10.6 By pass control
By pass control
In by pass control, by pass regulation or shunt regulation (see figure 10.6a) liquid is fed back from the delivery side of the pump into the suction side of the pump. Safety valves for displacement pumps work in the same way.
Operating points are most easily determined by combining the curves for a centrifugal pump and the by-pass line into a single curve.
Figure 10.6a By-pass regulation. The block diagram and curves for pump and by-pass line are reduced to a resultant curve.
The unwanted power loss with by-pass regulation is:
The difference in efficiency between points 2 and 3 can either reduce or increase the useful pump work.
where η3 is efficiency for flow Q3.
By-pass regulation is used for displacement pumps and, exceptionally, for centrifugal pumps. The energy consumption for these is generally greater with by-pass regulation than it is with throttle regulation, see for example the power curves for centrifugal pumps in chapter 3.
For centrifugal pumps, energy consumption is determined as explained in previous Section 10.5 >>>, energy consumption PE is power multiplied by time, or i.e. operating points on the Q-H curve are established, thereafter the power requirement of the turbine is derived from its power curve. The regulation efficiency for a centrifugal pump is generally lower with by-pass regulation than it is with throttle regulation, see figure 10.5e. In spite of this, by-pass regulation of a turbine pump may be justified because of the lower loss of head across the control valve with consequent lower noise levels and reduced risk of cavitation in the control valve. The operating points of the centrifugal pump should, however, always be checked with regard to cavitation and abnormal vibration levels.
In the case of displacement pumps using by-pass regulation, the power consumption is determined exclusively by the back pressure, i.e. by the system curve. In the same way as for throttle regulation, the valve must be given “authority” in the system loop. This is achieved by increasing the required flow, figure 10.6b.
Figure 10.6b By-pass regulation of a displacement pump. The system curve shown applies to a non-Newtonian liquid.
It is only in exceptional cases that the liquid characteristics for displacement pumps working with non-Newtonian liquids are so well known that the control characteristics can be analysed. It should be noted that the control valve through-flow area may not be adequate for liquids containing larger particles. The power requirement is determined by the flow ( =Qmax+ Qby pass) and the delivery head according to the system curve at the actual transported flow. Because of this, the regulation efficiency will be relatively high, figure 10.6c.
Figure 10.6c Regulation efficiency for by-pass regulated displacement pump with 20% by-pass at maximum required flow.
Overflow control, by-pass regulation without control valve.
With overflow the liquid is pumped into a reservoir at almost constant level. The required flow is taken from the reservoir by regulation of an overfall (weir) or as liquid at constant pressure. The surplus liquid goes back to the suction side of the pump or escapes in the waste pipe, figure 10.6d. The pump in such cases operates at constant flow and constant power requirement.
Figure 10.6d Principle of overflow control
Overflow control presents a simple and reliable method but uses a relatively large amount of energy. The regulation efficiency depends upon the proportion of static delivery head, figure 10.6e. The figure makes the assumption that there are no unwanted level differences.
Figure 10.6e Regulation efficiency with overflow control. HO = Hstat + hLO.