Pressurized Systems that Pump Downhill
Pumping downhill can give swings in pressure that are not found on systems that pump uphill or on a level plain. These swings in pressure happen as flow is increased or decreased at the lower level. With some of the pressure at the lower level made up by gravity and not the pump, changes in flow are not instantly seen as a change in pressure or flow at the top of the hill where the pump is located.
When there is a no flow condition and the system has stabilized, there may be 60 PSI at the bottom and only 20 PSI at the top of the hill. When there is a tap opened at the lower level, it may take some time before a drop in pressure or increase in flow is noticed at the upper level. Gravity is feeding the tap at the lower level and the water in the pipeline starts moving downhill to feed the tap. As this water falling downhill is replaced by water further uphill, it is similar to waiting for the slack to pulled out of a tow chain. Eventually the slack will be pulled out of the chain and the chain link at the top of the hill will feel the pull. Only when this last link feels the pull will pressure at the top of the hill begin to decrease and flow increase.
When the pressure at the top of the hill decreases a certain amount, the pump is started. Water in the line has been slipping downhill for sometime before the pump is started. There could have been several hundred gallons expressed from the tap before the pump was started. After the pump starts it will try to bring the pressure up at the top of the hill. The pump must replace the amount of water that has been used from the tap and push the slack back into the chain (so to speak) before the pressure will begin to increase at the top of the hill. Now the pump may be pumping several hundred gallons per minute, even though the tap at the bottom of the hill is only using 20 GPM for example. The water is moving downhill at a considerable velocity. Once this volume of water has replenished the amount that has already been used, a pressure wave will start up the hill as the slack is being pushed back into the chain. This pressure wave will increase in speed, as it gets closer to the top of the hill. The pump is still pumping downhill as the pressure wave is moving uphill. Only when the pressure wave reaches the pump will the pressure increase and the pump realize that no more water is needed.
Even if the pump controls are fast enough to quickly shut down flow from the pump, pressure will spike as the upward moving pressure wave hits the downward moving flow from the pump. The pressure wave runs into a dead end, pressure spikes, and the pressure wave bounces back downhill. At the bottom of the hill there is only a 20 GPM tap open. This is effectively another dead end for the pressure wave so it bounces back up the hill again. This process can be repeated quickly several times. The intensity of the wave usually decreases each time it bounces and will eventually stabilize. A pressure gauge at the top or the bottom of the hill could swing from very high, back to very low or even to a negative pressure. A surge tank placed at the top and bottom of the hill should help. However these tanks could absorb the pressure wave energy and then express this energy back the other way. This can reduce the pressure peaks but, might take longer to stabilize as the tanks continue to bounce energy back to each other.
The pump can also help continue the bouncing pressure wave. Every time there is a low pressure or negative wave at the upper level, the pump sees a demand and increases flow downhill. When the wave comes back uphill the pump cannot pump against that much pressure and flow is stopped. The pump is adding to the downhill wave each time, and the uphill wave is bounced back by check valves.
A surge tank at the top of the hill might help and could be used in conjunction with a fast opening, slow closing pressure relief. In many cases water would not even need to be pumped downhill if the water line was large enough that friction loss at high flow was avoided. Extremely slow closing taps at the bottom of the hill will also help. A pressure relief at the bottom of the hill will also help stop the pressure wave. The pressure relief at the bottom should also be fast opening but slow closing, and both pressure relief valves will undoubtedly continue to waste water for sometime until the pressure wave ceases.