The object of this technique is to deliver the same flow rate to all laterals in the terraced distribution without the need for balancing valves. In a conventional system the absence of balancing valves will allow more vertical head and flow rate to develop in the transfer pipes to laterals at lower elevations. The result is system imbalance.

Principle of Operation:
Consider two identical vertical cylinders, one is filled with water, the other with air. The pressure head at the bottom of the water cylinder will be the pressure at the top plus the added pressure from the weight of the water column in the cylinder.
The pressure at the bottom of the air cylinder will be exactly the same as the pressure at the top because the air has no weight to add to the pressure at the bottom.

Now, consider a small flow of water entering the top of the air cylinder. If the flow is such that it will just pour down the inside wall of the cylinder and flow out the bottom (without forming any fully flooded slugs), the pressure in the cylinder at the bottom will be the same as at the top. The trapped air in the cylinder will form a pressure bubble and prevent head build up in the cylinder as the flow descends.
Similarly, water flowing down an open channel at atmospheric pressure arrives at the other end still at atmospheric pressure.

The pressure bubble principle operates the same way except the ‘open channel’ is at a pressure higher than atmospheric pressure. The pressure is induced by the effluent flowing into a fully flooded transport pipe from a dosing tank at a higher elevation to the distribution manifold. The pressure at the manifold is commonly referred to as to the residual head.
The trapped bubble in the transfer pipes to laterals at lower level prevents the transfer pipe from flooding and building up static head. The pressure bubble effect is created by selecting a pipe diameter several times larger than the diameter of the laterals. This larger diameter transfer pipe allows the effluent to flow down the slope in the bottom of the pipe to the next lateral at the same residual head as the manifold.

Practical Considerations:

With the pressure bubble technique, the squirt height from the laterals further down the slope will be theoretically the same as the squirt height from the uppermost lateral thus requiring no balancing valves. In current practice, quarter turn ball valves or gate valves are installed on each lateral for balancing, rebalancing and maintenance.

Using pressure bubble natural balancing, quarter turn ball valves can be used for shut-off only as no rebalancing will be necessary when the valves are reopened.

Theoretically means the length of the pressure bubble must extend from the exit at the manifold down to the connection to the respective lateral. If there is a reduced air volume in the bubble, a short fully flooded section of the transfer pipe will appear at the lower end, “adding back” static head in the transfer pipe.

The entire paper, including a diagram and performance profiles, is available for download.

John Richardson
PREMIER PLASTICS INC.
8328 River Way,
Delta, BC. V4G 1C4