The reliability of a centrifugal pump is based on many simple factors that often are overlooked. Equipment reliability begins at the time the decision is made to purchase the pump, and it continues throughout the machine’s life cycle. To operate and maintain a centrifugal pump successfully, personnel should be familiar with not only the operation of the pump, but also its design, installation and preventive maintenance requirements.
DesignCasing. The pump’s casing plays many roles in the overall operation of the unit. It structurally supports the pump rotor and bearings to provide positive rotor alignment, and it also directs the flow of fluid into the impeller(s), collects the flow discharge from the impeller(s), directs the flow out of the discharge into the piping, and helps convert the velocity energy imparted to the liquid by the impeller into pressure. Pumps designed for “severe” service often are made of special materials and incorporate special designs and features to enhance wear life. For example, heat-treated alloys and hardened high chrome irons often are used for pumps that handle abrasive or high-temperature materials.
Impeller. The pump impeller, arguably the most important component affecting pump reliability, imparts energy to the liquid. For optimum impeller life, materials compatible with the fluid being pumped should be selected. Because materials with a similar hardness and composition tend to gall if they come in contact, a difference in hardness of from 50 to 100 points on the Brinell scale between the parts can help prevent galling. For this reason, it is common to see pumps designed with a 400 series stainless steel impeller and casing wear rings hardened to 475 and 450 Brinell hardness number (BHN), respectively.
Clearances. Close running clearances typically are provided for the impellers, interstage bushings (for multi-stage pumps) and casing bushings (for high head pumps) for high-efficiency operation. The clearance will vary with pump design, size and material of construction, but it generally runs about 0.0015" per inch of diameter wearing ring or open impeller face for pumps that handle relatively clear liquids. Interstage and casing bushings usually have a similar clearance as the impeller wearing ring.
Renewable impeller wearing rings, casing wearing rings, sleeves and bushings can contribute to optimum pump efficiency and reliability. Many single-stage end suction pumps with open and semi-open impellers also incorporate an impeller adjustment feature that allows the clearance to be adjusted to bring the performance back to near new levels.
Seals and Packing. Many different types of mechanical seals and packing are available to choose from, and the best choice can only be made by considering the requirements of the application and consulting with the pump and seal manufacturers. However, for applications operating under extreme vacuum or where the fluid being handled is of such nature that external leakage cannot be tolerated, a double mechanical seal lubricated by a compatible fluid and from an outside source is often
A flexible coupling allows operation should misalignment occur while the pump is running. However, it is not intended for misaligned operation. A misaligned coupling will cause pump vibration and can shorten the bearing life in either the pump or driver, or cause higher than normal bearing temperatures or even broken shafts.
InstallationLocation. Experience dictates that location is as important as any of the other aspects of reliability. If possible, locate the unit in a clean area with easy access, adequate lighting and reasonable dryness. Open space around the unit is also important to allow monitoring and maintenance of the pump/driver/seal combination.
Foundation. The pump, driver and base plate should be installed on a foundation rigid enough to support the weight of the unit, absorb the normal vibration forces inherent in a rotating machine and allow pump and driver alignment to be achieved and maintained. The base plate should be installed so that the pads supporting the pump and driver are level to within 0.005" per foot, co-planar. In other words, not only should each pad be level to within that limit, but also each pad in relation to every other pad should be level to within that tolerance. Although some specifications require even more accurate setting of the base plate, this level of accuracy is satisfactory for most process pumps.
After leveling but before tightening the anchor bolts or setting the pump and driver, the base plate should be completely grouted in place (unless it is otherwise designed). A good-quality, non-shrinking cement or epoxy grout should fill the base entirely, leaving no voids. Once hardened, the anchor bolts should be tightened to the manufacturer’s specification. If no specifications are provided, one half turn on the nut after it contacts the base should be sufficient.
The pump and driver must be rigidly bolted to the base with shimming for alignment. Typically the shimming is placed under the driver because it is easier to move than the pump. However, in some circumstances, depending on the arrangement of the equipment, it is acceptable to shim under both the pump and driver. Initial alignment should be made when the base plate is installed, leveled, grouted and bolted, and again just prior to initial operation. The system should be checked once more shortly after the initial operation of a fully loaded unit to ensure that good alignment is being maintained. A “hot” alignment check should be performed for units operating in continuous or nearly continuous service.
The driver alignment should be within the specified limits provided by the manufacturer for the pump and the coupling with respect to axial gap, parallelism and concentricity. If no specifications are available, experience dictates that a coupling aligned within a 0.001" (0.025 mm) tolerance (0.002" T.I.R.) allows the unit to run smoothly. Throughout the operating life of the unit, alignment should be checked periodically and a time cycle established to determine that good alignment is being maintained.
Piping. Piping to the pump should always match the flanges on the pump casing. Never allow the piping to be “sprung” to meet a pump flange. Piping-induced strain can distort or crack a casing and result in internal misalignment of the rotating assembly.
Using an expansion joint at or near a pump flange can prevent unsupported strains from being imposed on the casing. With some types of joints, it is recommended that joints be fixed in one direction after installation. Joints often are “rodded” to prevent expansion beyond the joints’ limits due to pressure in the system. The manufacturer of the joint should be consulted to determine the proper installation.
