ROUBLESHOOTING PNEUMA TIC CIRCUITS:STANDARD GRAPHICAL SYMBOLS

STANDARD GRAPHICAL SYMBOLS

FLUID POWER DYNAMICS-0463FLUID POWER DYNAMICS-0464

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Pumps:Other methods of flow control for PD pumps

Other methods of flow control for PD pumps Pumps in parallel PD pumps can generally be run in parallel without problems. This gives the ability to increase flow rate at the pressure rating of a single pump. The principal considerations are, the correct design of inlet and outlet pipework to avoid problems of NPIP/NPSH, overpressure in the discharge pipe and back flow through a non-running pump. Isolation valves for safe main­ tenance should also be considered. Pumps in series Rotary pumps in series can give increased pressure capability at the flow rate of a single pump, however it is not recommended that PD pumps be applied in this manner unless sophisticated speed control logic is used. Matching of the two pumps is extremely important and safety, in the event of failure of either unit, normally requires multiple overpressure protection devices and a means of ensuring the second pump is not underfed, creating low NPIP/NPSH and cavitation problems. Safety of the second pump has...
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Control principles for variable speed pumping:Methods of controlling a rotodynamic pump

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Methods of controlling a rotodynamic pump The methods of controlling a pump for varying duty are covered in more detail in other sections, but as an introduction to control principles a summary is presented. When the user wants to reduce the flow in a process, valve control (see Figure 8.1) can be used either to directly throttle, or control a bypass (see Figure 8.2). Alternatively, speed control (see Figure 8.3) can be applied using a VSD. Valve control will effectively reduce the flow, however, the consumed energy is often significantly lower if speed control is used. On-off control can be used to vary pumped volume in systems where an intermittent flow is acceptable . These systems often require a storage facility. If there is a static head in the pumping system, a substantial amount of energy may be used for lifting, compared with the energy required to overcome the friction in the pipes. Only small speed variations are possible and although energy savings can still sometime...
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BASIC DIAGRAMS AND SYSTEMS:Automatic Venting at End of Cycle.

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Automatic Venting at End of Cycle In systems where it is not necessary to hold pressure at the end of a cycle, it is possi­ble to unload the pump by automatically venting the relief valve. Figure 9-10 shows such a system using a cam-operated pilot valve to vent the relief valve. Midstroke Extending In view A of Figure 9-10, the machine cycle begins when the solenoid of the spring off­ set directional valve is energized. Pump output is to the cap end of the cylinder. The vent line from the directional control valve is blocked at the cam-operated pilot valve. Midstroke Retracting In view B of Figure 9-10, the limit switch has contracted the cam at the end of the extension stroke. This contact breaks the circuit of the solenoid. The directional control valve has shifted to retract the cylinder. The relief valve vent connection is still blocked. Automatic Stop View C of Figure 9-10 illustrates the circuit at the end of the retraction stroke. The cam on the cyl...
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