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what is a modulating valve and how does it work please?
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If you mean the rather old fashioned thermionic valves, Druiaghtagh, consisting of glowing hot stuff inside a glass bulb then their operation is surprisingly simple.
The essence is that the valve is sucked free of air, and a specially coated electrode, the cathode, is heated. Above this is another electrode at a high voltage positive with respect to the cathode, called the anode. When the cathode is heated and the high voltage applied, electrons flow through the vacuum from the cathode to the anode. Hey presto; a rectifier, a device allowing current in one direction only.
Next step, put a third electrode between the cathode and the anode with holes in it. With no potential on this third electrode, the control grid, the electrons flow unaffected through the holes. However, a very small potential on the grid causes a proportionately large variation in the electron flow. By varying the grid potential the cathode to anode current varies in proportion but to a greater extent. Hey double presto! An amplifier.
Finally, just to complicate things a bit further, more grids (screen and suppressor) are added to augment the attractiveness of the anode for the electrons in the former, and to stop the electrons bouncing off the anode and disrupting the electron flow in the latter.
Now comes the really clever stuff. Arrange for a high frequency signal to be added to the electron flow by forming an oscillator either by resonant feedback from anode to cathode and grid, or by injecting it from an external oscillator, then apply a low frequency signal, say an audio wave, to the grid. This will increase and decrease the amount of high frequency signal current present in the cathode/anode circuit in direct proportion to the grid signal voltage.
This is the basis of the simplest form of Amplitude Modulation. Even with the advent of robust solid state devices, some high power radio frequency transmission systems still use valves in the output sections.
The essence is that the valve is sucked free of air, and a specially coated electrode, the cathode, is heated. Above this is another electrode at a high voltage positive with respect to the cathode, called the anode. When the cathode is heated and the high voltage applied, electrons flow through the vacuum from the cathode to the anode. Hey presto; a rectifier, a device allowing current in one direction only.
Next step, put a third electrode between the cathode and the anode with holes in it. With no potential on this third electrode, the control grid, the electrons flow unaffected through the holes. However, a very small potential on the grid causes a proportionately large variation in the electron flow. By varying the grid potential the cathode to anode current varies in proportion but to a greater extent. Hey double presto! An amplifier.
Finally, just to complicate things a bit further, more grids (screen and suppressor) are added to augment the attractiveness of the anode for the electrons in the former, and to stop the electrons bouncing off the anode and disrupting the electron flow in the latter.
Now comes the really clever stuff. Arrange for a high frequency signal to be added to the electron flow by forming an oscillator either by resonant feedback from anode to cathode and grid, or by injecting it from an external oscillator, then apply a low frequency signal, say an audio wave, to the grid. This will increase and decrease the amount of high frequency signal current present in the cathode/anode circuit in direct proportion to the grid signal voltage.
This is the basis of the simplest form of Amplitude Modulation. Even with the advent of robust solid state devices, some high power radio frequency transmission systems still use valves in the output sections.
Well, I'd never heard of a modulating valve before, so I had a look on this site (might have to copy / paste if the link doesn't work):
http://www.mainpress.com/zonepumps/Pumps%20without
%20electricity.htm
The way it works sounds very clever - pity there wasn't a diagram. All that is required to power the pump is a supply of pressurised steam; the steam is channelled through a pipe to a box enclosing the pump mechanism, where it is blocked by a valve.
Water (to be pumped) flows into the pump box through a valve, that is to say, it cannot escape from the box the same way it got in. As it flows in, a floating part of the pump mechanism is forced upwards. When this floating part reaches a certain point, it snaps open the inlet valve for the pressurised steam, and closes the outlet valve for the steam.
[run out of space]
http://www.mainpress.com/zonepumps/Pumps%20without
%20electricity.htm
The way it works sounds very clever - pity there wasn't a diagram. All that is required to power the pump is a supply of pressurised steam; the steam is channelled through a pipe to a box enclosing the pump mechanism, where it is blocked by a valve.
Water (to be pumped) flows into the pump box through a valve, that is to say, it cannot escape from the box the same way it got in. As it flows in, a floating part of the pump mechanism is forced upwards. When this floating part reaches a certain point, it snaps open the inlet valve for the pressurised steam, and closes the outlet valve for the steam.
[run out of space]
[continued]
Obviously, high-pressure steam now rushes into the pump box (through the top of the box, I would think) and increases the pressure inside the box; the water inlet valve is now forced closed. Now both the water and the steam have only one exit from the pump: the water outlet valve. I imagine that this would be a finely adjusted valve, so that once the maximum pressure from the steam is reached inside the box, the valve will open, and out comes a jet of water (through, I suppose, the bottom of the pump).
As the water is squirting out, the floating thing is lowering back down towards the bottom of the pump box, and as it does it triggers the steam outlet valve to open and the steam inlet valve to close.
Now the steam rushes out of the newly opened valve into the atmosphere, and as it does the air pressure inside the box obviously falls back to normal. It was this high pressure that was in fact stopping water entering through the water inlet valve the whole time, so when the pressure goes, more water goes in and the whole thing repeats. Imagine inventing something like that!
Obviously, high-pressure steam now rushes into the pump box (through the top of the box, I would think) and increases the pressure inside the box; the water inlet valve is now forced closed. Now both the water and the steam have only one exit from the pump: the water outlet valve. I imagine that this would be a finely adjusted valve, so that once the maximum pressure from the steam is reached inside the box, the valve will open, and out comes a jet of water (through, I suppose, the bottom of the pump).
As the water is squirting out, the floating thing is lowering back down towards the bottom of the pump box, and as it does it triggers the steam outlet valve to open and the steam inlet valve to close.
Now the steam rushes out of the newly opened valve into the atmosphere, and as it does the air pressure inside the box obviously falls back to normal. It was this high pressure that was in fact stopping water entering through the water inlet valve the whole time, so when the pressure goes, more water goes in and the whole thing repeats. Imagine inventing something like that!
Phone a company called SEVERN GLOCON in Gloucester.. ask for the technical dep't. failing that, call Flowserve Ltd 01444 314400 and ask for the VALTEK division... they'll tell you everything you could ever want to know about modulating control valves.
( Sometimes referred to as Modulating Globe Valves or Control-Globes. You may also hear reference to Tiger-Tooth Valves and some other terminology. )
If you want to get some information on "modulating flow control valves" ( Larner-Johnson or Howell-Bunger types of design ), which are effectively energy-dissipation or anti-cavitation valves, e-mail me on [email protected], I'll be happy to help you.
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