types and their average life expectancy.
The primary cause of pump failure is material fatigue. Anything with moving parts will wear out over time due to friction- and heat-induced deformation. Friction and heat are proportional to workload. The greater the workload, the greater the material fatigue over time. The parts of the pump most susceptible to material fatigue include any that are subject to fluctuating or cyclic loading during operation. These include vanes which are subject to both suction and high pressure with each rotation, pistons subject to variable pressure as they move with each turn of the cam shaft, expansion and contraction of vibrating membranes, etc. The stresses imposed onto the materials by these cyclical loadings result in strains that propagate in opposite directions with each cycle. These microscopic cracks will get larger over time as they serve as pressure concentration points for subsequent loadings.
The key factor determining the extent of fatigue is the amount of the load versus the material’s maximum resistance. The closer each load is to this limit, the fewer cycles necessary to cause fatigue. This, simply put, is why higher workloads wear out pumps at a faster rate. The result is a significantly shortened operational lifetime. Maximizing pump lifetime depends on starting with an appropriate design configuration and operating the pump in good working conditions that avoid extreme workloads. Each pump model has a life rating assigned to it under normal operating conditions. A life rating is measured in terms of projected operating hours at a specific pressure range (measured in psi) and range of operating speeds (measured in rpm). This estimate is typically based on the pump’s shaft bearing life expectancy. It is up to the pump’s operator and the system’s designer to ensure that a pump does not have to exceed these operational limits.
Speed and pressure do not have equal effects on pump longevity. Pump operational lifetime is in direct inverse proportion to shaft rotating speed. Pump life can be doubled by reducing operating speed by half, or conversely it can be halved by doubling the rpms during operation. Pressure, on the other hand, has a much greater impact on pump longevity. Hydraulic pressure induces a side load on the shaft bearing. The side load is directly proportional to the hydraulic pressure. Pump life is inversely proportional to the cube of this resultant side load. So, doubling the hydraulic pressure of the pumping system would decrease to one eighth of its normal operational life. Or, halving the hydraulic pressure would increase the pump’s life by a factor of eight. This does not imply that pumps should be oversized and then run at lower-than-normal pressures and speeds to maximize longevity. This would be a false economy requiring more such oversized and under-run pumps to do the same amount of work.
In addition to pressure and speed, pump cycle time also affects pump longevity. The act of stopping and starting a pump induces several initial stresses into its material components. Constantly having to stop and start a pump results in significantly more wear and tear than continuous operation. Increased frequency of the start and stop cycle, can result in burning out the pump motor. While speed is an inherent characteristic of the pump, and pressure is a function of the pumping system configuration, cycle time is a consequence of the amount of liquid that is being moved and stored. Pumps can be configured to turn on once the water level in a reservoir reaches a designated elevation. This turn-on elevation is a function of the liquid inflow rate and the storage capacity of the reservoir, sump, or tank. There is also a turn-off elevation near the bottom of the reservoir whose depth is set to prevent pump cavitation that would result from having too little liquid to pump.
The duration of the “off” part of the pump cycle—the time needed to refill the reservoir—as determined by storage capacity (between the turn-off and the turn-on elevations) divided by the inflow rate. The pump is not operating while the tank refills. The duration of the “on portion” of the pump cycle—the time needed to empty the reservoir—is determined by the same storage volume, divided by the difference between the pump discharge rate and the liquid inflow rate (inflow is assumed to continue while the pump is operating).
OTHER FACTORS AFFECTING PUMP LONGEVITY
Excessive pressure and speed can cause pump failures by causing bearing shafts to prematurely reach the end of their useful life through material stress, wear, and tear. However, there are other extreme (and simply negligent) operating conditions that can reduce a pump’s operational lifetime. As mentioned above, cavitation can reduce pump longevity. Cavitation, the formation of bubbles in the liquid inflow is caused by insufficient head at the suction inlet point (due mostly to insufficient depth). It causes mechanical damage due to the shock of the bubbles’ impacts, excessive heat, etc. This can be avoided by maintaining the proper suction head at the inlet.
Misalignment of the pump rotating shaft can cause side bearing pressures like that of excessive hydraulic pressure. It also creates points of concentrated friction as the shaft rests against the casing sidewalls instead of rotating freely. It can also displace and ruin the shaft’s ball bearings. Proper care and maintenance should prevent this from occurring.
