Q.
What are the best materials to use in pump construction to minimize part corrosion?
A.
Pumps are produced in a wide variety of materials. Consider these factors in the material selection for wetted pump parts: the user's experience, expected pump life (temporary or long-term use), intermittent or continuous duty, pumping hazardous or toxic liquids and the liquid's condition. Some of the more frequently used materials include:
• Bronze-fitted pump: The casing is made of cast iron, and the impeller and impeller rings are made of bronze. This combination is commonly used on fresh water at ambient temperatures.
• All bronze pump: All parts of the pump in direct contact with the pumped liquid are made of manufacturer's standard bronze. This type is often used for pumping seawater.
• All iron pump: All pump parts in direct contact with the liquid pumped are made of ferrous metal (cast iron/ductile iron, carbon steel or low-alloy steel). This pump is commonly used in hydrocarbon services and some chemical applications.
• Stainless steel fitted pump: The casing is made of material suitable for the service. The impellers, impeller rings and shaft sleeves (if used) are made of corrosion-resistant steel with suitable properties for the specific application. The pump is also used in hydrocarbon and chemical services.
• All stainless steel pump: All pump parts in direct contact with the liquid pumped are made of corrosion-resistant steel with suitable properties for the specific application. This pump is commonly used in chemical applications.
• Rigid polymers/composites: All pump parts in direct contact with the liquid are made of rigid polymers or composites (plastics), either as coatings or as structural material. This type is commonly used in chemical services.
Q.
We checked the performance of our new vertical turbine pump (VTP) and found that it was not consistent with the manufacturer's published curve. The field test came close to the published head at shut-off, but the head fell below the curve as rate of flow increased. What is the most likely cause of this discrepancy?
A.
Manufacturers' published curves for VTPs are usually based on the bowl assembly performance. The pump discharge head is measured in the pump column pipe about two diameters downstream from the bowl discharge head or pipe increaser if one exists. The friction losses in the column pipe and the surface discharge elbow will reduce the bowl assembly head if discharge pressure is measured after the discharge elbow.
Manufacturers usually publish head loss values as a function of the flow rate for their column pipes. Locate this information for your pump's column pipe, and see if it accounts for your pump's losses. Most pumps can be built with a variety of column pipe sizes, and sometimes a small diameter column pipe is provided. Make sure you have a measurement of the actual column pipe dimensions to determine the correct loss values.
Pump performance is difficult to measure accurately in the field. ANSI/HI- 2.6 Vertical Pump Tests provides detailed procedures for testing VTPs. Carefully follow all detailed requirements.
Q.
How can we determine how much flow a mechanical seal requires to provide adequate flushing?
A.
Mechanical seal flushing flow should be as high as practical, based on the following considerations:
1. If from an external source, provide the amount that can be tolerated without excessive dilution of the pumped liquid. If the primary reason for the flush is to exclude the process from the seal chamber, then the flow rate should be sufficient to achieve a velocity of 15-fps past the throat bushing. On the other hand, if the process stream will polymerize, congeal or set up, the flush can be quite low and would be determined based upon experience.
2. If recirculating from the pump discharge, provide the amount that can be tolerated without excessive loss in pump performance since the recirculation flow is deducted from the pump outlet. A general rule of thumb is 1-gpm/inch, but if it is more than 3600-rpm or 500-psi, the seal manufacturer should calculate it. Depending on the flush arrangement, it should not be higher than 15-fps, especially if the fluid is not clean since it can erode seal components. On higher velocities (flow rates), the flush should not be directed toward the seal faces or designed with multi-port connections to lower the velocity. The flow rate is typically controlled by an orifice or a series of orifices sized to control flow and large enough to not clog.
3. The pipe size at the flushing connection should not be reduced. Flushing pipe should be kept the same size as the size of the flushing connection. The use of flexible tubing may result in an easily clogged passage.
4. Use a bell-shaped seal housing when practical. This housing provides the most appropriate flushing without dilution of the pumped liquid or reduction of pump performance. In the case of one paint manufacturer who installed ¼-in OD copper tubing to supply flushing liquid from the pump discharge, the small diameter tubing quickly became clogged with solidified paint and the seals failed. Conversion to a bell-shaped housing solved the problem.
Pumps & Systems, November 2008