MaintenanceDriver Rotation. A centrifugal pump must turn in the proper hand of rotation to produce the rated head, capacity and efficiency. During preventive maintenance checks, be certain that driver rotation is in accordance with the arrow on the outside of the pump casing or some other identifying feature. Many pumps require that the driver be “bumped” for rotation prior to being connected to the pump. This requirement is seen most often on pumps with threaded-on impellers because reverse rotation of the shaft can cause the impeller to unscrew and damage many of the components inside the pump. On other pump designs, if the impellers are removed from the pump shaft, be certain that they are replaced in the same orientation. While installing the impellers backwards may not necessarily damage anything, the pump performance will fall short of expectations.
Impeller Rings. Renewable impeller wearing rings, casing wearing rings, sleeves and bushings that are properly maintained can contribute to optimum pump efficiency and reliability. By rule of thumb, impeller rings should be considered for replacement when clearance increases two to three times the original clearance or when a noticeable loss of head or flow is recorded.
Gaskets. On split-case pumps, one of the frequently overlooked components is the gasket between the two casing halves. The gasket’s primary role is to seal the casing halves to prevent the pump from leaking. The gasket should be made of a material that is compatible with the intended service and should have a specific thickness as defined by the pump manufacturer. (Generally, a 1/64 or 0.015" thick gasket is used.)
Using a gasket that is thicker than the design calls for will cause the casing-joint halves to have greater separation than specified. This can allow the pumped liquid to cut into the gasket, weakening it and allowing leakage around the stationary parts of the rotating assembly. This leakage can damage the pump casing, which is one of the most expensive parts of the pump. By contrast, using a gasket that is too thin can allow the stationary parts to be compressed, resulting in rubbing and premature wear to the rotating and stationary members of the pump such as the wear rings. If the compression is significant, the parts can gall, and the unit can seize up.
Bearings. A number of different bearing arrangements are used in the various types of centrifugal pumps manufactured today, and each design has its own set of considerations and maintenance requirements. However, maintaining cleanliness and using the proper lubricant in the correct amounts are crucial for all bearings. The equipment manufacturer is the best source for this information, but lubrication consultants and lubricant manufacturers also can provide valuable guidance.
Alignment. Foundation changes and pipe strain can cause misalignment. Therefore, it is important to check for strain by loosening the pipe at the unit nozzles, as well as at the setting of pipe hangers and supports. This should be done each time the alignment of the unit is checked.
Whenever the casing is opened, all of the machine’s surfaces should be thoroughly cleaned of all corrosion. In addition, all of the internal surfaces of the pump casing should be cleaned of scale or product buildup to provide a smooth surface for efficient flow. Clean machine surfaces will permit the accurate positioning of the various stationary and rotating parts to ensure correct alignment of the pump rotor.
Stuffing Boxes. Another factor vital to reliability is the stuffing box or seal chamber, which can claim considerable maintenance time and contribute to downtime. Stuffing boxes usually are arranged for use with packing, while the seal chamber is designed to accommodate mechanical seals. Some stuffing box or seal chamber designs allow the use of either one. A stuffing box that uses packing and operates at an internal pressure less than atmospheric should have seal water injected at 10 to 15 psig pressure to provide adequate sealing and lubrication to the packing and reduce the likelihood of drawing air into the pump.
If the pressure at the stuffing box is above atmospheric, the seal water pressure should be maintained at 5 to 10 psig above the pump discharge pressure for single-stage end suction units and 5 to 10 psig above the suction pressure for double-suction pumps, in which the stuffing box is only subjected to the pressure on the suction side of the unit.
If a pump operates with a packed stuffing box above atmospheric pressure and handles a clean fluid that provides reasonable lubricity for the packing, additional seal water might not be necessary because it will only increase the pressure against which the stuffing box packing must operate.
Properly adjusted, the packing will leak a small amount as it is being cooled and lubricated by the seal water or pumpage. The amount of leakage varies with shaft size, shaft speed, the pressures present, and the condition of the packing and the shaft or sleeve on which it rides. It might be as little as a few drops per minute or as much as a small stream. The goal is to minimize leakage without overheating and burning the sleeve packing.
Mechanical Seal. The mechanical seal also is subjected to stuffing box pressure and the characteristics of the pumped fluid. A mechanical seal must allow a slight leakage across the rotating and stationary seal faces to be properly cooled and lubricated. Normally the majority of the leakage vaporizes as it crosses the seal face, so this leakage is not evident.
The proper selection of the mechanical seal requires knowledge of the acceptable pressure differential across the seal, temperature of the material being pumped and other physical characteristics of the liquid, as well as shaft speeds and other considerations. Again, a positive pressure above both atmospheric and the pressure seen at the seal chamber is required to ensure a flow of cooling, lubricating liquid across the seal faces. The actual pressure differential usually is prescribed by the seal manufacturer.
An infinite number of variables can affect pump reliability; this article has covered just a few. The installation, operation and maintenance instructions provided by the pump manufacturer are an important source for accurate information and should be referred to first if a question comes up. Many manufacturers today have web sites where the latest instructions can be downloaded quickly, easily and without cost.
Improving pump reliability is a continuous process. Specifying the right design for your application, ensuring the proper installation and following the correct maintenance procedures throughout the life of the pump can help facilities optimize their pump operation.