Pumps and Systems, May 2009
Q. What can be done to avoid the high-pressure differential from rupturing the diaphragm in reciprocating diaphragm pumps?
A.
A hydraulically coupled disc diaphragm does not have that problem.
The mechanically coupled design contains a flexible, round diaphragm, clamped at the periphery in direct contact with the process liquid being displaced. The diaphragm material is typically PTFE or a PTFE/elastomer composite. A connecting rod is connected directly to the diaphragm. The diaphragm is not pressure balanced as the process pressure is acting on one side of the diaphragm and atmospheric pressure is acting on the other side. This system results in higher stress levels in the diaphragm, so these pumps are typically used for lower pressure applications.
A hydraulic coupled disc diaphragm liquid end contains a flexible, round diaphragm, clamped at the periphery, and is in direct contact with the process liquid being displaced. See Figure 7.5A. This type of liquid end design is inherently leak free. The diaphragm material is typically PTFE, elastomer or a PTFE/elastomer composite. Liquid end designs featuring flexible metallic diaphragms are available and used in applications where severe operating conditions prohibit the use of PTFE or other elastomers.
Figure 7.5A
In operation, the diaphragm is moved by hydraulic fluid, which in turn is displaced by a reciprocating plunger or piston. The stresses in the diaphragm are minimal, as the process pressure acting on one side of the diaphragm is balanced by the hydraulic pressure acting on the opposite side. The process liquid is admitted through the suction check valves as the diaphragm moves rearward. As the diaphragm moves toward the wet end, the suction check valve closes and the discharge check valve opens, discharging liquid.
For more information, see ANSI/HI 7.1-7.5 Controlled Volume Metering Pumps for Nomenclature, Definitions, Application, and Operation.
Q. What are non-clog pumps, and what is unique about their design?
A.
Pumps designed to ensure maximum freedom from clogging when handling liquids containing solids or stringy materials are commonly called non-clog pumps, or trash pumps.
Non-clog pumps are recommended for handling raw or unsettled sewage, activated sludge, industrial wastewater containing solids and similar liquids where excessive clogging would otherwise be encountered. The largest solid sizes that the pump will be required to handle in normal operation must be specified. The term sphere size denotes the largest diameter ball that can be passed through the pump. Comminution and/or adequate bar screens must be provided to prevent large solids from entering the pump. When used, bar screen openings should be sized to prevent clogging from irregular-shaped solids.
To avoid clogging, impellers should be designed with only one or two vanes and with a wide space between the shrouds, so spherical solids of the specified size can pass through. See Figure 2 for an example of such an impeller.
Figure 2
Storm water and/or domestic sewage may be handled successfully by mixed flow and axial flow pumps, using the preceding guidelines.
For domestic sewage service, pumps built to the individual manufacturer's material specifications are ordinarily used. Corrosion-resistant and wear-resistant shaft sleeves and wearing rings are desirable for maximum life. Inspection openings in the casing or adjacent piping for access to the impeller are recommended. Stuffing-boxes may be furnished with mechanical seals or packing, either water or grease lubricated. When water is used for the stuffing-box, wearing ring lubricant or flush, the supply line must be isolated from any potable water system.
If the pumpage is corrosive and/or abrasive, the materials of construction for parts in contact with the liquid should be selected for resistance to the effects of the pumpage.
Q. What can be done to ensure better lubrication and increase the life of bearings in sealless pumps?
A.
Sealless pump bearings are generally lubricated and cooled by process liquid or separate flush liquid for canned motor pumps and magnetic drive pumps and are generally of the liquid-lubricated type. For satisfactory life, environmental control of bearings by proper design and application is essential to prevent flashing and air or vapor collection and to avoid solids.
Pressure and temperature at bearings shall be maintained to prevent flashing of the lubricating liquid at expected operating conditions. Consideration should be given to minimum operating flow, minimum NPSHA and temperature rise through the drive section.
When conditions of service require filtration of bearing lubricating liquid, a self-cleaning internal design may be used. If external filtration is required, the filter system should indicate when a filter change is required. Loss of flow to the drive section shall be avoided.
Bearings are to be designed and applied considering fluid characteristics, unit loading, corrosion, erosion, wear, heat transfer, fits and friction characteristics.
Bearing loading, alignment, shaft deflection, surface finish and wear-in characteristics of bearing materials shall be taken into account to prevent local surface failure.
Materials used for journal sleeves, thrust collars and bearings often have significantly different thermal expansion characteristics from shafts and other mating parts. Application guidelines and limits shall be established by the manufacturer for specific designs to avoid breakage or looseness under specified operating temperatures.
Several organizations are developing active magnetic bearing systems for use in sealless pumps. These systems maintain the rotor in a levitated state, so that no contact is made between rotating and stationary parts.
The design of a sealless pump with active magnetic bearings should also include an auxiliary bearing system to support the rotor when the active bearing system is de-energized, and for start-up and shutdown. The manufacturer shall specify the number of uncontrolled rundowns allowed before inspection of auxiliary bearings. Sensors used for positioning of bearings and the bearing components shall be protected from contact with the process liquid and shall not compromise pressure containment integrity.
Bearings should be selected to have a load capacity higher than static and dynamic loads under the most adverse operating conditions. Active magnetic bearings have a lower inherent transient overload capacity than hydrodynamic bearings, so attention shall be given to determine loads that will result from anticipated operating conditions.
Cooling shall be required to remove heat from electrical losses in windings and heat generated by windage losses.