Basic hydraulic circuit design: energy conservation, part 2.
Demand Flow Systems
Demand flow systems are defined as those that use fluid sources which are responsive to the spontaneous flow rate demand of output devices and use close-center directional control valves. Examples of fluid sources are: accumulators, charged by fixed displacement pumps which are bypassed through an unloading valve during standby periods; or pressure-compensated variable displacement pumps, which deadhead during standby or overload conditions.
The concepts for energy conservation previously discussed for constant flow systems apply equally to demand flow systems. The difficulty arises in providing a simple representation like that in Fig. 1 because [delta]p vs. Q characteristics change constantly.
[FIGURE 1 OMITTED]
Component sizing for a demand flow system should be made at maximum flow rates. The circuit designer may treat it as a constant flow system, with [Q.sub.c] = [Q.sub.max ] for sizing purposes, knowing that energy losses will be smaller for lower flow rates. If peak flow demand is short of duration, it may be more economical to size for lower flows.
Rethink The Circuit
Some system designers tend to follow a "cookbook" approach based on examples of existing circuits. The danger is that these so-called typical circuits may not properly match the requirements of the application cycle profile. Damaging inefficiencies can result. A hot-running system is the common consequence of circuit-to-load mismatches.
If analysis of the application indicates widely varying loads, the designer should be alert to watch for excessive inefficiencies. The design sequence can be summarized as follows: 1. perform the load analysis; 2. produce the cycle profile of the application; 3. select design pressure; 4. from force/torque profiles, size the cylinder or motor; 5. from the velocity profile, calculate flow rates using calculated cylinder and/or motor sizes; 6. calculate power profile base on P = K (Qp)/[e.sub.so], where [e.sub.so] is system overall efficiency.
The first two steps of the sequence are load orientated, that is, not discretionary and dependent on the machine cycle. The third, fourth and fifth are hydraulic circuit oriented. It is at these steps that the designer can control circuit parameters which affect efficiency--they are discretionary. The final step is the power cycle and is not discretionary. It depends on flow rate and pressure.
Clearly, selection of system design pressure [p.sub.d] is of prime importance because it virtually predetermines all other circuit design parameters.
Note that design pressure differs from component rate pressure. Design pressure is the acceptable pressure at the actuator corresponding to the maximum load reaction and would correspond to pf in the diagram in Fig. 1. To this value must be added the sum of the losses [delta]]p.sub.s] around the complete circuit which corresponds to the summation of differential pressure loses ([sigma][delta]p losses) shown in Fig. 1.
When selecting a component for a system, rated pressure PR, must be equal to or greater than design pressure Pd. The final decision is whether to use continuous rated pressure [p.sub.Ri]. The decision will depend on the nature of the load, i.e., whether it is steady, rapidly changing, high shock, etc.
Generally, the only discretionary decision left to the hydraulic system designer is the selection of system design pressure. Virtually, all other design decisions are compromises in the attempt to optimize dependent variable. If the circuit designer is also the machine cycle designer, or has inputs into the machine cycle design, he may have an opportunity to manipulate machine functions which yield a hydraulic system profile that will optimize energy efficiency.
For example, it may be possible to modify a machine cycle so that high velocities (which need higher flow rates) do not occur simultaneously with high load reaction, which need high pressures. Many machine performance specifications are arbitrary and may not be necessary.
Values for machine parameters are frequently picked out of thin air and can totally distort hydraulic circuit requirements. The cycle profile plotting technique helps emphasize these distortions and define legitimate machine functions.
What To Look For
The system designer should be alert to certain warning signs. They should:
* Watch for excessive flow rates caused by excessive velocities or speeds related to unrealistic, or unnecessarily fast, time cycles or oversized cylinders or motors due to low design pressure selected.
* Notice excessive pressures caused by undersized cylinder or motors for the machine's load reactions or by failure to account for loads caused by inertia which occur during the changes in velocity, or failure to adequately account for friction in hydraulic output devices and in machine loads or intermittent shock loading as a function of machine cycle.
* Watch for incompatibilities between circuit branch requirements such as flow rate matching problems with branches connected in parallel.
* Note branch pressure variation caused by non-uniform loading and/or different size output devices to match variable flow rate to velocity requirements.
* Be aware of pressure variation caused by use of flow or pressure control valves in circuit branches, especially in meter-out or counterbalance modes.
* Be cognizant of load changes from resistive to overrunning during work cycle.
In our next discussion, we'll look at the special class of hydraulic systems that has evolved to improve the efficiency of transmitting power from the prime mover to the load, load sensing systems.
Some information and illustrations for this article are from "Fluid Power Systems & Circuits," by Russell W. Henke, published by Penton.
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|Title Annotation:||HYDRAULIC SYSTEMS TRENDS|
|Publication:||Diesel Progress North American Edition|
|Date:||Apr 1, 2006|
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