The lack of a pump relief valve or a relief valve set at the wrong release pressure will result in excessive pressure buildup within the pump. Operating at maximum pressure, even if not actually pumping liquid, is effectively counted as operating time reducing the pump’s actual life.
Oil cleanliness and oil filtration are necessary to prevent internal friction grinding and the buildup of gunk on the pump’s moving parts. Usually a 150-uM pump suction strainer is required along with a return line filter of 10-uM rating or better. Using even finer strainers will result in longer operational lifetimes. However, the installation of very fine strainers may be prohibitively expensive compared to the cost of replacing the pump over time.
In addition to oil cleanliness, oil temperature needs to be maintained. Heated oil cannot be used to dissipate operating heat and thus indirectly adds to material heat stress. Oil temperature can be controlled by means of a heat exchanger or radiator attached to the oil circulation line.
Vertiflo Pump Company manufactures close-coupled horizontal end-suction pumps designed for the transfer of both water and chemical solutions. The Vertiflo Pump Company offers its Model 1312 Industrial, close-coupled, horizontal, end-suction pump for service in general pumping, chemicals, wash systems, deionized water, process and OEM applications. This pump is designed for pumping liquid chemicals from tank to tank, and into transport delivery trucks. The Model 1312 is designed for long life in extreme operating regimes with heads to 160 feet total operating head and flows up to 240 gpm. 1750 and 3500 rpm sizes are available. Its back-pull-out design construction allows rotating element to be easily removed since the casing remains in piping. Casing may be rotated in 90-degree increments to accommodate various piping and discharge orientation requirements. The close-coupled design saves installation space. Suction and discharge connections are threaded NPT. Construction material options include cast iron, 316 stainless steel fitted, or all 316-stainless steel. Pump volute, impeller, and mounting bracket are heavy cast metal. Vertiflo’s Model 1312 horizontal motor-mounted, end-suction pumps are designed for use with NEMA standard C-face electric motors. Its standard size mechanical seal is a self-aligning design. The pump’s semi-open impeller with balance hub is secured to the bearing shaft by taper and threads.
Peristaltic metering pumps are designed to dispense accurate amounts of chemical to a system. Positive displacement is created by rollers which squeeze the tubing in the pump head through which the chemical flows. The maximum pressure that a peristaltic pump can support varies from pump to pump. Blue-White Industries’ Proseries-M FlexPro Peristaltic Metering Pumps have a max pressure rating of 125 psi (8.6 bar). As pressure increases, the longevity of a pump may decrease.
High-pressure pump discharge over a long period of time can cause greater wear on a peristaltic metering pump. The main components affected by constant high pressure are the discharge tubing, the motor, and the roller assembly. The tubing’s service life may decrease with maximum discharge. For example, there may be a greater occurrence of pinhole failure. This is when the fluid finds the weakest point inside the tube and pushes out towards the exterior of the tube, creating a tiny pinhole. Once there is a hole in the tubing, most pumps will fail. Each Proseries-M FlexPro Pump is equipped with the exclusive patented Tube Failure Detection system, built right in. The TFD System can detect tube failure and will automatically shut off and energize a relay, permitting communications with external equipment, such as an alarm or a SCADA system. The chemical must be cleaned from the pump head before the pump will restart.
The motor also becomes more stressed when more pressure is placed on the pump. High pressure causes more heat which can cause oil to leak. FlexPro pumps are less inclined to be affected by this issue because they are equipped with brushless motors. Brushless motors are more efficient and do not get as hot as brushed motors. The motor shaft breaking due to wear or high pressures is another problem that can occur. The FlexPro motors have a sturdy design to compensate for this added pressure. Each FlexPro shaft is hardened through a heat treatment process to withstand these external forces.
Roller assemblies will possibly wear with the higher discharge pressure. The bearings in the pump’s roller assemblies need to overcome the increased pressure in the tubing. When more pressure is put on the bearings, they may fail. FlexPro has two CNC-machined squeeze rollers and two alignment rollers. The rollers are made in-house and undergo rigorous quality testing to ensure precise control over tolerances, resulting in the parts ability to withstand greater pressures for longer periods of time. All peristaltic metering pumps will show normal wear when left running for long periods of time at high pressure. There will always be general upkeep and maintenance. When deciding what pump to purchase for a particular application, it’s important to take into consideration quality, repair and maintenance cost, and proven success.
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