Pumps minimum flow through

Caplan, F., Estimated Minimum Required Flows through Pumps, Chemical Engineering, March 17,1975. 

Osmotic Pinch Ejfect Feed is pumped into the membrane train, and as it flows through the membrane array, sensible pressure is lost due to fric tion effects. Simultaneously, as water permeates, leaving salts behind, osmotic pressure increases. There is no known practical alternative to having the lowest pressure and the highest salt concentration occur simultaneously at the exit of the train, the point where AP — AH is minimized. This point is known as the osmotic pinch, and it is the point backward from which hydrauhe design takes place. A corollary factor is that the permeate produced at the pinch is of the lowest quality anywhere in the array. Commonly, this permeate is below the required quahty, so the usual prac tice is to design around average-permeate quality, not incremental quahty. A I MPa overpressure at the pinch is preferred, but the minimum brine pressure tolerable is 1.1 times H. 

The characteristics of the pump relate the applied pressure on the cake to the flowrate at the exit face of the filter medium. The cake resistance determines the pressure drop. During filtration, liquid flows through the porous filter cake in the direction of decreasing hydraulic pressure gradient. The porosity (e) is at a minimum at the point of contact between the cake and filter plate (i.e., where x = 0) and at a maximum at the cake surface (x = L) where sludge enters. A schematic definition of this system is illustrated in Figure 2. 

The minimum flow simply means that this flow must circulate through the pump casing (not recirculate with no cooling) back to at least the initial temperature of the feed, if excessive temperatures are not to dev elop. The best practice is to request the manufacturer to state tliis value for the... 

System components, such as pumps, valves and gauges, create both turbulent flow and high friction components. Pressure drop, or a loss of pressure, is created by a combination of turbulent flow and friction as the fluid flows through the unit. System components that are designed to provide minimum interruption of flow and pressure should be selected for the system. 

If a steam turbine generates electricity, then the flow through the turbine can be varied within the minimum and maximum flows allowed by the machine. If a steam turbine is connected directly to a drive (e.g. a back-pressure turbine driving a large pump), then there is likely to be no flexibility to change the flowrate through the steam turbine as this is fixed by the power requirements of the process machine. 

For spray-on applications, consult the manufacturer on minimum application temperature. Some materials do not flow through the pump or sprayer below 40°F to 50°F (4°C to 10°C) and some materials do not cure well below 30°Fto 40°F (-1°Cto4°C). 

A centrifugal pump handles 220°F (377 K) water and has a shutoff head (with closed discharge valve) of 3200 ft (975 m). At shutoff, the pump efficiency is 17 percent and the input brake horsepower is 210. What is the minimum safe flow through this pump to prevent overheating at shutoff Determine... 

Centrifugal boiler-feed pumps usually have a maximum allowable temperature rise of 15°F. The minimum allowable flow through the pump to prevent the water temperature from rising more than 15°F is 30 gal/min for each 100 bhp input at shutoff. 

Two types of measurements are made with the adsorption calorimeter, also previously described (3). In the batch mode a dry-solid surface is covered with a solution. In the flow mode the enthalpy changes result from a solution flowing through a bed of adsorbent. The flow system uses an LKB 10200 Perpex pump (reference to specific trade names does not imply endorsement by the Department of Energy) with a flow rate of approximately 12 g h 1. Because the silicone tubing on the pump may adsorb surfactant, the pump is placed at the output of the flow system and draws the solution through the cell. An Altex six-way valve is at the input of the flow system, and any one of six solutions can be selected to flow through the cell. Minimum detectable heat pulse is 4.5 x 10 Cal for the batch and minimum power output is 2.4 x 10 ca sec for the flow mode. Measurements reported for the adsorption study were made at 25° and 30° C 0.05° C. 

Design a centrifugal pump and control valve system so that a maximum flow rale ol 75 gpm and a minimum flow rate of 25 gpm are achievable with the control valve at 100 percent and 10 percent open, respectively. Liquid is pumped from a tank whose pressure can vary from 50 to 75 psia. The material is pumped through a heat exchangei (which takes 30-psi pressure drop at 50 gpm) and a control valve into a tank whose pressure can vary from 250 to 300 psia. Assume a flat pump curve. 

The attack tank is cooled by circulating the slurry through a low-level flash cooler, in which the reaction slurry is cooled by evaporation of water under vacuum. Circulating power requirements are kept to a minimum by using an axial-flow circulation pump and by the low elevation of the flash cooler above the attack tank. The temperature in the attack tank is controlled by varying the vacuum applied to the flash cooler. 

The installation of pumps requires special care to provide enough suction head to avoid cavitation. The net suction head can disappear if a pump generates too much heat in the condensate. It is important to maintain enough flow through the pump at all times to prevent this. Minimum flow can be provided by recycle controlled by an automatic valve or by placing an orifice assembly in a recycle line. As a rule of thumb, a flow of 1 hr is necessary for each 12kW of input if the temperature rise is to be kept to 10° C. The recycle flow must be cooled or go into a heat sink such as a condensate hotwell or boiler feed deaerator. A simple recycle into the pump suction line is inadequate. 

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