Q. Must multiple pumps operated in series or parallel be identical in size?
A. No, they do not have to be identical. However, pumps intended to operate in series must be close in size regarding flow rate-for example, lower speed booster pumps used to provide an increase in NPSHA for high-pressure pumps. Pumps for parallel operation must be close in total head developed; otherwise, one or more pumps may be forced to operate at shut off or at unacceptably low flows. A thorough review of the entire operating curve for each pump must be made.
Pumps operating in series produce head that is additive at the flow rate at which they would run individually. Two pumps, each capable of 1,000 m3/h (4,400 gpm) at 50 m (165 ft) of head, when connected in series could deliver 1,000 cubic meters per hour (4,400 gpm) at 100 m (330 ft) of head. Series operation is therefore used where higher pressures are required than the pressures that an individual pump can supply. Pumps used in series must be capable of the increased pressure levels that result from staging. See Figure 1.3.6.5.1a.
Pumps operating in parallel produce a flow rate that is additive at the head at which they would run individually. Two pumps, each capable of 600 m3/h (2,640 gpm) at 35 m (115 ft), when connected in parallel could deliver 1,200 m3/h (5,280 gpm) at 35 m (115 ft) of head. See Figure 1.3.6.5.1b.
In such cases the system curve will determine the final operation point. Two pumps operating in parallel will not automatically deliver twice the flow of one pump operating independently.
Q. Does the viscosity of the pumped liquid affect the Net Positive Suction Head Required (NPSHR) by a pump?
A. There is a dual influence of the pumped liquid viscosity on NPSHR. With increased viscosity the friction goes up, which results in an increase of NPSHR. At the same time, higher viscosity results in a decrease of air and vapor particle diffusion in the liquid. This slows down the speed of bubble growth and results in a thermodynamic effect, which leads to some decrease of NPSHR.
The effect of viscosity on NPSHR is substantially a function of the Reynolds number. However, this effect cannot be expressed by a single relationship for all of the different pump designs and types. As a general rule, larger size pumps and pumps with smooth and sweeping impeller inlets are less susceptible to changes in the pumped liquid viscosity.
When handling viscous liquids at lower shaft rotational speeds, the NPSHR has been observed to be higher than would be predicted by the Affinity Laws.
Overall the development of vaporization and gas release greatly depends on the time of exposure to lower pressure. In general, a cavitation test at constant flow rate and speed with variable suction conditions cannot be applied to viscous liquids if variation in suction pressure is obtained by lowering the pressure in the whole test loop. This is because, unlike water, the liquid in the tank will not be rapidly deaerated. Rather, air will gradually diffuse out of the liquid in the suction line and will cause blockage at the impeller inlet.
A generalized method for estimating NPSHR when pumping viscous liquids can be found in ANSI/HI 9.6.7 Effects of Liquid Viscosity on Rotodynamic (Centrifugal and Vertical) Pump Performance, section 9.6.7.5.3. This method is provided for approximation purposes, but the user is cautioned that it is based on an analytical approach and is not based on actual NPSHR test data. When pumping highly viscous liquids, ample margins of NPSHA over the NPSHR are required and the advice of the pump manufacturer should be sought.
Q. Must air operated pumps be protected against zero flow rate, and if so, what method of protection is recommended?
A. Air-operated diaphragm pumps are a class of displacement pump featuring flexible membranes in combination with check valves used to move liquids in and out of pumping chambers. These pumps are used extensively in transfer and metering applications requiring flows up to about 1,150 lpm (300 gpm). They are quite versatile, handling a wide variety of liquids including, but not limited to: chemicals, dry powders, food additives, glues, paints, pharmaceutical products, slurries, tailings and wastewater.
Air-operated pumps have unique operating characteristics. They are sealless pumps, have no dynamic seals or packing, are self-priming, can run dry indefinitely and can operate at infinitely variable flow rates and pressure within the pressure and flow rate range for the pump. The liquid discharge can be throttled to zero flow indefinitely.
The most common type is the double-diaphragm pump (duplex pump), which has two pumping chambers and two flexible diaphragms. The diaphragms are connected to each other through a connecting rod and are clamped at the outer edges of the diaphragm. Continuous reciprocating motion, along with internal check valves, creates an alternating intake and discharge of pumped liquid in and out of each chamber, which results in a nearly continuous pumping action from the combined chambers. See Figure 10.10A for details.
Figure 10.10A
There are two unique features of air-operated diaphragm pumps:
- With the pump shut off, there is no power consumption. Air consumption is approximately proportional to flow rate: zero air consumption at zero flow rate and maximum air consumption at maximum flow rate. This feature allows air-operated diaphragm pumps to be used in applications requiring the flow rate to be varied from 0 to 100 percent of full flow.
- The pump discharge pressure can remain the same for a given flow rate and air inlet pressure regardless of the specific gravity of the liquid pumped. The supplied air pressure, pump flow rate and NPSHA set the discharge pressure for a given air-operated diaphragm pump.
