ARAZA.ORG

Introduction to Centrifugal Pumps - Part 2

By E. M. Araza

Selection criteria for VS1, VS2, and VS3
 

VS1, VS2, and VS3 are vertically suspended, single-casing, with discharge through the pump column. VS1 is diffuser type, VS2 is volute, and VS3 is axial flow. These pumps are used in services with inadequate net positive suction head (NPSHA); their impellers are located below ground level to gain additional NPSHA. They are mostly designed for wet fit or sump installation and operate in an open-loop system, such as in irrigation system, dewatering, flood control, cooling tower, city water supply, etc.

VS2, being a volute pump, has either radial flow or mixed flow impeller. VS3, being an axial flow pump, has axial flow propeller.

VS4 is also vertically suspended, single casing pump but with discharge column separate from the main column. 

VS5 is similar to VS4 but is of cantilever design. A cantilever pump is one in which only the bowl and impeller are submerged; all other parts and joints are located out of the liquid. VS4 and VS5 pumps are commonly used in sewage or sump service. 

VS6 is similar to VS1 (diffuser type) but in double-casing design.

VS7 is similar to VS2 (volute type) but in double-casing design. VS6 and VS7 are not considered as in-line pumps.

Double-casing pumps are also known as barrel pumps. They are designed for installation inside a barrel and operates in a closed-loop system. It is used for pumping a variety of liquids including flammable, toxic, and other hazardous liquids. Depending on the design suction pressure, a VS7 pump may be made of heavy duty barrel and bowl assembly to handle high working pressure.
 

The many types of pumps available seems to make the task of pump selection difficult, but not so. Based on evaluating some operating parameters, such as pressure and temperature, the suitability or non-suitability of an OH or BB pump can be quickly identified in the selection process. On the other hand, the lack of NPSHA or the lack of ground space at the installation site may require the use of a VS pump. 

Service-specific group

Centrifugal pumps may also be grouped based on the specific service they are intended. These may consist of standard pumps that are modified to meet the special requirements of the service. 

Examples of service-specific pumps, and some of their special design features, are:

Slurry pumps – are those designed to pump liquid containing slurries. They may be provided with semi-open impeller to prevent it from clogging, or enclosed impeller with ribs to keep the solids suspended in the liquid to prevent them from the clogging the mechanical seal box. Slurry pumps run at lower RPM to keep the liquid velocity down to prevent the rapid erosion of impeller and casing that are typically made of material that are more resistant to erosion.

Coke-crusher pumps – are those provided with special parts or feature to break down coke particles to smaller sizes to allow them to pass freely through the pump. Earlier coke-crusher pumps included an open impeller design, matched with grooved liner as the cutting mechanism. This proved to be an inferior design because the open impeller, and the rapid wear of the liner, resulted in rapid pump performance degradation. Modern coke-crusher design incorporates enclosed impeller for higher efficiency, using an auger or screw as the cutting mechanism.

Sewage pumps – are those intended for pumping sewage, or flood water contaminated with dirt. They are provided with open-type impeller with few vanes (2, 3, or 4 vanes) to allow the passage of sewage or dirt without clogging the pump. The pump casing is usually provided with a cut-out for pump inspection or de-clogging. The pump is designed for low service application.

Fire pumps – in the U.S. and other jurisdictions, it is recommended that fire pumps comply with NFPA Standard 20. In some other jurisdictions, insurance companies require compliance with NFPA 20 as a condition for issuing insurance coverage, while others encourage compliance by offering reduced insurance premium for using UL listed, or FM approved, fire pumps.

Among other things, it is required that the performance curves of the fire pumps shall have a runout flow of 150% of the rated flow with a differential head not less than 65% of the rated head. The shutoff head shall not exceed 120% of the rated head. The pumps shall also have pressure relief valve, air release valve, and pressure gauges as minimum pump accessories.

Boiler feed pumps – are pumps used to supply feedwater to steam boilers. Because of  high pressure and temperature in steam boilers, boiler feed pumps are usually of multistage construction with centerline, or near centerline, case mounting to handle the radial thermal growth of the casing. Typical features include between-bearing (BB) construction to avoid the high axial thrust inherent with overhang (OH) pumps, high temperature running clearances, high temperature dowelling to permit the axial expansion of the casing, and materials of construction suitable for high pressure and temperature, etc. These pumps must meet certain hydraulic characteristics including a constantly rising curve from rated flow to shut-off, with a minimum of 10% head rise.

Bleeder pumps – are multistage pumps that are provided with integral interstage take-off (ITO) at a fewer stage volute location to bleed-off a smaller amount of flow rate at lower differential pressure that is intended to meet another service conditions. The performance curve of a bleeder pumps shows its performance when the ITO is opened or closed. In essence a bleeder pump is like two-pumps-in-one.