Ram Pumps revisited

I wonder why has my post Waterworks and Ram Pumps attracted more hits than any other page in my blog?  I plan to visit Alresford in a few weeks time and if I have opportunity to explore the sites I mentioned I shall report my findings here.

Perhaps it will not be so obvious to some of you as it is to me that the operation of the Ram Pump is much the same as that of the Boost Converter in electronics. Most folk are at least aware of the analogue between water and electricity.

The boost converter uses a low voltage d.c. supply to generate a higher voltage d.c. to drive a load.  Voltage in electricity answers to head or pressure with hydraulics. The output voltage of a boost converter can be perhaps as much as a hundred times the input voltage.  Of course you do not get "something for nothing" any more than with the Ram Pump because the output current is much less than the input current.

The switch shown is an analogue of the clack valve. It does not function automatically as in the Ram Pump but is operated by some control electronics not shown.  The switch is typically a MOSFET.

Initially the switch is closed (like the clack valve being open) and the supply V is connected.  The defining equation for an inductor is:

The interia of the water in the delivery pipe answers to the inductor. The diode answers to the non-return valve placed after the clack valve in the Ram Pump.V and L are constant therefore di/dt is constant which means that the current steadily grows. At a certain point the switch is opened (like the clack valve shutting) which means that the current i suddenly becomes zero, or di/dt becomes infinite.  Since L is a constant that means that an infinite voltage appears across the inductor. Whilst this voltage exceeds that across the load the diode conducts and delivers the output voltage.

The process now repeats so that charge is progressively "pumped" through the diode into the load.  The circuit thus generates spikes of high voltage.

In practice a capacitor is required across the load to smooth out these spikes, just as an expansion chamber is required in the Ram Pump.

Of course it isn't quite as good as that - it is not possible to turn the switch off that suddenly, inductors are not perfect, and so on.  The Ram Pump delivery pipe and the water itself will "give" (expand) with increasing pressure so will soften the water hammer.  And diodes are far from perfect.

The job of the control electronics is to pick the right time to operate the switch and to adjust the process to give the desired output voltage. There are many chips on the market that will do this function.