Pumps & Systems, August 2007
Q. Is there a way of predicting the maximum operating speed for a reciprocating plunger pump when pumping a viscous liquid?
A. Yes, but let's begin by setting the basic speed for such pumps when pumping cold water. The American Standard ANSI/HI 6.1-6.5 Reciprocating Power Pumps for Nomenclature, Definitions, Applications and Operation includes the following values for single acting plunger pumps:
It should be noted that these speeds are intended only as a reference point. Some manufacturers offer their pumps for operation above or below these speeds.
When pumping viscous liquids, the basic speed should be reduced by using Figure 6.46B.
When using this figure, the liquid velocity through the suction valve should be calculated as follows:
v =.642Q/MA
Where:
v = Average liquid velocity through the valve in ft/sec
Q = Rate of flow through pump in gpm
M = Number of suction valves
A = Suction valve flow area (each) in square inches.
The recommended speed is equal to the basic speed times the percent of basic speed as a decimal.
Q. How is the maximum allowable working pressure of a centrifugal pump established?
A. The maximum allowable working pressure (MAWP) of a centrifugal pump begins with the required pressure of the pump's application. It is then increased to match the MAWP of the next highest standard flange rating. It is common practice on standardized pump product designs to match pump working pressure with a standard flange rating so that the pump will be able to withstand hydrostatic pressure tests of the entire system. On engineered-to-order pumps, where this is impractical, the MAWP is derived to match the application limits, considering all potential variances in operating factors such as changes in speed, product density, etc.
The design of the pump casing and other pressure containing parts then follows the guidelines established by the appropriate design code (American Standard ASME Code for Pressure Vessel Design, Section VIII being one example). Such design codes include acceptable stress levels for materials of construction commonly used in pressure vessel construction.
Determination of metal thickness for pump casings and covers is not a simple matter because of the complex shape of pump volutes. For this reason finite element analysis methods are usually used. After that is done, additional metal thickness is added to allow for subsequent corrosion and erosion of the internal surface of the pump. For example, standard pumps designed for chemical applications usually have a corrosion allowance of .125-in added.
Q. What special requirements should we look for when ordering condenser circulating pumps for a large electric generating plant?
A. Condenser circulating water pumps take cool water from a river, lake, stream or cooling tower basin and circulate it through the condenser to condense exhaust steam from the main turbines. The pumps work against low to moderate heads typically from 80-ft to 150-ft and are installed to operate in parallel. These pumps can be added to or removed from service as head load and demand for cooling water varies (a function of ambient temperature and plant load).
Circulating pumps may be of either horizontal or vertical construction. The horizontal circulating pumps are low speed, axially split, single stage, double suction volute designs. They are located in a dry pit that allows full access for servicing, dismantling and inspection.
The vertical circulating pumps are of wet pit design, which means they are fully submerged in the water pumped. They use a long pipe column that supports the submerged pumping element, and the vertical driver is mounted on the top. Some vertical wet-pit designs allow removal of the pumping element without dismantling the pump casing or piping; these are referred to as "pull-out" designs.
Large circulating water pumps are very heavy and difficult to handle when maintenance is required. The figure below is a good example of a pull out design.
It is easier to handle the fewest number of parts during maintenance and this design allows that. The key to the design is a joint above the discharge elbow and in the diffuser that allows the pump rotating elements to be "pulled out" without removing the bowl, column, or discharge head. This reduces the weight of the assembly and provides ready access to the wearing parts.